\ Ooy^ vc^"^^ • THE MACMILLAN COMPANY NEW YORK • BOSTON • CHICAGO • DALLAS ATLANTA ■ SAN FRANCISCO THE INVERTEBRATA tA Manual for the Use of Students by L. A. BORRADAILE Fellonjo of Sekvyn College, Cambridge and F. A. POTTS Fellwo of Trinity Hall, Cambridge with Chapters by L. E. S. EASTHAM Professor of Zoology in the Uni-versity of Sheffield and J. T. SAUNDERS Felloe of Christ's College, Cambridge SECOND EDITION NEW YORK: THE MACMILLAN COMPANY CAMBRIDGE, ENGLAND: AT THE UNIVERSITY PRESS 1935 / Copyright, 1935, by THE MACMILLAN COMPANY All rights reserved — no part of this book may be reproduced in any form without permission in writing from the publisher, except by a reviewer who wishes to quote brief passages in connection with a review written for inclusion in magazine or newspaper. PRINTED IN THE UNITED STATES OF AMERICA PREFACE TO THE FIRST EDITION This book is intended for the use of students who have completed a year's study of the principles of zoology and of the anatomy and physiology of a series of invertebrate types such as is provided by any of several elementary textbooks in use in this country. The types commonly included in these books — various Protozoa, Hydra , Ascaris^ and the Liver Fluke, Earthworm, Leech, Crayfish, Cockroach, Pond Mussel, and Starfish — are not here described in detail. We have endeavoured to provide the student with a classification of the Invertebrata which proceeds as far as is usual in an honours course, with a concise statement of the characteristic features of each of the groups mentioned, and with a more detailed statement and discussion of matters of importance or interest concerning them. The choice of examples has been difficult, and we have not always been able to include all those we should have wished, but a fairly full account of certain representative genera has been given. The writing of the book has been shared among us as follows: Chapters I-IV, X, XI (except Onychophora), XII, XVIII and XIX have been written by L. A. Borradaile, Chapters V (except Cteno- phora), VII-IX, XIII, XV-XVII, and the Onychophora in Chapter XI by F. A. Potts, Chapter VI and the Ctenophora in Chapter V by J. T. Saunders, and Chapter XIV jointly by F. A. Potts and L. E. S. Eastham, but each of us has read and criticized the work of the others. We desire to express our grateful thanks for valuable advice and criticism to Dr S. J. Hickson, Professor D. Keilin and Dr S. M. Manton; for much care bestowed upon the illustrations to Messrs A. P. Hayle, J. F. Henderson, and C. F. Pond; and for valuable assistance in the preparation of the index and in other matters to Mr B. Newman. For permission to reproduce illustrations acknowledgment is due to Professor G. H. F. Nuttall; Messrs Geo. Allen & Unwin, Ltd. (Textbook of Zoology, Sedgwick) ; Messrs A. & C. Black, Ltd. (Treatise on Zoology, Lankester); the Council of the Cambridge Philosophical Society (Biological Reviews); Cambridge University Press (The De- termination of Sex, Doncaster, Plant Biology, Godwin, Ciliary Move- ment, Gray, Zoology, Shipley and MacBride, Primitive Animals, Smith, Palceontology , Wood); Herrn Gustav Fischer, Jena (Ergebnisse u. Fortschritte der Zoologie, Lehrbuch der Protozoenkunde) ; Herren Walter de Gruyter & Co. (Handbuch der Zoologie) ; the Council of the Linnean Society of London (Zoological Journal) ; Messrs Macmillan & Co., Ltd. (Cambridge Natural History , Harmer and Shipley, Human vi PREFACE Protozoology^ Hegner and Taliaferro, Textbook of Comparative Anatomy, Lang, Textbook of Zoology , Parker and Haswell); Messrs Methuen & Co., Ltd. (Textbook of Entomology , Imms); Oxford Uni- versity Press {The Animal and its Environment and Manual of Zoology, Borradaile). Acknowledgment to the authors of the works from which these illustrations are taken is made in the legends. THE AUTHORS CAMBRIDGE February, 1932 NOTE TO THE SECOND EDITION The book is now eighty pages longer. The additional matter is chiefly in Chapters IV and XIV. In other chapters a number of smaller additions and corrections have been made to the text and to figures. Each chapter has been revised by its writer, but the revision has been submitted to the other authors of the book. Our grateful thanks for various assistance are due to Drs A. M. Bidder, O. M. B. Bulman, S. M. Manton, and C. F. A. Pantin, and to Messrs L. E. R. Picken, J. D. Robertson, and P. Ullyott. Dr A.M. Bidder very kindly communicated to us certain facts, as yet unpub- lished, which are stated in the second half of p. 595. THE AUTHORS CAMBRIDGE February, 1935 TABLE OF CONTENTS! CHAPTER I Introduction: The Invertebrata page i CHAPTER n Subklngdom Protozoa 7 Phylum Protozoa] Class Mastigophora (Flagellata) 45 Subclass Phytomastigina 46 Order Chrysomonadina 50 Order Cryptomonadina 50 Order Euglenoidina 52 Order Chloromonadina 54 Order Dinoflagellata 54 Order Volvocina 56 Subclass Zoomastigina 58 Order Rhizomastigina 60 Order Holomastigina 60 Order Protomonadina 63 Order Polymastigina 65 [Suborder Polymastigina s.str'. [Suborder Diplomonadina [Suborder Hypermastigina Class Sarcodina (Rhizopoda) 68 Order Amoebina 68 Order Foraminifera 72 Suborder Monothalamia 72 Suborder Polythalamia 74 Order Radiolaria 76 [Suborder Peripylaea (Spumellaria)] [Suborder Actipylaea (Acantharia)] [Suborder Monopylaea (Nassellaria)] [Suborder Tripylaea (Phaeodaria)] Order Heliozoa 83 Order Mycetozoa 86 Class Sporozoa 87 Subclass Telosporidia ^ 88 Order Coccidiomorpha 88 Suborder Coccidia 89 Suborder Haemosporidia 91 ^ The names of groups which are mentioned in the text but not under a separate heading are here placed in square brackets. (^orn vm TABLE OF CONTENTS Order Gregarinidea page 93 Suborder Schizogregarinaria 93 Suborder Eugregarinaria 95 Appendix : Piroplasmidea 98 Subclass Neosporidia 100 Order Cnidosporidia 100 [Suborder Myxosporidia] [Suborder Microsporidia] [Suborder Actinomyxidea] Order Haplosporidia 102 Order Sarcosporidia 102 Class Ciliophora 102 Subclass Ciliata 102 Order Holotricha (Aspirigera) 106 Suborder Prociliata 106 Suborder Astomata 107 Suborder Gymnostomata 107 Suborder Vestibulata (Hymenostomata) 109 Order Heterotricha 109 Suborder Polytricha 109 Suborder Oligotricha no [Tribe Tintinnina] [Tribe Entodiniomorpha] Order Hypotricha III Order Peritricha III Order Chonotricha 114 Subclass Suctoria 114 CHAPTER III Subkingdom Parazoa 117 [Phylum Porifera] Class Calcarea 126 Class Hexactinellida 126 Class Demospongiae 126 [Order Tetractinellida] [Order Monaxonida] [Order Keratosa] [Order Myxospongiae] CHAPTER IV Subkingdom Metazoa 128 TABLE OF CONTENTS IX CHAPTER V Phylum Coelenterata page 146 Subphylum Cnidaria 152 Class Hydrozoa 153 Order Calyptoblastea 154 Order Gymnoblastea 154 Order Hydrida 154 Order Trachylina 164 Suborder Trachomedusae 164 Suborder Narcomedusae 164 Order Hydrocorallinae 165 Order Siphonophora 166 Order Graptolithina 169 Class Scyphomedusae (Scyphozoa) 172 [Order Stauromedusae] [Order Discomedusae] Class Actinozoa (Anthozoa) 180 Order Alcyonaria 180 Order Zoantharia 186 Subphylum Ctenophora 193 [Class Ctenophora] [Order Tentaculata] [Order Nuda] CHAPTER VI Acoelomate Triploblastica 197 Phylum Platyhelminthes 198 Class Turbellaria 213 Order Acoela 213 Order Rhabdocoelida 214 Order Tricladida 214 [Suborder Paludicola [Suborder Maricola] [Suborder Terricola] Order Polycladida 215 Class Trematoda 216 [Order Temnocephalea] Order Heterocotylea 218 Order Malacocotylea 220 Class Cestoda 223 Order Monozoa 225 TABLE OF CONTENTS Order Merozoa P^^^e 225 [Suborder Tetraphyllidea] [Suborder Diphyllidea] [Suborder Tetrarhynchidea] [Suborder Pseudophyllidea] [Suborder Cyclophyllidea] CHAPTER VII Phylum Nemertea ^33 Phylum Rotifera J^ Phylum Gastrotricha ^^ CHAPTER VIII Phylum Nematoda ^ Phylum Nematomorpha ^57 Phylum Acanthocephala 5 CHAPTER IX [Coelomata] ^^^ Phylum Annelida ^^^ Class Chaetopoda Order Polychaeta ^^^ Order Oligochaeta ^^^ Class Archiannelida ^94 Class Hirudinea ^^^ Class Echiuroidea 3 Class Sipunculoidea 3 4 CHAPTER X Phylum Arthropoda CHAPTER XI 305 Subphylum Onychophora 3^7 Subphylum Trilobita 3 3 CHAPTER XII Subphylum Crustacea ^2 Class Branchiopoda 353 Order Anostraca 35 [Order Lipostraca] TABLE OF CONTENTS xi Order Notostraca p(^g^ 360 Order Diplostraca 362 Suborder Conchostraca 362 Suborder Cladocera 362 [Tribe Ctenopoda] [Tribe Anomopoda] [Tribe Onychopoda] [Tribe Haplopoda] Class Ostracoda 368 Class Copepoda 370 Class Branchiura 376 Class Cirripedia 376 Order Thoracica 377 Order Acrothoracica 382 Order Apoda 382 Order Rhizocephala 382 Order Ascothoracica 385 Class Malacostraca 386 Subclass Leptostraca 390 Subclass Hoplocarida (Stomatopoda) 391 Subclass Syncarida 392 Subclass Peracarida 393 Order Mysidacea 393 Order Cumacea 393 Order Tanaidacea 395 Order Isopoda 395 Order Amphipoda 400 Subclass Eucarida 403 Order Euphausiacea 403 Order Decapoda 404 [Suborder Penaeidea] [Suborder Caridea] [Suborder Palinura] [Suborder Astacura] [Suborder Anomura] [Suborder Brachyura] CHAPTER XIII Subphylum Myriapoda 418 Class Chilopoda 418 Class Diplopoda 422 [Class Pauropoda] [Class Symphyla] xii TABLE OF CONTENTS CHAPTER XIV Subphylum Insecta (Hexapoda) page 425 Class Apterygota 463 Order Thysanura 463 Order CoUembola 463 Order Protura 465 Class Pterygota 466 Subclass Exopterygota 466 Order Orthoptera 466 [Suborder Cursoria] [Suborder Saltatoria] Order Dermaptera 468 Order Isoptera 469 Order Plecoptera 471 Order Embioptera 471 Order Psocoptera 471 Order Odonata 472 [Suborder Zygoptera] [Suborder Anisoptera] Order Hemiptera or Rhynchota 474 [Suborder Heteroptera] [Tribe Cryptocerata] [Tribe Gymnocerata] [Suborder Homoptera] [Tribe Auchenorhyncha] [Tribe Sternorhyncha] Order Ephemeroptera 481 Order Mallophaga 483 Order Anoplura 483 Order Thysanoptera 485 Subclass Endopterygota (Holometabola) 485 Order Neuroptera 485 Order Mecoptera 486 Order Trichoptera 486 Order Lepidoptera 487 [Suborder Homoneura] [Suborder Heteroneura] Order Coleoptera 492 [Suborder Adephaga] [Suborder Polyphaga] Order Hymenoptera 495 [Suborder Symphyta] [Suborder Apocrita] TABLE OF CONTENTS Xlll Order Diptera page 504 [Suborder Orthorrhapha] [Suborder Cyclorrhapha] Order Aphaniptera 512 Order Strepsiptera 5H CHAPTER XV Subphylum Arachnida 515 Class Scorpionidea 520 Class Eurypterida . 524 Class Xiphosura 526 Class Araneida 530 Class Acarina 533 [Order Notostigmata] [Order Cryptostigmata] [Order Prostigmata] [Order Stomatostigmata] [Order Heterostigmata] [Order Parastigmata] [Order Mesostigmata] [Order Metastigmata] Class Phalangida 537 [Class Palpigradi] [Class Pedipalpi] [Class Pseudoscorpionidea] [Class Solifugae] Class Pantopoda (Pycnogonida) 538 Incertae sedis: Class Tardigrada 539 Class Pentastomida 541 CHAPTER XVI Phylum Mollusca 543 Class Amphineura 547 Order Polyplacophora 547 Order Aplacophora 548 Class Gasteropoda 550 Order Streptoneura (Prosobranchiata) 563 Suborder Diotocardia (Aspidobranchiata) 563 (564)^ Tribe Rhipidoglossa 563 Tribe Docoglossa 563 ^ References in brackets are to the pages on which examples are described. Xll TABLE OF CONTENTS CHAPTER XIV Subphylum Insecta (Hexapoda) page 425 Class Apterygota 463 Order Thysanura 463 Order CoUembola 463 Order Protura 465 Class Pterygota 466 Subclass Exopterygota 466 Order Orthoptera 466 [Suborder Cursoria] [Suborder Saltatoria] Order Dermaptera 468 Order Isoptera 469 Order Plecoptera 471 Order Embioptera 471 Order Psocoptera 471 Order Odonata 472 [Suborder Zygoptera] [Suborder Anisoptera] Order Hemiptera or Rhynchota 474 [Suborder Heteroptera] [Tribe Cryptocerata] [Tribe Gymnocerata] [Suborder Homoptera] [Tribe Auchenorhyncha] [Tribe Sternorhyncha] Order Ephemeroptera 481 Order Mallophaga 483 Order Anoplura 483 Order Thysanoptera 485 Subclass Endopterygota (Holometabola) 485 Order Neuroptera 485 Order Mecoptera 486 Order Trichoptera 486 Order Lepidoptera 487 [Suborder Homoneura] [Suborder Heteroneura] Order Coleoptera 492 [Suborder Adephaga] [Suborder Polyphaga] Order Hymenoptera 495 [Suborder Symphyta] [Suborder Apocrita] TABLE OF CONTENTS Xlll Order Diptera page 504 [Suborder Orthorrhapha] [Suborder Cyclorrhapha] Order Aphaniptera 512 Order Strepsiptera 514 CHAPTER XV Subphylum Arachnida 515 Class Scorpionidea 520 Class Eurypterida . 524 Class Xiphosura 526 Class Araneida 530 Class Acarina 533 [Order Notostigmata] [Order Cryptostigmata] [Order Prostigmata] [Order Stomatostigmata] [Order Heterostigmata] [Order Parastigmata] [Order Mesostigmata] [Order Metastigmata] Class Phalangida 537 [Class Palpigradi] [Class Pedipalpi] [Class Pseudoscorpionidea] [Class Solifugae] Class Pantopoda (Pycnogonida) 538 Incertae sedis: Class Tardigrada 539 Class Pentastomida 541 CHAPTER XVI Phylum Mollusca 543 Class Amphineura 547 Order Polyplacophora 547 Order Aplacophora 548 Class Gasteropoda 550 Order Streptoneura (Prosobranchiata) 563 Suborder Diotocardia (Aspidobranchiata) 563 (564)^ Tribe Rhipidoglossa 563 Tribe Docoglossa 563 ^ References in brackets are to the pages on which examples are described. XIV TABLE OF CONTENTS Suborder Monotocardia (Pectinibran- chiata) p(^ge 563 (565) Tribe Rachiglossa 563 Tribe Taenioglossa 563 Subtribe Platypoda] Subtribe Heteropoda] Tribe Toxiglossa 563 Order Opisthobranchiata 567 Suborder Tectibranchiata 567 Suborder Nudibranchiata 567 Order Pulmonata 569 Suborder Basommatophora 570 Suborder Stylommatophora 570 Class Scaphopoda 572 Class Lamellibranchiata 573 Order Protobranchiata 579 (582) Order Filibranchiata 579 (583) Order Eulamellibranchiata 579 (585) Order Septibranchiata 579 Class Cephalopoda (Siphonopoda) 587 Order Dibranchiata 588 Suborder Decapoda 588 Tribe Belemnoidea 588 Tribe Sepioidea 588 Tribe Oegopsida 588 Tribe Myopsida 589 Suborder Octopoda 589 Order Tetrabranchiata 602 Suborder Nautiloidea 602 Suborder Ammonoidea 602 CHAPTER XVII Phylum Polyzoa 606 Class Endoprocta 612 Class Ectoprocta 613 Order Phylactolaemata 613 Order Gymnolaemata 613 Suborder Cyclostomata 613 Suborder Cheilostomata 613 Suborder Ctenostomata 613 Phylum Brachiopoda 613 Class Ecardines 618 Class Testicardines 618 Phylum Chaetognatha 618 Phylum Phoronidea 622 TABLE OF CONTENTS XV CHAPTER XVIII Phylum Echinodermata P^g^ 623 [Subphylum Eleutherozoa] Class Asteroidea 634 Class Ophiuroidea 638 Class Echinoidea 640 Order Endocyclica 647 Order Clypeastroida 647 Order Spatangoida 647 Class Holothuroidea 648 Order Aspidochirotae 652 Order Pelagothurida 652 Order Elasipoda 652 Order Dendrochirotae 652 Order Molpadida 652 Order Synaptida (Paractinopoda) 652 [Subphylum Pelmatozoa] Class Crinoidea 654 Class Amphoridea 658 Class Carpoidea 658 Class Thecoidea (Edrioasteroidea) 658 Class Cystoidea 658 [Order Diploporida] [Order Rhombifera] Class Blastoidea 659 CHAPTER XIX Phylum Chordata 660 Subphylum Hemichorda (Enteropneusta s.lat.) 662 [Class Enteropneusta (Balanoglossida)] [Class Pterobranchia] Subphylum Tunicata (Urochorda) 669 Class Larvacea 679 Class Ascidiacea 680 Class Thaliacea 681 Order Pyrosomatida (Luciae) 685 Order Salpida (Herhimyaria) 685 Order Doliolida (Cyclomyaria) 686 [Subphylum Cephalochorda] [Subphylum Vertebrata] CHAPTER I INTRODUCTION The Invertebrata have long since ceased to constitute one of the primary divisions in the scientific classification of the Animal King- dom. Their name is now no more than a convenience for designating a group of phyla with which it is often necessary to deal as a whole. The primary lines of real cleavage in the Animal Kingdom divide it, not into Vertebrata and Invertebrata, but into three unequal sec- tions, the Protozoa, Parazoa and Metazoa, which are ranked in the following chapters as subkingdoms. Between the Protozoa, which are without cellular differentiation and contain a large group of photosynthetic members, and the Meta- zoa, in which such differentiation is always strongly marked and photosynthesis is absent, there is a gulf which is in fact far deeper than that which sunders the Protozoa from the lower plants. The view, indeed, has been put forward that these two components of the Animal Kingdom are not, as is usually held, directly related to one another, but arose, with the Plants, as entirely distinct branches of an ancestral stock of living beings. The Parazoa or sponges — unique among many-celled organisms in possessing collared flagellate cells — are probably derived from the Protozoa by an origin distinct from that by which the latter group gave rise (if they did so indeed) to the Metazoa. Within the Metazoa, the most significant difference is that which exists between the Coelenterata or Diploblastica and the triploblastic phyla which constitute the rest of the subkingdom. The Coelenterata, which typically start life as a simple, two-layered, ciliate larva, the planula, either retain throughout life the two-layered condition, or depart from it only by the immigration, late in development, of cells from the two primary layers (ectoderm and endoderm, p. 128) into the space (blastocoele) between those layers. The triploblastic animals always possess a true third layer (mesoderm) which is early developed and forms important organs. They are the great majority of animals, and compose a number of phyla. The brigading of these phyla is a difficult task — one, indeed, which it is at present impossible to effect completely. Two main stocks, however, stand out fairly clearly. The Annelida, Arthropoda and Mollusca — by the plan of their central nervous system, the mode and position of origin of their mesoderm, the types of cleavage of the ovum (p. 281) and of larva (the ^roc^oj'/)^er^) which the Annelida and Mollusca 2 THE INVERTEBRATA share, and the presence of a cuticle and segmentation which the Anne- lida and Arthropoda have in common — constitute one of these stocks. The other comprises the Echinodermata and Chordata. Its members have a central nervous system which is not on the annelid plan and is peculiar in retaining its epithelium (p. 136); they exhibit a common mode of origin of the mesoderm, primitively as hollow pouches, from the gut wall ; they possess, or give indications of, three primary meso- dermal segments ; the cleavage of their ova is entirely different from that which is characteristic of the Annelida and MoUusca, and between the larvae of the lowest chordates (the Enteropneusta) and those of certain echinoderms there is a remarkable and detailed re- semblance. The remaining phyla, smaller and less important, are hard to relate either to the foregoing groups or to one another. By the type of cleavage of their ova and the possession of flame cells (p. 202), the Platyhelminthes and Nemertea seem to be akin to the annelid stock. Their lack of coelom is a difficulty in this respect. The structure of the adults of the Rotifera and of the larva of the Polyzoa, which has the character of a trochosphere, might link these groups to the same stock. Some other small phyla (Brachiopoda, Chaetognatha) have possibly distant relationship to the echinoderm-chordate grouping. Others, notably the Nematoda, are more difficult to place. In the great assemblage of triploblastic phyla, the backboned animals, or Vertebrata properly so-called, stand as a branch of one phylum, the Chordata. Yet their considerable numbers, the size, high organization, and intelligent activity of their members, and the fact that Man is one of them, give them an importance so great that they have always been the subject of a distinct department of zoo- logical study, and were at one time regarded as a primary branch of the Animal Kingdom. That standing they have lost; but it is still necessary for many purposes to treat them apart. The term " Invertebrata" is retained to cover all the non-chordate phyla and the chordates other than the Vertebrata. In that sense it is used in this book. Only the Cephalochorda (Amphioxus), which, though they are not vertebrates, have much in common with those animals, are left aside as best studied with them. The limits of the several phyla are, with one or two exceptions, agreed among zoologists. As much cannot be said for the lower grades of the classification. Different views upon phylogeny, and considerations of convenience, lead to many divergences as to the extent and rank of the various divisions in the systems preferred by different authorities; and even when there is agreement as to the limits of a group different names may be applied to it. In no two works will quite the same arrangement be found. This fact should ORGANISM AND ENVIRONMENT 3 be borne in mind by the student in using the table of classification which will be found as the Table of Contents at the beginning of this book. In surveying the diverse organisms which constitute the Inverte- brata, the student should bear in mind the following principles. The most fundamental of the characteristics of living organisms is the way in which, in the face of an environment which presents as many dangers as opportunities, they hold their own by making ad- justments within themselves. This statement applies equally to the struggle for existence of the individual and to the slow racial adjust- ments which we know as evolution. The term environment^ is a collective name for all the external things which affect any living being. Four principal factors constitute the environment — the ground or "substratum" (if any) upon which the organism stands, the "medium" (water or air) which bathes it, the heat and light which it receives from the sun's rays or can lose to its surroundings, and the other organisms in its neighbourhood. Of these factors the substratum has in most cases relatively little im- portance, and we may dismiss it now. The medium, on the other hand, is of enormous importance. Meet- ing all parts of the surface of objects that it contains, it exerts every- where a pressure upon them, supports them, may transport them, affects the movements they execute, and controls all exchange, whether of matter or of energy, between them and the world about them ; and from it animals obtain their supplies of free oxygen, often of water, and sometimes of food. If it be liquid, according as the concentration of substances dissolved in it be greater or less than that within the organism water and solutes will tend to pass to or from the body of any animal which is not covered by an impermeable cuticle. This exchange is of the utmost importance, both as a danger by up- setting equilibria within the body and as an advantage by facilitating the excretion of substances which are harmful in the organism. It is controlled by the surface layer of protoplasm, which either is, or owing to surface tension behaves as, a delicate membrane that has the power of actively regulating, to some extent, the passage of substances through itself, and by the activity of the organs of excretion. If the medium be gaseous, according to the amount of water vapour it contains water will tend to evaporate to it from the surface of the body. This is important owing to the necessity for the intake of water by the ^ The student may occasionally be puzzled by the phrase "internal en- vironment". This bizarre contradiction in terms is sometimes applied to what we shall presently call the "internal medium" (p. 132). 4 THE INVERTEBRATA mouth to compensate for it, and also because the latent heat of the evaporation lowers the temperature of the body. Whether the medium be liquid or gaseous, it offers, according to the free gases it contains, varying possibilities of interchange of oxygen and carbon dioxide with organisms. This has naturally extremely important effects upon respiration. The loss or gain of heat tends, of course, to affect the temperature within organisms, and with this the chemical processes of the latter vary, being, as is usual in such processes, slowed as the protoplasm becomes colder and quickened when it is warmed, and being brought finally to a stop when its minute organization is destroyed either by the coagulation of certain of its proteins by heat or by the freezing of its water. Every organism is tuned to work within a range of tem- perature peculiar to it. "Warm-blooded" animals keep their tem- perature within proper limits by active chemical and physical means; "cold-blooded" animals (to which all invertebrata belong) are in this respect at the mercy of their surroundings except in so far as they can circumvent them by their habits. Light, while it is essential for photosynthetic organisms, has chemical effects of importance in many others, and in all which possess sense organs capable of appreciating it is a source of stimuli from the external world. Relations between an animal and other organisms in its surround- ings are almost always based in the long run on nutrition. Either such organisms serve the animal for food, or they attack it to make it their food, or they are competitors for a common food-supply, or in rarer cases they assist it, or obtain its assistance, in the quest for food or in defence against enemies which would use it or them for food. Only between members of opposite sexes of the same species are there relations of another kind, namely those which are concerned with reproduction. The coming of organisms into relation with one another usually involves the receipt of stimuli and more or less complicated behaviour, with the use of organs of locomotion and prehension. The action of the environment upon the organism will be seen to be threefold: (i) it affects it mechanically, as by transporting it from place to place, by the impact of adjacent objects, or by the attacks of enemies ; (2) it affects the working of the living machine by the com- pulsory introduction or abstraction of materials (water, salts, etc.) or of energy ; (3) it directly stimulates it to activity, which may be an in- evitable response, such as the movement of certain organisms towards light (phototaxis) or be dependent upon conditions existing at the moment in the organism ; or it may inhibit such activity. Besides such action the environment may affect the organism negatively, by failure in respect of food, oxygen, or some other necessity which the organism is dependent upon obtaining from its surroundings. Where such ORGANISM AND ENVIRONMENT 5 failures occur from time to time the organisms have usually means of enduring them (reserve stores, resting stages, etc.). In proportion as the organism is unable to resist these influences of the environment it is liable upon occasion to be harmed by them. The process of evolution has been the development of organisms in such a way as to set them free from such influences in respect of their proper environments. Its results may be classed under three heads, (i) Some, such as the acquirement of a cuticle or of a habit of burrow- ing or of hibernation, merely fend off or avoid the action of the en- vironment : these involve the least increase in the complexity of the organism. (2) Others, such as the formation of organs for the ex- cretion of the excess of water, provide for remedial action: in these, as a rule, more complicated machinery is formed. (3) Others, such as the development of a nervous system or of organs of locomotion or weapons of oifence, bring it about that the action which results from the receipt of stimuli is turned to the best advantage by the organism: these cause a considerable, often a very great, complication of the living machine. Thus a general outcome of evolution is the forming of more com- plex, that is of " higher", organisms. But a relatively simple organism may, in its proper environment, enjoy as much autonomy as in other circumstances is possessed by one that is more highly organized. This is notably true of many parasites. Some of the results of evolution, as for instance the formation of a nervous system or of a cuticle, are such as to increase the independence of the organism in all circumstances. Others, however, such as the substitution of pulmonary for branchial respiration, or of absorption for ingestion of food, are of value only in particular environments or modes of life, and even unfit the organism for other ways of living. Thus two distinct phenomena underlie the diversity of the Animal Kingdom — an increase in the autonomy of the individual, and the specialization of animals for particular modes of life. Every species, however good a fight it maintains, is threatened with extinction owing to the continual loss of individuals, always by the action of its environment and usually also by that "natural death" which appears to await all organized protoplasm that is not periodic- ally reorganized.^ In reproduction^ howev^er, the individual provides by fission for the maintenance of its race. In the lower organisms the protoplasm of the body retains ascertain plasticity, and in these there is very often an asexual process of reproduction in which the new construction that is necessary to organize at least one of the products ^ See p. 27. It is possible that in some of the least highly organized metazoa natural death either is no more inevitable than in protozoa or is long delayed. 6 THE INVERTEBRATA of fission, and often goes on in both, is carried out with cells (or, in protozoa, with organized protoplasm) which existed as such in the parent. In more highly organized animals the only protoplasm which retains the required plasticity is that of the germ cells, and conse- quently such animals have only the sexual reproduction which these cells perform. The germ cells (gametes), before they reconstitute the adult body, normally undergo the process known as conjugation or syngamy, which is not an essential part of the reproductive process but a provision for heritable variation whereby the race becomes adaptable to its surroundings. Conjugation can only be performed by uninucleate individuals and therefore, while in protozoa it sometimes takes place between adults (hologamy, p. 31), in metazoa it always requires the production of uninucleate young (the ova and sperma- tozoa). The lower metazoa reproduce both by means of these gametes and also asexually . In the higher animals, as we have seen, reproduction is solely by gametes, though conjugation may be suspended for one or more generations by the development of unfertilized ova (partheno- genesis). CHAPTER II THE SUBKINGDOM PROTOZOA The Protozoa are sundered from the rest of the Animal Kingdom by a perfectly sharp distinction. The distinction consists in this : that in the body of a protozoon, whether there be one nucleus, or a few, or many, no nucleus ever has charge solely of a specialized part of the cytoplasm; whereas in other animals there are always many nuclei, each in charge of a portion of cytoplasm which is specialized for a particular function, such as contraction, or conduction, or secretion. Stated thus, the definition of the Protozoa is quite unambiguous. Unfortunately, ambiguity is usually imparted to this subject by the introduction of a concept, that of the "cell", which has a different extension for different authorities. If that concept, primarily of use in other connections, is to be introduced here, we must frame our definition in one of two ways, according to the meaning which we attach to the word "cell". If we apply this term to every nucleus together with its cytoplasm, we must define the Protozoa as " animals which consist of one cell or of several cells which are all alike, save sometimes the reproductive cells ". If, on the other hand, we give the term "cell" its earlier extension, applying it only to the specialized units of nucleus and cytoplasm which together compose the bodies of the higher animals and plants, we shall define the Metazoa as "cellular animals" and the Protozoa as "non-cellular". It will then be convenient to employ the term "energid" for application to any nucleus with its cytoplasm, whether they together constitute the body of a protozoon or a cell of a metazoon. In any case the facts remain the same, and they provide one of the main sources of the interest which the study of the Protozoa offers, namely the carrying out of the processes of life, and often of a complex life, by an organization which, though it may visibly be of correspond- ing complexity, is purely cytoplasmic. Considered in this light the structure of, for instance, the more complicated ciliates and flagellates is exceedingly instructive. In three other respects the Protozoa are peculiarly interesting. In their bodies dead " formed " material, how- ever plentiful it be as a covering or scaffold for the body, never as- sumes the importance which it has as ground substance or skeleton in the Metazoa, where the size of the body is such that the protoplasm cannot maintain its organization without support against forces that tend to deform it. Consequently, in observing the physiology and behaviour of a protozoon, we are seeing in the actual protoplasm of an 8 THE INVERTEBRATA intact organism processes which in an intact metazoon we observe as the activities of a complex in which protoplasm is masked and conditioned by other components of the body : in short, in the Protozoa we observe the normal activities of protoplasm more directly than in the Metazoa. Again, a life cycle comprising more than one generation, which is com- paratively rare among metazoa, is universal among protozoa, and its varieties are extraordinarily interesting. Finally, while every meta- zoon is thoroughly an animal, the Protozoa present an unbroken series from wholly plant-like organisms, through various intermediates, to members whose nutrition and behaviour are those of animals — or rather, as we shall see, there are several such series. The Protozoa are all of small size. Most of them are minute, ranging from a few thousandths of a millimetre to a little over one millimetre in length: a few reach dimensions of several, or even of many centi- metres, but these for the most part consist of a relatively thin layer of protoplasm (certain mycetozoa). With the small size of protozoa is probably to be connected, not only, as we have seen, the relative un- importance to them of dead skeletons, but also their characteristic type of organization. In larger organisms, the regions differentiated for special purposes must usually be correspondingly larger, and therefore require the services of nuclei of their own, the absence of which is the hall-mark of a protozoon. The actual size varies very much in each group. It is, on the average, least in the Mastigophora. The order of magnitude of certain representative species may be gathered from the approximate magnifications stated for figures below. The bodies of the Protozoa vary greatly in shape. Whereas in each of the metazoan phyla there is a fundamental type of body form to which the members of the phylum conform in essentials, however aberrant from it they be, the Protozoa have no such type. When the surface of the protoplasm is virtually fluid and is not retained by a shell, it takes, while it is at rest, a spherical form. When there is a firm surface layer, the individual tends, if it be a flagellate, to have an egg or spindle shape, if it be a ciliate to be bilateral with a spiral twist at one end, in the Suctoria to be cup-shaped. Concerning the body form of the Sporozoa, which are parasitic, no generalization can be made. Bodies of any of these shapes may be anchored, and have then usually a stalk, which may be of dead material as, for instance, in Acineta and Codosiga (Figs. I, 49), or a part of the living protoplasm. In the latter case it has generally a cuticular covering, as in Vorticella (Fig. 2), but may be naked (various flagellates). Stalked forms, and occasionally others, may be colonial; that is to say, a number of zooids, each having a nucleus and the shape and complete organization of an individual of related solitary species, are united by protoplasmic connections to form a single living being. The PROTOZOA ^vest. ^--myn Fig. 2. Fig. I. Fig. I. Two species of Actneta, x loo. After Saville-Kent. A, A. grandts. B, A. lemnarum. Fig. 2. Vorticella: a generalized figure of an individual, magnified, with a portion of the stalk further enlarged, c.vac. contractile vacuole; ci. outer ciliary wreath; ci.' inner wreath, of the peristome; cu. cuticle] f.vac. food vacuole; ?neg. meganucleus; mi. micronucleus ; myn. myonemes of bell; myn/ myoneme of stalk (containing elastic fibrils); pel. pellicle; ph. "pharynx" (terminal portion of vestibule); ppm. protoplasm of stalk; pst. peristome; res. reservoir; i/.?He. undulating membrane; vest, vestibule. lO THE INVERTEBRATA zooids of a colony are usually all alike, but differentiation may exist between them, in that certain of them are specialized for the produc- tion of new colonies, which is not performed by the other zooids (various volvocina. Figs. 3, 46 ; etc.). Colonies arise by the division of a single primary zooid, whose fission is not carried to completion, so that its products do not entirely separate. Their origin is therefore usually said to be a form of asexual reproduction. It may, however, also be looked upon from another point of view, as the repetition, within a continuous mass of protoplasm, of the nucleus and the other organs coincidentally. In this aspect, the colony is seen to have features in common with other multinucleate conditions of protozoa, such as (i) that oi Hexamitus (Fig. 4), etc., in which a unitary body has SomAttc cctU Fig. 3. Colonial volvocina. a, Eiidorina, a spherical motile colony of thirty- two similar zooids all capable of division, b, Pleodorina illinoiensis , a spherical motile colony consisting of thirty-two zooids, of which" four at one end of the colony constitute a "soma", which dies when the other twenty-eight zooids divide, c, Pleodorina californica. The "somatic cells" constitute about half the colony. After West and Fritsch. two similar sets of organs, one on each side of the body, or several sets, with a nucleus assigned to each, (2) that oi Polykrikos (Fig. 40 B), etc., in which there are several nuclei, and several sets of the other organs of the body, but the repetition (merism) of the nuclei and that of the other organs do not correspond, and (3) that of Opalina (Fig. 5), Actinosphaeriwn (Fig. 33), etc., in which there are numerous nuclei, but only one set of the other organs of the body. Multinucleate masses of protoplasm are known as syncytia. Syncytia which, like those cited above, arise by the division of an original nucleus in the mass of protoplasm are known as symplasts. An entirely different kind of syncytium arises by the union of uninucleate individuals, whose nuclei remain distinct in the resulting body. Such syncytia are known PROTOZOA II as Plasmodia. They are found in the Mycetozoa (Fig. 73) and occasionally elsewhere. Pseudocolonies, consisting of distinct individuals united only by stalks, tubes, etc., of dead material, are formed by various mastigo- phora (Fig. 38) and vorticellids. Fig. 5- Fig. 4. Hexamitus i?itestinalis, x 3800. After Dobell. axs. axostyle; ba.gr. basal granules of flagella; nu. nucleus. Fig. 5. Opalina ranarian, x 150. From Bronn. ecp. ectoplasm; 7iu. nuclei. The, protoplasm of living protozoa is often apparently homogeneous, apart from inclusions such as granules of reserves and other manu- factured substances, chromosomes, skeletal structures, and so forth. Sometimes, however, there is visible in it an apparent meshwork of 12 THE INVERTEBRATA a more viscid substance, with a more fluid constituent in its meshes. Actually, the structure is then that of a foam, and the meshwork is an optical section of the walls of bubbles or alveoli which contain the liquid constituent. In the nucleus the more viscid constituent is the linin meshwork, the liquid the karyolymph', in the cytoplasm the meshwork is the spongioplasm, and its contents the enchylema} The gelation to which this structure is due is produced by fixing reagents in many cases in which it does not exist in life. There is, perhaps, no fundamental distinction between the alveoli and the smaller of the spaces known as vacuoles of which so much use is made in the physio- logy of the Protozoa — for storage, as the site of chemical processes such as digestion, for drainage, for hydrostatic functions, etc. The largest vacuoles have often a definite wall of their own. The surface of the protoplasm is protected in various ways. {a) Sometimes, as in some amoebae, it is apparently quite fluid. Then, however, there exists upon it an extremely thin membrane, known as the plasmalemma, which has the power of regulating the exchange of materials between the organism and the watery medium in which it lives. Without this power the protoplasm would soon be poisoned or dissolved, {b) In other cases, the surface layer is semi- solidified (gelated) as a visible, firm, but living pellicle. This is often ** sculptured" in a pattern, as in Paramecium (Fig. 85 B, C). (c) Inter- mediate conditions connect the pellicle with the cuticle^ a. close-fitting dead membrane which may be nitrogenous, as in Monocystis, or of carbohydrate, as in many plant-like flagellates. In typical dino- flagellates (Fig. 40 A) it is composed of stout plates of cellulose. (d) Again, there may be a shell from which protoplasm can issue through an opening. Such a shell may be nitrogenous, as in Arcella (Fig. 59), etc., of a nitrogenous basis with foreign bodies built into it, as in Difflugia and Rhabdammina (Figs. 60, 6 A), of siliceous plates as in Euglypha (Fig. 7), calcareous, as in most foraminifera (Fig. 6 B, C), or of cellulose, as in the spores and sclerotium of the My- cetozoa. It is said that mineral shells always contain a groundwork of organic material. They are often composed of several chambers, and may be perforated by numerous small pores. Houses are loose- fitting, wide-mouthed shells (Fig. 38 C). Cysts are temporary shells without opening, {e) Finally, there may be an external lattice, which is pseudochitinous in Clathrulina (Fig. 8) and siliceous in the Silicoflagellata (Fig. 38 F), or a case of calcareous pieces (Coccolitho- phoridae. Fig. 38 E). The siliceous lattice of many radiolarians is part of an internal skeleton. The term ectoplasm is applied to any conspicuously differentiated outer layer of the protoplasm, and denotes very different conditions ^ The term hyaloplasm has been used in this, but also in other, senses. PROTOZOA 13 in different organisms — in Amoeba, a stratum which, save at its surface, is only unlike that below it in not containing granules; in various planktonic protozoa (Figs. 32, 33) a highly vacuolated layer Fig. 6. Shells of foraminifera. A, Rhabdarrmmia abyssorum, x 4-5. B, Nodo- saria hispida, x 18. C, Globigerina bulloides, x 55. After various authors. whose low specific gravity confers buoyancy; in the Ciliophora and many flagellates and sporozoa a stout pellicle with an underlying layer, the cortex, which is said to be stiffer than the internal protoplasm {endoplasm) and may exhibit differentiations of various kinds. H THE INVERTEBRATA Occasionally the protoplasm contains structures (trichocysts of ciliates and mastigophora, Fig. 9, so-called ''nematocysts" in certain dinoflagellates, Fig. 40, pole capsules of neosporidia, Fig. 82), from which threads can be shot out upon the surface of the body. The function of these threads is often doubtful, but it has been shown that the trichocysts of Paramecium are fixing organs, others which lie around the mouth of their possessor {Cyathomonas, Fig. 39 C; etc.) seize prey, and the pole capsules serve to anchor spores to the lining of the host's gut. The threads of " nematocysts " and pole capsules Fig. 7. Euglypha alveola ta, x 60. From Hegner and Taliaferro, after Calkins. are coiled up in vesicles before they are shot out ; those of trichocysts are formed by the stiflening of an extruded secretion. The motile organs of the Protozoa are of several kinds, each of which is mainly found in one of the classes of the phylum. Pseudopodia are temporary protrusions of protoplasm. They are of various types — blunt lobopodia (Figs. 54, 59), fine filopodia (Fig. 7), branching and anastomosing rhizopodia (Figs. 61, 65), and axopodia (Fig. 71) with an internal supporting filament. They are used in various ways and for various purposes. Their mode of formation is not fully understood, but it is clear that, at least in many cases, they do not arise, as has been PROTOZOA 15 alleged, by alterations in the surface tension of the protoplasm, and it is probable that the movement {amoeboid movement) in the course of which they are formed is not fundamentally different from the movements of muscles, or cilia, or flagella. Granules may often be seen to stream up and down the axopodia and rhizopodia. Flagella are lashes, long and usually few in number, which by a rowing (Fig. 10) or by an undulating motion (Fig. 11) draw or propel the body or attract particles to it. In the rowing stroke the flagellum is ^,^r«rjn. -^h'. -nuc. Fig. 8. Fig. 9. Fig. 8. Clathritlina. A, Ordinary individual, x 200. B, Binary fission within the lattice. C, Escaped fiagellate individual. After Leidy. Fig. 9. Part of a longitudinal section through a Paramecium showing the trichocysts at the end of the body discharged, and in the endoplasm material for the replacement of trichocysts. From Saunders, after Mitrophanov. 71UC. meganucleus; tr. undischarged trichocysts; tr.' discharged trichocysts; tr.m. material for replacing trichocysts. held rigid and slightly concave in the direction of the stroke; in recovering its position it bends as it is drawn back, so that less resistance is offered to the medium. When, as is usually the case, the flagellum beats obliquely, or the undulations pass around as well as along it, the body rotates as it advances, or if it be fixed a whirlpool is set up. Down each flagellum runs an internal thread, the axial filament^ which, on entering the body or at some distance within it, joins a basal granule.^ The latter is in most cases connected to the ^ This structure is sometimes called the blepharoplast, but as that name has also been applied to parabasal bodies its use is best avoided. .'-10 ^ -11 ■^--12 13 Fig. lo. Simplest type of movement of flagellum of Monas during rapid forward movement, a, 1-7. Successive stages in preparatory stroke. Note the flexure begins at the base and spreads to the tip. b, 8-13. Successive stages in the effective stroke. Note the rigidity of the flagellum. The arrow indicates the direction of movement of the organism. After Krijgsman. Fig. II. m Fig. 12. Fig. II. Peranema. a, Slow forward movement with undulations restricted to the tip of the flagellum. 6, Rapid forward movement with undulations along the whole length of the flagellum. After Verworn. Fig. 12. o, Flagellum of Tracheloynonos (Euglenoidina) showing axial filament and sheath. 6, Transverse section of the flagellum. After Doflein. PROTOZOA 17 nucleus by a thread or threads known as rhizoplasts (Fig. 47 A). Sometimes it lies against the nucleus. Rhizoplasts may connect it to other structures, notably in many parasitic flagellates to a body of unknown function called the parabasal body. The " kinetonucleus " of Trypanosoma (Fig. 47 E) is a body of this class, which possibly in- cludes structures of more than one kind. Sometimes, as in Trypano- soma^ a flagellum runs for some distance parallel with the surface of the body and is connected to it by a film of protoplasm known as an undulating membrane^ which must be distinguished from the structures of the same name which are formed by the fusion of cilia. When there are two flagella, it often happens that one is trailed behind the body and the other directed forwards (Figs. 47 D, 53 C, 70 B). Flagella are often used for anchoring, and sometimes appear to have a sensory function. Cilia are smaller and more numerous lashes which by a rowing action repeated by one after another of them in "metachronal rhythm " (Fig. 13) cause movements of the animal or of the water near it. Like flagella they have each an internal filament, a basal granule, and a rhizoplast, which, however, does not connect with the nucleus. Often cilia are united into compound organs, which may be conical am, paddle-like membranellae (Fig. 14), or undulating membranes (Fig. 90). Many protozoa which possess a definite body form are able temporarily to alter it by contractions of the protoplasm stretching the pellicle (metaboly), and in various cases this contractility is localized in fibrils, known as myonemes^ situated in the ectoplasm. Systems of fibres which ramify from a central mass known as the "motorium" and have been thought to be of the nature of a nervous system have been described in various ciliates ; in some of these cutting the fibres is said to destroy the co-ordination between different sets of ciliary organs. It is possible also that the rhizoplast system of flagellates may have a conducting function. Setise organs are possessed by various protozoa in the form of specialized flagella and cilia in which the tactile sense is highly developed, and by many of the plant- like flagellates as pigment spots (eye-spots), which may be provided with a lens. A chemical sense seems to be indicated by the fact that food is often recognized at a distance, and also probably in some of the cases of discrimination in ingestion (p. 19). Internal skeletal structures are found in many members of the phylum. They may be part of the living protoplasm, as the axial fibres of axopodia and the axostyles which lie in the midst of the body of various mastigophora (Trichomofias, Fig. 50; etc.) and probably the central capsules of the Radiolaria, or of dead inorganic matter, as the skeletons of the Radiolaria (Fig. 69). The Protozoa present every type of nutrition exhibited by organ- isms, except that of the "prototrophic" bacteria, which perform i8 THE INVERTEBRATA chemosynthesis by the use of energy obtained from reactions between inorganic substances. Holophytic nutrition/ however, is found, among protozoa, only in certain of the Mastigophora (see below). Of the holozoic members of the phylum, some feed by amoeboid action. Fig. 13- - - - vib. _«L .^ha.lam. ■- tl.fi. Fig. 14. ha.fi. Fig. 15- Fig. 13. Diagram illustrating the optical appearance given by a profile view of cilia beating in metachronal rhythm. After Verworn. Fig. 14. Three membranellae from the adoral wreath of Stentor. After Doflein. ha.fi. basal fibre of the rhizoplast system; ha.lam. basal lamellae; tl.fi. terminal fibres; vih. vibratile elements; the band beneath each of these represents the fused basal granules of the constituent cilia. Fig. 15. Paramecium, showing the motorium lying near the vestibule, and the fibrils which radiate outwards. After Rees. ^ The nutrition of an organism is said to be holophytic when it is eflfected, as in typical plants, by the building up of complex organic substances from simple inorganic ones by use of the energy of certain of the sun's rays (photo- synthesis). The radiant energy is obtained by means of the green, yellow, or brown structures known as " chromatophores " or "chromoplasts" (e.g. the PROTOZOA 19 Usually this can be done at any point of the surface, as in the familiar case of Amoeba^ but most of those flagellates which perform amoeboid ingestion do so in a particular region only. Other protozoa swallow through a permanent mouth. The true mouth is the spot at which the food passes below the ectoplasm. It may be (a) a bare patch of endoplasm, (b) the opening of an excavation (oesophagus) in the endoplasm, (c) the bottom of a depression (vestibule) in the ecto- plasm, (d) the junction of a vestibule and an oesophagus. Any passage, whether oesophagus, or vestibule, or compounded of both, through which food enters is called a gullet, though not all cavities to which this name is applied are actually used in feeding. The opening of a gullet is the cytostome, which when there is a vestibule is not the true mouth. Gullets are found in many of the Mastigophora and most of the Ciliata. In ciliates either of the kinds may be present (p . 1 04) . In the Mastigophora the gullet is at least sometimes ectoplasmic, but its morphology needs further investigation. A gullet may be supported by skeletal rods (Figs. 39 E, 89 A), and is then often dilatable: a vestibule may have ciliary apparatus, trichocysts, etc. for taking food (Figs. 39 C, 90). The Suctoria (Fig. i) draw the protoplasm of their prey into their bodies through tentacles. The details of ingestion into the protoplasm differ considerably in diflFerent organisms. In some amoeboid forms the cytoplasm comes into contact with the food at once, either by flowing over it or by its adhering to the surface and being drawn in ; others enclose the particles to be swallowed without touching them, either by arching over them, as Amoeba proteus does, or by excavating a vacuole for their reception. In some at least of the organisms whose food is driven into a gullet, a vacuole forms for it, apparently by the pressure of the water forced in, and on reaching a certain size nips off. Often, but by no means always, discrimination is exercised between particles which appear equally capable of being swallowed. It is doubtful whether this discrimination is concerned solely with such properties as the size and shape of the particles or also with their chemical qualities. Solid food is digested in food vacuoles, which usually contain visible fluid and in which the reaction is often first acid and then alkaline. Live food dies during the acid phase, and protein is digested during the alkaline phase. Protozoa have chloroplasts of green plants). In this mode of nutrition the simple materials of the food are absorbed through the surface of the body. In holozuic nutrition complex organic substances are swallowed through temporary or permanent openings as in the majority of animals. In saprophytic (or saprozoic) nutrition, practised by certain organisms, including among others various parasites, which are in contact with solutions of organic matter, relatively complex carbon compounds are taken, but these are absorbed through the body sur- face. The modes of nutrition classed under this head vary greatly in the complexity of the substances they require. 20 THE INVERTEBRATA not been shown to digest fat, but can usually dissolve starch, and some- times cellulose. The latter faculty becomes of great importance when they are symbionts in the alimentary canal of animals whose food consists of plant tissues (pp. 68, m). In a few cases {Balaiitidium, some Amoebae) contractions of the protoplasm divide large morsels into fragments. Often, but not, for instance, in foraminifera, the food vacuoles circulate in the cytoplasm ; sometimes they do this along a regular track. Their circulation is often due to streaming of the endoplasm, but sometimes (ciliates, etc.) it is brought about by peristalsis of the cytoplasm. Defaecation of the indigestible remains of food takes place at any part of the surface when there is no pellicle, but in pelliculate forms at a fixed spot. Sometimes there is a per- manent rectal passage lined by ectoplasm (Fig. 88 B, an). Saprophytic forms range from some which can subsist on mixtures of substances as simple as aminoacids and acetates (or even, as Polytoma can, upon ammonium acetate alone), to parasites whose food probably differs chemically but little from that of holozoic forms. Reserve materials^ for use at times when nutriment is not being taken or when some process, such as rapid multiplication, is making heavy demands upon the resources of the organism, are stored by most protozoa, and are often conspicuous, as granules, vacuoles, crystals, etc., in the cytoplasm. The carbohydrates starch, para- mylum (in the Euglenoidina), and leucosin (in the Chrysomonadina) are formed by holophytic organisms and by some colourless forms related to these (as by Polytoma, Fig. 24, Peranema, etc.). Glycogen is stored by parasitic and other anaerobic forms, in which it is per- haps split with evolution of energy, as in various anaerobic metazoa. Protein reserves are common in holozoic species. Nucleic acid ("volutin") is widespread, probably as a reserve for the nucleus. Oil reserves also occur in practically all groups — a rather remarkable fact, in view of the apparent inability of the Protozoa to digest fats. In phosphorescent forms (dinoflagellates, radiolarians) the oxidation of fats is the source of the emission of light. The nitrogenous excreta of the Protozoa appear to be most often ammonia compounds, less often urea, and occasionally urates. Ex- cretion doubtless frequently takes place from the general surface of the body. Sometimes there are recognizable in the cytoplasm granules or crystals of urates or phosphates, which may be expelled with the faeces but appear in other cases to be redissolved. Their material is then perhaps passed into the contractile vacuoles. The latter are spaces filled with water which periodically undergo collapse with expulsion of their contents to the exterior. In the simplest cases, as in the familiar laboratory types Amoeba, Chlamydomonas and Actino- sphaerium (Figs. 23, 33c.z;«c.), the contractile vacuoles are solitary, PROTOZOA 21 spherical cavities, one or more in number according to the organism; over these in pelliculate genera there is a soft patch in the pellicle through which discharge takes place. Sometimes, as in Eiiglena and Fig. 17. Fig. 16. Fig. 16. Paratnecium caudatiini from the ventral side, x 375. After Doflein. CV, contractile vacuoles: in the anterior one the main vacuole has just dis- appeared by discharging, and is about to be reconstituted from the accessory vacuoles, which are at their maximum size ; in the posterior one the accessory vacuoles having re-formed the main vacuole are themselves re-forming; f.v. food vacuoles ; me. meganucleus ; 7ni. micronucleus ; v. vestibule. Fig. 17. The cycle of action of the contractile vacuole and its canals in Para- mecium. From Lloyd, after Putter. Paramechifn, they are accompanied by accessory vacuoles by whose contents they are reconstituted (Figs. 39 D, 16, 17) and which in some ciliates (Fig. 89 B, C) extend as long canals through the cytoplasm. 22 THE INVERTEBRATA Another complication sometimes exists in the presence of a "reser- voir" through which the vacuole communicates with the exterior, either directly, as in Peranema (Fig. 39 E), or by way of the gullet, as in Euglena and Vorticella (Figs. 39 D, 2). At least some contractile vacuoles appear to have a lining membrane, and it is probable that they are not entirely abolished at systole. The fact that these organs are commoner in freshwater protozoa than in marine or parasitic species suggests that their primary function may be the discharge of water, which must enter the body when the surrounding medium has a lower osmotic pressure than the protoplasm. Possibly, however, they serve also as organs of excretion. Respiration no doubt takes place upon the whole surface of the body. It has been supposed that the contractile vacuoles subserve this function, but, while they no doubt remove carbon dioxide in solution, it is difficult to see how their activity could cause the entry of oxygen. Many protozoa either regularly or occasionally pass a period of their lives in a cyst. The cysts may be coats of jelly or stronger cover- ings, usually organic, but sometimes, as in the Chrysomonadina, chiefly composed of inorganic material. The function of the cyst is nearly always to shield the organism, either from unfavourable cir- cumstances or from stimuli which would interfere with some process, such as reproduction or digestion, but in a few cases it facilitates syngamy by keeping gametes together. Encystment is less common among species which live in the relatively equable conditions of the sea, than in freshwater and parasitic forms. Cysts which do not sub- serve reproduction may be resistance cysts, against drought, alterations of concentration, or the appearance of poisonous substances in the surrounding medium, either in the habitat in which encystment takes place or in those encountered in the course of distribution. They may on the other hand be resting cysts, which enable the organism to proceed undisturbed with digestion or photosynthesis or by quies- cence to conserve its energy during starvation. Cysts which subserve reproduction may be gamocysts, in which union of gametes takes place (gregarines. Figs. 76-78), oocysts, containing a zygote, or sporocysts containing several small individuals produced by fission. The oocyst frequently becomes a sporocyst by fission of the zygote. Reproductive cysts are often also resistance cysts. The nuclei of the Protozoa (Fig. 18) vary greatly in structure. They usually contain masses of some size composed of various materials. Such masses, when they consist only of the substance known as plastin (which takes acid stains), are known as nucleoli', if they also contain chromatin (basic-staining) they are artiphi- nucleoli. A single central mass is an endosome : it may be a temporary PROTOZOA 23 aggregation, as, for instance, in Actinosphaertum, but more often is permanent except, sometimes, at division. Such a permanent en- dosome is usually a nucleolus or an amphinucleolus, but is said sometimes to consist solely of chromatin or of achromatic matter. A permanent endosome consisting of plastin or chromatin, or both, is .nul kar.^ ^'kan nu. Fig. 18. Nuclei of Protozoa. A, Polystomella crispa. B, Amoeba proteus. C, Actinosphaerimn eichhorni. D, Naegleria bistadialis. E, Polytoma tivella. F, Ceratiiim fusus. G, Stentor coeriileus. All highly magnified, to various degrees. After various authors, cen.'i possible centriole; kar. karyosome, containing a centriole in D; nu.' nucleoli or amphinucleoli. known as a karyosome. Two principal types of protozoan nuclei — the dense and the vesicular — may be distinguished; there are, however, intermediates between them, and they do not characterize each a distinct branch of the phylum, but the dense appears to have arisen more than once from the vesicular. In nuclei of the dense type the achromatic part has a relatively firm consistency, and often exhibits, 24 THE INVERTEBRATA at least in fixed specimens, a fine meshwork. The plastin is in masses scattered through the nucleus, or occasionally in a single excentric mass. The shape is often not spherical (Figs. 87-90). The meganuclei of ciliophora and dinoflagellate nuclei belong to this type, which otherwise is rare. In vesicular nuclei the achromatic part is more fluid and its meshwork, if any, is coarse. The plastin may be in several masses under the nuclear membrane, but usually is in a karyosome. spd. .-cen. nn.mr.^ r^-, rr::^^'^=^^^Zll. ^ "-C^^ -^^ry^i^E—^^ • — =^^ \ ;'^^ ':A!^^=^ :r=^^p"^^ • i ' :. : ' Fig. 19. Mitosis (paramitosis) of the sporogony oi Aggregata eberthi. After Belar. A, Interphase between divisions. B, Early metaphase. C, Anaphase beginning. D, Later anaphase. E, Early telophase. F, Later telophase. cen. centriole; chr. chromosomes; 7iu.me. nuclear membrane; spd. spindle. The modes of division of protozoan nuclei are also very various. Many were formerly classed as amitoses but are now regarded as un- usual types of mitosis. True amitoses are rare, and perhaps occur only in the meganuclei of the Ciliophora. The mitoses are sometimes (Fig. 58) practically identical with those of the Metazoa, but are usually more or less aberrant. The "division centre" by which mitosis is initiated may be a centrosome consisting of centrosphere and centriole, or may be either of the latter two entities alone. The centrosphere often forms a plate or cap at each pole of the nucleus. Most often the nuclear membrane remains intact throughout the PROTOZOA 25 process. The division centre may be intranuclear or extranuclear ; when it is an extranuclear centriole, it is often identical or associated with the basal granule of a flagellum. Cases in which the chromosomes are distinct and on the whole behave like those of metazoa are known as eumitoses. Another set of mitoses, known as paramitoses (Fig. 19), differ from those of the Metazoa in that the chromosomes do not shorten in the metaphase, and are not symmetrically arranged on the equator of the spindle (if such be visible); and their longitudinal Fig. 20. Stages in the mitosis (cryptomitosis) of Haplosporidium limnodrili. After Granata. A, Resting nucleus : the spindle here persists. B, Metaphase. C, Anaphase. D, Telophase, clir. chromatin; kar. karyosome; spd. spindle. halves, when they separate, hang together to the last at one end so that they appear, though deceptively, to divide transversely. In a third set, known as cryptomitoses (Fig. 20), there are no distinct chromosomes but the chromatin merely concentrates as a mass upon the equator of a spindle, whose fibres may not be visible, and divides into two halves which travel to opposite poles. Intermediate cases connect cryptomitoses with eumitoses. Paramitoses occur in cocci- dians, dinoflagellates, and the spore formation of radiolarians, crypto- mitoses for the most part in parasitic and coprozoic forms, as in Haplosporidium and Naegleria. 26 THE INVERTEBRATA In certain cases mitoses repeated several times without dissolution of the nuclear membrane give rise to polyenergid nuclei which possess numerous sets of chromosomes, the sets being finally liberated to form each a daughter nucleus. The polyenergid condition is probably always a provision for spore formation, and may (as in the coccidian Aggregatd) occur only as a transient phase before sporulation, but in other cases (radiolarians) it persists for a long time, the nucleus dividing meanwhile as a whole by "giant mitoses'* in which all the chromosomes take part. A remarkable process in Amoeba proteus is possibly to be interpreted as the multiplication of units in a poly- energid nucleus by cryptomitoses. In the Ciliophora (other than the Opalinidae and the Chonotricha) the nucleoplasm, which in other protozoa is contained in one nucleus or in several which are all alike, is divided into two portions, a large amitotic meganucleus which breaks up periodically in *'endomixis" (see p. 35) and also at conjugation, and one or more small micronucleiy by division of one of which the pronuclei of conjugation are provided and the meganucleus replaced when the latter disintegrates. Indi- viduals without micronuclei have been observed and kept through several asexual generations. Thus the meganucleus is capable of conducting by itself the normal vegetative existence of the individual, though the absence of this nucleus at syngamy shows that it does not establish the characters of the race. That function must be performed by the micronucleus, but, since the latter does not exist without the meganucleus, save for a brief period during conjugation, it presumably does not regulate the life of the individual. The chromatin of the meganucleus is known as trophochromatin, that of the micronucleus as idiochromatin. A similar distinction between trophochromatin and idiochromatin is discernible in various other protozoa. In the Opalinidae (Fig. 21 C) and Chonotricha there are two sets of chromosomes, an outer and an inner, which divide successively at mitosis. The members of the outer set (megachromosomes), larger and less regular than those of the inner, are held to represent the meganucleus of other ciliophora : the material of which they are composed is known to be cast out of the nuclei of the Opalinidae before gamete formation. The members of the inner set {microchromosomes) represent the micronucleus. In various cases of gamete and spore formation by members of other classes, especially of the Sporozoa, there is a destruction (Fig. 21 A, see legend), or a casting out from the body (Fig. 21 B), of a portion of nuclear substance which is probably trophochromatin. It has been suggested also that the obscurity of cryptomitoses is due to a veil of tropho- chromatin dividing amitotically around the idiochromosomes. It may be that all protozoa contain chromatin in both these conditions; and PROTOZOA 27 it is perhaps in this respect, as well as in restriction of function, that the cells of metazoa differ from protozoa. From the fact that, in many cases at least, the trophochromatin is periodically destroyed and replaced, and from further facts which we shall cite in discussing the significance of conjugation, it would appear that trophochromatin, or some part of the protoplasm associated with it, in the course of its regulative activity eventually becomes effete and is replaced from the idiochromatin, which is not liable to that fate. Perhaps the possession by protozoa of the facility for this re- Fig. 21. Nuclear phenomena in protozoa. A, Extrusion of the germ nucleus froin the somatic nucleus in Gregarifia cimeata before gamogony. The somatic nucleus will break up and disappear. B, Extrusion of the substance of the endosome from the gamont of Eimeria schubergi, the germinal part of the nucleus remaining in the centre of the body. C, Mitosis in Opalina ranarinn, megachromosomes in anaphase in outer part of nucleus, microchromosomes in metaphase internal to them. A, after Milojevic; B, after Schaudinn; C, after Tonniges. placement, and the lack of such facility in the body cells of metazoa, is the explanation of the fact that protozoa are not subject to the "natural death" which eventually overtakes the body of a metazoon. The loss of trophochromatin during the formation of gametes is not to be confused with the reduction division of maturation. Reduc- tion divisions, however, have been seen in members of all classes of the Protozoa, and it maybe suspected that a process of this kind occurs in all species in which there is syngamy. Such divisions are sometimes {Actinophrys, etc.) strikingly similar to those of the Metazoa, but in other cases {Paramecium, etc.) are peculiar. The reduction division 28 THE INVERTEBRATA usually closely precedes syngamy, as in the Metazoa, but in the Telosporidia and Volvocina it takes place in the first division of the zygote, so that for the whole of the rest of its life history the organism is haploid. In many protozoa there are present in the cytoplasm, scattered or massed into a group, numerous granules which, like chromatin, take basic stains and are known collectively as the chromidium (Fig. 22 A). They appear to arise from the nucleus, and have been said, but probably incorrectly, in some cases to give rise to nuclei by con- densation. They may appear upon occasion or be present through the greater part of the life cycle. Their function is uncertain and probably not always the same. Th.Q. fission of the Protozoa takes place in several ways. Whether in Fig. 22. Arcella. After Swarczewsky. A, Vegetative individual. B, In- dividual in process of budding, chm. chromidium ; nu. nucleus ; op, opening of shell ; sh. shell. asexual reproduction or in the formation of gametes, it may be: {a) equal binary fission, the familiar mode of division of Amoeba, Paramecium, and a vast number of other cases; {b) budding, in which one or more small products separate from a parent body, as in Arcella (Fig. 22 B), the Suctoria (Fig. 96), etc.; {c) repeated fission, in which equal divisions give rise to four or more young which do not separate till the process is completed, as in Chlamydomonas (Fig. 23), the microgamete formation of Volvox (Fig. 44), etc.; or (d) midtiple fission, in which the nucleus divides several times without division of the cytoplasm, which finally falls into as many parts as there are nuclei, usually leaving behind some residual protoplasm, which may contain nuclear matter. Multiple fission is seen in the spore formation of numerous protozoa, as Amoeba, sporozoa (Fig. 74 C, D; Gg, G3; K, L), etc. The fission of multinucleate protozoa, such as Actino- sphaerium, Opalina (Fig. 86), etc., to form multinucleate offspring by PROTOZOA 29 division of the cytoplasm without relation to that of the nuclei, is known as plasmotomy. It is usually binary, but occasionally takes place by budding or is multiple. The plane of simple binary or of repeated fission is often transverse to the principal axis — if there be one — of the body, but in most flagellates it is longitudinal. Repeated longitudinal fission in which the daughter individuals remain in position is called radial; such fission is common in the green flagellates of the order Volvocina (e.g. some species of Chlamydomonas, Fig. 23 Fig. 23. Chlamydomonas. A, C angidosa, x 1000. B-D, the same, in fission (radial). E-H, C. /ow^wi/g'wa in fission (pseudotrans verse). Highly magnified. After Dill, with modifications, c.vac. contractile vacuole; cph. chromato- phore; cu. cuticle; e. eye-spot; nu. nucleus ; /)_yr. pyrenoid. A-D). Sometimes an individual in longitudinal fission shifts in its cuticle during the process, till the plane of division becomes transverse. Fission of this kind is said to be pseudotransverse : it is seen, for in- stance, in some Chlamydomonas (Fig. 23 E-H). In Polytoma (Fig. 24) the only vestige of longitudinal fission consists in a slight obliquity of the first division of the nucleus. Each type of fission takes place in some cases in a cyst and in others without encystment. BRARY MASS. 30 THE INVERTEBRATA The fate of flagella at fission varies. Sometimes, as in Chlamy- domonas and Polytoma^ they are lost, early or late in the process. In other cases they are retained. When this happens in an organism with a single flagellum, that organ has been said sometimes to be split longitudinally, but usually, if not always, a second flagellum grows out from the basal granule, which divides. When several flagella are present and persist, they are distributed between the ofi^spring, each of which grows new flagella to complete its equipment. Probably, a new flagellum always grows from a basal granule. Chromatophores divide, and if numerous may do so independently of the fission of the Fig. 24. Polytoma uvella, x about 1300. A, Ordinary individual, showing nucleus, eye-spot, contractile vacuoles, flagella with basal granules, and starch grains, the latter confined to the hinder part of the body. B-E, stages of the first fission. In C and D the flagella are omitted. After Dangeard. body. Contractile vacuoles and other organs rarely (Euglena) divide, but are usually shared as the flagella. Complex organs, however, are often destroyed (dedifferentiated) and remade by the individual that receives them. Conjugation^ or syngamy,^ the union of two nuclei, accompanied by the fusion of such cytoplasm as each may possess, is, so far as our ^ The name conjugation is by some authorities restricted to the peculiar process by which syngamy is accomplished in most ciliophora (p. 33). ^ The union of nuclei is karyogatny : in most cases of syngamy it is accom- panied by plasmogamy or the fusion of cytoplasm, but in typical ciliophora one of each pair of fusing nuclei has perhaps no cytoplasm ; and autogamy (p. 33) is said sometimes to occur between nuclei whose cytoplasm has never been divided. Plastogamy (p. 36) is plasmogamy without karyogamy. PROTOZOA 31 knowledge goes at present, by no means universal in the Protozoa. Especially among the Mastigophora, but also in other groups as in the Amoebina, there are many cases in which it appears not to occur. Probably, however, in the majority of species it either is known or may reasonably be inferred to take place. The energids by which it is performed, known here, as in all organisms, as gametes^ may be either merogametes, formed by special acts of fission and smaller than the ordinary energids of the species, or hologametes, not formed by special fissions, and as large as, or larger than, the ordinary energids. Syngamy between like gametes is known as isogamy, that between unlike gametes as anisogamy. The simplest cases of the process are those instances of isogamy in which two full-sized ordinary indi- viduals unite. Such unions are known as hologamy and are rare, though they occur in Copromonas (Fig. 39 F') and a few other species. The fact that nearly all protozoa in which it is certainly known to occur are coprozoic (p. 43) suggests that it is an adaptation to special conditions — perhaps to brief duration of the active stage — and is not, as might be assumed, the primitive form of syngamy. In all other cases the gametes are special individuals, and one at least is a merogamete. They may be isogamous or anisogamous, and in the latter case one — \ht female gamete — is less active than the other, which is the male gamete. Nearly always the female gamete (macrogamete) is larger than the male {microgamete), and often it is a hologamete. In the latter case the process is known as oogamy. As examples of isogamy of merogametes we may cite the syngamy of Polystomella (Fig. 66C-E) and of some Chlamydomonas (e.g. C. steini). Anisogamy occurs independently in many genera, and has more than once become oogamy. An interesting series of grades in this respect is provided by the Volvocina. Chlamydomonas euchlora ex- hibits the transition from isogamy to anisogamy. By undergoing different numbers (2-6) of divisions, its individuals form merogametes of several different sizes, but these pair indifferently, some unions being isogamous, some anisogamous. C. brauni and other species (Fig. 25) form merogametes of two sizes and are definitely ani- sogamous. Volvox (Fig. 46) and related forms have an anisogamy in which the female gamete is a hologamete (oogamy). A similar series is shown by the Sporozoa. The syngamy of some species of Monocystis, for instance, is isogamy of merogametes, that of others is anisogamy of merogametes with various degrees of unlikeness between the gametes, and that of the malaria parasite (Fig. 75) and its relations is anisogamy between a hologamete and a merogamete. Syngamy, whether isogamous or anisogamous, nearly always is exo- gamous, that is, takes place between the offspring of different parents. Since the male and female gametes are usually formed by distinct 32 THE INVERTEBRATA parents, sex may be said to exist among protozoa, but it is rarely that (as in the sporozoon Cyclospora, etc.) the sexes may be distinguished Fig. 25. Anisogamous syngamy of Chla?nydomonos. a-h, C. media, after Klebs. a, Vegetative individual. 6, Eight gametes produced inside the cuticle of the parent, c, Single gamete, d, Gamete before syngamy showing proto- plasm at one end of the cuticle, e, /, g, Syngamy between two gametes of unequal size : the individuals slip out of the cellulose wall at the time of fusion, the cilia withdraw and there is a complete fusion of the individuals, h, The resultant thick-walled zygote, z, j, k, C. braiini, after Goroschankin : in this species the gametes fuse whilst yet within their cellulose walls and there is marked anisogamy, the small gamete slipping into the cuticle of the larger gamete, j, Shows the nuclei, chloroplasts and pyrenoids of the two gametes still separate, k. Shows the fused nuclei. by other features. In many of the Telosporidia (e.g. Monocysfis, Fig. 78) sexual congress may be held to occur, in that individuals, male and female in cases of anisogamy, apply themselves together at PROTOZOA 33 the time of gamete formation, and their gametes unite each with one from the other parent. Hermaphroditism appears in the CiUophora^ (except the Opalinidae and Chonotricha). Here congress (Fig. 26) takes place between two individuals (conjugants) in each of which the meganucleus (see above) disintegrates, and the micronuclei divide to form a number of nuclei — perhaps a reminiscence of the formation of numerous merogametes. All but one of these nuclei disappear, and the survivor divides to form a male pronucleus, which passes over into the partner, and a female pronucleus which, in possession of the cytoplasm of the parent, awaits the arrival of the male pronucleus of the partner. Fusion now takes place between the male and female pronuclei in each of the pair of conjugants, the latter separate, and by the division of their zygote nuclei mega- and micronuclei arise. Two hermaphrodites have formed each a male and a female gamete and cross-fertilization has taken place .^ In the Vorticellidae (Fig. 26 B) the individuals which enter into congress differ, one being of the ordinary size and fixed, the other small and free-swimming. The smaller arises from an ordinary individual, as a bud or by repeated fission. After reciprocal fertilization of the type just described, the smaller partner perishes, its endoplasm being sucked into the larger. This curious simulation of sexual dimorphism by hermaphrodites occurs in a less marked form in other ciliates. A remarkable process known as autogamy, in which a nucleus divides into two which after maturation immediately reunite, occurs in Actinophrys and Actinosphaerium (see pp. 83-86), and possibly in some other cases. Parthenogenesis is known to occur in members of at least three of the four classes of the phylum. The clearest case is presented by Actinophrys, when gametes which have failed in attempt at cross- fertilization develop parthenogenetically (p. 86): it is interesting that one of these gametes is a functional male. Individuals of Polytoma which are potential gametes will grow and divide, and the same is true of the gametes of some species of Chlamydomonas and Haema- tococcus when syngamy has been missed. The endomixis of ciliates (p. 35) is a phenomenon of this kind. Since it is comparatively easy to observe the conditions which pre- cede and the results which follow syngamy in the Protozoa, many experiments and observations have been made upon those creatures, with a view to discovering the signifi^cance which the process has for ^ Actinophrys (p. 83) may be said to be hermaphrodite, and so perhaps are many of the Radiolaria. But it is not certain that the " gametes " of this group are not parasitic dinoflagellates. (See p. 80.) ^ Occasionally (Collinia, Dendrocofuetes) the conjugants also exchange halves of their meganuclei. The latter, however, always disintegrate. 34 THE INVERTEBRATA organisms in general. Most of these researches have been carried out upon ciliata. They have led to two theories: (i) that syngamy effects a periodical rejuvenescence of the organism ; (2) that it produces new Fig. 26. Conjugation of ciliates. A, Paramecium. B, Vorticella. Both at the moment of the exchange of male pronuclei, meg. fragments of disintegrating meganuclei ; mi. micronuclei : in each the male and female pronuclei produced by the division of a single nucleus still hang together by their spindle, but are parting, the male to pass into the other conjugant, the female to remain behind ; mi.' abortive division products of the original micronucleus. Fig. 27. A diagram of the behaviour of the micronuclei during the conjuga- tion of Paramecium caudatum. From Borradaile. The white circles represent the portions which degenerate. types of individual and therefore gives the species more chances of survival in the struggle for existence. (i) Cultures of protozoa in which conjugation is prevented are liable after a time to fall into an unhealthy condition known as de- PROTOZOA 35 pression, in which the nucleus^ is overgrown, the body stunted, divi- sion retarded, and the various organs and functions increasingly de- generate, until finally digestion ceases and the organisms die. From this condition conjugation will recover a culture which is not too far gone. It was held that depression was the senility of the organism — ultimately of the same nature as that which in the Metazoa destroys the parent body, while the gametes, after syngamy, continue the existence of the species — and the conclusion was drawn that in both cases the effect of the union of nuclei was rejuvenation. Now, how- ever, it is known that depression is a disease, which by more natural methods of culture can be avoided without conjugation. It is true that in cultures of ciliates there has been observed a periodical waxing <^ Cg5>>c2g><^ Fig. 28. A diagram of the nuclear changes in Paramecium aurelia during endomixis. From Robertson, after Jennings. The white circles represent degenerating nuclei. Fissions take place between D and E, and between H and I. and waning of vitality of which the low points in some cases coincide with conjugation ; but in other cases there occurs at these points not conjugation but a process known as endomixis, which closely resembles the procedure in conjugation, but takes place in solitary individuals and does not involve syngamy. In this process (Fig. 28) the mega- nucleus is destroyed and replaced by one of the products of the division of the surviving micronucleus, as in conjugation. It would appear from these facts that the invigorating effects of conjugation are due not to the true syngamy (union of nuclei) but to the accom- panying replacement of the meganucleus, which probably has become effete (see p. 27). If, as has been suggested (p. 26), those protozoa ^ In ciliophora the meganucleus. 36 THE INVERTEBRATA which have no meganucleus have in their nuclei trophochromatin which is destroyed at syngamy, this conclusion may be extended to them also. (2) Variety in a protozoan species is of three kinds : (a) that which results from the production of different combinations of genes at syngamy, and is permanent, forming races {pure lines) like those which exist in higher organisms in the absence of cross-fertili- zation; such pure lines have, for instance, been found in respect of body-length in cultures of Paramecium, each line in the culture breeding true so long as asexual reproduction continues; (b) that which results from the spontaneous appearance of mutations ; this also is permanent; it has been studied in Ceratium and other genera; (c) that which results from modification of the individual by the direct action of the environment; this, like mutation, produces differences between individuals of a pure line, but it is not permanent, though it may be inherited for several generations before it disappears. It would seem that, apart from the occasional appearance of mutations, the permanent varieties in a species are produced only by syngamy. Here may be mentioned the union of individuals by fusion of their cytoplasm, the nuclei remaining distinct, which is practised by the Mycetozoa (Fig. 73 F) and in some other cases. This process, which is not syngamy, is known as plastogamy, and its product as a Plasmodium. The life of a protozoon passes in the course of generations through a cycle in which individuals of different kinds succeed one another. The life cycles of various protozoa differ greatly, being related to the vicissitudes of the environment of the species and to the need for distribution as well as to the recurrence at intervals of conjugation, but it is possible to formulate a type of which all of them may be regarded as variants. After a period of "vegetative" existence and increase by asexual reproduction, during which the individuals are known as agamonts, there appears a generation known as gamonts because they produce gametes, the latter unite in pairs, and the zygote or sporont gives rise to a generation of sporozoites which, becoming agamonts, repeat the asexual part of the cycle. The table on p. 37 shows this typical life history. In comparing this table with the actual course of the cycle in any species, the student should remember: (i) that in each part of the cycle fission may take place in any of the modes described above, and that the agamogony of a species may proceed in more than one of these ways (as, e.g., that of Amoeba proteus by binary or multiple fission) ; (2) that in the cycle of most protozoa there is a point at which ad- justment must be made to unfavourable conditions, either recurring PROTOZOA 37 in the local habitat or met with in the course of the distribution of the species (e.g. freshwater forms and parasites); and that at this time (a) the ordinary agamogony is suspended, (b) the syngamy, if any, usually takes place, (c) there is often a phase of protective encystment, (d) there is often very rapid multiplication by multiple or repeated fission, which may be the sporogony, the gamogony (eugregarines), or an agamogony; (3) that any part of the cycle may be omitted ; in such cases it is most often the sporogony which is dropped, but many species appear to omit gamogony, and in a few (e.g. Monocystis and the other eugre- garines) there is no agamogony; Agamont (Schizont, Meront) Agamogony (Schizogony, Merogony) Agametes (Schizozoites, Merozoites) Growth of Agametes Agamont of second generation Agamogony repeated Gamont Gamogony Gametes Syngamy Zygote (Sporont) Sporogony QQ 0 0 0 © © © Sporozoites / 0 Growth of sporozoite ■ Agamont Fig. 29. A table of the life history of a protozoon. (4) that a reduction division may occur at either of two points in the cycle — shortly before syngamy (most cases), or directly after the formation of the zygote (the Telosporidia and Volvocina) — and that correspondingly either the diploid or the haploid phase may extend over the greater part of the life history. The term spore is applied to various phases of the life history in a way which is liable to cause confusion, {a) Strictly speaking, perhaps, it should be applied only to the products of repeated or multiple fission of a zygote (sporont). (b) Most often, however, it is used to denote the products of any repeated or multiple fission, {c) In a few 38 THE INVERTEBRATA cases (e.g. the " ciliospores " of the Suctoria) it is applied also to products of budding. A cyst in which several spores are enclosed is a sporocyst. Individual spores may be enclosed in spore cases^ when they are chlamydospores (as those of the Mycetozoa, Fig. 73 A), or naked, when they are gymnospores. The latter may be amoeboid (amoebulae or pseudopodiospores, e.g. Amoeba, Polystomella, Fig. 66 C), flagellate {flagellulae or flagellispores, e.g. Polystomella, Fig. 66 B, Chlamydomonas), or ciliate {ciliospores, e.g. the Suctoria, Fig. 87 H). Spores may be gametes (e.g. the Mycetozoa, Chlamydomonas), or serve for the distribution of the species, when, if they are motile, they are known as "swarm spores". The sporoblasts of many telosporidia (e.g. Plasmodium, Fig. 75, 16-18) are spore-like bodies which are not set free, but give rise under cover to another generation of spores. The so-called spores of such sporozoa as Monocystis (Fig. 78 G, H) are really minute sporocysts, enclosing several spores ("falciform young"). The life history of the individual protozoon usually exhibits little change save increase in size. Sporozoites and other spores, however, may diff^er considerably from the adults into which they grow. This difference reaches its height in the ciliospores of the Suctoria. The behaviour of a living being is that part of its life which consists in action upon the outer world. Like the rest of life it comprises activity of various kinds — mechanical, chemical, etc. — which in some cases, as in the direction of locomotion to or from the light or the shooting out of trichocysts, is immediately due to external circum- stances {stimuli), while in others, as in the beating of cilia which continues even when the organism is encysted, it is not. Both these sorts of activity are so ordered that in normal circumstances they conduce to the welfare of the organism. The reactions of the Protozoa to stimuli are at least superficially analogous to the reflexes of the Metazoa. Study of them has chiefly been directed to those which result in locomotion. Such reflexes are of two kinds, topotaxis and phobotaxis. In topotaxis the organism orientates itself in relation to the stimulus, and moves either towards or away from it. This is the less common mode of reaction in protozoa, but it appears to be per- formed by some in the neighbourhood of food, by gametes in their union, and by certain green flagellates {Volvox, sometimes Euglena, etc.) in approaching the light. In phobotaxis , which has been studied in many protozoa of various groups, the only circumstances which act as stimuli are those which are "unfavourable", that is from which the organism withdraws; and in doing this it is not repelled in a straight line, but turns away at an angle which has no necessary relation to the direction of the stimulus, and may again bring the individual into the unfavourable PROTOZOA 39 Fig. 30. A diagram of the path by which a protozoon is directed by phobo- taxis into the zone of optimum concentration of a substance diffusing from a particle. From Fraenkel, after Kiihn. C, Centre of diffusion. The arrows show the direction of movement of the protozoon, the circles define zones of equal concentration, the circles of dots and dashes enclose the optimal zone. J.-. . •-^ >' .■.. ; .^ J ...... ^ ' * • • , • •. • •'■.ife-'.' ■ ' <•■•■.•■• "r > ■ ■ r ^• • r 1 2 3 •it V- •v. /;• 1 \ N .••■•■ Y "\.>-S 5>y»;\^v-...»'. -.ly Fig. 31. Chemophobotaxis of Bodo sulcatus. From Fraenkel, after Fox. 1-5, Positions successively taken up by the members of a culture placed under a coverslip. The position in which the individuals gather in each case is that of the optimum concentration of oxygen, which alters as the supply of the element is lessened by the action of the flagellates. 40 THE INVERTEBRATA circumstances. The reaction is then repeated. Thus the organism is shepherded by its reactions in the direction of the optimum con- ditions. Fig. 30 shows the path of an individual in the neighbourhood of a particle whence is diffusing some substance of which a certain concentration is optimal for the species to which the individual belongs. Any departure from this concentration turns the moving individual, so that it is led to and kept in a zone in which the optimum exists. Fig. 31 shows how members of a culture of the flagellate Bodo sulcatus, when placed under a coverslip, find by this reaction the optimum concentration of oxygen, which is at first in the middle of the field and recedes as the organisms use up the supply of the element. The number of ways in which a protozoon can respond to stimuli is at most small, but the response to a stimulus by an individual in many cases depends not only upon the nature of the stimulus but also upon the condition of the individual at the moment (hunger, fatigue, etc.). The relation of protozoa to their environment is governed primarily by the fact that, owing to their small size, any cuticle which is thick enough to protect their protoplasm from loss of water or poisoning by substances in the medium has the effect of immobilizing the organism. Hence in the active phase they are only found in water or in damp places on land, and are peculiarly susceptible to variations in the composition of the medium. Purely holophytic protozoa are also dependent upon the presence of sunlight. Save for these restrictions, members of the phylum are found in every environment in which any other species of organism can exist. In the sea they are plentiful alike in the plankton and in the benthos, and occur at all depths. Their planktonic members are liable to possess the same peculiarities which appear in members of other phyla in the same conditions — spininess (Figs. 6C, 67, 69), phosphorescence, buoyancy, etc. In attaining a low specific gravity they often show an expedient of their own, namely the presence in their protoplasm of vacuoles of water of lower saline content than the medium in which they are suspended (radiolarians, Glohigerina, heliozoa ; Figs. 32, 33, 69, 71). In fresh waters their species have the same cosmopolitan distribution as other small freshwater organisms. Most of them, however, are severely restricted, in all the localities in which they are found, by the necessity for conditions which only occur in some one type of environment, and often even there only during certain seasons or (as in the case of the dung fauna) for yet siiorter periods. In this matter protozoa are particularly subject to the/)H of the medium, its dissolved organic contents, and its saline contents. Thus Po/_y/om« flourishes in an acid medium, ^S^zVoj-Zowwrn re- quires a slightly alkaline one, and Acanthocystis pronounced alkalinity. Euglena viridis and Polytoma live in highly nitrogenous mi\is\ons,Acti- PROTOZOA 41 B Fig. 32. Radiolaria without skeleton. A, Thalassicolla pelagica, x 20. After Haeckel. B, Collozoum inerme. C, A central capsule of the same, more highly magnified. After Doflein. cal. calymma; cps. central capsule; nu. nucleus; oil, oil globule; ^5. pseudopodium ; j^.c, "yellow cells" (Zooxanthellae). 42 THE INVERTEBRATA nosphaerium and Paramecium caudatum in less highly organic infusions, Volvox and Amoeba proteus in much purer waters, Haematococcus in rain water. As a rule the marine and freshwater faunas are restricted by conditions of salinity, but Polystomella ranges from the sea into brack- ish waters. For many holophytic protozoa the amount of sunlight is important. Others, as Euglena gracilis, bleach in the absence of light, but can still flourish if the presence of organic matter in solution makes c.vac. f.vac. Fig. 33. Actinosphaerium eichhorni, x 180. From Leidy. The endoplasm is crowded with food vacuoles containing diatoms, and nuclei are represented in the figure by the dark areas, c.vac. contractile vacuole ;/.z;<2c. food vacuole which has just swallowed a rotifer; ps. pseudopodia. saprophytic nutrition possible. Holozoic species must of course have their proper food; in infusions they appear as this becomes plentiful, first, after the bacteria, those whose diet is purely bacterial, such as Monas and Colpoda, then those, such as Stylonichia, that feed upon the first comers, and so on ; though some bacterial feeders, as Paramecium, are rather late to appear. Temperature has also an influence upon protozoan faunas. The powers, possessed by freshwater protozoa, of distribution across inhospitable regions and of surviving unfavourable PROTOZOA 43 conditions at home are no doubt due to the faciUty with which they form resistance cysts (p. 22). In various cases all the unsuitable con- ditions of the environment indicated above have been found to induce encystment, and encysted protozoa have been discovered in dust from the most remote desert regions. The protozoa which live in dung (coprozoic species) and in decaying bodies, and those of very foul waters, are branches of the aquatic fauna: they include many flagellates, Umax amoebae (p. 69), and ciliates, and the conditions in which they are in the active state may exist only for a very short period. These faunas merge on the one hand into that of intestinal parasites, and on the other into that of damp earth. In the latter there is a large population, some of whose members {Euglena, Arcella, Paramecium^ etc.) are of common occurrence elsewhere. It has important effects upon the fertility of the soil, by devouring valuable bacteria. Perhaps the only truly subaerial members of the phylum are certain mycetozoa. Parasitic members are included in nearly all the principal divisions of the phylum, but not in the Radiolaria or Volvocina. The Sporozoa are exclusively parasitic. The relations of parasitic protozoa to their hosts are of all degrees of intimacy : they may be merely epizoic (as Spirochona, p. 114), ectoparasitic (as Oodinium, P- 55)j inhabitants of internal cavities (as Opa- lina,p. 1 06), tissue parasites (as Myxobolus, p. 100), or intracellular (as Plasmodium, p. 91). They show, according to their de- gree of parasitism, the same peculiarities as other parasites — reduction of organs of locomotion, simplicity of form, means of fixation, the liberation of numerous young (in the Sporozoa), etc. Some, as Entamoeba histolytica, are harmful by destroying for their own nutriment the tissues of the host : more by secreting poisonous substances, as the malaria parasites do. Many are specific to a par- ticular host or hosts. Not infrequently there are two successive hosts belonging to difl^erent phyla: both of these may be invertebrates, as with Aggregata, which passes from the crab to the octopus, but more often one is a vertebrate and the other an invertebrate. In such cases it is often possible to decide which was the original host, and this proves sometimes to be the vertebrate and sometimes the inverte- Fig. 34. Oodinium poucheti, parasitic on Oikopleura. A, An Oikopleura bearing the parasites. B, A free spore of the parasite, pst. parasite, on the tail of the host. The trunk of the Oikopleura is enclosed in the newly-secreted and not yet expanded "house". 44 THE INVERTEBRATA brate. It is interesting that the two most dangerous protozoan para- sites of Man, the sleeping-sickness and malaria parasites, differ in this way (pp. 63, 91). Symbiosis^ of various kinds is practised by both holophytic and holo- zoic protozoa. Instances of this are described below, on pp. 47, 68, m, 193. The division of the phylum into the four classes, Sarcodina, Mastigo- phora, Ciliophora, and Sporozoa, characterized by the presence or absence in the predominant phase of the life history of the several types of motile organs, will be familiar to the student. Two attempts have been made to brigade these classes into subphyla. One con- trasts the Sarcodina under the name of Gymnomyxa with the other classes, or Corticata^ on the ground that the latter possess a firm ecto- plasm. The other contrasts the Ciliophora with the rest of the classes (Plasmodroma), which lack cilia and a meganucleus. Neither of these systems is satisfactory, for each is probably grounded, not upon a fundamental cleavage of the phylum, but upon the specialization of one branch of it. The ancestral group of the Protozoa is probably the Mastigophora. This is fairly evident as concerns the Sporozoa — a class highly adapted to parasitism, and often possessing a flagellated phase — and the Ciliophora, also a greatly specialized group, which possesses in the cilia organs easy to derive from flagella. The Sarcodina, on the other hand, were formerly held to be ancestral to all protozoa, on account of the supposedly primitive condition of their protoplasm. But neither the structure nor the behaviour of amoeboid organisms is really simple; their holozoic nutrition is a less easy process and is much less likely to be primitive than photosynthesis, which is common in the Mastigophora ; the sporadic occurrence of amoeboid forms in various groups of the Mastigophora probably indicates that the latter have more than once given rise to organisms resembling the Sarcodina; and, finally, the Sarcodina very commonly have flagellate young, but the Mastigophora do not have amoeboid young. The Mastigo- phora, indeed, are probably not only the basal group of the Protozoa but also' not far removed from the ancestors of all organisms, for they alone present (and often can alternate) the modes of nutrition both of plants and of animals ; and their characteristic organ, the flagellum, occurs in the zoospores of plants, in bacteria, and in the spermatozoa of metazoa. ^ The term symbiosis has been used in various senses. It is here applied to all cases of partnership between two organisms of which one lives within the body of the other and both derive benefit from the association. It is sometimes restricted to cases, such as those described on p. 47, in which the infesting partner is photosynthetic. PROTOZOA 45 The connection between the Protozoa and the Metazoa in the family tree of the Animal Kingdom is an interesting but a very obscure problem. Concerning it three theories are held. The first, supported by the morphological resemblance of the uninucleate protozoon to a cell in the body of a metazoon, and of Volvox to the blastosphere stage in the development of such a body, holds that the metazoon is a colony of protozoa, each differentiated as a whole for some function in the body which they compose. The second, based on the fact that the protozoon, which performs equally all the processes of life, is thus physiologically equivalent not to one cell but to the whole body of a metazoon, holds that the Metazoa arose from multinucleate protozoa by the nuclei taking in charge each a local, differentiated portion of the cytoplasm. The third, based on the fact that, save for their mode of nutrition, the Metazoa have — in their cellular structure, nuclear division, maturation of gametes, etc. — more in common with multicellular plants than with the Protozoa, holds that the earliest organism we can as yet envisage was multinuclear and photosynthetic, and gave rise independently to the Metazoa and, by reduction of the body, to flagellates, and so to the Protozoa, which on this view are not truly members of the Animal Kingdom. Class MASTIGOPHORA (FLAGELLATA) Protozoa which in the principal phase possess one or more flagella; may be amoeboid, but are usually pelliculate or cuticulate; are often parasitic but rarely intracellular; have no meganucleus; and do not form very large numbers of spores after syngamy. The reproduction of the Mastigophora is in most cases by equal longitudinal fission. The way in which in many of the solitary Volvocina this becomes transverse has been described above (p. 29). In the Dinof^agellata fission is oblique or transverse. The fission may be simply binary or repeated. The number of fissions often varies in the same species, and is usually greater in the formation of gametes than in asexual reproduction. Binary fission in forms which have not a stout cuticle usually occurs in the free-swimming stage, but may take place in a cyst or jelly case, as, for instance, occasionally in Eiiglena viridis. In forms with a stout cuticle, as in the Volvocina, the protoplasm shrinks from the cuticle, which serves as a cyst. Repeated fission usually occurs in a cyst. The^fate of the flagella at fission has been dealt with on p. 30. The mitoses (see p. 25) in this group range from beautiful eumitoses to the extremest cryptomitoses, the latter generally in parasitic forms. Paramitosis occurs in the Dinoflagellata. In many genera syngamy is not known to occur. Among those in which it does, all degrees of difference between gametes are found, 46 THE INVERTEBRATA and in particular among the Volvocina there are interesting cases intermediate between hologamy and merogamy, and between isogamy and anisogamy. Thus in Polytoma the age at which the products of fission unite varies in a species, so that some are merogametes while others, delaying, become hologametes ; in Pandorina (p. 58) isogamy and anisogamy are facultative ; and various species of Chlamydomonas (see p. 31) make up a series in which there is a transition from com- plete isogamy to a pronounced anisogamy which rises to oogamy in Volvox and other colonial forms. The zygote is very commonly encysted. The Mastigophora fall into a number of fairly well-defined orders. It is convenient to group these by their nutrition into two subclasses — the Phytomastigina, containing orders most of whose members are holophytic (see p. 18), and the Zoomastigtna, which have no holo- phytic members — but all the orders of the Phytomastigina contain some colourless members, whose nutrition is purely saprophytic, and all except the Volvocina include colourless holozoic forms. Owing to this fact it is impossible to frame a definition which will enable every member of each subclass to be recognized as such with- out comparison with other species. Certain characteristics, however, distinguish most members of the Zoomastigina from most of the colourless Phytomastigina. These characteristics are stated below, in the section which deals with the Zoomastigina. Subclass PHYTOMASTIGINA Mastigophora which possess chromatophores, and species without chromatophores which closely resemble such forms. There can be no doubt, for reasons which have been given above, that this subclass contains the most primitive members of the phylum. Its nutrition is extraordinarily interesting from that point of view. Some of its species, notably among the Volvocina, are purely holo- phytic. Others are normally also saprophytic, and some of these, like Euglena, can upon occasion practise this mode of nutrition alone. Yet others, like Polytoma, have become colourless, and are purely sapro- phytic. Others again are both holophytic and, by amoeboid in- gestion, holozoic. These lead insensibly to similar forms, members of the Zoomastigina (Monas, etc.), which, being without chromato- phores, have not the faculty of photosynthesis, but are purely animal in their nutrition. Some of the coloured forms which possess a pit that is called a gullet are said to take food with it, and thus to combine holophytic and holozoic nutrition. In any case certain of their relatives which have lost the chromatophores {Cyathomonas, Peranema, etc.) take solid food through a similar gullet. Most of the holozoic forms MASTIGOPHORA 47 are probably also saprophytic. Certain species {OchromonaSy etc.) are known to make use of all three modes of nutrition. Thus all ways of obtaining nutriment meet in this group. The species which practise photosynthesis do so, like plants, by means of chromatophores, of which they may possess one, two, or many. The chromatophores are plate- or cup-shaped masses of proto- plasm of a green, yellow, or brownish colour, owing to the presence in various proportions of the pigments chlorophyll, xanthophyll, carotin, etc. The chlorophyll absorbs the rays of sunlight whose energy is used in photosynthesis. The green chromatophores are known as chloroplasts, the yellow as xanthoplasts . Often there are to be seen in or on the chloroplasts the protein bodies known as pyrenoids , which act as centres of starch formation. A red pigment, haemato- chrome, is frequently present, diffused through the cytoplasm. In bright light it spreads over the surface and is believed to shield the chloroplasts from excess of certain rays. A small red spot of carotin, sometimes darkened by another pigment, is generally present in photosynthetic species, and probably acts as a rudimentary eye, making the organism sensitive to light, which is of such importance in its nutrition. The holophytic forms are usually capable of passing into a resting phase, in which the flagella are withdrawn, the body rounded off, a cyst or jelly case secreted, and the organism closely resembles a plant cell. Division may take place in that condition, establishing a pseudo- colonial stage known as the palmella, and from this there may be built up a branched body (Fig. 38 D, D^) which simulates those of the lower algae. Plant-like forms of this kind occur in every order of the group. It is indeed impossible to define the Phytomastigina from the Algae, and the members of this subclass are regarded both by botanists and by zoologists as coming within the scope of their sciences. Many of the coloured species are liable to produce colourless In- dividuals. This happens in two ways: the chromatophores may be- come bleached owing to the animal living in darkness ; or the rate of division of the chromatophores may lag behind that of the body, so that eventually there are produced offspring in which there are no chromatophores (" apoplastid " individuals). These facts show how the colourless species may have arisen. Members of various orders of the Phytomastigina (cryptomonads, a chrysomonad, a chlamydomonad, and perhaps dinofiagellates) are known to live in the resting stage as symbionts in holozoic organisms (other protozoa, sponges, coelenterates, worms, etc.). Nearly all are yellow or brown (Zooxanthellae) ; most green symbionts {Zoochlorellae) are algae belonging to the Protococcaceae. An exception to this is the 48 THE INVERTEBRATA chlamydomonad of the genus Carteria which lives as a zoochlorella in the tissues of the turbellarian worm Convoluta roscoffensis (Figs. 35, 36). The photosynthetic partner in these symbioses benefits by a supply of carbon dioxide and the nitrogenous excreta of its host ; the latter has waste matters removed, is supplied with oxygen, and sometimes draws on the supply of carbohydrates manufactured by the guest, though it is rarely, as Convoluta, unable to dispense with this nutri- ment, and often, as the reef corals (p. 193), makes no use of it. If kept in the dark it is apt to devour the guest. A photosynthetic organism is specific to a particular host species. In some cases the two partners are capable of living apart; in others, they are mutually dependent. a. epd. ■pyr. Fig. 35- Fig. 36. Fig. 35. A section through a portion of the superficial tissues of Convoluta roscoffensis, showing symbionts belonging to a species of Carteria (Chlamy- domonadidae, Volvocina). From Keeble. ci. cilia of epidermis; epd. epi- dermis; gr.c. "green cells" (symbionts); nu. nucleus of symbiont; pyr. pyrenoid. Fig. 36. A free individual of the species of Carteria which is symbiotic in the resting stage with Convoluta roscoffensis. From Keeble. chl. chloroplast ; e. eye-spot; nu. nucleus ; ^jyr. pyrenoid. The plant organism usually enters the host by being ingested but not digested. It may be passed on from one generation to the next in asexual reproduction or even, as with the green Hydra, in the ovum, but is often lost in the gametes of its host, so that the zygote must be reinfected. Protozoan hosts in symbiosis are usually members of the Radiolaria (Figs. 32 A, 37, 69 A) or Foraminifera, but various ciliates, Noctiluca, etc., also harbour holophytic symbionts. Zooxanthellae are commonest in marine hosts, zoochlorellae in fresh water. The amoeboid faculty possessed by some members of the group may be limited to ingestion, but is often exhibited also in locomotion. Certain forms with such locomotion lose their flagella for shorter or longer periods : some may have done so altogether. When species with PHYTOMASTIGINA 49 nu amoeboid movement become colourless they are only to be separated from the Sarcodina by certain features (of their nuclei, cysts, swarm spores, etc.) which prove them to be related to various mastigophora. Of the orders of the Phytomasttgina, that which contains the most highly organized members is the large and protean group Dinoflagellata, characterized by the posses- sion of two flagella, one longitudinally di- rected and the other transverse, usually in a groove around the body but in a few cases twisted about the base of the longitudinal flagellurrL Three of the remaining orders differ from the rest in the possession, in the anterior part of the body, of a pit ("gullet ") Fig- 37- Lithocircus annu- r 1 • u\u a 11 11 laris. After Lankester. c/)s. or groove, from which the flagella usually ^^^^^^^ ^^^^^^^. ^^ /^. arise. One of these, the Cryptomonadina, cleus; ^or. pore plate; j^.c. has simple contractile vacuoles and its carbo- "yellow cells ". hydrate reserves are of starch : it is held by some authorities to be related to the ancestors of the dinoflagellates. The second, the Euglenoidina, has a more complex contractile vacuole system, and its reserves are of paramylum. The third is the little group Chloromonadina, which differs from the Euglenoidina in having oil reserves only and in the delicacy of its pellicle. The orders without groove or gullet are the Volvocina^ the most plant-like of the Masti- gophora, with green chromatophores (except in a few colourless genera) and starch reserves; and the Chrysomonadina, by some re- garded as the most primitive members of the class, which have yellow or brown chromatophores and no starch reserves and are often capable of becoming amoeboid. Each of these groups exhibits most or all of the varieties of nutri- tion and motility which have been mentioned above. Each of them possesses {a) coloured, flagellate, solitary forms which constitute most of its membership, {b) coloured species, whose individuals pass most of their time in a non-flagellate condition, as a palmella, which is sometimes of branched, plant-like form, {c) colourless saprophytic forms, and {d), except in the Volvocina, colourless holozoic forms. More than one order has purely amoeboid members, non-flagellate throughout the greater part or all of-their existence. The support which this versatility gives to the view that the Mastigophora, and in particular the Phytomonadina, are near the base of the genealogical tree of organisms has already been mentioned. 50 THE INVERTEBRATA Order CHRYSOMONADINA Yellow, brown, or colourless phytomastigina ; without starch reserves, but usually with leucosin and oil ; without gullet or transverse groove ; often amoeboid. The genera briefly mentioned under this and the following orders illustrate the range of variety within the group. Chrysamoeba (Fig. 38 A, A^). One flagellum; two yellow chromato- phores; no skeleton. Egg-shaped when swimming, but on the sub- stratum becomes amoeboid and may lose flagellum. Ingests food by pseudopodia. In fresh waters. Ochromonas (Fig. 38 B). As Chrysamoeba^ but with two unequal flagella; and usually one chromatophore. Dinobryon (Fig. 38 C). Two unequal flagella; two yellow chro- matophores. Secretes a flask-shaped house, which in some species adheres to those of other individuals to form a pseudocolony. In fresh waters. Hydrurus (Fig. 38D-D2). One flagellum; one chromatophore. Passes most of its life in the resting stage, which by division forms a plant-like growth (see p. 47). In fresh waters. Rhizochrysis. Flagella normally lacking; one chromatophore; body naked and permanently amoeboid. Leucochrysis . As Rhizochrysis, but colourless. Silicoflagellata (or Silicoflagellidae) . One flagellum; numerous yellow chromatophores ; a lattice-work case of hollow, siliceous bars. Marine, planktonic, e.g. Distephanus (Fig. 38 F). Coccolithophoridae . One or two equal flagella; two chromatophores (sometimes green) ; a case composed of calcareous plates {coccoliths) or rods (rhabdoliths) enclosing the body. Marine, planktonic, e.g. Syracosphaera (Fig. 38 E). Order CRYPTOMONADINA Green, yellow, brown, or colourless phytomastigina; with starch (and occasionally also oil) reserves ; with gullet or with longitudinal groove, without transverse groove; very rarely amoeboid. Many of the yellow members of this group live in the resting stage as symbionts in other organisms.^ Cryptomonas (Fig. 39 A). Two flagella; two chromatophores, usually green; a gullet. Marine and in fresh waters. ^ Owing to certain features of their nucleus and its mode of division these symbionts have been held to be related to the Dinoflagellata. Their other features, however, are those of the Cryptomonadina. Fig. 38. Chrysomonadina. A, ChrysamoSa radians in the flagellate phase, X 1250. Ai, The same in the amoeboid phase. B, Ochromonas sp., x iioo. C, Dinobryon sertularia, x 750. D/' Flant" oi Hydrurus. Dj , Tip of a branch of the same. D2 , Flagellate stage (" s warmer ") of Hy drums. E, Syracosphaera pulchra, x 2000. F, Distephanus speculum, x 800. After various authors, with modifications, cph. chromatophore ; cth. coccolith; leu. leucosin; nu. nucleus. 52 THE INVERTEBRATA Chrysidella (Fig. 39 B). Two flagella; two yellow chromatophores ; a groove anteriorly. Symbiotic in foraminifera, radiolarians, etc. Cyathomonas (Fig. 39 C). Two flagella; chromatophores absent. Holozoic, seizing food by trichocysts in the gullet. In fresh waters. Chilomonas. Two flagella ; chromatophores absent ; gullet very deep and narrow. Saprophytic. In foul fresh waters. Phaeococcus. Normally in the palmella phase. Marine and in fresh waters. Order EUGLENOIDINA Phytomastigina which have numerous green chromatophores or are colourless ; with reserves of paramylum and sometimes also oil ; with gullet; with contractile vacuole opening by a "reservoir", usually into the gullet; without transverse groove; with stout pellicle, usually with metaboly ("euglenoid movement"). Euglena (Fig. 39 D, D'). A typical member of the group, with chromatophores ; one flagellum, arising from the bottom of the gullet, double at base, and connected by two rhizoplasts to a basal granule behind the nucleus ; pyrenoids present only in a few species ; paramylum reserves; and contractile vacuole fed by accessory vacuoles. The nutrition is interesting. Most species, at least, can live and multiply, with purely holophytic nutrition. All, however, flourish better if traces of aminoacids be present. If the medium be rich in organic substances, the use which is made of these varies with the species. Most, including E. viridis, can take in organic combination nitrogen, but not carbon ; a minority, including E. gracilis, can also obtain carbon in that way. In the dark, if suitable compounds, especially peptones, be present, the latter set of species bleach and live as saprophytes. It has not been established that Euglena uses its gullet to take solid food. Fresh waters, and infusions. Peranema (Figs. 11, 39 E). Without chromatophores; gullet sup- ported by rods and can open or close. Saprophytic and holozoic. Paramylum reserves formed. In infusions. Copromonas { = Scytomonas , Fig. 39 F, F'). Without chromato- phores; body pear-shaped; no metaboly; gullet long and narrow. Nutrition holozoic, chiefly by bacteria. Coprozoic in dung of frogs. After some days of binary fission syngamy takes place between ordinary individuals (hologamy), the nuclei first throwing out two "polar bodies". Some zygotes encyst; others continue to divide. Finally all encyst. The cysts are washed away and swallowed by a frog or toad with its food. They pass uninjured through the gut and hatch in the moist faeces, where alone the active stage exists. Colacium. Normally in the palmella phase, forming branched, plant-like growths. Fig. 39. Cryptomonadina and Euglenoidina. A, Cryptomonas ovata, x 900. B, Chrysidella schaudinni, in the resting stage. C, Cyathomonas truncata, X 1000. U, Euglena viridis, x 400. D', A longitudinal section of the anterior end of the same, more highly magnified. E, Pera?tema trichophorum, x 850. F, Copromonas siibtilis, x about 1700. F', A pair of the same, beginning to conjugate, less highly magnified. After various authors, with modifications. c.vac. contractile vacuole ; c.vac' accessory contractile vacuole ; cph. chromato- phore; e. eye-spot ; /.z^«c. food vacuole;^, flagellum; ^j/. gullet; nn. nucleus; pmy. paramylum grains; res. reservoir; rod. stiflfening rods of gullet; stch. starch grains; tri. trichocysts. 5^. THE INVERTEBRATA Order CHLOROMONADINA Phytomastigina which have numerous green chromatophores or are colourless ; with reserves of oil ; gullet ; and complex contractile vacuole ; without transverse groove ; possessing a delicate pellicle, or amoeboid. Vacuolaria. Typical, bright green members of the group, which pass much of the life history in the palmella stage. In fresh waters. Order DINOFLAGELLATA Phytomastigina which have numerous yellow, brown, or green chromatophores or are colourless; with reserves of starch or oil or both; with complex vacuole system; with two flagella, one directed backwards and usually in a longitudinal groove (sulcus) and the other transverse, usually in a more or less spiral groove (annulus) ; usually with an armour of cellulose plates, but sometimes amoeboid. The complex vacuoles of dinoflagellates are not, as was held, con- tractile, but contain water driven into them through their external pores by the action of the flagella. Their function is unknown. Possibly they are hydrostatic, or alimentary, or both. The plane of fission is oblique, but resembles the longitudinal fission of other Mastigophora in passing between the two flagella. Fission may be within or without a cyst: in either case it may be simply binary or repeated ; within a cyst it is sometimes multiple. The products of repeated binary fission of pelagic forms sometimes hang together for a considerable time as a chain. The occurrence oisyngamy is suspected but has not yet been proved beyond doubt. The typical members of this order are free-living and highly organized, but it includes forms which are greatly degenerate and only recognizable as belonging to it while they are spores. The members may be holophytic, saprophytic, or holozoic, feeding in the latter case by pseudopodia either from a spot on the sulcus or at any point. They are usually pelagic, sometimes parasitic, and for the most part marine. Ceratium (Fig. 40 A). Typical, armoured, holophytic species; with three long spines. In freshwater forms the chromatophores are green ; in marine species they are yellow or brown. Dinophysinae. Pelagic genera, often of bizarre form, with the annulus at one end of the body, and the shell in two lateral plates. Polykrikos (Fig. 40 B). Soft-bodied species; colourless and holo- zoic; with the flagella and other external features repeated several times along the axis of the body, and the nucleus also repeated, but not in correspondence with the other features (see p. 10). The protoplasm contains peculiar nematocyst-like organs. Holozoic. PHYTOMASTIGINA 55 Oodinium (Fig. 34). Thin-cuticled ; pear-shaped; colourless; living as an ectoparasite on marine pelagic animals, and possessing the typical dinoflagellate organization only in the spore stage. Dinamoehidium. Colourless and holozoic; completely Amoeba-like in the ordinary phase, but forming dinoflagellate swarm spores in a fusiform cyst. Noctiluca (Fig. 41). (Formerly placed in an independent order — Cystoflagellata.) Large, peach-shaped forms; colourless and holo- zoic; with highly vacuolated protoplasm; a stout pellicle; and, in nil. tr.fl.- t^-ann. ntc.--i Fig. 40. Dinoflagellata. A, Ceratium macroceras, x about 300. B, Polykrikos schwarzi, x 250. C, A discharged "nematocyst" of Po/j'/en'i^o^. After various authors, with modifications, ann. annuli; cph. chromatophore ; cu. cuticle; In.fl. longitudinal flagellum; ntc. "nematocyst" ; nu. nucleus; sul. sulcus; sut. suture between plates of cuticle ; tr.fl. transverse flagellum. the groove of the peach, an elongate mouth, a small flagellum, a structure known as the tooth which is said to represent the transverse flagellum, and a strong tentacle, homologous with a similar structure in certain more normal dinoflagellates. Noctiluca is phosphorescent. Like other dinoflagellates it reproduces by binary fission and by spore formation after multiple fission. The spores are more dinoflagellate- like than the adult. Marine, pelagic. Dinothrix. Normally in the palmella phase, forming thread-like growths. Marine. 56 THE INVERTEBRATA Order VOLVOCINA Phytomastigina which have usually a flask-shaped, green chromato- phore, with one or more pyrenoids, but are sometimes colourless, though never holozoic; form starch reserves, even when colourless; have no gullet or transverse groove ; possess usually a cellulose cuticle and often haematochrome; and regularly undergo syngamy. Of all the Mastigophora, the members of this order most closely resemble the typical plants. Fig. 41. Fig. 41. Noctiluca, x 65. A, Ordinaiy individual. B, Spore formation. C, A spore. After various authors, with modifications, fl. flagellum; nu. nucleus; ten. tentacle; tth. tooth. Fig. 42. Haematococcus lacustris, x 475. After West. A-C, Individuals in ordinary phase, showing strands of protoplasm from body to cuticle. D-F, Successive stages in fission. G, H, Individuals in resting phase. Chlamydomonas (Figs. 23, 25). Typical solitary members of the order, with two flagella ; an eye-spot ; a close-fitting cellulose cuticle ; and one pyrenoid. The various species exhibit isogamy, anisogamy, and intermediate conditions (see p. 31). In fresh waters. Polytoma (Fig. 24). A colourless Chlamydomonas', retaining the eye-spot (usually) and the habit of starch formation; but with the VOLVOCINA 57 cuticle composed of some substance which does not give the cellulose reaction. Nutrition saprophytic by means of simple substances (fatty acids, aminoacids, etc.). Syngamy is facultatively hologamy or merogamy, isogamous or anisogamous, according to the age of the gametes. In infusions of decaying animal substances. Carter ia (Figs. 35, 36). Differs from Chlamydomonas in having four flagella. It is probably a species of this genus that is symbiotic in the turbellarian Convoluta roscojfensis . Fig. 43. Pandorina. From Godwin, a. The adult colony of sixteen similar flagellated zooids, x 200. h, A colony undergoing asexual reproduction, X 450 — each zooid has divided to form a daughter colony which still remains within the parent body. Some of the colonies have already produced flagella, and will shortly break out of the wall which enclosed the parent, b-g, Stages in sexual reproduction — b, Motile gametes, c, Stage immediately after fusion of two gametes, d, Later stage showing flagella withdrawn, e, Later stage showing resting zygote with thickened wall. /, Motile individual produced by the zygote on germination, g. New colony produced by vegetative division of the motile individual. Haematococcus ( = Sphaerella, Fig. 42). Differs from Chlamy- domonas in that there is a wide space, traversed by protoplasmic threads, between body and cuticle; several pyrenoids. Much hae- matochrome is often present. Isogamous. Common in collections of rainwater. Pandorina (Fig. 43). Spherical, fi:ee-swimming colonies of 16 or 32 green pear-shaped zooids, each with the organization of the solitary members of the order, closely pressed together with the narrow end inwards and the flagella outwards. An additional cellulose envelope containing mucilage encloses the whole colony. The colonies are re- produced in two ways: (i) asexually, by the repeated fission of each 58 THE INVERTEBRATA zooid to form a group of 16 like the parent colony, the dissolution of the colonial and zooid envelopes, and the setting free of 16 young colonies ; (2) sexually, by the division of each zooid and the setting free of its products as gametes which, except in size, resemble ordinary zooids. Since the number of fissions in the formation of gametes differs in different colonies, the gametes differ in size. They unite in- differently, so that some of the unions are isogamous, though most are anisogamous. The zygote, after a period of encystment, becomes a free flagellate and divides to form a colony. In fresh waters. Eudorina (Fig. 3 a). Colonies which differ from those of Pandorina in that : {a) the zooids are spaced on the inside of the common en- velope, though connected by strands of protoplasm; {b) the sexual reproduction is strongly anisogamous, since in some colonies the zooids do not divide but, becoming somewhat larger, act as macro- gametes, while in others each zooid divides into a bundle of 16-64 slender individuals (microgametes), which are set free and fertilize the individuals of a macrogamete (female) colony. Pleodorina (Fig. 36, c). Rather larger colonies which differ from those of Eudorina in that some of the zooids do not perform repro- duction. These zooids, which are smaller than the rest, are termed "somatic". Volvox (Figs. 44-46). Large, subspherical colonies resembling in general features those of Pleodorina but with smaller and more numerous zooids, of which a much smaller proportion is reproductive. Those zooids which perform asexual reproduction are known as parthenogonidia : the plates of young zooids which arise by their radial fission, curving into spheres to form the new colonies, bulge into the hollow of the parent colony, where they remain for a time before they are set free. The clusters (antheridia) of microgametes arise in the same way. In some species the microgametes are considerably modi- fied, being pale, very slender, and bearing their flagella in the middle of their length. Male, female, and asexual reproductive zooids may be found in any combination in a colony. Details of the structure of the colonies are shown in Figs. 45, 46. Subclass ZOOMASTIGINA Mastigophora which do not possess chromatophores and are not otherwise practically identical with coloured forms. By one or more of the following peculiarities of the Zoomastigina most members of the group are distinguished from most colourless members of the Phytomastigina. I. The Zoomastigina never have starch or other amyloid re- serves. MASTIGOPHORA 59 2. They often have more than two flagella. This is very rare in the Phytomastigina. Fig. 44, Volvox aureus. After Klein, a (x 180), A medium-sized colony showing as round black dots the numerous "somatic cells" of which it is made up; the protoplasmic connections between them, and the cell-walls, can only be made visible by staining. The colony contains three types of re- productive units : daughter colonies (d.c.) produced asexually by division of a single zooid; ripe macrogametes or young zygotes (z); and young "an- theridia" (an) whose contents are dividing up and will eventually form microgametes. b, A colony of microgametes which has just escaped from the antheridium. c, Mature antheridia as seen in surface view of a colony; in two the microgametes are seen sideways, and in two endways. 3. With a single exception,^ it has not yet been established that syngamy occurs in any of them. 4. Many of their parasitic members possess parabasal bodies. ^ Helkesimastix, a coprozoicmember of the Protomonadina, performs hologamy. 6o THE INVERTEBRATA Order RHIZOMASTIGINA Zoomastigina with one or two flagella, and the whole surface of the body permanently amoeboid. Mastigamoeba (Fig. 47 A). One flagellum; numerous, finger-like pseudopodia. In fresh waters. ^Mm -(mo)— I V.Cc.l. V42. Fig. 45. Diagrams to show the structure of the colony of two species of Volvox. After Janet. V.a.i. Surface view of a small part of the colony of V. aureus. V.a.z. Section through a similar region. V.g.i. and V.g.2. show V. globator in the same way. The zooids are very different in shape in the two species, but in both they have been separated by the formation of mucilage {mu) by the cell-walls; the unaltered middle layer of the walls {m.l.) is still visible. Protoplasmic strands (p.c), fine in the one species and thick in the other, connect the zooids. Each zooid, with its curved chloroplast (ch) often containing more than one pyrenoid, its eye-spot, and two flagella, has the structure of a Haematococcus. Order HOLOMASTIGINA Zoomastigina with numerous flagella, and the whole surface of the body capable of amoeboid action. Multicilia. Spherical, with 40 or 50 flagella scattered evenly over the whole surface, at any point on which food can be ingested by amoeboid action. A marine species with one nucleus; freshwater species multinucleate. Fig. 46. Volvox. After Janet and Klein, a, V. aureus, a daughter colony of small size seen through the layer of zooids of the parent colony; the opening left in the young colony at its formation is shaded, b, V. aureus, a single macrogamete among the ordinary somatic zooids; abundant protoplasmic filaments connect it with surrounding zooids and it contains large nucleus (a) and chloroplast (ch). c, V. aureus, a plate of mature microgametes just liberated from an antheridium and now beginning to separate. Each contains nucleus (n), eye-spot (y), flagella, a chloroplast (ch), and pyrenoid (p). d, V. globator, diagrammatic section through the middle of an old colony showing three large daughter colonies projecting into the interior of the parent colony which is full of thin mucilage with a radiating structure, e, V. globator, similar section to d, showing three antheridia {an) in different stages of maturity and three large macrogametes {e). Both types of organ have been formed from a single zooid of the parent sphere into the interior of which they now project. In d and e, the flagella of the somatic zooids have been omitted. 62 THE INVERTEBRATA f-vac.-/- Fig. 47. Zoomastigina. A, Mastigamoeba aspera, x about 300. B, Oikomonas termo, x 2000. C, Monas vulgaris, x 2000. D, Bodo saltans, x 2000. E, Try- panosoma hrucei, x 2800. F, Crithidia sp., x 2300. After various authors, with modifications, ba.gr. basal granules of flagella ; bri. bristle-like processes borne by the surface of the protoplasm; c.vac. contractile vacuole ;/.z;ac. food vacuole; fl. flagellum; fl.' trailing fiagellum; M. position of mouth-spot; nu. nucleus; p.by. parabasal body; ^5. pseudopodium ; rh. rhizoplast; u.me. undulating membrane. ZOOMASTIGINA 63 Order PROTOMONADINA Zoomastigina with one or two flagella; amoeboid movement, if present, not active over the whole surface of the body ; and no extra- nuclear division centre. Monas (Fig. 47 C). Two unequal flagella. Ingestion at base of flagella. Except for absence of chromatophores much resembles Ochromonas among the Phytomastigina and is probably related to that genus. In fresh waters and infusions. Bodo (Fig. 47 D). Two rather unequal flagella, of which one trails freely behind and is used for temporary anchoring. Ingestion at a spot near the base of the flagella. In infusions and coprozoic. —fl- % -u.me. -nn. B ~~p.by. Fig. 48. A diagrammatic comparison of various Trypanosomidae. A, Her- petomonas. B, Leishmania. C, Crithidia. D, Trypanosoma, ba.gr. basal granule;^, flagellum; nu. nucleus; p.by. parabasal body; u.me. undulating membrane. Oikomonas (Fig. 47 B). One flagellum. Ingestion of food as in Monas. This genus bears the same relation to certain uniflagellate Chrysomonadina that Monas bears to Ochromonas. In fresh waters and soil. Trypanosomidae (Fig. 48). Parasites, with one flagellum; a slender, usually pointed shape; a strong pellicle without ingestion spot; a parabasal body; and no contractile vacuole. This family, which con- tains many dangerous parasites of man and domestic animals, appears to have originally infested invertebrates and to have obtained access 64 THE INVERTEBRATA to vertebrates owing to the latter being subject to attack by the original hosts. The original mode of infection was by faeces. The species of each genus assume, in certain circumstances, the forms characteristic of other genera. The following are the principal genera. Herpetomonas { = Leptomonas) . Basal granule and parabasal body at one end, near the origin of the flagellum. Parasitic in the gut, principally of insects, but also of other invertebrates and of reptiles. Leishmania. Oval bodies containing a nucleus, parabasal body, basal granule and rhizoplast, but with no flagellum, infesting the tissues of vertebrates, and transferred by flies of the genus Phleboto- muSy in whose gut they assume the form of Herpetomonas. In Man they cause kala-azar and Oriental sore. Crithidia (Fig. 47 F). Flagellum starts from a basal granule near the middle of the long, slender body, to which the flagellum is united by an undulating membrane; parabasal body placed between the basal granule and the nucleus. Parasitic in the gut of insects. Trypanosoma (Fig, 47 E). As Crithidia^ but the basal granule of the undulating membrane and the parabasal body are beyond the nucleus, towards the non-flagellate end. Many species, all parasitic in the blood and other fluids of vertebrates, and nearly all (not T. equiperdum) distributed by a second, invertebrate, host, which is usually an insect for terrestrial species and a leech for aquatic species. In the invertebrate the trypanosome passes for a time into a condition in which it resembles Crithidia^ and during which it is incapable of reinfecting the vertebrate. Reinfection is in some species (e.g. T. lewisi in the rat, transmitted by a flea) by the invertebrate or its faeces being swallowed by the vertebrate ; this is probably the original mode of obtaining entry to the vertebrate host. Other species (e.g. T.gambiense^ transmitted by a tsetse fly) are reintroduced to the vertebrate by the bite of the invertebrate. T. equiperdum, parasitic in horses, in which it is the cause of " dourine", is transmitted by coitus and has dispensed with the invertebrate host. Most, if not all, of the pathogenic species have a wild host with which they are in equilibrium and in which they are non-pathogenic. T. lewisi, non-pathogenic in the blood of the rat, has a period of intracellular multiple fission in the stomach of the flea and then passes into the rectum of the latter, where it changes from the crithidial to the trypanosome form and becomes capable of reinfecting the verte- brate, which it accomplishes in the manner mentioned above. T. cruzi, the cause of Chagas' disease in Man in South America, is non-patho- genic in the armadillo. It is transmitted by the bug Triatoma, in which it probably has an intracellular stage, and becomes infective in the faeces. In the vertebrate host, it passes most of its time, and reproduces, as a Leishmania form, in the tissues. T. gambiense and T. rhodesiense^ ZOOMASTIGINA 65 causes of sleeping sickness in man when they have passed into the cerebrospinal fluid, and T. brucet, the cause of African cattle sickness, are non-pathogenic in antelopes. Their crithidial stage is passed in the salivary glands of the tsetse (Glossina), reproduces by binary fission, and is not intracellular. They are transmitted to the vertebrate host by the bite of the fly. Choanoflagellata (or Choanoflagellidae) . Uniflagellate, generally fixed, forms; with a protoplasmic collar around the base of the flagel- lum. Ingestion by attraction of particles by the flagellum to the outside of the collar, adherence to this, and transference by streaming of protoplasm to the base of the collar, where they are received by a vacuole which is formed between the cuticle, if present, and the proto- '■■f-vac. Fig. 49. Choanoflagellata. A, Monosiga brevipcs, x 1200. B, Codosiga um- bellata, x 310. Both after Saville-Kent. C, Ingestion in Codosiga. f.vac. food vacuole. The dotted lines show the currents set up by the flagellum, the small arrow the transport of the food particles on the collar. plasm (Fig. 49 C): defaecation within the collar. There is usually a stalk, generally not of living matter. This may branch, and thus unite numerous zooids. Examples are Monosiga (Fig. 49 A), solitary, with protoplasmic stalk; Codosiga (Fig. 49 B), branched, with cuticular stalk. Order POLYMASTIGINA Zoomastigina with two to many, generally with more than three, flagella, and an extranuclear division centre. The genera here placed in one order are usually separated as Poly- mastigina, Hypermastigina, and Diplomonadina. They are the most highly organized members of the Mastigophora. Trichomonas (Fig. 50). (One of the Polymastigina sensu stricto.) Body roughly egg-shaped ; with four flagella, of which one is directed 66 THE INVERTEBRATA backward and united to the body by an undulating membrane ; a cyto- stome near the broad anterior end; and an axostyle which projects from the posterior end. The united basal granules act as a division centre, possibly in virtue of a centriole concealed among them. The cytostome is used for ingestion. A staining body which follows the base of the undulating membrane has been regarded as the parabasal body, but a deeper-lying structure is now asserted to represent that Fig. 50. Trichomonas niuris, semidiagrammatic. From Hegner and TaHa- ferro, after Wenrich. axs. axostyle; ce. compound basal granule which acts as a centriole; ch.gr. inner row of chromatic granules; ch.gr.' outer row of chromatic granules; ch.rd. chromatic basal rod of undulating membrane; ch.rg. chromatic ring at the emergence of the axostyle ; fl. anterior flagella ; fl,.' posterior flagellum; kar. karyosome; M. mouth (cytostome); nu. nucleus; p.hy. parabasal body; u.me. undulating membrane; u.me.' posterior flagellum lying along the edge of the undulating membrane. organ. In the cytoplasm, a number of " chromatic granules " are also present. Several species, parasitic in various cavities of vertebrates, including the mouth, intestine, and vagina of man. Hexamitus { = Octomitus, Fig. 4). (Diplomonadina.) Body elon- gate ; without gullet ; presenting strong bilateral symmetry ; and bear- ing on each side four flagella, three anterior and one posterior, the basal granules of the foremost being united; and an axostyle. Two nuclei are present, one on each side of the body, near the anterior POLYMASTIGINA 67 group of basal granules, with which they are connected. Intestinal parasites of vertebrates. Giardia { = Lambliay Fig. 51). (Diplomonadina.) Shaped like a half- nu Fig. 51. Fig. 52. Fig. 51. Giardia intestinalis, from the intestine of man. Semidiagrammatic. axs. axostyle (axoneme) ; ba.gr. basal granules ; ce. centriole ; cone, ventral con- cavity (" sucker "); yz. fibre around concdivity ; fl., fl.' , fl.'\ fl.'" anterolateral, posterolateral, ventral, and caudal flagella; kar. karyosome; la.sd. lateral shield, the thickest part of the body; nu. nucleus; p.by. parabasal body; rh. rhizoplasts. Fig. 52. A diagram of the structure of Trichonympha campanula, showing a portion of each layer. From Hegner and Taliaferro, after Kofoid and Swezy. alv.l. alveolar layer; ant.fl. anterior flagella; ba.gr. rows of basal granules; ce. point at which the spindle arises in division; chr. chromatin granules in nucleus; ecp. ectoplasm; enp. endoplasm; f.b. food bodies; lat.fl. lateral flagella; long.my. longitudinal myonemes; nu. nucleus; obl.f. oblique fibres (rhizoplasts) ; /)e/. pellicle ; /)05f.yZ. posterior flagella; ^wr/.rd'^. surface ridges of pellicle; tr.niy. transverse myonemes. pear, broad end forwards, with, on flat side, a concavity for adhesion. Organization as Hexamitus but all flagella in middle or hinder region. Parasitic in intestine of man and other mammals. Trichonympha (Fig. 52). (Hypermastigina.) Body narrower in 68 THE INVERTEBRATA front than behind; provided with very numerous flagella arranged in three distinct sets; without gullet. At the front end is a papilla. The ectoplasm, thin behind, is strong and complex in the fore part of the body, where it is composed of the following layers: (i) a pellicle, sculptured into longitudinal ridges, (2) a layer containing longi- tudinal rows of the basal granules of the flagella, (3) a layer containing a network of rhizoplasts ("oblique fibres"), (4) an alveolar layer, (5) a layer of transverse myonemes, (6) a layer of longitudinal my- onemes. In the conical front region on which the first set of flagella stand, the rhizoplasts and basal granules are merged to form con- verging strands with which the flagella are connected. At division this conical apparatus acts as a division centre, dividing first and forming the spindle between its halves as they separate. Possibly it does so in virtue of a concealed centriole. Trichonympha is symbiotic with ter- mites, in whose gut it lives (p. 435). The termite devours wood but is unable itself to digest it. The digestion is performed by the protozoon, which obtains in return food and lodging. Wood particles are con- tained in the endoplasm of the hinder part of the body of Tricho- nympha, into which they are ingested by the cupping-in of this region under the action of the myonemes of the forepart. Class SARCODINA (RHIZOPODA) Protozoa which in the principal phase are amoeboid, without flagella; are usually not parasitic; have no meganucleus; and, though they may have a phase of sporulation, do not form large numbers of spores after syngamy. With the exception of the Amoebina and Foraminifera, which are undoubtedly closely related, the orders of this class have much less affinity with one another than have those of the Mastigophora. In all of them flagellate young and gametes are common. Order AMOEBINA Sarcodina which have no shell, skeleton, or central capsule; whose pseudopodia never form a reticulum and are usually lobose ; and whose ectoplasm is never vacuolated. Thus defined, the group excludes forms such as Arcella which differ from its members practically only in the possession of a shell. These forms, however, are also connected with the typical Forami- nifera by intermediates (as Lieberkuhnia and Allogromia) . There is, indeed, a continuous series from naked amoebae to such foraminifera as Polystomella, and the drawing of a boundary line between the groups of which they are typical is a matter of convenience. PROTOZOA 69 Naegleria (Fig. 53). Small amoebae which live in various foul in- fusions ; possess a contractile vacuole ; and in certain conditions pass into a biflagellate phase. Naegleria is placed here rather than among the Rhizomastigina because it is most often in the non-flagellate con- dition, its flagellate phase, though fully grown, is not known to per- form reproduction, and the general features of the amoeboid phase are those of the amoebina of the Umax group, most of whose members appear to have no flagellate phase. These organisms form one or two _ fcon.vac. Fig. 53. Naegleria bistadialis, X 800. Partly after Kiihn, in Doflein. A, Amoeboid condition. B, Transition to flagellate condition. C, Flagellate condition, coti.vac. contractile vacuole; rh. rhizoplast. broad pseudopodia, are given to assuming a slug-like shape with one pseudopodium at the foremost end, and have a very simple nucleus with a large karyosome. Vahlkampfia, also found in foul infusions, is a typical member of the Umax group. Amoeba (Fig. 54). Typical amoebae, with numerous pseudopodia; contractile vacuole; and no flagellate phase. Various species, of which the commonest three are shown in the figure. The true A. proteus is the largest of the common Amoebae, has a lens-shaped nucleus and yo THE INVERTEBRATA longitudinal ridges on the ectoplasm, forms spores endogenously in the unencysted condition, and does not normally feed on diatoms, which form a great part of the food of A. duhia. Entamoeba (Figs. 55, 56). Parasitic amoebae; without contractile vacuole. Reproduction during most of the life history is by binary fission. Finally encystment takes place and in the cyst the nucleus divides several times. The cysts pass out of the host and infect a new ^■f£-..5>;.*" Jia-tf'- '..' ■-'■' 1^ ><^- ■\^^ -••'■•'''?; ._.;:® #?; Fig. 54. Amoebae. From Hegner and Taliaferro, after Schaeffer. A, A. pro- teus. a^, Equatorial view of nucleus, a^, Polar view of nucleus, a^. Equatorial view of nucleus in the folded condition often seen in this species, a*, Crystal of the kind found distributed in the endoplasm of the species. B, A. discoides. 6^, 6^, Equatorial and polar views of nucleus. 6^, Crystal. C, A. dubia. c^,c^. Equatorial and polar views of the nucleus, c^-c^", Crystals and concretions. Dimensions in microns : A, 600 in length. B, 450 in length. C, 400 in length. a^, 46 X 12. b^, 40 X 18. c\ 40 X 32. a*, maximum 4-5. b^, maximum 2-5. ^3_^io^ maxima 10 to 30. individual, in which they are dissolved and set free their contents, which divide into uninucleate young. The cysts must remain in a fluid medium if they are to cause reinfection. Several species exist, occur- ring in various vertebrates and invertebrates. E. coli is a harmless commensal in the colon of man, feeding on bacteria, etc. E. histolytica { = E. dysenteriae), a parasite which often causes dysentery and oc- casionally abscesses of the liver and other organs, differs from E. coli in having a distinct ectoplasm, in the central position of the karyo- ecp. enp. B /• vac. Fig. 55. Entamoeba, x about 2000. After Dobell and O'Connor. A, E. histo- lytica. B, £". to/t. ^.c. ingested red blood corpuscles; ecp. ectoplasm; enp. endoplasm ; f.vac. food vacuole ; kar. karyosome, eccentric in E. coli ; nu. nucleus ; ps. pseudopodium. or / Fig. 56. Fig. 57. Fig. 56. A diagram of the life cycle of Entamoeba histolytica, a-f, Encystment and formation of amoebulae. I-III, Binary fission (in gut of host). Fig. 57. Pelomyxa palustris. Partly after Doflein. f.p. undigested particles swallowed with food ; ^(y. glycogen granules; nu. nuclei (stained). 72 THE INVERTEBRATA « some and in certain other features of the nucleus (Fig. 55), and in forming only four, instead of eight, nuclei in the cyst. This species breaks up by digestion cells of the intestinal epithelium and other tissues, absorbs the soluble products, and ingests portions of the destroyed cells and also red corpuscles. Pelomyxa (Fig. 57). Large, multinucleate species, living in, and feeding by ingesting, the mud of stagnant fresh waters rich in vegetable debris. The cytoplasm contains glycogen granules (see p. 20). Order FORAMINIFERA Sarcodina w^hich have either a shell or reticulate pseudopodia or, usually, both these features ; and in pelagic species a vacuolated outer layer of protoplasm. The shell may be of one or of several chambers, and is composed in different cases of different materials, nitrogenous, calcareous, siliceous, or of foreign particles. The pseudopodia may be lobose, filose, reticulate without streaming of particles along them, or reticulate with streaming. The latter type alone is found in the Polythalamia. The reproduction of the single-chambered forms (Monothalamia) is both by binary and by multiple fission. In binary fission, Lie- herkUhnia and Trichosphaerium divide the shell. In the rest, a portion of the protoplasm emerges from the old shell and secretes a new one (Fig. 58), the nucleus or nuclei divide, one of the products of each passing into the protruded protoplasm while the other remains in the old shell, and the two portions of protoplasm break apart. Multiple fission usually produces amoebulae, sometimes flagellulae. The latter are known or suspected to be gametes. In these forms there does not usually appear to be a regular alternation of sexual and asexual repro- duction. In the Polythalamia binary fission does not occur, and in some of them, perhaps in all, there is a more or less regularly alternate production of asexual amoebulae and flagellate gametes. Suborder MONOTHALAMIA Foraminifera, usually of freshwater habitat; with non-calcareous, single-chambered shells; whose pseudopodia are rarely reticulate; and whose protoplasm does not extend as a layer over the shells. Arcella (Figs. 22, 59). Shell pseudochitinous, shaped like a tam-o'- shanter cap, finely sculptured; pseudopodia lobose; two or several nuclei and a chromidium present. Gas vacuoles in the protoplasm are said to contain oxygen and to have a hydrostatic function. Re- production by binary fission, or by budding to form amoebulae with fine pseudopodia {Nucleariae). In fresh waters. Fig. 58. Binary fission of Eiiglypha alveolata, x about 450. From Hegner and Taliaferro, after Schewiakoff. A, B, C, D, Successive stages in the mitosis, with formation and occupation of a new shell. att. ah. Fig. 59. Arcelladiscoides, x 500. From Leidy. A, Seen from above. B, Seen from the side, optical section, ott. thread attaching animal to inner surface of shell; f.vac food vacuole; g.vac. gas vacuole; nu. nucleus; op. edge of opening into shell ; ^^. pseudopodia; j/?. shell. 74 THE INVERTEBRATA Diffiugia (Fig. 60). Shell of sand grains, etc., united by organic secretion, pear- or vase-shaped; pseudopodia lobose; one or two nuclei and a chromidium present. Gas vacuoles sometimes formed. In fresh waters. Euglypha (Figs. 7, 58). Shell resembling that of Difflugia but formed of siliceous plates secreted by the animal; pseudopodia filose. In fresh waters. Trichosphaerium. Flat, encrusting forms, with a jelly coat; finger- like pseudopodia protruding through separate openings in the coat; and numerous nuclei. Reproduction alternately by escape of amoe- bulae and of biflagellate isogametes ; but both generations can perform plasmotomy. Marine. sh. Fig. 60. Fig. 61. Fig. 60. Difflugia urceolata, x 100. After Leidy. sh. shell composed of particles of sand containing body of the animal ; ps. pseudopodia. Fig. 61. Lieberkuhnia wagneri. After Verworn. Lieberkiihnia (Fig. 61). Shell thin, flexible, egg-shaped, with mouth directed to one side; pseudopodia reticulate. Shell divided at binary fission. Marine and in fresh waters. Suborder POLYTHALAMIA Foraminifera, nearly always of marine habitat; usually with a shell of several chambers, which is most often calcareous, but sometimes with one chamber or no shell ; whose pseudopodia are reticulate ; and whose protoplasm extends as a layer over the shell. The external layer of protoplasm can be withdrawn into the shell. FORAMINIFERA 75 The shells of this group are typically many-chambered and cal- careous, but a fair number are one-chambered, and most of these and some of the many-chambered shells are composed of foreign particles {arenaceous). Either kind may be imperforate or perforate by numerous small pores, but most of the non-calcareous shells are imperforate. The one-chambered shells are of various shapes. They usually grow A B Fig. 62. A, Section of a foraminifer in which each septum is formed of a single lamella. B, One in which the septum is formed of two lamellae and a supplemental layer is present. After Carpenter, a, passages between the chambers; h, septum; c, anterior wall of last chamber; d, supplemental skeleton. A B Fig. 63 Selsea. form, Dimorphism of Numniulites laevigatus, Bracklesham Beds (Eocene), From Woods. A, Section of the entire shell of the megalospheric X 9. B, Section of the central part of the microspheric form, x 9. by extension at their openings. Shells with more than one chamber grow by the addition of chambers. The protoplasm bulges from the mouth of the shell and there secretes around itself a new chamber into which opens the previous mouth. The chambers may be arranged in a straight line, as in Nodosaria (Fig. 6B), or in a spiral, as in Poly- stomella, etc. (Figs. 62, 63, 65), or occasionally irregularly; and the shell may be strengthened by the deposition, upon their original 76 THE INVERTEBRATA walls, of a supplemental layer (Fig. 62 B). The nuclei, where there is more than one, bear no constant relation to the chambers. In many species the shells are dimorphic, the two forms (Figs. 63, 66) being distinguished by the size and arrangement of the first formed chamber, which is small in one (the microspheric form) and larger in the other (megalospheric) . These forms correspond to the alternation of generations in the life cycle (Fig. 66), the microspheric form, which usually becomes multinucleate at an early stage, reproducing asexu- ally by multiple fission, while the megalospheric form, which remains uninucleate till it is about to reproduce, produces gametes. Most foraminifera are creeping organisms, but the Globigerinidae are planktonic and have, correspondingly, vacuolated ectoplasm and long slender spines on the shell. The shells of such forms, falling to the bottom, form an important constituent of many deep-sea oozes. Allogromia (Fig. 64). Shell one-chambered, egg-shaped, pseudo- chitinous. Marine and in fresh waters. Rhabdammina (Fig. 6 A). Shell one-chambered, straight or forked, tubular, composed of foreign particles. Marine. Nodosaria (Fig. 6B). Shell perforate, calcareous, consisting of several chambers arranged in a longitudinal row, the mouth of each chamber opening into the next younger and larger. Marine. Polystomella (Figs. 65, 66). Shell perforate, calcareous, consisting of numerous chambers, arranged in a flat spiral, and complicated as follows in the details of their architecture: each whorl is equitant, i.e. overlaps the previous whorl at the sides and thus hides it; the mouth is replaced by a row of large pores ; backward pockets {retral processes) stand along the hinder edge of each chamber; the sup- plemental layer contains a system of canals filled with protoplasm. Marine. The life cycle of this genus, which shows the alternation of generations described above, has been followed in detail (Fig. 66). Nummulites (Fig. 63). As Polystomella but with more chambers. Marine. Includes, besides recent forms, large fossil species in Eocene limestones. Globigerina (Fig. 6C). Shell perforate, calcareous, chambers fewer and less compact than in Polystomella, arranged in a rising (helicoid) spiral, and bearing long spines. External layer of protoplasm frothy, with large vacuoles by which the specific gravity is reduced. Marine, pelagic. Its shells are common in oceanic oozes and in chalk. Order RADIOLARIA Marine, planktonic Sarcodina, which have no shell but possess a central capsule and usually a skeleton of spicules ; whose pseudopodia are fine and radial and usually without conspicuous axial filament; and the outer layer of whose protoplasm is highly vacuolated. Fig. 64. Allogromia oviformis, x 230, but the pseudopodia less than one- third their relative natural length. From M. S. Schultze. sh. shell; ppm. pro- toplasm surrounding shell; ps. pseudopodia, fusing together in places and surrounding food particles such as diatoms, which have adhered to the pseudopodia owing to the stickiness of the latter, and are digested in situ, without the formation of visible food vacuoles around them. 78 THE INVERTEBRATA Fig. 65. Polystomella crispa, x 45. After M. S. Schultze. sh. shell ;/)/)m. a mass of protoplasm formed by the fusion of pseudopodia ; ps. pseudopodia. The retral processes are darkly shaded : the external protoplasm is not visible. FORAMINIFERA 79 Fig. 66. Stages in the life cycle of Polystornella. Semidiagrammatic. A, Me- galospheric form, decalcified and stained. B, Shell of the same surrounded by escaping gametes. C, D, Conjugation. E, Zygote. F, Microspheric form, decalcified and stained. G, Shell of the same surrounded by escaping amoe- bulae. H, Young megalospheric individual with three chambers, gam. gametes ; i.wh. inner whorl of spiral ; o.wh. outer whorl ; nu. nucleus, ret.pr. re- tral processes; i, first chamber. 8o THE INVERTEBRATA The pseudopodia branch, and to some extent join: they are said to contain an axial filament and they show streaming of granules. The central capsule is a pseudochitinous structure, of varying shape accord- ing to the species, which encloses the nucleus and some cytoplasm containing oil globules. It is perforated by pores, which by their arrangement characterize the suborders, being evenly distributed in the Peripylaea {Spume llaria), gathered into groups in the Actipylaea (Acantharta), concentrated into one *'pore plate" in the Monopylaea (Nassellaria), and represented by three openings or "oscula" in the Tripylaea {Phaeodaria). The spicules are usually siliceous, but in one group (Acantharta) they are said to be of strontium sulphate. They are rarely absent, occasionally loose, but usually united into a lattice-work 1/ V Fig. 67. Fossil Radiolaria. From Woods. A, Lithocampe tschernyschevi, Devonian. B, Trochodisciis longispmus, Carboniferous. C, Podocyrtis schom- burgki, Barbados Earth (Tertiary). A and C, Nassellaria; B, Spumellaria. (Figs. 67, 68), which is often very complicated, with projecting spines. The latter may be radial but do not meet at a central point except in the Acantharia. The outer layer of the body differs from that of the pelagic Foraminifera in that the vacuoles are contained in a layer of jelly (calymma) traversed by strands of protoplasm, which secrete it and the vacuoles, and in that it cannot be withdrawn. There is no contractile vacuole. The Radiolaria reproduce by binary fission and by spore formation. The spores found in them are sometimes alike (isospores) and some- times of two kinds (anisospores) . The latter are held to be gametes, and it is said that union between them has been observed. On account of their resemblance to the Dinoflagellata it has been suggested that they belong to parasitic members of that group. It is possible, on the other RADIOLARIA 8l hand, that the Radiolaria have an alternation of generations like that of the Foraminifera. Peculiarities of the mitoses in this group have been mentioned above (pp. 25, 26). Symbiotic flagellates, known as "yellow cells" {Zooxanthellae, see pp. 47, 50), are present in large numbers in the cytoplasm of many of the Radiolaria. Thalassicolla {Fig. 32 A). (Suborder Spumellaria.) Skeleton absent or represented by some loose siliceous spicules; one nucleus; yellow cells in extracapsular protoplasm. -Hn. nu. Fig. 68. A, Heliosphaera inermis, x 350. After Hertwig. B, The skeleton of Actinomma. After Biitschli. sk. skeleton; cps. central capsule; 7iu. nucleus. The yellow cells are shown, but not labelled, in A. Collozoum (Fig. 32 B). As Thalassicolla, but with central capsules united by their extracapsular protoplasm into a colony; and each capsule contains several nuclei. Heliosphaera (Fig. 68 A). As Thalassicolla, but the skeleton has the form of a lattice-work on the surface of the body. Actinomma (Fig. 68 B). As Heliosphaera, but the skeleton consists of several lattice spheres, formed successively at the surface as the animal grows, with radial struts joining them. Ultimately the inner- most sphere may lie in the nucleus. Acanthometra (Fig. 69 A). (Suborder Acantharia.) A skeleton of radial spicules of strontium sulphate meeting centrally in the central capsule; nuclei numerous; yellow cells intracapsular. Remarkable structures known as "myophrisks", surrounding the spines of this 82 THE INVERTEBRATA sp. osc.~. Fig. 69. Radiolaria. A, Acanthometra elastica, after Hertwig. B, Aulac- tinium actinastrum, after Haeckel. cps. central capsule; mph. myophrisk; nu. nucleus; osc. oscula of central capsule; phae. phaeodium; ps. pseudo- podium; sp. spine; y.c. yellow cells. SARCODINA 83 genus at their junction with the calymma, are contractile and are used in the regulation of the diameter of the body. Lithocircus (Fig. 37). (Suborder Nassellaria.) A siliceous skeleton in the form of a ring, bearing spines. Yellow cells extracapsular. Aulactinium (Fig. 69 B). (Suborder Phaeodaria.) A skeleton of hollow, radial, compound, siliceous spicules, not meeting in the centre; nuclei two; central capsule with three oscula, one of which is surrounded by a mass of coloured granules (the phaeodium^ from which the suborder is named). Like the rest of the Phaeodaria this is a deep-sea form and does not possess yellow cells. Order HELIOZOA Sarcodina, generally of floating habit and freshwater habitat ; without shell or central capsule; sometimes with siliceous skeleton; with spherical bodies; typical axopodia; and usually a highly vacuolated outer layer of protoplasm. The locomotion of members of this group, in the ordinary phase, is effected as rolling, due to contraction of successive pseudopodia in contact with the ground so that the body is pulled over. The pseudopodia usually show streaming of granules. When they bend, which they do to enclose prey which has adhered to one of them, their axial filaments are temporarily absorbed at the bend. Protoplasm from the pseudopodia then surrounds the prey and streams with it inward to the endoplasm, where a food vacuole is secreted around it. Contractile vacuoles are present. Asexual reproduction is usually by binary fission (or plasmotomy in multinucleate forms), sometimes by budding. Sexual processes have only been thoroughly investigated in Actinophrys and Actinosphaerium, where they take the form of autogamy (see below). Dimorpha (Fig. 70), one of the Helioflagellata, a small group of organisms which is usually appended to the Heliozoa, bears somewhat the same relation to that order that Naegleria bears to the Amoebina. It has a biflagellate and a heliozoan phase, and can pass from one to the other. In the latter it retains the flagella, whose filaments share a common basal granule with those of the axopodia, and has no vacuolated layer or protecting case. In fresh waters. Actinophrys (Figs. 71, 72). Unprotected ; with one nucleus, against which the central filaments of the axopodia end; no skeleton. Auto- gamy (or more correctly paedogamy^) takes place as follows: the pseudopodia are withdrawn and a jelly cyst formed. Binary fission now takes place, so that two individuals lie side by side in the cyst. ^ Paedogamy is a kind of autogamy in which not only the nucleus but also its cytoplasm divides and reunites. Fig. 70. Dimorpha mutatis. Partly after Blochmann. A, In the flagellate phase, alive. B, In the heliozoan phase, stained, with pseudopodia as if alive. ba.gr. basal granule ; es^n. chromatic matter which will condense to form the endosome', f. vac. food vacuole ;yZ. flagella; nu. nucleus ;^s. pseudopodia. Fig. 71, Actinophrys sol, x about 800. From Bronn. ecp. ectoplasm; enp. endoplasm; c.vac. contractile vacuole ; /.I'ac. food vacuole; nu. nucleus; ps. pseudopodium. Hl'LIOZOA 85 Each divides mitotically twice, throwing out as a polar body one product of each division. The first of these two divisions is a reduction division. The two individuals now fuse, one behaving as a male by T^;^^W. .-»*a«=*^ Fig. 72. A-F, Successive stages in the autogamy of Actinophrys sol. After Belar. ecp. ectoplasm; enp. endoplasm; ./.ielly coat;^^. pseudopodium put out by 3, the male gamete, towards ?, the female gamete. sending out a pseudopodium towards the other, and a strong inner cyst forms around the zygote. After a while the latter undergoes binary fission and the two products escape from the cyst. Occasion- ally two individuals enter a jelly cyst together and then either the two 86 THE INVERTEBRATA gametes of each undergo cross-conjugation with those of the other, or there is one cross-conjugation and the remaining gamete of each of the two original individuals performs parthenogenesis. In fresh and marine waters. Actinosphaeriutn (Fig. 33). Unprotected; with many nuclei, against which the central filaments of the axopodia do not end. In preparation for autogamy the nuclei are reduced in number and the cytoplasm divides into as many corpuscles as there are nuclei. Each of these then undergoes a process similar to that which occurs in Acttnophrys, forming a zygote which hatches as an independent individual. In fresh waters. Clathrulina (Fig. 8). Animal enclosed in a stalked, pseudochi- tinous lattice sphere; one nucleus. At binary fission, one product becomes a biflagellula and swims away. In fresh waters. Order MYCETOZOA Plasmodial Sarcodina, living usually in damp places on land ; which have in the active phase no shell, skeleton, or central capsule, but in the quiescent phase a cyst of cellulose; possess numerous, blunt pseudopodia; and are usually distributed by air-borne, cellulose- coated spores. The life history of a typical mycetozoon is as follows. The adult Plasmodium is a sheet of protoplasm containing many thousands of nuclei and numerous contractile vacuoles. In it there are to be seen veins along which streaming takes place, alternately towards and from the periphery. It feeds in a holozoic manner, usually upon de- caying vegetable matter, sometimes (Badhamia) on a living plant. In drought it breaks up into numerous multinucleate cellulose cysts which constitute the sclerottum. It prepares for reproduction by condensing at certain points, at each of which it forms a cellulose spo- rangium, often stalked. In the sporangium is a capillitium of cellulose threads and entangled in the capillitium are uninucleate, cellulose- coated spores, whose formation is preceded by a reduction division. When the sporangium is ripe it bursts and the spores are dissemin- ated by wind, etc. In damp surroundings they open and liberate each an amoebula which becomes a flagellula. The flagellulae perform syngamy and the zygote again becomes an amoebula. The amoebulae tend to fuse and form small plasmodia. By multiplication of their nuclei the adults arise. Chondrioderma (Fig. 73). On bean stalks. Badhamia. On fungi, especially Stereum. Plasmodiophora. In turnips, causing " finger-and-toe " disease. No sporangia. Distribution by flagellulae in soil. PROTOZOA Class SPOROZOA 87 Protozoa which in the principal phase have no external organs of locomotion or are amoeboid ; are parasitic, and nearly always at some stage intracellular; have no meganucleus; and form after syngamy large numbers of spores, which may be sporozoites or undivided zygotes. The two subclasses, Telosporidia and Neosporidia, of this class have little in common, and their association in classification is a matter of convenience. m-nu- nu Fig. 73. Various stages of Chondrioderma difforme. From Strasburger. A, A spore hatching. B and C, Flagellulae. D, Young and E, Older amoe- bulae. F, Amoebulae fusing to form plasmodium. All x 540. G, Portion of Plasmodium, x 90. nu. nucleus. Though upon analysis the type of life history characteristic of the Telosporidia is found to differ profoundly from those of the Neo- sporidia, all sporozoan life histories are complicated. Usually they comprise all the phases indicated in the scheme on p. 37, though in the Eugregarinaria (and perhaps in the Actinomyxidea) agamogony is omitted. Each phase, moreover, is liable to be elaborated. The term sporoblast is applied to certain stages in various life histories, but un- fortunately the stages so named are not all comparable with one another. In the Telosporidia it denotes either the zygote or the 88 THE INVERTEBRATA products of the first of two successive multiple fissions whereby the sporozoites and other spore-like stages often arise. In the Neosporidia it denotes the syncytia (of different origins in different groups) from which by differentiation of cells complex spores are formed. Subclass TELOSPORIDIA Sporozoa in which the adult of the vegetative stage has only one nucleus; and comes to an end with spore formation; and the spore cases, if present, are simple structures, which nearly always contain several sporozoites. The vegetative stage {trophozoite) has usually a definite shape, but in some haemosporidia is amoeboid. Its fission (agamogony), if such occur, is multiple, and is usually known as schizogony^ the term schizo- zoites or merozoites being applied to the offspring. Its single nucleus only divides to form those of the young into which this stage breaks up, but owing to such division the body may be for a while multinucleate. The trophozoite of one of the two orders (the Coccidiomorpha) remains intracellular: in the other order (the Gregarinidea) it after a time outgrows its cell host. Save in one suborder (Eugregarinaria), it passes through the usual phase of agamogony before giving rise to gamonts, but in the Eugregarinaria agamogony is omitted, and the members of the single vegetative generation become gamonts, which provide for the increase of the species by the formation of many gametes in both sexes. The gamonts may be free or intracellular. Free individuals are often able to adhere by a sticky secretion, form- ing what is known as a syzygy. When gamonts so adhere (Figs. 76, 6; 77 B) they do so in pairs^ whose members are to be the parents of gametes that will unite reciprocally. Syngamy is isogamous in a few of the Gregarinidea, but is usually anisogamous, and in the Cocci- diomorpha becomes an oogamy (p. 31). In some cases, perhaps in all, the first division of the zygote is a reduction division, so that nearly the whole of the cycle is haploid. The little group Piroplasmidea, whose members in some respects resemble the Telosporidia, are best placed as an appendix to this subclass. Order COCCIDIOMORPHA Telosporidia in which the adult trophozoite remains intracellular; and the female gamete is a hologamete. Typically the members of this order are parasites of the gut, but more than once they have come to infest the blood. One such invasion gave rise to the suborder Haemosporidia. The rest of the group con- stitute the Coccidia. ^ The term syzygy should perhaps be restricted to such pairs. SPOROZOA 89 Suborder COCCIDIA Coccidiomorpha, for the most part gut parasites ; of which the zygote is non-locomotory ; the sporozoites are nearly always encased; and the gamonts often form a syzygy. Eimeria (Fig. 74) is parasitic in the intestinal epithelium of various vertebrates and invertebrates. E. schubergi, from the intestine of the centipede Lithobtus, may be described as a type of the suborder. The spherical trophozoite (agamont) undergoes schizogony (agamogony) by multiple fission within the epithelial cell which it inhabits. The spindle-shaped schizozoites (agametes) being set free into the cavity of the organ, each infects another cell in which it grows like its parent. After some days of this there occur fissions in which the young on invading a host cell grow into adults unlike their parents and of two kinds — male and female gamonts. Each female gamont extrudes stainable matter from its nucleus and thus becomes a female holo- gamete. In the male gamont the nucleus divides several times, and the daughter nuclei are set free with portions of the cytoplasm as biflagellate male gametes, which are thus merogametes. The gametes leave the host cell and unite while free in the gut cavity. The zygote nucleus undergoes what is probably a reduction division and encysts. Within its cyst (the oocyst) it divides by multiple fission into four sporoblasts each of which forms a cyst of its own (a secondary sporo- cyst) in which it divides into two sporozoites. Thus sporogony takes place in two stages. In each of these there is some residual proto- plasm. Meanwhile the oocyst has passed out of the host in the faeces. Infection of a new host takes place by contamination of food by the encysted spores, which hatch in the intestine. Aggregata is remarkable among coccidians for having two hosts. Its agamogony takes place in crabs and involves a generation of sporo- blasts, but is not repeated. A cuttlefish, devouring a crab, ingests the agametes, which in the new host proceed to become gamonts. After gamogony with flagellate male gametes, fertilization, and sporogony, the spores, containing four or more sporozoites, are passed with the faeces of the mollusc and swallowed by a crab. Adelea is parasitic in the epithelium of the gut of Lithobius. Its life history resembles that of Eimeria, but the gamonts, which differ con- siderably in size, the male being the smaller, become free and form a syzygy in the gut, though without encystment. The male gametes are consequently not under the necessity of reaching the female by swimming, and are not flagellated. Haemogregarina has become completely a blood parasite, and has a life history closely resembling that of the Haemosporidia, with the sexual process in an invertebrate host (see below). Since, however, it 90 THE INVERTEBRATA undergoes syzygy, the organism would appear to belong to the Adelca stock, whereas the Haemosporidia are probably related to Eimeria. Fig. 74. A diagram of the Hfe cycle of Eimeria schubergi. A, Infection of a cell of the intestinal epithelium of the host. B, Growth of the agamont. C-E, Agamogony (schizogony). F, G, Gamogony. H, Conjugation (syn- gamy). I-L, Division of the encysted zygote into sporoblasts. M, Division of each sporoblast within its cyst into two sporozoites. The oocyst containing the sporocysts is passed out of the host and swallowed by another. N, Escape of the sporozoites in the intestine of the new host. Schellackia and Lankesterella, which have no syzygy, are transitional to the Haemosporidia, under which (on p. 91) their life histories are described. COCCIDIOMORPHA 9I Suborder HAEMOSPORIDIA Coccidiomorpha, always true blood parasites; which have naked sporozoites ; a locomotory zygote {ookinete) ; and no syzygy. The members of this suborder are intracellular blood parasites of vertebrates and contain granules of pigment (melanin) derived from the haemoglobin of the host — a feature which is lacking in the blood parasites that belong to the Coccidia. They are transmitted from one vertebrate host to the next by a blood-sucking invertebrate. Their agamogony and the formation of their gamonts take place in blood cells of the vertebrate host, but their gametes are formed, and ferti- lization takes place, in the invertebrate. A series of intermediate cases shows how this condition may have arisen. (i) Schellackia (suborder Coccidia), parasitic in the gut of a lizard, leaves the gut epithelium after schizogony and completes its cycle in the subepithelial tissues. In order to reach a new host it has therefore to rely on transference by a carrier instead of passing out with the faeces. To accomplish this, the sporozoites enter blood vessels, get into red corpuscles, and are sucked up by a mite. The blood- sucker, however, does not inject the parasite into the new vertebrate host, but is swallowed, so that the parasite infects the host through the gut epithelium, in which its schizogony is still performed. (2) Lankesterella (suborder Coccidia), parasitic in frogs, passes its whole cycle in the epithelioid lining of blood vessels, the sporozoites being transferred, as in Schellackia^ in red corpuscles, which are sucked up by a leech. Infection is still through the gut of the verte- brate, whose wall the sporozoites pierce on their way to the blood vessels. (3) Haemoproteus (Haemosporidia), parasitic in birds, has its schizogony alone in the blood vessel walls, the sexual part of the cycle being remitted to the invertebrate host. The parasite enters the red corpuscles not as a sporozoite but earlier, as the young stage of the gamont, which grows up in the corpuscle. At the same time a change in the mode of infection has taken place, the blood-sucker injecting the sporozoites into the blood vessels of the vertebrate host. Thus the parasite has completely abandoned the gut wall and become a true blood parasite. (4) Plasmodium (Haemosporidia), the cause of malaria and ague in man, is parasitic in the red blood-corpuscles of mammals and trans- mitted by the mosquito Anopheles. Its schizonts (trophozoites), as well as its gamonts, inhabit red corpuscles. The trophozoites of Plasmodium (Fig. 75) are amoeboid. In the young stage they are rounded and each contains a large vacuole which gives it the appearance of a ring. They undergo schizogony in the 92 THE INVERTEBRATA Fig. 75. A diagram of the life cycle of Plasniodimn vivax. After Borradaile. 1-7, Schizogony (Merogony), asexual reproduction which takes place in man. 8—13, Gamogony and syngamy, which take place in the stomach of a mosquito. 14-20, Sporogony by the zygote (sporont), which takes place in the body cavity of the mosquito, i, Infection of a red corpuscle. 2, Signet- ring stage. 3, Amoeboid stage. 4, Full-grown schizont preparing to divide. 5, Multinucleate stage. 6, Rosette stage, corpuscle breaking up. 7, Free schizozoites. 8, Infection of red corpuscles by young gamonts. 9, Full- grown gamonts free in the mosquito's stomach. 10, 11, Formation of gametes. 12, Conjugation. 13, Zygote in the ookinete condition. 14, Invasion by zy- gote of endoderm cell of mosquito. 15, Encystment. 16, Sporoblasts formed by division of zygote (sporont). 17, 18, Formation of sporozoites. 19, In- vasion by latter of salivary gland. 20, Sporozoites injected into blood of a man. TELOSPORIDIA 93 red corpuscles, which then break up, setting free the schizozoites (merozoites) and also products of the metaboUsm of the parasite which cause fever. After some generations, gamonts similar to those of Eimeria appear, but remain quiescent unless sucked up by a mosquito, in whose gut the female gamont becomes a spherical macrogamete, the male gamont throws off whip-like microgametes, and syngamy takes place. The zygote becomes elongate and active (an ookinete), and bores its way through the wall of the mosquito's stomach, on the outside of which it becomes encysted (oocyst). Here its nucleus divides and it breaks up into sporoblasts which in turn produce spindle-shaped sporozoites. The oocyst now bursts, setting the sporo- zoites free in the blood of the insect. They make their way into the salivary glands and are injected with the saliva into a mammalian host, where they give rise to trophozoites which infest the red corpuscles. Three species of Plasmodium infest man — P. vivax which sets free a generation of schizozoites in forty-eight hours, P. malariae which does so in seventy-two hours, and P. falciparum whose schizogony occurs at more irregular intervals. Since the attacks of fever take place when the corpuscles break up and set free the toxins formed by the parasites, the fever caused by P. vivax returns every third day and is known as "tertian ague", and that caused by P. malariae ("quartan ague") recurs every fourth day, while P. falciparum causes irregular (quotidian) fevers which are more or less continuous. These latter are the "pernicious malaria" of the tropics. The morphological differences between the species are small, but P. vivax is distinguished by the active movement of its pigment granules and the large number (15-24) of its schizozoites, P. malariae by the sluggishness and often quadrilateral form of its amoeboid stage, P. falciparum by the paucity of its pigment and by its curved, sausage-shaped gamonts. Order GREGARINIDEA Telosporidia in which the adult trophozoite becomes extracellular; and the female (as well as the male) gametes are merogametes. Intestinal and coelomic parasites of invertebrates, especially of arthropods and annelids. Suborder SCHIZOGREGARINARIA Gregarinidea which undergo schizogony. Schizocystis (Fig. 76). Parasitic in the intestine of the larvae of dipterous flies. The young trophozoite attaches by one end to the gut epithelium of the host. Its nuclei multiply. When ripe it undergoes multiple fission. The products (schizozoites) either repeat asexual 94 THE INVERTEBRATA reproduction or become gamonts. These undergo syzygy, coencyst- ment, and gamogony. The gametes unite, and the zygotes form small oocysts ("spore cases") within the gamocyst. In its case each zygote divides into a bundle of sporozoites. The spores are set free and swallowed by new members of the host species, in whose intestine the spore cases are digested and the process repeated. Fig. 76. A diagram of the life cycle of Schizocystis. 1-4, Schizogony. 5, Gamonts. 6, Syzygy. 7-9, Gamogony in a cyst (gamocyst). 10, 11, Syn- gamy. 12, Freed spore case containing sporozoites resulting from sporogony. Ophryocystis (Fig. 77). Parasitic in the Malpighian tubules of beetles. The cushion-shaped trophozoites are attached to the host's cells by branched processes. After several generations of schizogony, they become free gamonts, enter into syzygies, encyst, and within the gamocyst undergo two divisions, whereby each forms one definitive gamete and a binucleate enveloping cell which perhaps represents abortive gametes. Syngamy then takes place, and the zygote divides GREGARINIDEA 95 to form within the enveloping cells a parcel of eight sporozoites in a case. Thus each syzygy produces only one pair of gametes and results in only a single spore. Suborder EUGREGARINARIA Gregarinidea which have no schizogony. The adult trophozoite has a stout cuticle and the ectoplasm con- tains myonemes, longitudinal or transverse, or both. Partitions of the ectoplasm without myonemes may (Fig. 80 F) divide the body into «?* >*«%'"«'*; 'r.Va •. fr V, -V >* i'l*.' •-"»\' *' % str. B -gam. n^-^yg- spz.-j Fig. 77. Stages in the life history of Ophryocystis mesnili. A, Agamont, on the epithelium of a Malpighian tubule of the host. B, Syzygy. C, Formation of a cyst (gamocyst) and multiplication of nuclei. D, Formation of gametes. E, Zygote. F, Spore case with sporozoites, still enclosed in residual proto- plasm of gamonts. gam. gamete; nu. nuclei of agamont; nu.' gamete nucleus; nu." nuclei of enveloping (residual) protoplasm; spz. sporozoites; str. striated border of epithelium of Malpighian tubule; zyg. zygote. three segments — the epimerite or fixing organ, protomerite^ and deuto- merite, which latter contains the nucleus. When ripe the trophozoites become gamonts, joining in syzygies of two which together form a gamocyst and give rise to gametes (iso- or anisogametes according to species) by multiple fission in which residual protoplasm remains. Syngamy takes place within the cyst between the gametes of one parent and those of the other. The zygotes secrete small oocysts {pseudonavicellae) of their own, and within these divide into several sporozoites ("falciform young"). Passing out of the host these are 96 THE INVERTEBRATA swallowed by another of the same species, within which their cysts are digested and a new infection begins by the sporozoites invading cells of the host. These they eventually outgrow, and lie in a cavity of the host, either entirely free or attached by an epimerite. Fig. 78. A diagram of the life cycle of Monocystis. A, Trophozoite adhering to the seminal funnel of the host. B, Encysted syzygy. C, Formation of gametes. D, Conjugation. E, Encystment of zygotes. F, Multiplication of nuclei of the same. G, Formation of sporozoites (only four of the eight are shown). H, Release of sporozoites in intestines of new host. I, Infestation of sperm morula, ext. external coat of gamocyst; gam. gametes; int. internal coat of gamocyst; res. residual protoplasm; spc. cells of sperm morula; spe. tails of withered spermatozoa adhering to parasite; spz. sporozoites; zy zygote. In comparing this life cycle with that of Etmeria, given above, it should be noted that in the gregarines, whose female gametes are merogametes and numerous, the "spores" (small sporocysts each EUGREGARINARIA 97 containing several sporozoites) are each the whole product of a zygote (i.e. are oocysts), whereas in the coccidians, where the female gamete Fig. 79. Monocystis. From Borradaile. A, M. magna, x 25. B, M. lumbrici, X 85. The latter is covered with the tails of spermatozoa, the offspring of the sperm mother-cell in which it was embedded. Fig. 80. Gregarina longa, from larva of Tipula, the Daddy-long-legs. Highly magnified. After Leger. A, B, C, D, E, Stages of the development of G. /ow^a at first within and then pushing its way out of one of the cells of the intestine of the Tipula larva. F, Mature form. c. cell of intestine of host; nu. its nucleus ; pst. parasite. is a hologamete, the zygote forms, by means of a generation of sporoblasts, several such spores in its oocyst. Monocystis (Fig. 79). Without divisions of the body. Parasitic in 98 THE INVERTEBRATA seminal vesicles of earthworms. Several species, some isogamous, others anisogamous. The spores escape either down the vasa defer- entia of the host or by the latter being eaten by a bird, whose faeces contain them intact. Swallowed by another worm, their cases are digested and the sporozoites traverse the intestinal wall to reach the vesiculae seminales, where they enter sperm mother-cells, in which they pass their earlier stages. Gregarina (Fig. 80). All three divisions of the body present. Parasitic in the alimentary canals of cockroaches and other insects. The gamocyst develops into a complicated structure with ducts for the discharge of the pseudonavicellae. Appendix to the Telosporidia Order PIROPLASMIDEA Protozoa, parasitic in red blood-corpuscles and transmitted by ticks ; which have no external organs of locomotion; perform agamogony by binary fission; conjugate as hologametes; and after syngamy become motile zygotes which divide in a cyst into numerous, naked sporozoites. The members of this group are of doubtful affinity. In the general course of the life-cycle they resemble the Telosporidia, but in the possession by the trophozoite of part of a flagellar apparatus, and in that the gametes are both hologametes, they diff^er from the other members of that subclass. An interesting feature of their life history is that they are transmitted in the ovum from one generation of the invertebrate host to the next. They are at present only known from mammals and ticks. Piroplasma { = Babesia). Infests various mammals (cattle, dogs, monkeys) and causes the red-water fever of cattle and other diseases. The trophozoites, in red corpuscles, are pear-shaped and unpig- mented, and have a rhizoplast and basal granule as if for a flagellum. When taken into the alimentary canal of a tick they become gametes and form zygotes, which are ookinetes (p. 91), bore through the gut wall of the host, and reach its ovary. There they enter ova in which they are transmitted to the next generation of the tick. They encyst in the ova and divide into amoeboid sporoblasts (sporokinetes) which are distributed as the cells of the host divide and by their own active migration. Thus some reach the salivary glands. There they become multinucleate and break up into sporozoites, which are injected with the saliva into a new mammalian host. SPOROZOA 99 Fig. 8i. A diagram of the life cycle of Piroplasma. After Dennis, with modifications. 1-5, agamogony in red corpuscles ; 6, gamete ; 7, 8, conjugation in gut of ticks; 9-12, passage of zygote through walls of gut and oviduct into ovum and encystment there; 13, 14, formation of sporoblasts ; 15, migration of sporoblast; 16, sporoblasts, now multinuclear, in cell of rudiment of salivary gland of tick of second generation; 17 (less magnified), acinus of salivary gland containing sporozoites formed by break-up of sporoblasts. al. wall of gut; od. wall of oviduct; ov. ovum. lOO THE INVERTEBRATA Subclass NEOSPORIDIA Sporozoa in which the adult of the vegetative stage is a syncytium ; which usually forms spores continuously within itself; and the spore cases are usually complex structures, which, except in the Actino- myxidea, contain only one germ. Order CNIDOSPORIDI A Neosporidia whose spores possess pole capsules. The formation of the spores in this group is a complex process of which the details and the relation to the typical life cycle of the Protozoa have not yet been completely elucidated. The following scheme provisionally co-ordinates the facts that have been estab- lished concerning it. In the syncytium (Fig. 82), which is the agamont and which often multiplies by plasmotomy, there arise, perhaps by the coming together of nuclei, bodies known 2iS pansporo- blasts^ each composed of a couple of envelope cells with one or more cells known as sporoblasts. The nucleus of each sporoblast divides and the sporoblast gives rise to a complex, multicellular spore, composed of a case of two or three pieces, each with an underlying nucleus, one to five nematocyst-like pole capsules ^ each with a nucleus, and one or more germs. In most cases the germ is single and at first has two nuclei, which later fuse. Here we may regard the sporoblast as a gamont and the products of its division as homologues of gametes, of which some become the accessory cells of the spore and two (those which the germ at first possesses) the definitive gametes. In one group, however (the Actinomyxidea), there are several germs (often as a syncytium), and syngamy takes place not between nuclei in a germ but at an earlier stage, between pairs of cells in the pansporo- blast, each zygote becoming a sporoblast. Here the sporoblast is a true sporont, and the products of its division are homologues of sporozoites, of which some become the accessory cells of the spore and the others (the germs) are the definitive sporozoites. It is a remarkable, but apparently an established, fact, that syngamy thus takes place at different stages in the formation of essentially similar spores. Infection of new hosts is by the mouth, and the function of the pole capsules is, by discharging their threads, to anchor the spore to the gut wall. A schizogony may precede pansporoblast formation. Of the three suborders of the Cnidosporidia, the Myxosporidia have two or four pole capsules in the spore, the Microsporidia one, and the Actinomyxidea three. The latter group also differ from the other two in respect of the germs, as mentioned above. Myxobolus (Myxosporidia, Fig. 82). Large syncytia in the tissues of various freshwater fishes. Some species are harmless, others dangerous pests. Fig. 82. A diagram of the life cycle of a typical member of the Myxosporidia. The schizogony shown here (D-F) probably often does not occur. A, Escape of germ. B, Migration within host. C, Infection of a cell of the latter. D-G, Schizogony and reinfection. H, Multiplication of nuclei. I, Appearance of first pansporoblast. J, Appearance of mare pansporoblasts and multiplica- tion of syncytium by plasmotomy. K, L, Development of spores in a pan- sporoblast. M, Fully formed spore, before conjugation. N, Ripe spore after conjugation, env. envelope cell; nu.' nucleus of spore case; nu." nucleus of pole capsule ; spb. undifferentiated sporoblast with nuclei which will become those of the germ, spore case, and pole capsules; vac. vacuole containing glycogen sometimes found; zy.nu. zygote nucleus. 102 THE INVERTEBRATA Nosema (Microsporidia). The syncytium early breaks up, first into binucleate forms and finally into single sporoblasts. In the intestinal epithelium of insects. A serious pest of the silkworm, causing the disease known as pebrine, and of the bee. Sphaeractinomyxon (Actinomyxidea). The whole body is reduced to a single pansporoblast, as in all members of the suborder. The spores are without the spines found in related genera. In annelids. Order HAPLOSPORIDIA Neosporidia whose spores possess cases with a lid, but have no pole capsules. This order contains certain parasites which infest aquatic in- vertebrates. They are perhaps derived from the Cnidosporidia by loss of the pole capsules. Haplosporidium^ parasitic chiefly in annelids, is the typical genus. Order SARCOSPORIDIA Neosporidia whose spores do not possess cases or pole capsules. These organisms are tubular syncytia with a radially striped ecto- plasm, parasitic in the muscle fibres of mammals, and reproducing by simple, sickle-shaped spores. Sarcocystis (Fig. 83). In various mammals, occasionally in man. Class CILIOPHORA Protozoa which, at least as young, possess cilia ; are never amoeboid ; if parasitic are very rarely intracellular ; nearly always possess a mega- nucleus; and do not, after syngamy, form large numbers of spores. This class, though some of its parasitic members are of compara- tively simple structure, contains the most highly organized Protozoa. Facts concerning sundry of the organs and processes in its members (the ciliary apparatus, p. 17 ; the contractile vacuole system, pp.21 , 22 ; the nucleus, p. 26; conjugation, p. 33 ; etc.) have been stated above. The life history, except for the remarkable process of conjugation undergone by most of the class, is relatively uncomplicated. In particular, though the nuclear peculiarities of the typical members of the group render inevitable certain special features in the metagamic divisions, there is no true sporogony. Subclass CI LI AT A Ciliophora which as adults possess cilia; and which do not possess suctorial tentacles. The morphology of this group is much aff"ected by the disposition of the apparatus used in obtaining nutriment. The food may be ab- PROTOZOA 103 sorbed through the surface: the shape of the body is then simple (Figs. 5, 87 A). Nearly always, however, there is a mouth. In some of the lower genera this is anterior and terminal, or nearly so (Fig. 89 A), but usually it is removed to one side of the body (Fig. 87 E). This side is then said to be "ventral", and that opposite to it is ** dorsal". »' III ;S^;".-J : * >-l^'^ ■,-■1 \mM}m m U— m. ^\:r:\::m A ■>- B Fig. 83. Sarcocystis lindemanm, from the vocal cords of man. After Baraban and Saint-R^my. A, Longitudinal section of muscle fibres, showing the parasite in situ in a fibre, x 300. B, Enlarged portion of outer region of para- site, showing the striated wall, and some of the compartments which contain the spores. C, A single spore, x 1600. m. muscle fibre; w. wall of parasite. The mouth, either is merely a soft patch of exposed endoplasm or possesses a gullet (p. 19). In a relatively few cases (including all those in which the mouth is terminal and a few of those in which it is 104 THE INVERTEBRATA ventral) the mouth is at the surface of the body : in such cases the gullet, if there be one, is an oesophagus, excavated in the endoplasm and capable of being opened and closed to seize the prey which is of some size. Most often, however, there is a vestibule. This, to which also the name '* gullet" is often applied, is a depression leading to the mouth, incapable of being closed, lined by inturned ectoplasm, and containing a ciliary apparatus, which usually includes one or more undulating membranes. By this apparatus the minute objects which constitute the food of all ciliates that have a vestibule are drawn in, being meanwhile, in some cases at least, entangled by a mucous secretion. At the bottom of the vestibule lies the true mouth; some- times an oesophagus is present (Stentor) or is represented by a cleft in the endoplasm (Paramecium). The inner part of the vestibule may be free from cilia, and so simulate an oesophagus {Paramecium, Vorti- cella).^ The vestibule is usually approached by 2i peristome. This is a groove, of varying dimensions, which leads from the front end along the ventral side to the opening (cytostome) of the gullet. It is not straight, but runs in a longer or shorter spiral round the body, so that the anterior end of the latter is spirally deformed (Figs. i6, 84 A). The higher forms have along what is primarily the outer edge of the peristome a food-gathering row of cirri or membranellae, the adoral wreath (Fig. 90, ad.mae.). Typically, the spiral is open, as in Para- mecium, but in some cases, as in Stentor (Figs. 84B, 89 C), it has con- tracted, so that it lies coiled as a crown at the anterior end. In such cases the animal is usually fixed temporarily or permanently by the opposite end. The members of one order (Hypotricha) are depressed dorso- ventrally, and have a flat ventral side, along which the peristome runs and which is usually provided with a complex apparatus of cirri (Figs. 90, 91). The animal applies this side to the substratum, in locomotion upon which certain of the cirri are used. The dorsal side is naked save for a few " sensory " cilia. It is probably from such forms that the familiar bell-animalcules and their relations (Peritricha) are derived. In these, the shape of the body and the position of the peri- stome at first suggest that the morphological peculiarities of the group are due to an evolution similar to that by which such forms as Stentor came into being— but the fact that the peristome, which in all other ciliates that- possess it curves clockwise, is in the Peritricha twisted in the opposite direction, makes this view impossible. The origin of the Peritricha may be explained as follows (Fig. 84). In ^ It is possible that this is a true oesophagus. Other regions are sometimes to be distinguished in the vestibule: in Paramecium, for instance, its outer section has trichocysts and cilia but no membranes, its middle section membranes only. CILIATA 105 hypotrichous forms which had taken to fixing themselves to the sub- stratum by that (ventral) side which they applied to it, the mouth, being no longer of use in its ventral situation, moved to the left side. The peristome accordingly came to run along the edge of the body, around which it became continued on the dorsal surface. In dorsal aspect its direction is of course reversed; and the adoral wreath has come to be internal. The body, in correspondence with the changed habit of life, has shortened, till its outline, seen from above, is circular, and has deepened. Thus the oral-aboral axis of the Peritricha is not anteroposterior as in Stentor, but dorsoventral. per.-- \UM4UUA Jj^r-vesL vest Fig. 84. A diagram of the disposition of the peristome in various ciHata. A, Ventral view of a typical heterotrichous form. B, Similar view of Stentor. C, Ventral (aboral) view of a peritrichous form without stalk, such as Tricho- dina (Fig. 86 D). C, Dorsal (oral) view of the same. ad. adoral wreath of membranellae ; vest, vestibule ; per. peristome. The general surface of the body is in the lower and in some of the higher genera uniformly covered with cilia, but most of the more highly organized forms are naked save where there stand certain special pieces of ciliary apparatus. The ectoplasm (Fig. 85) has a definite and often complicated structure. There is always a tough pellicle, which is frequently sculptured. Under it is often an alveo- lar layer of minute, regular vacuoles. When there are myonemes, these lie on the inner walls of larger canal vacuoles of this layer. Under it again is usually a layer, the cortex, whose firm consistency prevents the granules, vacuoles, etc., of the endoplasm from entering it, though it may possess small granules of its own. The basal granules of the cilia lie immediately below the alveolar layer; trichocysts are im- bedded in the cortex. Either the cortex or both it and the alveolar io6 THE INVERTEBRATA layer may be absent. In Paramecium the cortex is covered by a thick pellicle which possibly contains a minute alveolar layer. rid. tri. fl! B Fig. 85. Details ofthe ectoplasm of ciliates. After Wetzel. A, Frontonia leucas (Vestibulata). B, Paramecium. C, The same, surface view. alv. alveolar layer; ba.gr. basal granules; cor. cortex; enp. endoplasm ; yZ. flagellum;^.' insertion of flagellum ; pel. pellicle ; rid. ridges of pellicle ; tri. trichocyst. Order HOLOTRICHA (ASPIRIGERA) Ciliata which do not possess an adoral wreath ; and which nearly all have uniform ciliation of the whole surface of the body. This order is a collection of relatively simply organized ciliates, some of which are primitive while others are degenerate through parasitism. Suborder PROCILIATA Holotricha without mouth ; and without differentiation of meganuclei from micronuclei. Opalina (Figs. 5, 21 C, 86). With several, usually many, nuclei, which are all alike. In each nucleus, however, there can be dis- Fig. 86. Opalina ranarum. From Borradaile. A, Ordinary individual in longitudinal fission. B, The same in transverse fission. C, Small encysted individual (distributive phase). D, Gamete. E, Encysted zygote. CILIATA 107 tinguished two kinds of chromosomes, which are held to represent the chromatin of the mega- and micronuclei of other ciUates. The life history differs from that of other members of the class in that syngamy is of the normal type. The agamont, parasitic in the rectum of a frog or toad, reproduces by binary plasmotomy. In the spring the plasmo- tomy outruns the nuclear divisions so that there arise small individuals with few nuclei. These encyst and pass out of the host. Swallowed by a tadpole, they hatch, and give rise to uninucleate gametes, of two sizes (anisogamous). After fusion of the gametes the zygote encysts for a while, issues, and by nuclear division becomes the adult agamont. Suborder ASTOMATA Holotricha without mouth; but with mega- and micronuclei. Unlike Opalina, the members of this group are probably not primitive but degenerate through parasitism. Collinia. Parasitic in the blood-spaces of the gills of Gammarus and other crustaceans. Anoplophrya (Fig. 87 A). Reproduction by repeated budding at one end of the elongate body, forming a chain. Parasitic in various annelids. Suborder GYMNOSTOMATA Holotricha with a mouth, whose gullet, if any, is without ciliary apparatus (i.e. an oesophagus); and with mega- and micronuclei. Ichthyophthirius . Subspherical, with a mouth at one pole and short gullet ; numerous contractile vacuoles near the surface of the body ; and a meganucleus, but no micronuclei visible in the adult. Para- sitic in various freshwater fishes, where it lies in blisters in the skin. When it is full-grown, it falls out of the host, encysts, and forms by repeated fission a number of small ciliospores, each of which has a mega- and a micronucleus, the latter having appeared during the process, perhaps from within the meganucleus. The spores infect new hosts. A sexual process of the nature of autogamy has been described, but is very doubtful. Prorodon (Fig. 89 A). Ovoidal, with mouth at one pole, a deep gullet which is supported by skeletal rods and is capable of opening and closing; one mega- and one micronucleus. In fresh waters. Loxodes. Compressed, with mouth as a mere slit in the pellicle on the ventral edge of the body, overhung by the beak-like anterior end; numerous mega- and micronuclei ; a row of vacuoles containing ex- creta along the dorsal border, and a contractile vacuole at the hinder end. In fresh waters. axL. c.vac/ F' G H Fig. 87. Various Ciliophora. A, Anoplophrya prolifera, x 200, after Saville- Kent. B, Entodinium caudatum, after Schuberg. C, Tintinnidium inqidlinwn, after Faure-Fremiet. D, Trichodina pedicidus , x 450, after Biitschli. E, Col- poda steini, x 1300, after Wenyon. F, Sphaerophrya sol, in the free stage, X 170, after Biitschli. F', The same, dividing subequally to form a ciliated bud. G, Dendrocometes paradoxus, x 250, after Biitschli. H, Free bud of Tocophrya quadripartita , after Biitschli. ad. adoral wreath; c.vac. contractile vacuole; ci. rows of body cilia; ci.' aboral ring of cilia; ci." girdle of cilia; cu. cuticle of host; esm. endosome of meganucleus (an unusual feature); f.vac. food vacuole; hk. hooks; meg. meganucleus; mi. micronucleus ; ten. tentacles; vest, vestibule. CILIATA 109 Suborder VESTIBULATA (HYMENOSTOMATA) Holotricha with a mouth and a gullet (vestibule) which is permanently open and usually possesses an undulating membrane ; and with mega- and micronuclei. Colpoda (Fig. 87 E). Kidney-shaped; with large vestibule on con- cave side; but no undulating membrane; and no peristome. Fission, binary or repeated, takes place in a cyst. Common in infusions, fresh- water and marine. Colpidiiim. As Colpoda\ but with undulating membrane. Common in infusions, freshwater and marine. Paramecium (Figs. 9, 15, 16, 17, 26, 85). Slipper- or pear-shaped according to species; with undulating membranes^; and peristome. Common in infusions, freshwater and marine. mrp Fig. 88. Ciliata from the rectum of the frog. From Borradaile. A, Balan- tidium entozoon, x 65. B, Nyctotherus cordiformis, x 130. a«. anus ; cy. con- tractile vacuole ; meg. meganucleus ; mi. micronucleus ; v. vestibule. Order HETEROTRICHA Ciliata which possess a gullet, permanently open and provided with undulating membrane; an adoral wreath, curving clockwise; and most often on the rest of the body a uniform covering of cilia ; and whose body is not depressed. Suborder POLYTRICHA Heterotricha which retain the uniform ciliation of the general surface of the body. Balantidium (Fig. 88 A). Egg-shaped; the peristome a deep groove at the anterior end. Parasitic in the rectum of frogs, the intestine of man (where it is occasionally harmful), etc. Nyctotherus (Fig. 88 B). Kidney-shaped; with permanent anus. Parasitic in the rectum of frogs, the intestine of man, etc. ^ There are three of these, each composed of four rows of cilia. See p. 104, no THE INVERTEBRATA Spirostomum (Fig. 89 B). Rod-shaped; with the peristome as a long groove; meganucleus beaded; several micronuclei. In fresh waters and marine. Stentor (Fig. 89 C). Long and funnel-shaped ; attached by the base, but often frees itself to swim ; meganucleus beaded ; several micro- nuclei. The animal is very highly contractile. In fresh waters. M. ,rods mi.-Wti^'r-ri-; -meg. ■vac! c.vac. c.vac.' cvac? Fig. 89. Ciliata. After various authors. A, Prorodon teres, x 500. B, Spiro- stomum ambiguum, x 150. C, Stentor coeruleus, x 50, ad.zv. adoral wreath; c.vac. contractile vacuole; c.vac' accessory vacuole; c.vac." accessory canal; f.vac. food vacuole ; ecp. ectoplasm ; M. mouth ; meg. meganucleus ; mi. micro- nucleus ; per. peristome ; rods in protoplasm around gullet. Suborder OLIGOTRICHA Heterotricha of shortened form; with the body cilia reduced to a few rows or absent. This suborder contains two tribes of very different habits, the pelagic Tintinnina, and the Entodiniomorpha, forms of bizarre shape parasitic in the alimentary canal of mammals, chiefly in the stomach of ruminants. Both suborders have an anterior peristome with very strong membranellae, and are naked on the rest of the body, save sometimes for a few cilia or patches of cirri. Tintinntdium (Fig. SyC). (Tintinnina.) Cup-shaped ; anchored by an aboral process into a chitinoid case. In fresh waters and marine. CILIATA III Entodinium (Fig. 87 B). (Entodiniomorpha.) With three posterior processes, of which the largest is said to serve as a rudder. In the rumen and reticulum of sheep and oxen. Like others of the tribe, these organisms are present in such numbers that they are believed to be symbionts which play a part in the nutrition of the host, render- ing the vegetable food more easily assimilable by feeding on it and being in turn digested further on in the alimentary canal. Infection of the host is probably by cysts on grass. Order HYPOTRICHA Ciliata with depressed body; a gullet, permanently open and pro- vided with undulating membranes ; an adoral wreath, curving clock- wise ; the dorsal cilia represented only by a few stiff hairs ; and on the ventral side usually an elaborate system of cirri and other ciliary organs. The animals can swim but spend much of their time crawling over solid objects by means of the cirri. Stylonichia (Figs. 90, 91). A typical example. Common in in- fusions. Kerona. With a less highly developed ciliary system than Styloni- chia. Ectoparasitic on Hydra, Order PERITRICHA Ciliata, for the most part permanently fixed by the aboral surface; with a gullet, permanently open and provided with undulating mem- brane; an adoral wreath, curving counter-clockwise; and on the rest of the body no cilia, save those of an aboral ring in the free-swimming species and stages. The conjugation of members of this group has. been discussed on p. 33, their morphology on pp. 104, 105. The anus and contractile vacuole open into the deep vestibule, perhaps owing to an extension of the depression of the ectoplasm which forms the latter. The mega- nucleus is horseshoe-shaped. Trichodina (Fig. 87 D). Dice-box shaped; with aboral ring of cilia for swimming, enclosing a ring of hooks for temporary attachment. Ectoparasitic on Hydra and other animals. Vorticella (Figs. 2, 92). Shaped like a solid, inverted bell, with, in place of the handle, a stalk which consists of a prolongation of the body, and is clad in a cuticle and contractile by means of a myoneme. Solitary. In fresh waters and marine. Carchesium (Fig. 93). As Vorticella, but colonial. In fresh waters. Epistylis. As Carchesium, but the stalk is purely cuticular and non- contractile. In fresh waters and marine. 112 THE INVERTEBRATA fr.cir.'^'-zZ ^u.me. ^nd.ci. -jyrexi. int.u.me. f.vac- ahxin-^t: se.ct.^-- - (^ - -^j-^A n.cir. pt.cir. Fig. 90. Stylonichia mytilus, in ventral view, x 200. After various authors. ab.cir. abdominal cirri; ad.mae. adoral membranellae ; An. position of anus (on dorsal side) ; An.cir. anal cirri; c.vac. contractile vacuole; c.vac/ accessory canal of the same; end.ci. endoral c\\i2i\ f .vac. food vacuole ;/r.aV. frontal cirri; gu. gullet; int.u.me. internal undulating membrane; lip. projecting lower lip of peristome; mar.cir. marginal cirri; meg. meganucleus; mi. micronucleus ; pre.ci. preoral cilia; pt.cir. posterior cirri; se.ci. "sensory" cilia of dorsal surface ; ii.me. preoral undulating membrane (another undulating membrane is present but is omitted, to simplify the figure). Fig. 91. Stylonichia mytilus , from the left side. After Biitschli. CILIATA "3 Fig. 92. ■?, Fig. 93. Fig. 92. A group of individuals of Vorticella in various phases of the life history. From Borradaile. a, Ordinary individual, b, The same contracted, c, Ordinary fission, d, A later stage of the same, e. Free-swimming individual produced by ordinary fission. /, /', Two modes of fission to form microcon- jugants (/, budding;/', repeated fission of one product of a binary fission). gy Conjugation. Fig. 93. Carchesium epistylidis, x 100. After Saville-Kent. 114 THE INVERTEBRATA Order CHONOTRICHA Ciliata, permanently sessile by the posterior end upon the bodies of Crustacea; with the peristome represented by a spiral funnel at the anterior end, coiled clockwise, ciliated inside, and leading to the mouth; and the rest of the body naked. A small but very interesting group which shares with the Prociliata two characteristics not found elsewhere in the class, namely (i) that their nuclei are of one kind only and at mitosis form two sets of chromosomes (see p. 26), (2) that they form numerous gametes, mi.-if--^ Fig. 94. Fig. 95. Fig. 94. Spirochona gemmipara, x 520. ach. achromatic part of nucleus (centrosphere) ; chr. chromatin ; mi. micronucleus (which divides within mega- nucleus, where it appears when division is impending). Fig. 95. A diagram of the formation of an internal bud by one of the Suctoria. which unite in the same way as those of members of the other classes of the phylum. In the Chonotricha the reproduction, both sexual and asexual, is carried out by buds. The nucleus contains a large achro- matic mass which acts as a division centre. Spirochona (Fig. 94). Shaped like a slender vase. On the gills of Gammarus, etc., in fresh and marine waters. Subclass SUCTORIA Ciliophora of which all but a few primitive forms lose their cilia in the adult; and which possess one or more suctorial tentacles. A few members of the group are free ; a few are endoparasitic ; most CILIOPHORA 115 are attached, and these have usually a cuticular stalky which is often expanded at the end to form a shallow cup in which the animal sits or a deep one which encloses it. The suctorial tentacles contain a tube, lined by ectoplasm, which opens at the end, where there is often a knob. In some species there are also solid, sticky tentacles, used to capture prey. Reproduction by simple binary fission does not occur. In a few cases fission is equal or almost so (Podophrya, Sphaerophrya, Fig. 87 F'), but here one of the products differs from the parent in losing its tentacles and acquiring cilia and thus resembles the buds of other species. This happens whether the parent be a stalked or a floating form. Most species multiply by typical budding. The buds may be external (Fig. 96 B) or formed in brood pouches (Fig. 95) from which they Fig. 96. Ephelotagemmipara. After Hertwig. A, Ordinary individual, x 150. B, Budding individual, c.vac. contractile vacuoles ; nu. meganucleus (stained) ; processes of this form the meganuclei of the buds, as in all the budding of the Suctoria (cf. Figs, 87 F', 95). escape when they are ripe. External budding is the more primitive, internal the commoner process. In either, one bud or more than one may be formed at a time. The buds (Fig. 87 H), whether external or internal, are usually ciliated and at first without tentacles; the cilia form a girdle round the body, with sometimes the vestige of an adoral wreath. Certain species form also unciliate and often tentaculate oflF- spring by external budding. Some species will, in unfavourable cir- cumstances, resolve practically the whole body into one internal bud which swims away, leaving the pellicle and stalk behind. Conjugation is of the same nature as in the Ciliata. Two individuals become united by pseudopodia-like processes of protoplasm, their meganuclei break up, and their micronuclei form pronuclei which unite reciprocally. Often, however, the conjugants do not break apart, but one detaches itself from its stalk to unite permanently with Il6 THE INVERTEBRATA the other. It is not known what happens to the two zygote nuclei in these cases. The arrangement of the larval cilia in rings, the prevalence of a sessile habit, the frequent inequality of conjugants, and sometimes the absorption of one of these by its partner, suggest the derivation of this subclass from a form which resembled the Peritricha. Sphaerophrya (Fig. 87 F, F'). Spherical species; which are at first free and provided with knobbed tentacles on all sides ; afterwards be- come endoparasites in ciliates ; and are then without tentacles. Fission equal or somewhat unequal ; in the parasitic stage it is repeated before the young escape. Parasitic in Paramecium^ etc. Ephelota (Fig. 96). Stalked; not seated in a cup; bearing tentacles distally. Reproduction by external, usually multiple, budding. Marine. Acineta (Fig. i). Stalked; the stalk expanding to form a shallow cup. Reproduction by internal budding. In fresh waters and marine. Dendrocometes (Fig. 87 G). Body lens-shaped; without stalk; with branched arms which end in several pointed tentacles. Reproduction by formation of one internal bud. Sessile upon the gills of Gammarus. CHAPTER III spi.2 THE SUBKINGDOM PARAZOA (PORIFERA) Multicellular organisms ; invariably sessile and aquatic ; with a single cavity in the body, lined in part or almost wholly by collared flagellate cells; with numerous pores in the body wall through which water passes in, and one or more larger openings through which it passes out; and generally with a skeleton, calcareous, siliceous, or horny. The members of this phylum are the sponges. The simplest sponge is a little creature, osc. known as the Olynthus (Fig. 97), which is found only as a fleeting stage in the development of a few of those members of the group which possess calcareous skeletons ; but the bodies of all sponges may be regarded as derived from it, even though it may not appear as a stage in their life history. It is a hollow vase, perforated by many pores, and having at the summit a single large opening, the osculum. Through the pores water con- stantly enters it, to pass out through the osculum. Herein it and its kind difl^er from all the Metazoa, using the principal opening not for intaking — as a mouth — ■ but for casting out. The wall (Fig. 98) of the vase consists of two layers, (a) a gastral layer, composed of collared flagel- late cells resembling the Choanoflagellata (p. 65) and known as choanocytes, stand- ing side by side but not touching, which lines the internal cavity or paragaster except for a short distance within the Fig. 97. The Olynthus of a rim; and (b) a dermal layer, which makes simple calcareous sponge, with 1 ^ ' r 1 1 • 1 r part of the wall cut away to up the greater part of the thickness of ^^p^^^ ^^e paragaster. osc. os- the wall and is turned in a little way at culum ; po. pore ; spi. spicule, the rim. This layer again consists of two parts, (i) a covering layer of flattened cells, known as pinacocytes, rather like those of a pavement epithelium, but with the power of changing their shape; and (ii) the skeletogenous layer, between the covering layer and the gastral layer. The skeletogenous layer consists ii8 THE INVERTEBRATA of scattered cells, with a jelly in which they are imbedded. The most numerous of these cells are engaged in secreting spicules of calcium carbonate by which the wall is supported. They wander from the covering layer into the jelly, and then each divides into two, and the resulting pair secrete in their protoplasm, which is continuous, a needle-like spicule which presently outgrows them. Most often the original spicule cells come together in threes before this process, so that the three spicules which they secrete become the rays of a three- rayed compound spicule. This lies in the wall with two rays towards the osculum and one away from it. Sometimes a fourth cell joins the others later, and forms a fourth ray which projects inwards towards the paragaster. Often there are simple spicules which project from the surface of the sponge. Other cells, known asporocytes, of a conical shape, extend through the jelly, having their base in the covering layer am Fig. 98. Part of a longitudinal section of the wall of an Olynthus, including a portion of the rim of the osculum. From Borradaile. a.m. amoeboid cell ; ch. choanocyte; e.' flat covering cells (pinacocytes) of dermal layer; e.'' similar cells lining the rim of the osculum ;;. jelly; por. pore; pc. young porocyte; pc' fully developed porocyte; sp. spicule; sp.c. spicule cell. while their apex reaches the paragaster between the choanocytes. Each is pierced from base to apex by a tube, which is one of the pores. Be- sides these cells of the dermal layer, there are in the jelly wandering amoeboid cells which appear, in some cases at least, to belong neither to the gastral nor to the dermal layer, but to be descended inde- pendently from blastomeres of the embryo. Some of them become ova; others, it is believed, give rise to male gametes; the rest are occupied in transporting nutriment and excreta about the sponge. There are no nerve or sense cells in this or any other sponge. The current which flows through the body is set up by the working of the flagella of the choanocytes. It carries with it various minute organisms which serve the sponge for food, being swallowed, in some way which is still in dispute, by the collar cells. These digest the food, rejecting the indigestible parts into the space within the collar; and passing on the digested food to amoebocytes, which visit them to obtain it. PORIFERA 119 No sponge remains at this simple stage throughout its life. At the least the body branches and thus complicates its shape, and then often new oscula appear at the ends of the branches (Fig. 99). A higher grade is reached when, as in the calcareous sponge Sycon (Fig. 100), the greater part of the vase is covered with blind, thimble-shaped out- growths, regularly arranged, and touching in places, but leaving inh.c. Fig. 100. Fig. 99. A branched calcareous sponge of the first (Ascon) type. From Sedgwick, after Haeckel. Fig. 100. A semidiagrammatic view of a simple Sycon, opened longitudinally, with a portion of the wall enlarged. i7ih.c. inhalent canal; fl.c. flagellated chamber. between them channels, known as inhalant (or afferent) canals, whose openings on the surface of the sponge are often narrowed and are known as ostia. The thimble-shaped chambers are known as flagellated chambers, and are lined by choanocytes, but these are now lacking from the paragaster, where they are replaced by pinacocytes. Water enters by the ostia, passes along the inhalant canals and through the I20 THE INVERTEBRATA pores, now known as prosopyleSy into the excurrent canals, leaves these through the openings, known as apopyles^ by which they com- municate with the paragaster, and flows outwards through the osculum. A third grade is found in sponges such as the calcareous sponge Leucandra (Fig. loi), where the wall of the paragaster is folded a second time, so that the flagellated chambers, instead of opening direct into the paragaster, communicate with it by exhalant (or efferent) canals lined with pinacocytes. The three grades of sponge structure (Fig. 102), in which suc- cessively the choanocytes line the whole paragaster, are restricted to flagellated chambers, or are still further removed by the presence of exhalant canals, are known as the *' Ascon ", *' Sycon ", and *' Leucon " Fig. 1 01. Diagram of a section of the wall of the sponge Leucandra aspersa, showing the direction of the currents. After Bidder. grades. In many of the sponges whose canal systems are of the third grade, the flagellated chambers are no longer thimble-shaped, but small and round. As the canal system has grown more intricate, com- plication has taken place also in the skeletogenous layer. It has grown thicker, forming outside the flagellated chambers a layer known as the cortex, in which the inhalant canals ramify; and there appear in it branched connective tissue cells which can change their shape. The sponges which we have so far considered have skeletons com- posed solely of calcareous spicules, and their choanocytes are re- latively large. They constitute a comparatively small group, the class Calcarea. The majority of the phylum are without calcareous spicules and have relatively small choanocytes. They have usually siliceous PORIFERA 121 spicules, of which there exist many different types (Fig. 103), cha- racteristic of various groups of sponges, while minor differences distinguish those of the species, which are often only separable by this means. A horny substance, spongin, may occur as a cement uniting spicules, as fibres in which spicules are imbedded, or as a fibrous skeleton from which spicules are absent. The sponges in which the skeleton is in the latter condition constitute the horny f\ 0 far por OHQ fit ^^ exh c Oit lO^^ CP ost ost r I ink c ink c 3 4 Fig. 102. Diagrams of the canal systems of sponges. Partly after Minchin. I, Ascon grade. 2, Sycon grade. 3, Leucon grade. 4, Leucon with small, round flagellated chambers, exh.c. exhalant canal; inh.c. inhalant canal; fl.c. flagellated chamber; osc. osculum; ost. ostium ;^ar. paragaster ; /)or. pore. sponges (Keratosa), of which the bath sponge {Euspongia, Fig. 104) is an example. Foreign bodies (sand grains, etc.) are often imbedded in the spongin fibres. In a few cases (Myxospongiae) there is no skeleton. The choanocytes of non-calcareous sponges are always restricted to flagellated chambers. Almost without exception these are arranged as in calcareous sponges of the Leucon type, and in most cases the system is made still more intricate by ramifications of the paragaster, the irregular appearance of numerous oscula, which 122 THE INVERTEBRATA Fig. 103. Various types of sponge spicules. From Woods, a- e, are from Demo- spongiae, /, from a hexactinellid, g and h, from extinct groups of sponges, J, from Calcarea. a, With one axis (monaxon). b and c, With four axes w-^u^^'u"' ^ '^ ^ "calthrops", c a "triaene" spicule), d and e, Irregular. /, With three axes (triaxon ; four six-rayed spicules united as part of a continuous skeleton by additional deposits), j, A three-rayed compound spicule formed by the union of monaxons. PORIFERA 123 put it into communication with the water at many points, and the appearance of "subdermal cavities" and other complications in the outer part of the body. The non-calcareous sponges fall into two very distinct classes — the Hexactinellida, in which there is always a siliceous skeleton of six- rayed spicules (Fig. 103/), the jelly is absent, and the flagellated chambers are thimble-shaped, as in the simpler Sycons; and the Demospongtae, in which the skeleton, if present, does not contain six- rayed spicules of silica, jelly is present, and the flagellated chambers are almost invariably small and rounded (Fig. 106 C). ff.c. Fig. 104. A diagram of the structure of a bath sponge (Euspongia). From Borradaile. exh.c. exhalant canal; inh.c. inhalant canal; fix. flagellated chamber; osc. osculum; ost. ostia; sd.c. subdermal cavity; sk. one of the principal pillars of the skeleton, containing imbedded sand grains; sk.' minor fibres of the skeleton. Sponges have free larvae, of several different kinds, but all covered, wholly or in part, with flagellate cells, by which they swim. The re- markable feature of the metamorphosfes by which these larvae become the fixed adults is that the flagellated cells pass into the interior, develop collars, and become the choanocytes (Fig. 106). Asexual reproduction is found throughout the group. It takes place by the outgrowth and separation of external buds, or by the formation of internal buds or gemmules, enclosed in stout coats. In 124 THE INVERTEBRATA some cases (Spongillidae) the gemmules are remarkable in that they originate as clumps of the amoeboid cells of the parent. They will stand freezing or drought, and carry the species through unfavourable conditions. The power of regeneration and repair is possessed by sponges in a high degree, and they can be propagated artificially by cuttings. Sponges are found in all parts and at all depths of the sea. Only one family, the Spongillidae, occurs in fresh water, but its members are plentiful and widespread. ost. ih.ch. *'^' i ih.ch. ih.ch. ost. spi— Fig. 105. Section of a portion of Grantia extusarticulata. Highly magnified. From Dendy. ost. openings of the inhalant canals (ostia) ; ih.ch. inhalant canal ; prp. openings of inhalant canals into flagellated chamber (prosopyles) ; fl.c. flagellated or collar ceils (choanocytes) ; y?.c/z. flagellated chamber; spi. spicules ; ap. exhalant opening (apopyle) of flagellated chamber. The affinities, and therefore the systematic position, of the phylum Porifera have been the subject of much dispute. In that their bodies consist of many "cells", they might seem to be metazoa. But they differ from all members of that group in several important respects. In no metazoon are choanocytes found. In none is the principal opening exhalant. In none is there during development an inversion whereby a flagellated outer covering becomes internal. Lastly, and perhaps most significantly, in a sponge the *' cells" are far less special- ized and dependent upon one another than the cells of a metazoon. Many of them can assume various forms, becoming amoeboid, PORIFERA 125 collared, etc. Many are isolated in the jelly, and when they touch they are often not continuous. No nervous system co-ordinates their activities. Even the choanocytes, though the sum of their efforts produces a current, do not keep time in their working. In short, the Porifera are practically colonies of protozoa. Moreover, it would Fig. 106. A, Larva (Amphiblastula) of Sycon raphanus. B, The same with flagellated cells invaginating. After Schulze. ca. segmentation cavity; der.c. dermal cells ; fl.c. flagellated cells. C, Section of flagellated chamber of Spongilla lacustris. From Vosmaer. ap. apopyle ; nu. nucleus ; vac. vacuole. seem that they took origin from choanoflagellate mastigophora. Now opinion is, as we have seen, not unanimous that the Metazoa arose as colonies of protozoa, and in any case it is unlikely that they sprang from choanoflagellates. Thus the sponges, in spite of certain super- ficial resemblances to the Metazoa, have no real similarity to, and probably no genetic affinity with, that subkingdom. For this reason 126 THE INVERTEBRATA it is best that, in a classification of animals, they should be given, under the name of Parazoa, the same rank as the Protozoa and the Metazoa. Class CALCAREA Sponges with skeletons consisting solely of calcareous spicules ; and with large choanocytes. Clathrina. A meshwork of Ascon tubes. The nuclei of the choano- cytes are at the bases of the cells. British. Leucosolenia. A clump of erect Ascon tubes, each of which may be branched, connected at their bases. The nuclei of the choanocytes are apical. British. Sycon (Fig. loo). A simple vase with a canal system of the second type, having the thimble-shaped outgrowths little adherent to one another. The nuclei of the choanocytes are apical. British. Grantia. Differs from Sycon in that the outgrowths which contain the flagellated chambers adhere in many places and are covered by a cortex (Fig. 105). British. Leucandra. Canal system of the third type (Fig. loi). Nuclei of choanocytes basal. British. Class HEXACTINELLIDA Sponges with a purely siliceous skeleton composed of six-rayed spicules; with small choanocytes and thimble-shaped flagellated chambers ; and without jelly, the soft parts of the body being united solely by a meshwork of trabeculae furnished by branching cells of the dermal layer. A deep-sea group. Euplectella^ Venus' flower basket, and Hyalonema, the glass-rope sponge, have both been dredged in British waters. Both harbour various commensal crustaceans. On the rooting-tuft of long, fine spicules, which is the "glass-rope" of Hyalonema, grows an epizoic anemone of the genus Epizoanthus. Class DEMOSPONGIAE Sponges whose skeleton, if present, does not contain six-rayed spicules of silica, and may be purely siliceous, or composed of silica and spongin, or of spongin alone ; whose flagellated chambers have small choanocytes and are usually small and rounded; and which possess jelly. PORIFERA 127 Cliona (Monaxonida^). A cosmopolitan genus, which bores into the shells of molluscs and into calcareous rocks. Halichondriay the crumb-of-bread sponge (Monaxonida). A com- mon British littoral form, usually of encrusting growth. Spongilla (Monaxonida). A member of the family of freshwater sponges mentioned on p. 124. Cosmopolitan. Euspongia, the bath sponge (Keratosa). Medit., W. Indies, etc. Hippospongia (Keratosa). A sponge of the same kind with a coarser texture due to the inclusion of much foreign matter in its skeleton. Oscarella (Myxospongiae). British, has no skeleton. ^ Orders of Demospongiae : Tetractinellida, with tetraxon spicules (Fig. 103); Monaxonida, with monaxons; Keratosa, with spongin skeleton; Myxo- spongiae, without skeleton. CHAPTER IV THE SUBKINGDOM METAZOA The fundamental difference in histology which distinguishes the Metazoa from the Protozoa has already been described in Chapter ii. Something must here be said concerning the main features of the organization of the Metazoa. The simplest type of bodily architecture in this subkingdom is that with which the student is familiar in Hydra, where the body consists of a sac with one opening, and with the wall composed of two cellular layers and a layer of secreted jelly between them. The inner layer is the endoderm. It consists of cells specialized for the processes of digestion, and the cavity which it lines is for the reception of food. The outer layer is the ectoderm: by its cells relations with the en- vironment are regulated. Some of these cells form a protective and retaining sheet ; among them stand others which are sensitive ; others — nerve-cells — lying below the sheet, are branched so as to serve for the transmission in various directions of the stimuli received by the sense cells : together they form a nerve-net. At the base of both ectoderm and endoderm there lie muscle fibres — which in Hydra are elongate contractile processes of the retaining cells but in other animals of this type are often whole cells that have left the surface. Lastly, from certain undifferentiated cells at the base of the ectoderm there are formed the generative cells. When we compare this organization with that of a protozoon we observe that the cellular structure of the metazoon, primarily, perhaps, necessitated by its size (p. 8), has the following result : by isolating the units specialized for the performance of particular functions it (a) re- moves most of them from the direct action of the outer world, (6) makes it possible that groups of them should constitute independent organs, and {c) enables the relations of such organs, both with the environ- ment and with one another to be regulated by intervening cells and internal media. Already in the simple case we have examined these facts are turned to advantage. Under the protection of the layer which remains in contact with the outer world there are established a special organ of digestion and a system for distributing stimuli which are received by distinct units on the surfaces. Other ele- ments (muscular, genital) are beginning to separate. In the following pages we shall see this process of separation and differentiation carried much further. Its result is that the activities of the organism are less and less liable to interference from or suppression by the environ- METAZOA 129 ment, either through the unregulated exchange of substances or by unregulated stimuli. We shall see also, how the machinery which is fashioned in this way varies in correspondence with the environment. In the phylum to which Hydra belongs, the Coelenterata, the body is always of the type just described, whatever form the sac or its layers may assume, though the jelly may contain cells, sometimes plentiful, of various kinds — muscle fibres, skeleton forming cells, and amoeboid corpuscles — which have migrated into it from the ectoderm or endo- derm. In all other metazoan phyla there is between ectoderm and endoderm a third layer, the mesoderm, which usually is more bulky than either of the other layers and forms the greater part of the body. The phyla which possess this layer are known as Triploblastica — three-layered animals — while the Coelenterata are Diploblastica. It is true that the mesoderm is partly foreshadowed by the cells which are present in the jelly of many coelenterates, but mesoderm is more plentiful than the cells in the jelly generally are, it contains important organs and usually definite systems of spaces (see p. 131), and its rudiment appears very early in the development of the individual. Every triploblastic animal, however, passes through a stage — the gastrula — in which it consists only of ectoderm and endoderm. Save in this essential feature, the gastrulae of diff'erent animals may be extraordinarily unlike, and, especially when the animal is developed from a very yolky tgg, they are sometimes very difficult to recognize as such; but where the gastrula is well formed, as in the familiar de- velopment of Amphioxus or in that of a starfish (Fig. 438), its two- layered wall may always be found to contain a cavity, the archenteron, which possesses a single opening, the blastopore. The ectoderm and endoderm are separated by a space, which is often a mere crack, but may be much wider, and contains a fluid or a slight jelly. This space is known as the blastocoele, and when, as in the cases cited above, the gastrula arises by the dimpling-in (invagination) of the wall of a one- layered hollow vesicle or blastula, the blastocoele begins as the cavity of the blastula. The mesoderm, whose appearance converts the gastrula into a triploblastic body, is not a single entity, but contains components which originate in two diff'erent ways, namely: {a) Cells which migrate from ectoderm or endoderm, or from mesoderm of the other kind, into the blastocoele; this kind of mesoderm (Fig. 438, mch.) is known as mesenchyme, and is comparable to the cells which invade the jelly of coelenterates. {b) Cells which constitute the wall of the cavity known as the coelom. This kind of mesoderm is called mesothelium. In some cases, as in Amphioxus, the starfish, Sagitta, and the Brachiopoda (Figs. 462, 438, 430, 427 A), it arises as pouches of the archenteron which separate 130 THE INVERTEBRATA from the latter, their cavity becoming the coelom and their wall the mesothelium. In other cases it arises as solid outgrowths or layers shed off from the wall of the archenteron, and coelomic cavities after- wards appear in it. This happens, for instance, in the tadpole. In yet other cases a single pole cell or teloblast^ as in annelids (Fig. 196) and molluscs, or a group of a few cells, as in arthropods, separate, on each side of the embryo, from the rudiment of the endoderm, and multiply so as to form a band of cells in which coelomic cavities appear. A coelom which arises as a pouch from the archenteron is known as an enter ocoele ; one which arises in a mass of mesothelium is a schizocoele. In the lower triploblastic phyla (Platyhelminthes, Nemertea, Nematoda, etc., p. 197) there is no mesothelium. Chaetognatha have no mesenchyme. In most phyla, both kinds of mesoderm develop. ^ We must now consider the organs formed by each of the three layers. i. Endodermal organs. After giving rise to mesoderm, the archen- teron becomes the rudiment of the alimentary canal. Except in Platyhelminthes, its blastopore is in various ways replaced by two openings,^ so that it has both mouth and anus. Its wall, the endoderm, forms the lining of the alimentary canal, except in those regions, known as fore gut or stomodaeum and hind gut or proctodaeum, which are formed by a tucking-in of the ectoderm at the mouth and anus. The endoderm also gives rise to the various diverticula of the mid gut, such as the liver and other digestive glands, the lungs of vertebrata,etc. A true stomach is an enlargement of the mid gut. Digestion was perhaps originally entirely intracellular in the endo- derm cells, and many of the lower animals still have intracellular digestion, though this is usually preceded by an extracellular process which by dissolving certain components of the food enables the re- mainder to be reduced to particles small enough to be taken up by the cells. In the annelids, arthropods (except certain ticks, p. 534), cuttlefishes, and Chordata digestion is entirely extracellular. The enzymes secreted vary with the food : in carnivorous animals such as cephalopods and starfishes they are principally proteases, in feeders on vegetable tissues they are largely carbohydrases, in omnivores such as the crayfish and cockroach and holothurians they are adapted to deal with all classes of food-stuffs. Considering the importance of cellulose both as a potential food-stuff and as cell walls which enclose more valuable foods, it is remarkable that cellulases should be rare (PP-435, 559>587)-, Both intracellular ingestion and absorption are not always confined ^ Mesenchyme is scanty in the lower Chordata. ^ The most primitive way is probably that of Peripatus (p. 319), in which the middle of the blastopore closes and the ends become mouth and anus. METAZOA 131 to the alimentary canal proper but may take place in digestive glands or "livers", as for instance in those of the mussel, the snail, and the crayfish, but not in those of cuttlefishes or vertebrates. It is said that in various bivalve molluscs and in holothurians amoeboid corpuscles pass through the endoderm, take up particles in the gut, digest them, and, returning, distribute the products. The presence of a cuticle in the ectodermal portions of an alimentary canal does not always prevent absorption there (e.g. in the fore gut of some insects). Finally it should be noted that some animals perform a part of their digestion externally to the body J as the starfish by extruding its stomach (p. 636), and various insects, mites, earthworms, etc. by pouring out saliva ; and that in other cases bacterial or protozoan symbionts (pp. 68, iii) play a part in the digestion of food — particularly of celluloses — in the gut. The food of all animals contains amino acids, usually as protein, for the manufacture of the proteins needed in the repair and growth of protoplasm. Much amino acid, however, is deaminated^ the car- bonaceous residue being oxidized, together with the carbohydrate and fat which the food usually also contains, for the liberation of energy, and the ammonia excreted in various forms by various organs presently to be mentioned. ii. Mesodermal organs. Since mesothelium gives rise to mesen- chyme, it is often difficult to distinguish between the two and to decide what part each plays in the formation of organs ; but broadly speaking it can be said that the connective and endoskeletal, the vascular, and some muscular tissues arise from mesenchyme, while in coelomata the peritoneum and the organs derived from it — gonads (ovaries and testes), mesodermal kidneys, etc. — and the principal muscles arise from mesothelium. Within the massive layer of mesoderm, cavities are necessary for sundry purposes. Channels must be provided for the transport of various materials — the products of the digestion of food, the gases of respiration, water, the waste products of metabolism, which are usually eliminated with the excess of water, and the substances known as hormones which are secreted by certain organs as messengers to regulate the activity of others. The germ cells, which are sheltered in this layer, must be given access to the exterior. Often there must also be spaces to give play to movements of the viscera. Such facilities are provided by two systems of cavities, the primary and secondary body cavities^ of which either or both jnay be present. {a) The primary body cavity, sometimes known as the haemocoele, is to be regarded, morphologically, as representing that part of the blastocoele which is not obliterated by the mesenchyme cells or by a solid matrix or fibres secreted by them. Its fluid contents, containing free mesenchyme cells ("corpuscles"), are the blood and lymph, and 132 THE INVERTEBRATA it has usually the form of a branching system of vessels (''vascular system") through which the fluid is caused to circulate by the con- traction of muscular fibres in the wall of some portion of it which is known as a heart. In some cases, however, the haemocoele forms large "perivisceral" sinuses around the internal organs. It never contains germ cells or communicates with the exterior. Since the haemocoele fluid is in intimate relation with the tissue, its composition is a matter of very great importance to the animal. It bears to the tissues much the same relation that the external medium bears to the body as a whole and is on that account often spoken of as an internal medium} If it be changed the working of the organism is influenced. It is liable to be fouled by poisonous waste products of metabolism and these must be removed from it and ex- creted or so changed as to be harmless. It is liable to alteration by diffusion between it and the external medium, and in proportion as this can take place the animal will be at the mercy of its surroundings. To maintain it in a constant condition in respect of the substances which it might exchange with a particular external medium two agencies are at work — the guardianship, active or passive, of the pro- tective sheet of ectoderm and of any cuticle or other covering which the latter may secrete, and the activity of the excretory organs, especially in the excretion of water. The effectiveness of these agencies varies. The independence of the body fluids from the external medium is least in some marine animals, such as echinoderms and certain molluscs: in these the fluids closely resemble sea water both in the ions present and in the total osmotic pressure. In a series of others, independence grows, and it is highest, in the sea, in teleostean fishes. In fresh water animals the composition of the blood is kept entirely different from that of the external medium. In land animals there is of course no question of the exchange of solutes, and unless the loss of water were reduced to a minimum life would be impossible. It is an interesting fact that, though the resemblance of the body fluids in fresh water and land animals to sea water is much less than that of marine animals, something of it still remains, no doubt because protoplasm came into being in sea water and still requires to be bathed by a fluid which somewhat resembles the latter. The principal differences are an increase in potassium and a decrease in magnesium and SO4 ions and a lower total osmotic pressure. The blood is the principal means of transport within the body. A very important part of its freight is oxygen. Its capacity for ^ The fluids of the secondary body cavity (coelomic fluids) are also internal media, but less intimate and therefore chemically less important than the blood. In echinoderms, tiowever, they are probably more important than the fluid of the vestigial haemocoele (lacunar system, p. 629). METAZOA 133 this gas, however, would be quite insufficient for it to maintain the metabolism of an active animal if the gas were carried in mere solution. This deficiency is met, when necessary, by the presence in the blood of respiratory pigments. These bodies are compounds of a protein with a nitrogenous pigment which contains a metal. They are re- lated to one another, to chlorophyll, and to the colourless substance cytochrome which is very widely distributed in the protoplasm of animals and plants, where it plays a part in the regulation of oxida- tions. They form very labile addition compounds with oxygen, which they can thus take up in the organs of respiration and carry to the tissues, where they yield it up by dissociating under the lower oxygen tension, undergoing at the same time a change in colour. The most important of them are haemoglobin, which contains iron and is red, chlorocruorin, also containing iron, which is green, and haemocyanin, containing copper, which is blue when oxygenated. Haemoglobin is present in Vertebrata, where it is carried in the "red corpuscles", and sporadically in many invertebrates, as in the earthworm, where it is in solution in the plasma. Chlorocruorin is found in solution in the blood of various polychaete worms, haemocyanin in solution in the blood of the higher Crustacea, the king-crab (Limulus), and various molluscs. Both haemoglobin and haemocyanin are slightly different compounds in different animals, and with these differences are as- sociated differences in the pressure at which they take up or yield oxygen. Broadly speaking, the blood pigments of animals which live under conditions of low oxygen pressure take up the gas at a lower pressure than those which live under high oxygen pressure. On the other hand they do not maintain so high a pressure in the tissues. Independently of such differences, the haemocyanins are less efficient oxygen carriers than the haemoglobins. In tracheate arthropods, where air is brought direct to the tissues by a system of tubes, there are no blood pigments. The blood of the higher invertebrates contains in solution a con- siderable amount of protein, of which the respiratory pigment, if present, is only a part. This protein is comparable with the organic ground substance of a skeletal tissue. It is not a food for the tissues but by maintaining the osmotic pressure of the blood it is of im- portance in regulating the distribution of water between that fluid and the tissues, and, since proteins combine with both acids and alkalies, it helps to neutralize excess of either of these. In vertebrates some of this protein provides the material for clotting, by which loss of blood or injury is prevented; but invertebrates, when they form a clot, do so from material furnished by corpuscles. (b) The secondary body cavity or coelom is from the first completely surrounded and separated from the blastocoele by the mesothelium, 134 THE INVERTEBRATA which is derived, as we have seen, from the endoderm. This cavity has various forms, but is rarely tubular and never possesses a heart. Usually it constitutes one or more large perivisceral spaces around the heart, alimentary canal, and other organs. It will be noted that the perivisceral cavity which surrounds the internal organs of most triplo- blastic animals, so that these organs are unaffected by the movements of the body wall and are able freely to perform movements of their own, may be either coelomic or haemocoelic, but is usually coelomic (Fig. 107 a-c). In the Arthropoda, where the perivisceral function of the coelom is entirely usurped by the haemocoele {d~g), the former space is reduced to small cavities in the gonads and excretory organs. In animals which possess a coelom, the gonads are derived from its walls, and either the germ cells are shed into a coelomic perivisceral cavity or the gonad itself contains a cavity which is a separated portion of the coelom. The coelom communicates with the exterior. The communication is usually made through organs belonging to one or other of the types known as "nephridia" and "coelomoducts", though it occasionally takes place through openings of other kinds, such as the dorsal pores of the earthworm and the abdominal pores of fishes. Nephridia and coelomoducts are organs which meet the need for the passage to the exterior of products of organs derived from or imbedded in the mesoderm. Their characteristic features are as follows : (a) The nephridial system is primarily an organ which serves the mesenchyme, though it may come to lie in the coelom, and in certain annelids communicates with that space. It is for the most part intra- cellular, and consists of tubes, often, at least, of ectodermal origin, usually branched and bearing at the end of each branch a solenocyte or flame cell (see p. 202). It may be continuous or divided into seg- mental units, the nephridia. Water, probably containing excreta, is shed by the protoplasm of the tubes, and passes out in the current set up by the action of the flame cells or by cilia. {b) Coelomoducts are mesodermal passages which open at one end to the exterior and at the other usually into the coelom, though the coelomic opening may lead only into a minute vesicle of the coelom, or even be lost altogether. They may (i) be solely excretory, the excreta being shed into them by gland cells in their walls, or borne into them by a current of fluid from the coelom through the coelomic opening of the organ, or derived from both these sources (see p. 141) ; (2) combine excretion with the function of conducting the germ cells to the exterior; (3) be simply gonoducts, which was perhaps their original function. Many annelida possess compound excretory organs formed by the METAZOA 135 — • -H CO s_^ -M • J> C 5 u • ^ " ^ ^ 5 feiii^^. ""? u M ^ "O " ^ es u. D ^ ._ d *-| cl CO h i_t a, ^ JO a >^ c ^^ G< C ^ -M CO CO S^-g-d c d ^ o'S)g-.S <"'aS-C'2coL."'^«3 ^ «-^ a 8 3 tj.-g^«^ w) l^o+j-Qco'^ca P »-; "> CO g^ g S g f 1 i_ •r-' C3 CO ^3-5 d^'-^-sS'^ ^ > o CO "" o ^ V ^ u-i 1^*- (S-n "^o^ ■^ «SOco3.^A,co - rn— ,. .-hT3 V Q 4) C ,, .i:*-^ w;S "^-a o"^-*^ fa JJ z:; o a.i=! „ „ G p c a, « <« ._ p^-c coGoca«<«.2o ^ ^-^ 2 t ^ ^ 8 .. J3 S o ^ r2o«5.S,„co«i5'*-,^3 CO C Q V S -^ t^ •> •£ ^ ^ .« i; fl - "^ "- 1 J2 ^ 0 S ^"J -rf — 1- ^^ Tl ^ • -H 1*^ r* '"^ C .^ C3 ^ Lh ^^ t^ ,, ^S Oi-icocvO bcC'0'-tX'',r« IhotCmSr^ cells into gono- 164 THE INVERTEBRATA processes which engulf decaying polyps, epizoic organisms like diatoms and protozoa and larvae of other epizoic forms. Sertiilaria (Fig. 122 B) with a creeping hydrorhiza, more or less branching stems which bear opposite hydrothecae; hydrothecae large, so that the polyps can completely retract within them. The following genera of Gymnoblastea may also be mentioned : Cordylophora, living in fresh or brackish water (Norfolk Broads), polyps with scattered filiform tentacles. Pennaria (Fig. 122C) with two kinds of tentacles, oral capitate and aboral filiform; nematocysts of very large size; medusae de- generate but become free when gonads are mature. Hydr actinia^ with spreading plate-like perisarc covered by naked coenosarc, very often found coating a shell inhabited by a hermit crab; with spiral dactylozooids and sessile gonophores. Podocoryne, as Hydr actinia, but with free medusae. The polyp forms of many medusae, both Antho- and Lepto- medusae, are unknown. Order TRACHYLINA This group consists of forms in which the medusoid develops directly from the egg and the polyp has either been reduced to a minute fixed individual or is represented only by the planula larva which metamorphoses into a medusa. The possession of sense tentacles with endodermal concretions is an important character. There are two suborders : Trachomedusae. Trachylina with sense tentacles in pits or vesicles and with gonads situated in the radial canals ; with marginal tentacles on the edge of the umbrella. Examples: Geryonia, Limnocodium, Carmarina (Fig. 120 II), Limnocnida. Narcomedusae. Trachylina with sense tentacles not enclosed and marginal tentacles inserted some distance aborally from the edge of the umbrella ; with gonads on the oral wall of the stomach. Example : Cunina. The inclusion of the following freshwater forms in the order is provisional : Limnocnida is a remarkable freshwater form found in the Central African lakes. Up till the present only male medusae have been found in Lake Tanganyika and female in Victoria Nyanza. Asexual repro- duction by budding takes place from the margin of the bell. Other species occur in Rhodesia and the Indian rivers. Craspedacuta (Limnocodium) w^s first known from the Victoria Regia tank in the Royal Botanic Gardens at Kew, but has now been dis- covered in various North American rivers and has even colonized HYDROZOA 165 ponds and canals in England. It has a polyp -like stage, Mtcrohydra, which has a certain likeness to Hydra. Order HYDROCORALLINAE The forms included in this group are mostly associated with reef corals in tropical seas. The main part of the colony consists of a much branched hydro rhiza with frequent anastomoses. Instead of secreting a horny perisarc as the Calyptoblastea and the Gymnoblastea do, the ectoderm lays down an exoskeleton consisting of calcareous grains, which becomes bulky and solid. It may be either massive or encrust- ing or branching. From pits in the surface of the colony arise the AacL tab. Fig. 123. Diagrammatic section through Millepora showing a gastrozooid with a dactylozooid (dact.) on each side of it and an ampulla (amp.) with a medusa enclosed in it; can.i, the living canals, shown in black, and can.2, the degenerating canals, shown as lines, constitute the hydrorhiza, and the skeleton is represented by stippling ; med. a medusa just liberated ; tab. tabulae in a gastropore. Slightly altered from Hickson. polyps. These are of two types (Fig. 123). First there are the in- dividuals of normal structure with a mouth surrounded by tentacles {gastrozooids) : these nourish the colony. Then there are the dactylo- zooids which are much longer and more slender. They have no mouth but they possess scattered capitate tentacles and may form a ring round a gastrozooid, in which case it is readily observed that their function is to catch prey and hand it to the central gastrozooid for digestion. Besides the polyps there are the medusae, which, as in Bougainvillea, are budded directly off from the coenosarc: they are lodged in pits of the skeleton called ampullae^ but their liberation has been observed in Millepora. It is supposed, however, that their free- living existence is very brief. l66 THE INVERTEBRATA Order SIPHONOPHORA The Siphonophora are colonial animals which exhibit the maximum development of polymorphism found in the Coelenterata or indeed in any group of the Animal Kingdom. They are pelagic and each colony originates from a planula which metamorphoses to form a single medusiform individual (Fig. 124B nec.^ which later drops off from the colony), from the exumbrellar side of which springs a coenosarcal tube budding off all the other members of the colony (Fig. 124 B gst. etc.). It usually happens that those which are de- veloped first are needed to buoy up and propel the young colony. Consequently the first individual is either medusiform or else forms an apical float or pneumatophore, the epithelium of which secretes gas (Fig. 124 A/)w., B nee}). There may also be formed from the ectoderm of the first formed individual an oleoeyst containing a drop of oil. The succeeding medusiform individuals resemble the bell of an antho- medusa, with velum, musculature and canal system but lacking the manubrium, and they are called neetoealyees: while the most primi- tive siphonophores have only a single one there may be a series of them. Following these the coenosarc in one type of colony (Fig. 124 A) grows to a great length and buds off at intervals along its length similar assemblages of individuals. Such an assemblage is known as a eormidium, and may consist of (i) a shield-shaped hydrophy Ilium which covers the rest of the cormidium, (2) a gastrozooid resembling the manubrium of a medusa, with a mouth, and a tentacle usually branched, (3) a mouthless individual, the daetylozooid^ with a tentacle usually of great length and provided with strong longitudinal muscles, and (4) a gonozooid (or individual bearing gonophores) which may or may not have a mouth. The gonophores often resemble those found in some of the Gymnoblastea like Tubularia. Such forms as those described above are the genera Halistemma, Diphyes and Muggiaea. In other cases the coenosarc is not a linear stolon but a massive body from which are budded off innumerable cormidia, in which gastrozooids, dactylozooids and gonozooids are all crowded together to form a compact colony. In Physalia (Fig. 125 B), the ** Portuguese man-of-war ", there is an enormous cap-shaped pneumatophore which floats above the surface of the water. There are no neetoealyees, but the colony is borne hither and thither by the wind and countless numbers are cast up on the lee shores. The dactylozooids of Physalia hang suspended from the colony and form a drift net ; when they are touched by a fish the nematocysts discharge and the fish is captured. The tentacles contract and the prey is drawn up until the gastrozooids can reach it. The lips of these are spread out over the surface of the fish until it is enclosed in a sort of bag in which it undergoes the first SIPHONOPHORA 167 ^>can.r. cor. —med. Fig. 124. Development of the siphonophore colony. A, Diagram of the possible combinations of individuals in a colony. The continuous gastro- vascular system is shown in black, pn. pneumatophore ; nee. nectocalyx; hyd. hydrophyllium ; gst. gastrozooid; dac. dactylozooid with its tentacle; gnz. gonozooid ; cor. cormidium ; esc. coenosarc ; ten. branched tentacle, sometimes springing from the base of the gastrozooid. B, Early stage of colony of Muggiaea, showing two generations -of nectocalyces, nec.^, nec.^ can.r. the radial canals of the first nectocalyx. Other lettering as in A. nec.^ is lost later and nec.^ becomes the single permanent nectocalyx of the colony ;/)w. is really an oleocyst and not a pneumatophore. C, Sarsia, an anthomedusan, for comparison, showing budding of daughter medusae from the end of the radial canals, mnb. manubrium; med. daughter medusae. A, altered from Hertwig; B, after Chun; C, after Allman. i68 THE INVERTEBRATA Stage of its digestion. Physalia can catch and devour a full-grown mackerel, and the poison of its nematocysts is so virulent as to en- dac/-~- Fig. 125. Examples of Siphonophora. A, Velella. Altered from Haeckel. Vertical section, showing the cavity of the pneumatophore (stippled) and produced into branching gas tubes, the tracheae (tra.), and a network of endodermal tubes (black), which arise from the cavity of the gastrozooid and gonozooids (black); ?ned. medusa buds. Other letters as in Fig. 124. B, Phy- salia showing the "drift net" arrangement of the tentacles of the dactylo- zooids. danger human life. In Velella (Fig. 125 A) the disc-shaped colony has a superficial resemblance to a single medusa. The pneumato- HYDROZOA 169 phore consists of a chitinous disc containing a number of chambers and raised into a vertical ridge which forms a sail. On the under surface there is a single large gastrozooid in the centre, a larger number of gonozooids surrounding it and a fringe of dactylozooids at the margin. The gonozooids produce buds which actually escape as free medusae. The coenosarc consists of a mass of tissue which is traversed by endodermal tubes placing in communication the cavities of the gastrozooid and the gonozooids, and ectodermal tubes (tracheae) which are prolongations of the gas cavity of the pneumatophore. This tropical form is often brought in large numbers to the shores of Devon and Cornwall by the Gulf Stream. The medusae and nectocalyces of the Siphonophora are very similar to the Anthomedusae. Medusae like Sarsia (Fig. 124C) may bud off other medusae either from the bell or the manubrium, but the Siphonophora are probably not to be regarded simply as a colony of medusae connected by coenosarc. A further change has gone on in which organs have been displaced from their original position. The manubrium has come to lie outside the primary medusa bell, forming a gastrozooid (Fig. 124 B, gst.) at the beginning of the main coeno- sarcal axis. No manubria corresponding with the medusa bells of the nectocalyces are present. In the cormidia the hydrophyllium which may be a modified bell, the gastrozooid and the tentacles may be quite separate from one another while the complete medusoid form is shown only by the fixed gonophores (Fig. 124 A and B). In more specialized siphonophores owing to the shortening of the main axis the displacement of parts is more extreme and the com- ponent parts of the cormidia no longer recur in the typical groups, all kinds of organs being crowded together. Lastly, with the great development of the gas-secreting pneumatophore, the medusa bell is suppressed. While the above description gives an impression of the order re- garded as colonial animals the siphonophores must be primarily considered as coelenterates exhibiting growth variability to such an extent that the identification of the component structures as organs or individuals is difficult and of purely academic interest. Order GRAPTOLITH INA Extinct, probably planktonic, animals ; if related to the Hydrozoa, the polyp generation is dominant, the medusoid generation unfossilized or possibly represented by the prosicula; the individuals are budded oflF from one another and remain in contact with the parent ; there is no definite coenosarc; and the perisarc is produced round the polyps as hydro thecae. 1 70 THE INVERTEBRATA Graptolites are represented in the earliest fossiliferous rocks, the Cambrian, Ordovician and Silurian. Though we know nothing of their soft parts, the exoskeleton was horny or chitinous and so may be well preserved. It resembles in general development that of the colonies of the Calyptoblastea, in that it was produced round the polyps to form definite hydrothecae. The graptolites, however, differ from calyptoblast hydroids because new individuals have the appear- ance of being budded off directly from older ones rather than from m. h.2 )ThA A « C ^'^- -^- Fig. 1.7. Fig. 126. A. Young colony of Monograptus, showing prosicula (/)), meta- sicula (m) and developing hydrothecae i and 2, with growth-lines. After Kraft. X15. B. More mature colony of Mowo^rop/M^. X2-5. C. Early part of the colony of Climacograptus, showing sicula {s) partly enclosed by the early hydrothecae. After Wim an. X15. Fig. 127. A, Didymograptus \-fractus. Ordovician. Early part of the poly- pary. After Elles. si. sicula ; cr.c. crossing-canal ; i, first hydrotheca ; 2, second hydrotheca. X5. B, Tetragraptus similis. Lower Ordovician. Young form with virgula and disc. After Ruedemann. x 4. a common coenosarc. Each colony originates from a conical body called a sicula^ the exoskeleton of the first formed individual, consist- ing of the pro- and meta-sicula (Fig. 126 A). From the side of the metasicula a bud is formed, which develops into the first hydrotheca, and from this is produced the second, and so on. In this way a linear series of polyps is produced which are arranged in a slender lamella (stipe) y the hydrothecae being in contact and the cavity of the colony being continuous. This is the simplest arrangement, and is seen in Monograptus (Fig. 126 A and B) where the hydrothecae are all on one GRAPTOLITHINA 171 side of the stipe. In Didymograptus (Fig. 127) the second hydrotheca grows across the sicula to open on the opposite side, and the first and second hydrothecae go on budding independently, so that we have a colony with two stipes or branches. By another modification later, hydrothecae bud off two individuals instead of one, and colonies like Tetragraptus (Fig. 128) and Bryograptus are formed. In Diplograptus and Climacograptus (Fig. 126 C) there is a biserial stipe, either formed of two uniserial stipes growing back to back and thus separated by a xi. Fig. 128. Fig. 129. Fig. 128. Tetragraptus. Ordovician Rocks, a, central disc. Fig. 129. Diplograptus foliaceus from the Utica Slate, New York, x f , After Ruedemann. For description see text. median septum, or as a result of alternate budding throughout the colony. In the absence of a coenosarc the graptolites were not attached by a creeping hydrorhiza, such as occurs in Calyptoblastea. There was, however, a thread coming off from the end of the sicula which ended in a disc, by which it is supposed that the graptolites were attached to floating seaweed (Fig. 127 B). It is also possible that some grapto- lites were independent planktonic organisms with a pneumatocyst or other kind of float. Such a pneumatocyst appears to be shown in Diplograptus (Fig. 129) as a square central body from which a number of stipes radiate. There is also a circle of round bodies which are possibly gonophores, as they contain siculae. In any case the grapto- lites were true pelagic organisms and their floating habit gave them 172 THE INVERTEBRATA a universal distribution in the Palaeozoic oceans. A series of life zones may be traced in the rocks which were there laid down, each characterized by a definite assemblage of graptolites, and these may be traced throughout the world. By a careful consideration of these graptolite successions the main line of evolution of the group has been worked out. It is now concluded that actual genetic relationship is best traced by the characters of the hydrothecae. The earlier forms have very simple hydrothecae, but the shape becomes gradually more complex. On the other hand the genera were usually founded on the number of branches or stipes in the colony, such as Bryograptus with many stipes in the Cambrian, Tetragraptus with four in the Lower Ordovician, and Didymograptus with two in Lower and Middle Ordovician. These genera succeed each other in geological age, and so we may suppose that they constitute an evolutionary series. In reality they constitute not one but several series. Thus there is the same type of hydrotheca (which we will call A) in Bryograptus callavei^ Tetragraptus hicksi and Didymograptus affinis, while another type (B) is common to B. retroflexus, T. denticulatus and D . fasciculatus . The genera of graptolites as at present constituted are thus open to criti- cism ; it would be more correct to classify all the species into hydro- thecae of type A as one genus, and those into type B as another. In the genus Monograptus, which is the last and most abundant of the graptolites, though the form of the colony is simple, the hydrothecae vary tremendously, and it is obvious that we have here grouped to- gether the descendants of many different genera undergoing com- paratively rapid evolutionary changes. Certain forms, whose relationship is not clear, occur very com- monly at certain horizons in the Cambrian and Ordovician and less commonly in later rocks up to the Carboniferous and are grouped together as *' dendroid" graptolites. It is possible that they are closely related to the Calyptoblastea. They differ from the "true" graptolites in showing polymorphism, the thecae being generally interpreted as having enclosed feeding individuals (corresponding to the thecae of the true graptolites), gonozooids (or perhaps protective individuals) and budding individuals. Class SCYPHOMEDUSAE (SCYPHOZOA) This class contains the common jellyfishes of temp>erate and colder seas, some of which are of extraordinary size, like Cyanea arctica^ the diameter of whose disc is a couple of yards. The simplest type of Scyphomedusae is found in the division known as the Stauromedusae, two members of which, Haliclystus and Lucernaria (Fig. 130), are not uncommon on the British coasts. SCYPHOZOA 173 adhering to the blades of Zoster a or Laminaria. It has a narrow stem arising from its exumbrellar surface, by which it attaches itself temporarily to seaweed. The edge of the bell is divided into eight Fig. 130. Longitudinal sections through A, Lucernaria and B, a strobilizing scyphistoma of Chrysaora. In A the section passes through an interradius, on the left on the exact line of the mesentery so as to show the subumbral pit and on the right to one side so as to show the face of the mesentery. In B only the right side of the section passes through an interradius. C, Transverse section through Lucernaria along the line x x in A. D, Ephyra larva of Aurelia. can.c. circular canal ; g. gonad ; g.f. gastral filament ; e.s. exumbrellar stalk ; 7nes. mesentery ; ni.L' longitudinal muscle of mesentery ; su.p. subumbral pit; ten.s. tentacle becoming later a tentaculocyst ; I.R. interradius; P.R, per- radius; A.R. adradius with first indication of canal. A and C, altered from Bourne; B, after Heric; D, original. lobes, on each of which are several short tentacles and the adhesive organs which are called marginal anchors} There is no velum and tentaculocysts are absent. The manubrium is well developed and the ^ Absent in Lucernaria. , THE INVERTEBRATA I opens into a spacious gastric cavity which is divided by four |ns, the interradial mesenteries^ into four broad chambers which to be perradial. The mesenteries are vertical walls projecting frpnii tke body wall and composed of endoderm with an internal layer ofimaiogloea. They have a free edge centrally, while on each side a vdrtlcal series of gastric filaments project into the enteron, and a parallel series of gonads stand nearer the body wall. The perradial chambers do not quite extend to the edge of the bell : a circular canal is'cut off from the rest of the enteron. Also in the interradial position ) and penetrating the whole length of the mesentery is an ectodermal 'invagination, the subumbral pit. The Stauromedusae only exist as individuals of this structural type, superficially more like a polyp than a medusa, but usually sup- posed to be a medusa, and the egg develops into an individual exactly resembling the parent. The vast majority of the Scyphomedusae belong to the subdivision Discomedusae, which includes our type Aurelia aurita (Fig. 131), the commonest British jellyfish, but one whose distribution is world wide. It has a similar external appearance to that of Obelia^ save for the difference in size, the margin of the bell being surrounded by very numerous short tentacles. The manubrium is well developed and the corners of the mouth are drawn out into four long frilled lips along the inside of which are ciliated grooves leading into the gullet. The gullet is very short and opens into the endodermal stomach. This is produced into four interradial pouches in the lining of which the genital organs develop as pink horseshoe-shaped bodies. Parallel to the internal border of the gonads there is a line of gastric filaments which project freely into the lumen of the pouch. The endodermal cells of which they are composed contain batteries of thread cells which kill any living prey taken into the stomach. The gastric pouches of Aurelia occupy the position of the mesenteries of Lucernaria\, and the subgenital pits occurring underneath the gonads and lined by ecto- derm correspond to the subumbral pits of the simpler form. The broad perradial pouches in Lucernaria have disappeared owing to the great growths of the mesogloea and the restriction of the gastric cavity to a central position. There is, however, an extensive canal system running from the gastric cavity to the circular canal which is all that represents the former extension of the gastric cavity. It consists of eight branched and eight unbranched canals : four of the branched canals are interradial and four perradial: the eight alter- nating unbranched canals are called adradial. In this elaborate "vascular" system there is a circulation of fluid produced by the cilia of the lining epithelium working in definite SCYPHOZOA 175 directions (Fig. 132). The water drawn in by the mouth passes first into the gastric cavity and then the gastric pouches ; thence by the adradial canals to the circular canal. It returns thence by the branched interradial and perradial canals to exhalant grooves on the oral arms. The whole circulation takes about twenty minutes, and it serves to maintain a constant supply of food to all parts of the body. Food Fig. 131. Aurelia aurita. Somewhat reduced. From Shipley and MacBride. M. mouth; oa. oral arm; tn. tentacles on the edge of the umbrella; p.cn. one of the branching perradial canals ; there are four of these, and four similar interradial canals ; the perradial canals correspond to the primary stomach pouches of the hydratuba, the interradial to the pouches of the medusa; a.cn. one of the unbranched adradial canals; c.cn. the circular canal; tct. marginal lappets hiding tentaculocysts ; g.fil. gastral filaments, just outside these are the genital organs. undergoes its preparatory digestion in the stomach : the half-digested fragments are swept by the cilia on the round described above and may be ingested by any of the endodermal cells of the canal system and become available for local needs. The gastrovascular system thus at once fulfils the functions of the digestive and circulatory systems of higher animals. 176 THE INVERTEBRATA The neuromuscular system is further developed than in even the medusoid individuals of the Hydrozoa. The muscles are ectodermal, and each cell is almost entirely converted into contractile protoplasm vi^ith a cross-striated pattern forming an elongated fibre; physio- logically they are capable of rapid rhythmic contraction and not of slow- tonic contraction like the muscle of a sea anemone (p. 193). The fibres p,cn. I. en. a,cn p.en. x.cn. Fig. 132. Diagram showing the course of ciliary circulation (see arrows) in the genital pits and other organs of an adult Aurelia. After Widmark. A, interradius ; B, perradius ; gen. gonad ; gg.cn. gastrogenital canal ; gst.p. gastric pouch; i.cn. interradial canal; o.o.a. opening on oral arm. Other letters as in Fig. 131. are arranged as a circular musculature over the peripheral part of the subumbrella. The nerve net is also confined to the ectoderm and is concentrated in the neighbourhood of the tentaculocysts. There is no true velum, but a pseudovelum consisting of an internal flange which is not occupied by muscles and a nerve ring as in the Hydrozoa. The tentaculocysts are the characteristic sense organs of the SCYPHOZOA 177 Scyphomedusae (but are present also in some Trachylina in the Hydrozoa). They are minute tentacles which project at the end of the interradial and perradial canals, which are continued into them. The edge of the bell projects over them as a hood. In each apical endoderm cell of the tentacle there is a crystal which according to some authors is calcium oxalate. On one side of the tentacle is a pigment spot which may be an ocellus, and near it are two pits lined with sensory epi- thelium and said to be olfactory. In the neighbourhood of these tentacles, then, all the senses appear to be localized. The tentaculo- cyst (Fig. 133) is made up of two parts, a club-shaped projection heavy at its distal end, and a pad of sensory epithelium immediately beneath it. If the medusa is tilted from the normal horizontal position the club of the highest tentaculocyst will press more firmly against its sensory pad, and the club of the lowest tentaculocyst less firmly. Fig. 133. Diagram of tentaculocyst of Aurelia: A, in horizontal position; B, with medusa tilted, the tentaculocyst t being pressed down upon the sensory epithelium se. ; h, hood. Whatever tentaculocyst is highest produces greatest stimulation : this alone controls the rate of beating of the bell, which has been shown to be 50 % greater than normal when the animal is tilted through 90°. Further the state of excitation of the highest tentaculocyst does not allow complete relaxation of the musculature of the section of the bell nearest to it between successive beats. This means that less water is driven downwards at each beat from the uppermost half of the bell than from the lower half, with the result that the bell automatically rights itself. The Scyphomedusae are excellent subjects for experi- ment, and if cut into ribbons will still live and their muscles function. If the tentaculocysts are cut out one by one the rhythmic movements of the bell continue until the last is removed when they suddenly cease. After that, drastic stimulation, tactile or chemical, is necessary to make the muscles contract. The gonads are situated, as has been already stated, in the floor of the stomach, and the ripe gametes are liberated into the genital pouch. 178 THE INVERTEBRATA The eggs are fertilized as soon as they become free by spermatozoa from another individual which are drawn^into the mouth along with the food. They pass through the canals to the opening on the oral arms (Fig. 132, o.o.a.) and undergo the first stage of their development enclosed in pouches at the side of the oral grooves. Little opaque patches along the side of the lips are to be seen with a lens, and when dissected out they prove to be masses of planula larvae. The planula^ is eventually set free, but soon attaches itself to stone or weed and develops into a small polyp, without perisarc, the hydratuba, which Fig. 134. Strobilation of Aurelia aurita. From Sars. A, Hydratuba on stolon which is creeping on a Laminaria. The stolon is forming new buds at I and 2. B, Later stage or scyphistoma, x 4. The strobilation has begun. C, Strobilation further advanced, x 6. D, Free-swimming ephyra stage, showing first appearance of unbranched adradial canals, x 7-5, seen from below. E, The same seen in profile, X7'5. eventually grows sixteen long and slender tentacles. Internally this stage has the same structure as Lucernaria with four interradial mesenteries, which are invaded by vertical ectodermal pits, and form perradial pouches between. At the base of the hydratuba a horizontal stolon grows out, and off this fresh hydratubae may be budded (Fig. 1 34 A). They may separate from the parent as in Hydra. During the winter the whole hydratuba is segmented by transverse horizontal furrows. This process is termed strobilation (Fig. 134 B). In each of the disc-like segments so produced, marginal growth at once begins, ^ In Aurelia the formation of the planula sometimes takes place by in- vagination of the blastula. SCYPHOZOA 179 eight notched lobes being formed, four of which are interradial and four perradial. In each notch there is a short tentacle and this becomes a tentaculocyst. Each lobe is provided also with two short lateral tentacles, but these disappear. A prolongation of the gastric cavity into each lobe indicates the beginning of the branched perradial and interradial canals, and at a little later stage the adradial canals also appear (Fig. 134 D). The gastric filaments are also seen as four pairs in the interradial mesenteries. The Scyphistoma is the name given to the segmented body and each of the segments is an Ephyra larva (Figs. 130 D, 134). They lie upon each other like a pile of saucers, connected, however, by strands of tissue in which run the muscles of the interradial mesenteries con- tinuous throughout the pile of individuals. These muscles contract violently at intervals until the communicating strands snap and one by one the ephyrae swim away. The ephyra develops into the adult by the filling up of the adradial notches in the margins as well as by the growth of the bell as a whole. The mesogloea increases enorm- ously in thickness, causing the two layers of the endoderm to come together as a solid lamella except where the canals occur. The mesen- teries lose their attachment and cease to exist as partitions with the collapse of the enteron, but their position is marked by the gastric filaments. The basal part of the scyphistoma remains and grows new tentacles, and after a resting period as a hydratuba may strobilate again. The life history of the sessile form may thus be summarized. The hydratuba feeds and buds in the summer, continues to feed and stores food in the autumn but ceases to bud, strobilates in the winter, grows new tentacles in the spring and feeds and buds again. In this the Scyphomedusae show features in common with the life history of the hydroid colonies and the freshwater Hydra. The Rhizostomeae are a division of the Scyphomedusae in which the four lips around the mouth are vastly developed and folded, and the central mouth itself is narrowed and in a number of forms en- tirely closed. It is replaced by thousands of small "sucking mouths " which lie along the course of the closed-in grooves of the lips. These lips now constitute organs of external digestion. Small copepods and even fish are enclosed by the lips, digested and the fluid absorbed through the "sucking mouths" which are too small to admit solid particles of any size. The young medusa of Rhizostoma still has a central mouth, but in the adult of that and other forms, e.g. Pilema here figured (Fig. 135), it is entirely closed. Cassiopeia is a semi- sedentary form, which lies with its exumbrellar surface upwards on the mud of mangrove swamps. The bell pulsates gently and brings in a constant stream of plankton organisms which are seized by the lips. i8o THE INVERTEBRATA The mode of development described above is typical in the Scypho- medusae. There are, however, certain exceptions. In the genus Pelagia the medusa develops directly from the egg into an ephyra larva, and in Cassiopeia the hydratuba only produces a single ephyra at a time, a condition which is obviously primitive compared with Aurelia, "polydisc" strobilation being a secondary adaptation for the more effective spread of the species. Fig- 135- Diagrammatic longitudinal section through Pilema. Enteron and its branches shown in black, many "sucking mouths" along the lips, can.r. radial canal ; sg.p. subgenital pit. Class ACTINOZOA (ANTHOZOA) Solitary or colonial coelenterates with polyp individuals only: coelenteron divided by mesenteries: stomodaeum present: genital cells derived from endoderm. They are divided into the two orders Alcyonaria and Zoantharia. Order ALCYONARIA Actinozoa with eight mesenteries and eight pinnate tentacles ; stomo- daeum with a single siphonoglyph (ciliated groove) ; skeleton internal, consisting of spicules in the mesogloea, occasionally supplemented by an external skeleton ; longitudinal muscles on the ventral faces of the mesenteries. As a type of the order we will describe Alcyonium digitatum, *' Dead men's fingers", a colonial form which occurs below low-tide mark, attached to stones, in various sizes and shapes, but usually in broad- lobed masses. A small portion or lobe of a colony is shown in Fig. 136, and it is seen that the polyps project in life from the general surface of the colony. The ectoderm, mesogloea and endoderm of the polyps ALCYONARIA I«I are of course continuous with the same layers in the coenosarc of the colony, but while the ectoderm is only a thin skin composed of a single layer of cells spread over the surface of the whole colony, the mesogloea is expanded to form a bulky mass of jelly which is traversed by the endodermal tubes of the polyps. These run parallel with each other without joining for considerable distances, but they are con- nected by other endodermal tubes which are much more slender, so that, like a hydroid colony, the alcyonarian colony has a common coelenteric system. Fig. 136. Diagram of section through colony of Alcyonium showing ex- tended polyps with pinnate tentacles and coenosarc. Original. The direction of water-circulation is shown by arrows. The mesogloea is indicated by dots and the spicules it contains by small crosses. D, dorsal and V, ventral sides of polyp ; bd. endodermal bud which will give rise to a new polyp ; ect. ecto- derm ; end. endoderm ; end.s. and sol. solenia, solid endoderm strands ; mes.d. the two dorsal mesenteries; mes.' the other mesenteries; std. stomodaeum. The polyps are delicate and withdraw on the slightest stimulus, the oral disc with its crown of tentacles being pulled inside the enteron by the contraction of longitudinal muscles running in the mesenteries and attached to the oral disc. By a continuation of this contraction the whole column of the polyp is introverted ("turned outside in", as with the finger of a glove). This is the condition in which preserved colonies oi Alcyonium are nearly always found, and tangential sections l82 THE INVERTEBRATA through the superficial layers of the colony are rather difficult to interpret in consequence. There is no oral cone in the actinozoan polyp, but the mouth is an elongated slit and is situated in the middle of a circular flattened area, the oral disc, which is surrounded by the tentacles. It does not open directly into the enteron but into a tube lined with ectoderm, the stomodaeum or gullet^ which communicates with that cavity. The whole of the stomodaeum is ciliated, but at one end of it there is a groove which is lined with specially strong cilia which draw water in at the mouth. This is the siphonoglyph, and it is said to occupy a ventral position, but the student must be warned that there is no homology between surfaces so termed in the coelenterates and in the higher Metazoa. , Internally the enteron is divided up by eight vertical folds of the body wall, the mesenteries, which project so far into the cavity of the enteron that their upper parts join with the stomodaeum. Below the level of this organ they end in an enlarged free edge, the mesenteric filament. The foundation of the mesentery is the mesogloea, which is not much thicker here than in the body wall but is folded in the muscular region of the mesentery. On both sides it is covered with endodermal epithelium. While in the hydroid polyp there is little differentiation into regions, in the actinozoan polyp the endodermal cells specialized for various functions are arranged in strips of tissue occupying definite positions on the mesenteries. This may be seen in the sections of a polyp in Figs. 140 and 141. It must in the first place be explained that the presence of the siphonoglyph and the elongation of the stomodaeum are an indication that on the original radial sym- metry of the polyp a bilateral symmetry has been imposed, and on each side of the axis of the stomodaeum the mesenteries correspond exactly in arrangement. Now the muscular endodermal cells are concentrated on the ventral side of each mesentery and into a narrow part of it to form a longitudinal retractor muscle. In the section below the siphonoglyph the ipesenteric filament is seen, and this consists of different elements in the different mesenteries. One pair of mesen- teries, which are " dorsal " in position, are distinguished from the rest in having a filament which is flattened in cross-section, and is covered by very large ciliated cells (Fig. 140 F). They work in concert with the cells of the siphonoglyph to produce a current of water which is drawn in at the mouth and flows right along the ventral side of the tubes through the system, bearing with it oxygen and food for the tissues which are contained in the depths of the colony. The cilia of the dorsal mesenteries are responsible for the return current which makes its way out of the polyp by the dorsal side of the stomodaeum. These two mesenteries are much longer than the rest, as may be seen ALCYONARIA 183 in Fig. 136, mes.d., and their persistence throughout the endodermal tubes is necessary for the maintenance of the exhalant current. In contrast with this the remaining six mesenteries have rounded filaments covered with an epitheUum consisting largely of gland cells. Also the germ cells arise near the free border (Fig. 140 F). Small organisms caught by the tentacles and introduced into the enteron are embraced by these mesenteric filaments and held fast while the fluid from the glands brings about a disintegration and partial digestion of the tissues. Solid fragments of food resulting from this are ingested by individual endodermal cells and the diges- tion completed. Not only do the dorsal mesenteric filaments differ from the others in function but they are ectodermal while all the rest are endodermal. The mesogloea of Alcyonium is invaded by cells from the ectoderm which form in their cytoplasm aggregations of calcium carbonate with a characteristic shape which are called spicules. As the spicules develop the secretory cells migrate into the deeper parts of the colony. They are present in such numbers as to give a certain quality of solidity to the colony, and on its death the spicules it contains remain behind as a not inconsiderable mass. The part which alcyonarians consequently play in the formation of coral reefs, though secondary, is not unimportant. The mesogloea, as has been mentioned above, is traversed by hollow strands of endoderm (solenia) which communicate between the polyp tubes and also by solid endodermal strands which may play some part in the secretion of the jelly of the mesogloea. From the solenia, where they approach the surface, small buds are formed which develop into new polyps. The gonads are developed at the breeding season, from groups of endodermal cells near the filaments, but they only occur on the six ventral mesenteries. The eggs are comparatively large and pass very slowly up the enteron and out of the stomodaeum, being fertilized outside the polyp and developing into a planula larva. After a free- swimming period this fixes and becomes a single individual which by budding gives rise to a colony. Variation in the Alcyonaria occurs mostly in the method of forma- tion of the colonies and the skeleton. The simplest form is found in Cornularia and Clavularia. From the original polyp a creeping stolon with a single endodermal tube is given oflF, and this gives rise at in- tervals to polyp buds, which may in turn produce fresh stolons. The coenosarc of the colony thus forms a network like a hydroid colony. In Alcyonium, as already described, the elongated polyps are crowded together in bundles and fused along nearly the whole of their length, the ectoderm and mesogloea of adjacent polyps being continuous, and the endodermal tubes in frequent communication. The mesogloea 184 THE INVERTEBRATA thickens enormously. In the red coral Corallium rubrum (Fig. 137) there is an upright branched colony with a rigid axis composed ojf spicules compacted together which is the precious coral of commerce. This is clothed by the delicate tissue of the coenosarc from which the short polyps arise and which contains a network of endodermal tubes, some of which run along the parallel grooves which are sometimes to be seen on the surface of a piece of precious coral. The mesogloea contains spicule-forming cells derived from the ectoderm, and these travel inwards and add their secretion to the central skeleton. This form occurs at considerable depths in the Mediterranean and the seas of Japan. Dimorphism, as described below for Pennatula, also occurs here. Fig. 137. Section transverse to the axis of Corallium. After Hickson. A, autozooid; Ax, skeletal axis; S, siphonozooid without tentacles. The ectoderm is indicated by the outer line, the mesogloea by stippling and the endodermal network (solenia) by the irregular spaces in the mesogloea. The gorgonians (suborder Gorgonacea) also have upright branching colonies. The supporting axis has, however, an origin, different to the last, being horny and not calcareous and secreted by the ectoderm on what is really the outer surface of the animal. As secretion is con- fined to an invagination of the basal epithelium which burrows into the whole length of the colony, it appears to be an internal skeleton. The gorgonians are a remarkable feature in shallow tropical seas, forming groves and thickets which challenge comparison with the plant forms of the land (Fig. 138). In Pennatula and its relations (suborder Pennatulacea) a single axial polyp grows to a relatively enormous length, sometimes as much ALCYONARIA 185 as three or four metres , and contains a long horny axis which is possibly endodermal. The secondary polyps are budded off from endodermal tubes which ramify in the much thickened mesogloea of the body wall of the primary polyp, and belong to two types of individuals, the normal autozooids which feed the colony and the stphonozooids, with reduced mesenteries and enlarged siphonoglyph, whose only function is to maintain the circulation of water in the canals of the colony. The autozooids in Pennatula are arranged in rows side by side to form Fig. 138. Gorgonians (two species on the left) and hydrocorallines (on the right) growing on a coral reef in Florida. From an underwater photograph by Professor W. H. Longley. equal and regular lateral branches on each side of the axis giving the colony its feather-like form, and the siphonozooids are mainly found on the back of the axis. A colony has a limited but remarkable power of movement and can burrow into sand or mud by its basal stalk. In two genera, Tubipora (the organ-pipe coral) and Heliopora (the blue coral), which are widely distributed on coral reefs, a continuous calcareous skeleton is developed resembling that of reef corals. The polyps of Tubipora are elongated and parallel and connected by stony platforms which are traversed by the endodermal tubes. But while i86 THE INVERTEBRATA in Tuhipora there is an internal skeleton developed as in Cor allium, by the fusion of spicules in the mesogloea, in Heliopora the skeleton is secreted by a layer of ectodermal cells and not composed of spicules. In Heliopora (Fig. 139) there are on the surface of the colony larger pits (thecae) occupied by the polyps and smaller pits which lodge tubular processes of the network of solenia : the same skeletal cha- racters also occur in the fossil Heliolites which closely resembles it and was a dominant type in Palaeozoic coral reefs. Tubipora too has a Palaeozoic representative in Syringopora} Fig. 139. Diagrammatic section through the edge of a colony of Heliopora. After Kukenthal. The skeleton is shown as deep black, the ectoderm and endoderm as lines and the mesogloea by stippling, pol. polyp ; sol. network of solenia parallel to the surface of the colony ; tub. vertical tubules arising from this network ; th. theca. Order ZOANTHARIA Actinozoa with mesenteries varying greatly in number, typically arranged in pairs, the longitudinal muscles of which face each other, except in the case of two opposite pairs, the directives , in which the muscles are on opposite sides; tentacles usually simple, six or some multiple of six in number; mesenteric filaments trefoil-shaped in section; stomodaeum with two ciliated grooves; typically a calcareous exoskeleton, but this may be entirely absent. The coelenterate animals which are included in this group fall into two apparently different categories, the sea anemones, which are usually single individuals and never possess any kind of skeleton, and the madreporarian corals, which are usually colonial animals and always have an ectodermal exoskeleton. The polyps, however, may all be referred to the same type of structure, and the presence or absence of a skeleton or of the colonial habit are matters of secondary importance compared with this. ^ The relationship between these recent alcyonarians and the Palaeozoic corals is denied by some authors. ZOANTHARIA 187 In its main structural lines the zoantharian polyp resembles the alcyonarian type. The stomodaeum is elongated in the same plane but possesses two siphonoglyphs instead of one. There are tentacles which are hollow, unbranched, and often very numerous. The mesenteries are like those of Alcyonium, but their arrangement and the structure of the mesenteric filament is very different. Numbers and grouping of mesenteries vary greatly within the limits of the Zoantharia itself. The simplest form, and that most like Alcyonium (Fig. 140 A), is found in the small burrowing sea anemone, Edwardsia (Fig. 140 C). Here there are eight mesenteries with bilateral symmetry, as in Alcyonium. In six of these the longitudinal muscles are on the same side, facing ventrally, while the remaining pair have the muscles facing outwards and dorsally, so that the arrangement is different from that in the Alcyonaria. In the typical sea anemone, such as Actinia, and in coral polyps, the mesenteries are arranged in cycles (or generations). There are six couples of primary mesenteries in the first cycle, and these are the largest and alone reach as far as the stomodaeum. In four of these pairs the muscles face each other ; in the other two pairs, the directives, they face away from each other. The secondary mesenteries, which are much smaller, are situated in the spaces between two adjacent pairs (exocoeles), never between two members of a pair (entocoeles). Finally, there may be tertiary and even quaternary mesenteries, al- ways in exocoelic spaces of the generation preceding, making third and fourth cycles. This *' hexactinian" type, in which the mesenteries are present in multiples of twelve, is derived from that in Edwardsia, as may be seen in the development of some of the Zoantharia, for example another small burrowing anemone, Halcampa. In this there is first of all an Edwardsia stage (Fig. 140 C) with eight mesenteries. From this the hexactinian type is derived quite simply by the sub- sequent growth of four additional mesenteries with muscles on their dorsal faces. These belong to the first cycle and join up with the stomodaeum, and they arise in such positions as to complete, with pre-existing mesenteries, four pairs with muscles facing each other. These four mesenteries in Halcampa never develop a mesenteric filament, but the complete adult arrangement, as seen for instance in Actinia mesembryanthemum, the commonest of our British anemones, is given in Fig. 140 E. In such a form as Peachia, often used in labora- tories on account of its simplicity, there are slight deviations from the type. There is no second siphonoglyph (sulculus) and the second cycle of mesenteries is incomplete, none of them having a mesenteric fila- ment, while the pairs in two exocoeles are completely absent (Fig. 140 B). The mesenteric filament of the Zoantharia (Fig. 141 B, C) is tre- i88 THE INVERTEBRATA mes.dir. ^ -m. I. mes! mes.din --mes.dir. mes.fi I. -ov. Fig^i40. Diagrammatic transverse section of corals and sea anemones AAlcyomum. BPeacha. CEd^oardsia. T^, Aulactinia. E, Typical a^fan m the region of the stomodaeum. F. Alcyonium below the stomodaeum mes. primary mesentery; mes.dir. directive mesenteries; mes.^ mes^ mes *' filamen't?^' '^TV ^."^^^^f ^^"^^ mesenteries; mes.fil. ciliated mesenteric filaments m./. longitudinal muscle; ov. ovaries; sip. siphonoglyph ; sip.' sulculus. The direction of the top of the page is dorsal ^ ZOANTHARIA 189 foil-shaped in section, and while the functions of digestion and water-circulation are in the Alcyonaria performed by different filaments, here they are performed by different parts of the same fila- ment. Thus, near the stomodaeum, the central part of the filament of a sea anemone or coral is crowded with digestive gland cells and also with nematocysts, while the wings are covered with strongly ciliated epithelium which maintains a current. In the lower part of the me- sentery the filament is exclusively digestive in function : the cells of Fig. 141. A, Vertical section through a sea anemone showing primary (right) and secondary (left) mesenteries (dotted) from the endocoelic side. ac. acontia ; ap. aperture in mesentery ; g. gonads ; x. ciliated region, and y. di- gestiv'e region, of mes.f. mesenteric filament ; M. mouth ; or.s. oral sphincter ; retr. longitudinal retractor muscle ; ten. tentacle. B, Transverse section through the ciliated region of mesenteric filament at x. C, Similar section through the digestive region at y. c.ph. phagocytic cells filled with carmine and fish fragments; gl.c. gland cells; nem. nematocysts; z. zooxanthellae. After Stephenson. the wings are phagocytic, as is shown by feeding with carmine. From the central part of the filament free threads called acontia are produced in some anemones, which are loaded with nematocysts and may be shot out of the mouth or of special-pores in the body wall when the polyp is stimulated. In the corals the skeleton is secreted by the ectoderm, but only by that part of it which forms the basal disc, A flat plate of calcium car- bonate is laid down first of all by the whole of the disc, but almost at once the epithelium is thrown into radial folds and into a circular fold 190 THE INVERTEBRATA which encloses them, and in these are formed vertical walls which rise from the plate ; the circular wall is called the theca and the radial wall septa (Fig. 142 A). The latter are formed in spaces between the Fig. 142. Skeleton formation in the Zoantharia. A, Oral view of a young coral polyp with the beginning of the skeleton seen through the transparent tissues. B, Vertical section through a later stage. C, Development of a colony showing budding from the extrathecal zone. D, Division of a polyp, pol. polyp before division; pol.' polyp after division and subsequent growth: skeleton of pol. shown in black (as in earlier diagrams) and that of pol.' by stippling ; th. theca ; sep. first formed part of septum ; ex.th. extrathecal portion of polyp or colony. E, Lophohelia. Skeleton of colony, soft parts indicated by dotting, pol. polyp ; pol.' polyp about to divide ; th. theca with septa indicated ; cch. coenenchyme. F, Astroides. After van Koch. Tangential section of young form fixed on cork {ck.). ect. ectoderm ; end. endoderm ; cal. granular secretion of calcium carbonate forming the basal disc ; mes. mesentery ; sep. septum. mesenteries. The continued secretion by such a form as the English solitary coral Caryophyllia produces a cup of limestone, of which the tapering basal portion is solid but which has a shallow apical de- ZOANTHARIA ^ 191 pression, which is traversed by the radiating vertical septa and con- tains in the centre a more or less regular vertical rod, the columella. The depression always tends to become filled up by the secretory activity of the general surface of the basal disc, but the building up of the theca and septa keeps pace with this. It is difficult at first to realize that this is an exoskeleton and that in a massive structure like a brain coral the actual living tissue is a mere film on the surface of a great hemispherical mass of calcium carbonate which it has secreted. It is not surprising to learn that such colonies with a diameter of a yard or more have a life span of a hundred years or so. With regard to the actual mechanism of lime secretion the view most generally held is that illustrated by Fig. 142 F, which shows a coral larva which has fixed upon a piece of cork. The skeleton as shown in a section is, when first laid down, a series of spheroidal masses of calcium carbonate, which thus appear to be a secretion of the ectoderm cells, issuing from the cells as a solution and immedi- ately crystallizing out as irregular masses. Another suggestion is that ammonium carbonate excreted by the coral meets the calcium salts of the sea water and carbonate of lime is precipitated round the ecto- derm; and still another, that calcium carbonate is stored up in the ectoderm cells and when the cells are full they drop out of the epithelium and are added to the skeleton. Coral colonies exist in the most diverse shapes and forms (Fig. 143), from the slender tree-like colonies of many Madrepora to the massive rounded forms like Pontes. Each colony is formed from a single planula which settles down and forms a polyp. From this first in- dividual the hundreds of thousands of polyps in a large colony are formed by division or gemmation. An example of division is given in Fig. 142D. In such a case when the polyp has reached a certain size the oral disc becomes elongated in the direction of the long axis of the mouth, tentacles and mesenteries increase in number, and finally a transverse constriction divides first the mouth, then the disc and lastly the whole polyp. The division of the polyp is followed by that of the theca. In the Meandrine corals (brain corals) the polyp elongates enormously and the mouth divides but not the theca, and so we get the curious thecae running more or less parallel to each other which recall the convolutions of the human brain. In Lopho- helia (Fig. 142 E) division is equal, but while one of the polyps re- sulting from it continues to grow the other marks time; the axis of growth changes sides at each division and the result is a colony showing cymose branching. In Fig. 142 B it is shown that part of the coral polyp overlaps the theca. It is this extrathecal zone which gives rise to young polyps when a colony is formed by gemmation (Fig. 142 C). The bud and the 192 THE INVERTEBRATA parent remain connected by their extrathecal portions, and this con- stitutes the coenosarc of the colony. The gaps between the thecae of the colony are filled up by calcareous material secreted by the coeno- sarc and called coenenchyme , P. Mo. Fa Fu. M. Fig. 143. Photograph of a pool on a coral reef (Great Barrier Reef), showing various types of zoantharian corals. Fa. Favia, with circular thecae ; Fu. the free coral Fungia, a single polyp ; M. Meandrina, the brain coral ; Mo. Monti- pora, a branched coral; P. Pontes. (Photograph by Dr S. M. Manton.) The polyps of the Zoantharia attain a higher physiological grade than those found elsewhere in the coelenterates. The sea anemones, like Hydra, in the absence of any external skeleton, are capable of locomotion, especially in the case of burrowing forms. The muscles of the body are arranged in such a way as to bring about many different kinds of movements. Thus, while the longitudinal muscles of the COELENTERATA I93 mesenteries cause a longitudinal retraction of the polyp, the transverse muscles of the mesenteries in the neighbourhood of the stomodaeum open the mouth when they contract, and the longitudinal muscles of the tentacles when these are touched by particles of food contract so that the tentacle bends towards the mouth and helps to push the food inside it. The muscular system is for the most part under the control of the nerve net. Although there is no central nervous system the amount of contraction produced is proportional to the strength of the stimulus. If a sea anemone is violently stimulated, e.g. touched by a glass rod in any part the stimulus is transmitted to every muscle and the whole animal shrinks to a shapeless lump. The process of feeding is extremely complex and involves the action of the muscles, the cilia and the glands. In a sea anemone like Metridium^ which lives on the minute animals of the plankton, when these approach the oral disc they are stunned by the nematocysts, snared by the mucus of the glands of the tentacles, transported by cilia to the tips of the tentacles, and pushed by the tentacles towards the mouth, which gapes to receive them. Most remarkable of all, the cilia of the lips, which normally maintain the outwardly flowing respiratory current, reverse their beat to sweep the food into the enteron. While there is this remarkable co-ordination of activities in feeding the nerve net preserves the individuality in action of the parts so that the severed tentacle of a sea anemone is able to execute movements just as if it was still in place on the appropriate stimulation. In another common anemone, Tealia, there are no cilia on the tentacles and oral disc, and feeding takes place entirely by the muscular movement of the tentacles . Sea anemones and corals are often nocturnal, remaining contracted by day, expanding and feeding at night. In such corals as Lobo- phy Ilium the tentacles are capable of enormous extension. In the forms which feed by day like Fungia the tentacles are shorter and the food is collected more by the action of cilia on the tentacles and oral disc and less by the seizing of organisms by the arms and withdrawal to the mouth. A remarkable biological feature is the frequent presence of commensal algae (compare Hydra viridis) in the tissues (Fig. 141 B, z.). This is especially the case in reef corals, in which the most recent investigations show that the algae are of no nutritive value while the oxygen they liberate in the tissues has no relation to the needs of the coral. On the other hand the fact that they remove excreta from the coral tissues is of importance. SuBPHYLUM CTENOPHORA Free and solitary Coelenterata ; whose active locomotion takes place by ciliary action ; which are not reducible either to the polyp or to the medusoid type ; and are without nematocysts, but possess '* lasso cells ". 194 THE INVERTEBRATA The Ctenophora, apart from certain aberrant forms, are globular, pelagic, transparent animals living in the surface waters of the sea. They are usually classed with the Coelenterata, but they differ from other members of that phylum in several important respects, notably in the entire absence of nematocysts. Two British forms are easily procurable, Pleurohrachia pileus and Hormiphoraplumosa. Pleurohrachia pileus is about the size of a small Fig, 144. Hormiphora pliimosa. After Chun. Side view. M. mouth leading via stomodaeum into infundibulum ; ab.p. aboral pole with sense organ ; ab.fu. aboral funnel of infundibulum ; pa.can. paragastric canal running to- wards oral pole; 8, one of the eight meridional comb plates; ca. one of the eight canals running towards 8; tn.po. a tentacular pouch; tn. a tentacle; gel. gelatinous material. hazel nut, while Hormiphora plumosa (Fig. 144) is rather smaller. They are transparent and ovoid. At one pole is the mouth; the only other openings into the alimentary canal are two small pores near the sense organ. At the other pole is the sense organ marked as a small CTENOPHORA I95 Spot lying in a slight depression. The surface of the body is beset by eight meridional rows of comb plates formed of strong cilia borne upon modified ectodermal cells. The general surface of the body is not ciliated. On opposite sides of the body are two tentacles set in pouches. The tentacles have muscular bases and are capable of being protruded from the pouches or withdrawn again. They are usually about half as long again as the body when fully extended. The tentacles are armed with cells of a special type called "lasso cells" or coUoblasts, which take the place of nematocysts. Each colloblast consists of a sticky head having at its base a spiral thread wound round a stiff central filament. The tentacles are used for catching the prey which is entangled by the sticky heads of the colloblasts. The mouth leads through a stomodaeum lined with ectoderm into a space, the infundibulum, lined with endoderm. From the in- fundibulum four canals radiate outwards ; each of these divides into two and then runs under the comb plates as the subcostal canals. Two more canals lead out from the infundibulum and run directly without branching to the base of the tentacles. There are also two paragastric canals running alongside the stomodaeum. At the opposite pole to the mouth, the aboral pole, is the elaborate sense organ formed of small round calcareous bodies united into a morula. This morula is supported on four pillars of fused cilia and is covered by a roof also formed of fused cilia. Ciliated furrows lead out from the sense organ to the comb plates and are believed to assist in carrying stimuli to the comb plates from the sense organ. The comb plates are the locomotor organs. When at rest the tip of a plate is directed towards the oral pole. In movement a rapid beat of the plate is directed aborally and the cilia then return slowly to rest. The ctenophore therefore moves slowly through the water with the oral end in front. Each plate of the comb beats in succession, the first plate to beat being the one at the aboral end and the remainder following in succession. This type of beating, which is common in ciliary movement, is termed "metachronal" (see p. 17). It gives the appearance of waves travelling down the comb from the aboral to the oral pole. Ordinarily all the eight rows of plates beat in unison, but interference with the aboral sense organ destroys this unison. The main substance of the ctenophore, which fills the space be- tween the ectoderm and the endoderm, is a gelatinous material in which are found strands of muscle. Immediately beneath the ecto- derm lies a subcuticular layer of muscle and nerve fibres which, in appearance, closely resembles the arrangement found in the Turbel- laria. It is important to note that the whole musculature of the Ctenophora is derived from the mesenchyme. There are no musculo- epithelial cells. 196 THE INVERTEBRATA Ctenophores are hermaphrodite ; the male and female gonads occur close to each other in the subcostal canals. Self-fertilization probably occurs. It is a remarkable fact that, if the first two segments of the dividing egg of a ctenophore be separated a half larva will develop from each segment. In the egg, therefore, the organ forming sub- stances must be localized. If these half larvae be kept until generative organs develop, the missing half is then regenerated. In contrast to this behaviour in the Ctenophora, the separated blastomeres of the cnidarian egg as far as the sixteen-celled stage will develop each into a complete animal. The Ctenophora are divided into two orders: (i) Tentaculata^ possessing tentacles, to which the majority of forms belong ; (ii) Nuda, without tentacles, to which belongs only the genus Beroe. Most of the Tentaculata have the ovoid shape, similar to that seen in Pleurobrachia, but some are flattened in a peculiar manner. Cestus Veneris, Venus' Girdle, is flattened laterally and the body is drawn out into a narrow band, two inches wide and nearly a yard long. It is found in the surface waters of the Mediterranean. The Platyctenea, a group of Tentaculata to which belong the forms Coeloplana and Ctenoplana, are flattened dorsoventrally. The flatten- ing is produced by the expansion outwards of the stomodaeum so that the whole of the ventral surface corresponds to the stomodaeum of the normal types. Ctenoplana lives in the surface waters of the sea and retains traces of the swimming plates, but Coeloplana crawls over the rocks and seaweed, and resembles a turbellarian. It has lost the swimming plates and developed pigment, but it still retains the sense organ and the two tentacles. The gut system is irregularly branched and the muscular system is highly developed for crawling purposes. One member of the group, Gastrodes, is a parasite in the body of Salpa. Its chief interest, however, is in the larva, which is a planula, found nowhere else among the Ctenophora, and thus provides the strongest piece of evidence for the close relationship of the Cteno- phora with the Coelenterata. CHAPTER VI THE ACOELOMATA: PLATYHELMINTHES Under this title are grouped the phyla Platyhelminthes, Nemertea, Rotifera, Nematoda, Gastrotricha, Acanthocephala and Nemato- morpha (the three last of which are very small groups). The animals contained in these are unsegmented forms with mesenchyme (p. 129) and the space between the gut and the body wall (when it exists) is a primary body cavity filled with fluid (e.g. Rotifera). The turgor of the body cavity fluid when present has a determining role in the preservation of the form of the body (e.g. Nematoda, and Rotifera). Generally speaking this space with its contained fluid plays the part of a circulatory system, but in the Nemertea the body cavity is re- duced to a series of canals which constitute the first vascular system in the animal kingdom. This primary body cavity has no definite epithelial boundaries and so can be easily distinguished from a true coelom. It tends to be invaded by mesenchyme cells; in the Platy- helminthes these completely fill it, forming a characteristic tissue (parenchyma), and in the Nematoda the cavity appears to be also completely occupied by a very few enormous vacuolated cells: the vacuoles simulate a body cavity. The excretory organ is of nephridial type (or it may be derived from this as in Nematoda). It is a canal, closed at the internal end, intracellular or intercellular, with some hydromotor arrangement which maintains a flow of fluid to the exterior. In the simplest cases there is a continuous ciliation of the inner wall of the canal (some Turbellaria). Usually, however, the ciliation has disappeared over most of the canal but is strengthened and diflFerentiated in others; the characteristic units of the system, the flame cells, being now found. Flame cells may be situated in the course of the canal in some forms but usually constitute the terminal organ (Fig. 149). This system though usually spoken of as "excretory" is primarily concerned with the regulation of fluid content and is often absent in marine forms (e.g. Turbellaria Acoela, p. 213). A nerve net is usually present and from this are differentiated an anterior "brain" and some longi- tudinal nerves. The reproductive system is that in which differences between and within the groups principally occur: these differences are to be regarded as adaptations to the varying conditions of life. 198 THE INVERTEBRATA PHYLUM PLATYHELMINTHES Free-living, bilaterally symmetrical, triploblastic Metazoa; usually flattened dorsoventrally ; without anus, coelom or haemocoele; with a flame-cell system; and with complicated, usually hermaphrodite, organs of reproduction. The name Platyhelminthes is given to a division of that hetero- geneous collection of animals which in Linnaeus' time were called Vermes. The Vermes included everything that looked like a worm, but appearances have since been found to be deceptive and the collection has been broken up into separate phyla, one of which is the Platyhelminthes or flatworms. Of all the worm-like animals the flat- worms are undoubtedly the most primitive, for they alone show relationships to the Coelenterata. The phylum Platyhelminthes falls naturally into three classes: (i) Turbellaria, (ii) Trematoda, (iii) Cestoda. Of these the Turbellaria are with few exceptions free-living, while the Trematoda and Cestoda are all, without exception, parasites. It is in the Turbellaria that we see most clearly the typical organization of a platyhelminth, for in the Trematoda and Cestoda the parasitic habit has induced a considerable departure from the structure of the free-living ancestor. In shape the Platyhelminthes are flattened, they are not segmented and do not possess a coelom. The ectoderm is ciliated in the Turbellaria, but the ciliation is lost in the two parasitic groups and there are further modifications. The gut, which is present only in the Turbellaria and Trematoda, has but one opening which serves both as mouth and anus, and in this respect reminds us of the Coelenterata. Between the ectoderm and the endoderm which con- stitutes the lining of the gut there exist a large number of star-shaped cells with large intercellular spaces forming a mass o{ parenchymatous tissue. The nervous system consists essentially of a network as in the Coelenterata, with the important diflference that there is an aggre- gation of nerve cells at the anterior end which, in the free-living forms almost always takes the form of a pair of cerebral ganglia, and that certain of the strands of the network stretching backwards from these cerebral ganglia are often more distinct than others and merit the name of nerve cords (Fig. 145). There is, therefore, the beginning of a definite central nervous system. There are no ganglia other than the cerebral, but in the general nervous network nerve cells and nerve fibres are mixed together. By operating on the animals in different ways it is possible to show what functions the different parts of the nervous system have. If the cerebral ganglion of a Polyclad is removed, the body of the animal PLATYHELMINTHES 199 remains permanently quiescent after the operation. This state of quies- cence is not however due to a loss of co-ordination in the motor system. Stimulation of the anterior end can evoke all the normal ce.ga.-/-- M.-l--, ?— Fig. 145. The nervous system of Acoela, to show the nerve strands of the network. After Steinmann. ce.ga. cerebral ganglion (brain); M. mouth; (S and ?, male and female openings respectively. forms of locomotion, and this shows that the nerve net and not the cerebral ganglion is responsible for the correlation of the different parts of the musculature. The primitive central nervous system which 200 THE INVERTEBRATA here takes the form of a cerebral gangHon is best regarded as a development in connection with the special sense organs, from which it receives stimuli. The cerebral ganglion functions as a relay system in which the stimuli received from the special sense organs are re- inforced, often extended in time, and then passed on to the nerve net. When this sensory relay has been destroyed by removing the cerebral ganglia, the nerve net is no longer excited to bring the muscular system into action, although this may still be done by artificial stimuli. Sense organs occur in adults only in the free-living Turbellaria, where they may take the form of eyes, otocysts, tentacles and ciliated CB <^ \ H n.net. Fig. 151. Transverse section through the outer layer of pharynx of a triclad. Altered from Steinmann. ba.mefnb. basal membrane ; circ.m. layer of circular muscles; ect. ectoderm; gl. glands; long.m. layer of longitudinal muscles; n.net. nervous network; nu. nuclei of ectoderm cells; ra.m. radial muscles. and the Cestoda, the ectoderm cells have all sunk into the paren- chyma, and the body is covered by a thick cuticle secreted by the ectoderm cells. The parenchyma (also called the^mesenchyme), which fills the interior of the body, is of very different structure in different Platyhelminthes. It is generally formed of cells with long irregular processes and much intercellular space. Within these cells are small granules and particles, which stain readily. Their appearance and number vary according to the state of health of the animal, whether it is starved or fed, and they 2o6 THE INVERTEBRATA are probably, therefore, products of secretory activity formed after the assimilation of food and destined eventually to be converted into rhabdites or the slime which flows from the slime glands. The parenchyma is no mere padding tissue. It probably serves for the transport of food materials, and certain cells in it provide for the repair of lost parts of the body. These free cells of the parenchyma retain their embryonic condition and do not become vacuolated or branched. They are smaller than the branched cells of the paren- chyma and scattered among them in normal circumstances, but when an injury occurs they migrate to the cut surface, where they collect in large numbers and proceed to regenerate the tissues lost by injury. The digestive system of the platyhelminth differs entirely from that of the higher animals in that it is a sac with one opening only, which serves both for the entry of the food and the exit of the faeces, and not a tube with a mouth and anus serving separately for the entry and exit of food. In the simplest forms, in many of the Rhabdocoela, the sac is a straight wide tube with no diverticula (Fig. 152), while in others the gut is branched. In the Tricladida the gut has three main branches. A muscular structure lined by an inturning of the ecto- derm surrounding the mouth forms the pharynx. The pharynx itself may lie in a pit of the ventral body wall, called the pharynx pouchy from which it can be protruded or withdrawn. The epithelial lining of the gut cavity consists of large cells without cilia, the cell walls of which are often difficult to distinguish. A muscular wall to the gut is present, but is so exiguous as to avoid identification in many forms, and it appears therefore as if nothing separates the cells of the gut from the parenchyma. It is possible for food substances to pass not only from the lumen of the gut into the cells lining it, but also from the parenchyma. Thus when Turbellaria are starved they can consume certain organs lying in the parenchyma (ovaries, testes, etc.) by passing these into the gut cells or into the lumen of the gut for digestion. The Turbellaria are carnivorous and will eat small living Crustacea or worms which are caught by the protrusion of the pharynx. A sticky secretion, derived from the slime glands and perhaps the rhab- dites, is immediately poured over the prey, which is thus wrapped up in sHme. If the object is small enough it is ingested whole into the gut. Here digestion proceeds. Fat is digested in the lumen of the gut, but the digestion of other substances takes place in vacuoles in the cells of the gut wall. Animals which have recently died are also eaten by Turbellaria, and an effective trap can be made by placing a freshly killed worm or a Gammarus or two in a jampot and lowering it to the bottom of the stream or pond. The Turbellaria are able to "scent out" the food, and all those within a wide area collect in the pot for PLATYHELMINTHES 207 ves.sem. -rec.sem. Fig 152. Dalyelliaviridu, dors^Uie^. From Bresslau. ^^^ b"^"'^^^^;^;'-;^ MTha^^xfri-' -S«-iul'senfinis; <. test.s; ...... ves.cula seminalis ; vit. vitellarium. 208 THE INVERTEBRATA the feast. When the animal is too large to be ingested whole, the pharynx is attached to the prey and worked backwards and forwards with a pumping motion, while at the same time a disintegrating digestive fluid is poured out from the walls of the pharynx. Particles of food are thus pumped up into the gut cavity and digested in the same way as the living prey. In the Trematoda, also, the cells lining the gut have a certain limited power of amoeboid movement at their exposed edges, and intracellular digestion is apparently the usual method. The Turbellaria are able to go without food for long periods, but during starvation they grow smaller and smaller. Stoppenbrink starved Planaria alpina, keeping them entirely without food, while as a control he kept a similar collection supplied with food. His results are given in the table below. The measurements are in millimetres. Date Fed Starved Largest Smallest Largest Smallest Lgth Bdth 2 2-5 2-5 2-5 Lgth Bdth Lgth Bdth Lgth Bdth i6. iii. 03 15. vi. 03 15. ix. 03 15. xii. 03 13 17 17 17 10 12 13 14 I 2 2 13 10 7 3h 2 I 10 6 4 2i I t T 3 This reduction in size is accompanied by the absorption and digestion of the internal organs, which disappear in a regular order, the animal using these as food in the manner already described. The first things to go are the eggs which are ready for laying, then follow the yolk glands and the remainder of the generative apparatus. Finally the ovaries and the testes disappear, so that the animal is reduced to sexual immaturity. Next the parenchyma, the gut and the muscles of the body wall are reduced and consumed. The nervous system alone holds out and is not reduced so that starved planarians differ in shape from the normal forms in having a disproportionately large head end, the bulk of which is the unreduced cerebral ganglion. On feeding these starved forms will regenerate all the lost organs and return to the normal size, like Alice when she ate the right half of the mushroom. It is in the generative organs that the Platyhelminthes show the greatest complexity of organization (Figs. 165, 166). With rare excep- tions the Platyhelminthes are hermaphrodite. The generative pore is variably placed but it is usually to be found in the middle line of the ventral surface not nearer to the anterior or posterior end than PLATYHELMINTHES 209 v.d. — ves.scm.- f/en.at.^ Tpfnini of^enl r„ *''^^^"■"'' " "^ " p\^naH.n. After Steinmann. g.o. oviduct °/ovar' 1 ^^""-"'-^ ',"? '""'°''' "■•"■°'- '""^<="'^'- organ; orf. deferens , ws.jem. vesicula seminalis ; vit. vitellarium. 210 THE INVERTEBRATA one-quarter or one-fifth the length of the body. This pore leads into a space known as the genital atrium. Into the genital atrium open the separate ducts leading from the male and female portions of the generative system, together with other accessory organs. The homo- logies of the various accessory portions of the generative organs in the three different groups are difficult to ascertain. Names are often used which were applied to organs before their homologies were ascertained, and this increases the confusion. In studying the generative systems in actual specimens elaborate reconstruction from sections is often necessary, as the heavy pig- mentation obscures them when the animal is viewed by transmitted light. In transparent specimens careful staining will bring to light most of the parts but it often requires considerable skill and practice to identify these parts. The organization of the platyhelminth generative system may be reduced to a general plan as follows. The testes are round bodies, often very numerous, having a lining of cells which give rise to the sperma- tozoa. From the testes lead out ducts, the vasa ejferentia, which, uniting, form the vas deferens. There are usually two vasa deferentia collecting the sperm from the testes on either side of the body. The ends of the vasa deferentia are often distended and act as vesiculae seminales. The vasa deferentia unite and lead into a pear-shaped bag with very muscular walls. This is the penis. At rest it opens into the genital atrium, but during copulation it is extruded through the genital pore to the exterior and pushed into the genital pore of another in- dividual. The penis is usually seen very easily, being one of the most conspicuous parts of the genital apparatus. The female portion of the generative system consists of the ovary, which produces the ova, and the vitellarium, which supplies the ova with yolk and a shell. The shell substance is liquid and hardens later. This division into ovarium and vitellarium (or "yolk gland" as it is sometimes called) occurs throughout the Platyhelminthes, but it is probably an elaboration of the more usual arrangement of forming the yolk in the ovary, an arrangement which occurs in the primitive Acoela and in the Polycladida. The ovaries discharge their ova into an oviduct which is enlarged near the point of this discharge and thus forms a receptaculum seminis. Here fertilization occurs. The oviduct next receives the opening of the vitelline ducts. After the opening of the vitelline ducts the duct continues as the ductus communis, and leads into the genital atrium. At the junction of the oviducts and vitelline ducts there is a thickening of the walls of the duct and certain glands, the "shell" glands, pour a secretion on to the egg which probably assists in hardening the shell. This thickening is indistinct in the Turbellaria but is very marked in the Trematoda, PLATYHELMINTHES 211 and the structure there receives the name of ootype, because it is the place where the egg is shaped before being passed into the uterus for storage. In the Trematoda the ductus communis is long and coiled and serves for the storage of eggs. It is called the " uterus ", but it is not of course homologous with the "uterus" of the Rhabdocoelida which will be described shortly, nor with the ''uterus " of the Cestoda which is again probably a different organ. The genital atrium receives not only the openings of the male and female organs but also certain accessory organs. In the Rhabdocoelida, of which Mesostoma is an example, there open out from the genital atrium on either side the paired uteri (Fig. 165, i), in which the eggs are stored before laying. In Dalyellia (Fig. 152) the fertilized eggs pass into the parenchyma. There is another opening which leads into a short muscular receptacle, the bursa copulatrix. The bursa copulatrix receives the penis of another individual during copulation. Sperm is deposited here but remains only for a short time before being expelled by muscular contractions and received into the oviduct where it is collected near the ovary in the true receptaculum seminis. In the Tricladida the uterus and the bursa copulatrix are replaced by organs, the homologies of which are doubtful. These are the unpaired stalked gland organ and the unpaired muscular gland organ. The stalked gland organ is often called the "uterus" but it has not been observed to contain eggs. It is regularly present, whereas the muscular gland organ is often absent. It has recently been shown that the stalked organ serves as a bursa copulatrix and receives tem- porarily the penis and the sperm of another individual. During copulation the ventral surfaces of two animals are applied together so that the genital openings lie opposite to each other. The penes are extruded through the genital opening of one copulant into the genital opening of the other. There is a mutual exchange of sperm. Since the ova are ripe at the same time as the sperm, and as, in many forms, there is only one common genital opening to the exterior, special precautions are necessary to prevent self-fertilization. To ensure that cross-fertilization shall take place a great elaboration of the structures surrounding the genital atrium has occurred, resulting in that complication of the genitalia, which is so characteristic of the Platyhelminthes . In freshwater Tricladida copulation occurs fairly freely among animals kept in glass jars, where they are easily observed. When the penis is retracted its lumen is closed so that sperm cannot escape into the genital atrium, whence it might find its way up the oviduct (Fig. 154). When the penis is thrust out through the genital opening during copulation it is dilated on extrusion, so that the lumen is opened. This dilation also causes the penis to fill completely the 212 THE INVERTEBRATA genital atrium and opening, so that the opening of the oviduct into the genital atrium is blocked and no sperm can enter or ova escape. At copulation the penis of one animal is squeezed past the penis of the other into the genital atrium. It cannot enter the oviduct, since this is blocked and so it is received into the stalked gland organ, where the sperm is temporarily deposited. After copulation is finished, the penes are withdrawn and the sperm is transferred from the stalked gland organ to the oviduct. The arrangement of the organs round the genital atrium in the Tricladida varies considerably. In Bdellocephala, for example, the penis is reduced and, when extruded, does not fill the genital atrium sufficiently to block the opening of the oviduct. In vc's^em. mini.0K g.o. Fig- 154- Longitudinal vertical section through region of the genital atrium in Dendrocoelum lacteum. After Ullyott and Beauchamp. c.o. common opening to exterior; g.o. opening of genital atrium (gen.at.); fl.p. flagellum of penis; mus.or. muscular organ; od. oviduct; p. penis; sp. stalked gland organ (bursa copulatrix) ; ves.sem. expanded portion of vas deferens forming a vesicula seminalis. this case a flap of skin has developed which is drawn over the opening of the oviduct, when the penis is extruded. After the sperm is transferred to the oviduct, it moves up to the receptaculum seminis at the top, near to the point of discharge of the ova. The ova are fertilized in the oviduct and then move down towards the genital atrium, receiving on the way the products of the vitellaria. On arrival in the genital atrium a cocoon is shaped and made ready to be deposited. When laid it is usually attached to weeds, sometimes by a stalk. The parasitic Trematoda and Cestoda are unaffected by the sea- sons and are perpetually producing eggs. But in the Turbellaria the season of egg-laying varies. In some, for example Dendrocoelum lac- teum, the generative system is in full working order all the year round, PLATYHELMINTHES 213 in others, for example Planaria alpina, the eggs are only produced during the winter months. Mesostomum produces two kinds of eggs which are called ''summer" and "winter" eggs. The "winter" eggs have a thick shell and are well supplied with yolk; they remain in the uterus and escape only with the death of the parent. The "winter" Qg^ can remain dormant for a long period. The "summer" egg is very thin-shelled and has very little yolk. The development is very rapid and the young embryos are seen moving in the uterus of the parent seventy-two hours after the appearance of the eggs. They escape by the genital pore and their formation does not involve the death of the parent. The term "winter" and "summer" egg is not entirely apposite, for "winter" eggs are often found in midsummer. The "winter" egg is a method of carrying the species over unfavourable conditions which may develop in winter or in summer. The ' ' summer " egg is a means for rapid multiplication when conditions are favour- able. Asexual reproduction occurs commonly in the Turbellaria. In Microstoma lineare the hinder end buds off new individuals which re- main attached for some time so that chains of three or four individuals in different stages of development are often seen. Planarians undergo autotomy, cutting themselves in two by a ragged line which traverses the middle of the body. Lost parts are easily regenerated in the Tricladida and the group is a favourite one for experimental work on regeneration. Having thus provided the reader with a general account of the organization of a platyhelminth it will now be possible for us to follow the systematic arrangement of the phylum, to define the divisions and to point out features of interest in various forms and life histories. Class TURBELLARIA The Turbellaria may be defined as Platyhelminthes which are nearly all free living and not parasitic, which retain the enteron; which have a cellular, ciliated outer covering to the body; which usually have rhabdites; and which do not form proglottides. Suckers are very rarely present. The systematic arrangement of the Turbellaria is based primarily on the structure of the gut. There are four orders : (i) Acoela, (ii) Rhab- docoelida, (iii) Tricladida, (iv) Polycladida. Order ACOELA In these the gut is not hollow but consists of a syncytium formed by the union of endodermal cells. There is no muscular pharynx. Primitive features are the nerve net and the fact that the germarium and vitellarium are not separated. Convoliita roscojfensis is the best 214 THE INVERTEBRATA known member of this division. It lives between the tidemarks on sandy shores. Imbedded in the parenchyma are algal cells which live in a symbiosis (p. 47) with the Turbellarian. The photo-synthetic products of these algal cells provide a source of nourishment for the animal. Convoluta henseni, another member of this order, is a rare platyhelminth that has adopted a planktonic habitat. Order RHABDOCOELIDA In these forms (Fig. 152) the gut is straight and the mouth is near the anterior end. The gut may or may not have lateral pouches. In the more primitive members of this order, oivjhich. Microstomum lineare is a common example, found in fresh water, the germarium and the vitel- larium are not separated. Another well-known member of this group is Dalyellia viridis, common in freshwater ponds in Britain and remarkable for the elaborate chitinous structure of the penis. Mesostoma ehrenbergi and M. quadr angular e, the latter X-shaped in cross-section, both occur in freshwater ponds. They are large and transparent and form the best objects for studying the structure of the group. Plagiostomum lemani is a form with side pouches to the gut.^ It occurs at the bottom of deep lakes in temperate regions. Otoplana also has side pouches to the gut but is chiefly remarkable for possessing an otocyst overlying the brain. The Rhabdocoelida occur in both fresh and salt water; marine forms are, however, very small. Order TRICLADIDA In this group the gut is divided into three main divisions with numer- ous lateral diverticula from each division. The mouth has shifted back- wards to the middle of the body. There are three well-recognized divisions of this order, separated according to habitat : the Paludicola or freshwater forms, the Maricola or marine forms, and the Terricola or land forms. The Paludicola are all fairly large forms in contrast with the Maricola which are small, no more than 2-4 mm. long. To the Paludicola belong the three commonest freshwater Turbellaria in Britain: Dendrocoelum lacteum, a white form, Planaria lugubris^z. black form, and Polycelis nigra, a rather smaller black form easily recognized by the ring of eyes round the anterior edge of the body. Perhaps the best known member of the Maricola is Procerodes lobata ( = Gunda segmentata) in which the side diverticula of the gut are regularly arranged, with testes and excretory openings between them, giving the appearance of a segmented animal. The Terricola often reach a very large size — as long as 50 cm. They are often brightly ^ These forms, with side pouches to the gut, are sometimes placed in a separate order called Alloiocoela. TURBELLARIA 215 coloured with stripes down the dorsal surface. Bipalium kewense is a cosmopolitan tropical form that often turns up in greenhouses. It is often a foot long and is easily recognized by the axe-shaped head. Rhynchodemus terrestris, a small form 6-8 mm. long, is a British representative of this division. It is found in damp situations under the bark of decaying trees and fallen timber. Order POLYCLADIDA These are entirely marine. The gut has many diverticula leading out from a not very conspicuous main stem. The mouth has shifted to the ciL' — ciV Fig. 155. Miiller's larva of a Polyclad, Cycloporus papillosus Lang. Ventral view, cil.^ large cilium at anterior end; e. eyes; l.^ L^ l.^ projecting lobes, the edges of which have cilia longer than those on the general body surface (there are eight of these lobes, there being one similar to 1} and another pair similar to /.^ on the dorsal surface); m. mouth; cil} large cilium at posterior end. (Altered from Kiikenthal.) posterior end. The germarium and the vitellarium are combined into one organ but there are separate male and female openings. The ovum is entolecithal, i.e. it has the yolk inside it as in the Acoela. In all other Platyhelminthes the ovum is ectolecithal, i.e. it has no yolk inside it but is surrounded inside the egg shell with yolk cells, which break down when development begins. The early embryological stages in the development of the Polycladida resemble, as might be expected, those of the Acoela, but there are however four macromeres instead of two as in the Acoela. A further point of difference is that in the 2l6 THE INVERTEBRATA Polycladida the entry of the ovum by the sperm takes place after the extrusion of the polar bodies, whereas in other Turbellaria this follows the entry of the sperm. These facts have inclined modern authorities to the belief that the Polycladida are more nearly related to the primitive Acoela than to the Rhabdocoelida and Tricladida. A further point of interest in this group is that development is not direct. It leads to the production of a larva, known as "Miiller's larva" (see Fig. 155), which is characterized by projecting processes and a band of ciha. As we have seen (p. 145), projecting processes (arms) and bands of cilia are characteristic of the larvae of many forms belonging to several phyla ; but their presence is probably an adaptive feature and it is unwise to base phylogenetic speculations on them. "Miiller's larva" is a planktonic, and therefore a dis- tributive stage, in the life history. At metamorphosis, when the animal adopts the crawling progression of the adult, the larva loses the projecting arms and the bands of cilia, while at the same time it loses its rotundity, becoming flattened and elongated. Some members of this group attain a considerable size, six inches or more in length. A small sucker is found in some forms behind the genital pore. Thysanozoon, a member of this order, has the dorsal surface covered with papillae into which run coeca from the intestine. In Yimgia there are similar papillae also containing diverticula of the gut, some of which open to the exterior. Class TREMATODA The Trematoda may be defined as Platyhelminthes which are para- sitic (or, in Temnocephalea, epizoic) ; which retain the enteron ; which in the adult have outside the ectoderm a thick cuticle; which have suckers; usually, but not always, a sucker on the ventral surface in addition to one surrounding the mouth; the ventral sucker is sub- divided in some forms and may also be stiffened with a ringlike chitinous skeleton. The Trematoda are linked to the Turbellaria by the little group of animals which constitutes the order Temnocephalea containing the genus Temnocephala and one or two others. These animals have a very discontinuous distribution and live attached to the surface of fresh- water animals,chiefly Crustacea. They do not feed on their host but use it as a resting place from which they catch rotifers, Cyclops, and other small water animals for food. The possession of five tentacles at the anterior end makes the group easily recognizable (Fig. 156). The epi- dermis is retained as a nucleated syncytium which secretes outside it a thick cuticle. In the region of the tentacles rhabdites occur. The mouth is anterior, the gut has the same shape as in the Rhabdocoela. There is a large sucker at the posterior end with the common male and female PLATYHELMINTHES 217 opening in front of it. The nervous system is of the primitive network type, but the ovary and vitellarium are separate. Many authors place the Temnocephalea with the Turbellaria, basing their claims to be associated with this class rather than the Trematoda on the presence of some scanty cilia, rhabdites, a basal membrane and the absence of any chitinous thickening to the sucker and the absence of Laurer's canal. They are symbionts rather than parasites, which further dis- tinguishes from the Trematoda, but their thick cuticle and their syncytial ectoderm are undoubtedly Trematodan in character. l-v£ sue CU. circ.in. I lovg.m. ( ect.c. -par.c. — vesx. Fig. 156. Fig. 157- Fig. 156. Temnocephala minor, x 12. After Haswell. g.o. genital opening; M. mouth; sue. sucker; ten. tentacles. Fig. 157. Transverse section through body wall of a trematode. After Benham. ba.memh. basement membrane; cire.m. circular muscle layer; CU. cuticle; ect.c. ectoderm cell; long.m. longitudinal muscle layer; par.c. parenchyma cell; sp. spine; ves.c. vesicular cell (present in many trematodes). The rest of the Trematoda are all parasitic but they resemble in general shape the Turbellaria. They have retained the mouth, which is anteriorly placed, and the gut, which, however, is bifid, a shape not found in the Turbellaria. As in the Turbellaria, the gut may have lateral diverticula which branch freely. The Trematoda have, how- ever, lost the external ciliation of the Turbellaria (Fig. 157). The 2l8 THE INVERTEBRATA ectoderm is represented by cells sunk into the parenchyma in much the same way as nuclei of the ectodermal cells in the pharynx of the Tricladida. But the outer portion of the cell is lost in the Trematoda and its place is taken by a thick cuticle, which is often armed with spines. Suckers are always present for attachment to the host and are of large size. The presence of these suckers and their shape makes it possible to divide the Trematoda proper into two orders : (i) Hetero- cotylea, (ii) Malacocotylea. Order HETEROCOTYLEA In the Heterocotylea there is a large posterior sucker stiffened with chitinous supports. It is often subdivided, as in Octohothrium or Polystomutn (Fig. 158). In the Malacocotylea the sucker is not always posterior, it often moves forward on the ventral surface so that, as in Fasciola, it comes to lie one-third of the body-length from the anterior end. It is never provided with chitinous supports. All the Hetero- cotylea are ectoparasites with the single exception of Po/y^^owwm which occurs in the bladder of the common frog, of which from 3 to 10 per cent, are infected by it. They are confined to one host only. The Malacocotylea are all internal parasites and pass from one host to another at certain stages in their life history. In the Heterocotylea the excretory pores are paired and lie near the anterior end of the body, whereas in the Malacocotylea the excretory system discharges to the exterior through a single median pore placed at the posterior end of the body. In the Heterocotylea there are separate openings for the male and female portions of the generative system, while in the Malacocotylea there is but one common opening. In the Heterocotylea there is a pair of ducts leading from the ootype to the exterior indepen- dently from the male and female ducts, usually called the vaginae. The vaginae are inconspicuous as a rule, but in Polystomutn their openings are very clearly marked by two prominences on either side of the body about one-fifth of the body-length from the anterior end (Fig. 158). Corresponding ducts do not occur in the Malacocotylea. The nervous system of the Heterocotylea is more primitive than that of the Malacocotylea, but in both groups it is stereotyped and does not vary as it does in the Turbellaria. In both groups it consists of a cerebral ganglion with six cords leading posteriorly. In the Hetero- cotylea there are irregular commissures between the cords, while in the Malacocotylea the commissures are few in number and regular. Life history of the Heterocotylea. The usual habitat of this order is on the gills of fishes where they often live isolated. Self-fertilization must therefore be practised, but copulation has been observed in Polystomutn and also in Diplozoon, where it is permanent. The members of this order probably cause considerable inconvenience TREMATODA 219 to their hosts, but the numbers infesting one host is seldom very considerable and they have no economic importance as parasites. The eggs when laid are normally attached to the body of the host, Poly- stomum being exceptional in laying the eggs in the bladder whence they vas def. Fig. 158. Polystomum integerrimum, ventral view, showing the reproductive system. After Zeller. g.i. genito-intestinal canal; g.o. common genital open- ing; ho. booklet; M. mouth; oot. ootype; ov. ovary; p. penis; sue. sucker; t. testis; ut. uterus with eggs inside it; vag. vagina; vag.po. vaginal pore; vas def. vas deferens ; vit. vitellarium ; vit.d. vitelline duct. pass out to the exterior into water. The egg hatches as a larva with eye- spots and a large ventral posterior sucker. It swims by means of cilia which are arranged in bands round the body. These larvae make their way to some particular spot on the host after being free-swimming for a time. As soon as they attach themselves the ciliary covering is cast off and the generative organs develop. The larva of Polystomum 220 THE INVERTEBRATA seeks out a tadpole, dying within twenty-four hours if one is not found. If a tadpole is reached, the parasite fastens itself on to the gills, where its ciliary covering is cast and it then creeps into the bladder to wait for three years before becoming sexually mature. The larvae may, how- ever, attach themselves to the external gills, where a copious supply of nourishment induces such rapid growth that the animal becomes sexually mature in five weeks and produces eggs. But it dies when the tadpole metamorphoses, and thus it never reaches the bladder. In Dtplozoon, which lives attached to the gills of the minnow, the larvae attach themselves to the gills of the host, but they do not develop generative organs until they meet another larva. If such a meeting occurs the larvae fuse across the middle. After fusion the generative organs develop and the animals grow in such a manner that the vas deferens of one form is permanently connected to the genital atrium of the other. They thus remain throughout their lives in permanent copulation. Another form which displays a variation of the usual type of history is Gyrodactylus which occurs on the gills of freshwater fish. In Gyrodactylus the ovary and the vitellarium are not separated, as is the general rule in the Trematoda, but constitute one organ. A single egg ripens at a time and, after fertilization, develops into an embryo in the uterus. Before the first embryo leaves the mother a second younger one appears inside it so that we thus have a con- dition of three generations one inside the other, and the conditions are such that the youngest embryo must develop without fertilization. This feature of the development of one larva with another without the agency of fertilization is common in the life histories of the Malacocotylea but Gyrodactylus is the one member of the Hetero- cotylea in which it occurs. Order MALACOCOTYLEA The life history of Fasciola (Fig. 159) may be taken as the type of life history commonly found in the group. For details of this life history the reader is referred to elementary textbooks. In the Malacocotylea the adult is always, with rare exceptions, parasitic in some vertebrate host, the sporocyst and redia stages are always parasitic in a mollusc. Two hosts are always, and three may be necessary for complete development. Divergence from the type of life history recorded for Fasciola may come about by (i) a generation, the redia stage, being omitted, (ii) the sporocyst forming by budding a second generation of sporocysts within which the cercariae arise, (iii) the cercaria requiring to encyst in a host and to await this host being eaten by the final host before reaching sexual maturity as in the case of Gasterostomum fitnbriatum, where the sporocyst develops in ves.sm.^^ Fig. 159. Diagram of reproductive and nervous system of Fasciola hepatica, X about 8. From Leuckart. M. mouth ; ph. pharynx ; n. nerve ring ; In.n. chief longitudinal nerve; al. beginning of alimentary canal; p. opening of penis; ves.sm. vesicula seminalis ; iit. uterus ; ov. ovary ; sh.gl. shell gland ; a.t. anterior testis; pt.t. posterior testis; y.gl. yolk glands; vas de. vas deferens. 222 THE INVERTEBRATA the liver of Anodoriy the cercaria encysts in the roof of the mouth of the roach and only reaches sexual maturity when the roach is swallowed by a perch. In Distomum macrostomum^ which is parasitic in the gut of thrushes, there is no free-living stage in the life history. The eggs, passed out with the faeces of the bird, are eaten by a snail, inside which the sporo- cyst develops. The sporocyst finds its way into one of the tentacles. It there develops pigment, being brightly coloured in bands of green and red, while its presence stops the snail from withdrawing this tentacle. Presumably this brightly coloured object attracts the bird which devours the snail and infects itself by setting free the cercariae from the sporocyst. -e.po. Fig. i6o. Schistosoma: the male (cJ) is clasping the female (?) in the gynae- cophoral groove (gr.). e.po. excretory pore; M. mouth; v.suc. ventral sucker. After Fritsch. Schistosoma ( = Bilharzia) is a parasite of man, living as an adult in the abdominal veins (Fig. i6o). It is long and thin and well adapted for this habitat. It is one of the rare examples of dioecious trematodes. The male, however, does not lose touch with the female once he has found her, but carries her permanently in a fold of the ventral body wall. The eggs are laid in the blood vessels and, being provided with a sharp spike, they lacerate the walls of the capillaries and pass into the bladder. Immediately the urine is diluted the miracidia hatch, but they wait for dilution before hatching. The second host is a water snail. The cercariae swim freely in the PLATYHELMINTHES 223 water, and in districts in China and Egypt where the disease is com- mon they swarm. Bathing, washing or drinking the infected water allows the cercaria to enter the final host. The cercariae penetrate the skin with great rapidity and, entering the blood system, make their way to the abdominal veins where they become mature. The disease can be prevented by strict sanitary measures in regard to water, and it can be cured by the administration of compounds of antimony to infected patients. That the disease is a very old one in Egypt is shown by the discovery of Schistosoma eggs in the kidneys of mummies of the twentieth dynasty (i 250-1000 B.C.). The hatching of miracidia from the egg of Schistosoma is dependent on the dilution of the urine by fresh water and this serves to emphasize the fact that the stages in the life history of all parasites are ultimately connected with environmental conditions. The egg ofFasciola hepatica does not hatch unless the pYi of the water in which it is deposited is below 7-5, the optimum point apparently being about pYL 6-5. If the eggs are kept in water more alkaline than pYi 7-5 the embryo remains within the shell and eventually dies. The identification of a cercaria with an adult is a task which requires great patience, and many cercaria are known which have not been as yet connected with an adult. Almost any mollusc, if dissected care- fully under a hand lens, will provide specimens of rediae and cercariae, although infected specimens may be more common in some localities than in others. The tail of a cercaria is often an elaborate structure. Some have rings and chitinous stiffenings, while the well-known Bucephalus larva of Gasterostomum is a cercaria with a forked tail (Fig. 161). Class CESTODA The Cestoda may be defined as endoparasitic Platyhelminthes in which the enteron is absent and the ciliated ectoderm has, in the adult, been replaced by a thick cuticle. In the parenchyma lime cells occur (see Fig. 162). Proglottides are usually formed. The Cestoda as a group have felt the influence of the parasitic habit more than the Trematoda. They have dispensed altogether with a gut, there is no mouth, and they absorb their food through the skin. As they live always in the alimentary canal of vertebrates they are con- veniently situated for this purpose and the amount of food available to them probably counterbalances the difficulties attendant on dis- pensing with the usual method of digesting and assimilatiiig food. The ectoderm cells have sunk into the parenchyma after secreting a cuticle as in the Trematoda, but this cuticle is thicker and divided into layers. Immediately beneath the cuticle are the longitudinal muscles. The circular muscles are incomplete at the edges. In transverse 224 THE INVERTEBRATA yl'Ory. Fig. i6i. Bucephalus larva (cercaria) of Gasterostomum fimbriatum. After Benham./.^a//, forked tail ; gl.org. glandular organ ; M. mouth ; phar. pharynx. ectc. c.rn. ^^^^^ u,t. cut, I m.fi.: /^^^ ^^ ca. Fig. 162. Transverse section through a mature proglottis of Taenia, x about 12. From Shipley and MacBride. cut. cuticle; ect.c. ectoderm cells sunk into the parenchyma; m.fi. longitudinal muscle fibres cut across; cm. layer of circular muscles; lime c. lime cell; ov. ovary; t. testis with masses of germ cells forming spermatozoa; In.exc.ca. longitudinal excretory canal; In.n. longitudinal nerve cord; iit. uterus; od. oviduct. CESTODA 225 sections the circular muscles appear to divide the parenchyma into two regions, an outer cortical zone, where occur the cut ends of the longitudinal muscle together with calcareous bodies, and an inner or medullary zone, where the generative system lies (Fig. 162). The Cestoda may be divided into two orders : (i) Cestoda Monozoa, (ii) Cestoda Merozoa. Order CESTODA MONOZOA These are small forms which live in the gut of fishes, usually Elasmo- branchs. They resemble a trematode in shape and in the fact that they do not form proglottides, but they have no gut. They have at one end a "frilled" organ which serves for attachment, and a small sucker at the other end. An example of this order is Amphilina. It is difficult from the structure to say which end is the anterior and which the posterior, for the nervous system consists of two cords running down either side of the body with a single similar commissure at either end. But when the animal moves it has the ''frilled" organ in front so that is spoken of as the anterior end. Order CESTODA MEROZOA These are distinguished from the Cestoda Monozoa by the fact that they all have the power of budding and so reproducing asexually, re- sembling in this respect the turbellarian Microstoma lineare. The adult worm has a scolex which is provided with organs of fixation such as hooks, suckers or folds (Fig. 163). The scolex is usually buried in the intestinal mucosa of the host. Behind the scolex comes the neck, the most slender portion of the body, which may or may not be sharply marked off^ from the scolex. It is in the neck that asexual reproduction occurs, fresh segments being continually cut off and, as they grow larger, pushed by the formation of new segments away from the scolex. The segment so formed is called a proglottis. The proglottis is not truly comparable with the new individuals pro- duced in Microstoma lineare. Through each proglottis run the ex- cretory canals and the nervous strands which are common to all (Fig. 162). The proglottis when first cut ofiF from the neck region is devoid of generative organs, but these develop as it becomes more mature. When the generative organs are mature, fertilization of the ova occurs, the ovaries and the testes disappear, and the uterus alone remains to store the eggs. When the proglottis reaches this stage it is "ripe " and breaks off to pass out with the faeces (Fig. 164). Despite its connection with the scolex, each proglottis must be regarded as an individual for it contains a full set of generative organs both male and female. sue: Fig. 163. Taenia solium. Slightly magnified. From Shipley and MacBride. A, Entire worm, showing head and proglottides, sue/ sucker on head; g.po. genital pores; prog, ripe proglottis. B, Head. rost. rostellum; ho. hooks; sue. suckers; stroh. commencement of strobilization. C, Ripe proglottis broken off from worm. od. remains of vas deferens and oviduct; ut. branched uterus crowded with eggs. CESTODA MEROZOA 227 The Structure of the scolex is of importance for it forms the basis of the classification of the Cestoda Merozoa. In the tapeworms occurring in the gut of fishes the scolex may have two or four suckers and the neck may be sharply separated from the region where bud- ding occurs. In these tapeworms the scolex is often armoured with chitinous projections and hooks, and the number of the proglottides is usually small. The tapeworms occurring in the mammals {Cyclo- phy Hided) are, with one exception, characterized by a head which bears four suckers at the sides, and, on a projection at the top, called the rostellum, is a crown of hooks. ln,exc.ca, tr.excca vasde Fig. 164. Diagram of a ripe proglottis of Taenia solium, x about 10. From Cholodkowsky. In.exc.ca. longitudinal excretory canal; tr.exc.ca. transverse excretory canal ; vas de. vas deferens ; vag. vagina ; ov. ovary ; y.gl. yolk gland ; sh.gl. shell gland ; ut, uterus ; t. testes ; ln,n. longitudinal nerve. As a general rule the more primitive cestodes are found in the lower vertebrates, while the advanced types are found in the mam- mals. The evolutionary stage of the parasite is therefore closely related to that of its host. A notable exception to this rule is Diboth- riocephalus latus, the Broad Tapeworm of man, which belongs to a group of tapeworms occurring more commonly in the guts of fishes. The scolex of Dibothriocephalus has two suckers on either side of the head. These suckers are of the nature of flabby folds sharply distinct from the well-defined cuplike suckers of the Cyclophyllidea. The generative organs are of the same type as is found generally throughout the Platyhelminthes. There is a single opening for both male and female organs. From the ootype there leads out a duct which 228 THE INVERTEBRATA is called the uterus and is used for the storage of eggs, but it is doubtful whether it is homologous with the uterus of the Trematoda. The life history of a cestode is a complicated combination of sexual and asexual reproduction. One, two or three hosts may be necessary. The egg passes to the exterior with the faeces. It contains inside it an embryo armed with six hooks called an "onchosphere". The egg case takes diiferent shapes ; in Dibothriocephalus latuSy which is a more primitive type of cestode, the covering of the embryo is ciliated. In the Cyclophyllid tapeworms, which constitute the most advanced group of the Cestoda the ciliary covering is lost. In Dipylidium caninum, the adult of which occurs in the alimentary canal of the cat or dog, it is replaced by an albuminous coat with a chitinous Uning inside, while in most of the other forms only the chitinous covering persists. The egg hatches as an onchosphere after being swallowed by the first host. The onchosphere then penetrates the wall of the alimentary canal using its hooks for this purpose and lodges somewhere in the peritoneal cavity of the host. Here it develops suckers and a scolex. In primitive forms such as Dibothrio- cephalus, the larval cestode rests inside the first host, a Cyclops, at a stage of its development known as the plerocercoid stage. This stage is ovate in shape and the generative organs are undeveloped and there are no signs of proglottides. The Cyclops is then eaten by a freshwater fish, after which the larva, or plerocercus, bores through the wall of the alimentary canal and rests in the body cavity where it grows still further, reaching the metacestode stage. Proglottides can be distinguished in the metacestode stage but the generative organs are not fully mature. Growth now ceases but the metacestode stage is often inconveniently large for the body cavity, causing it to bulge. Sticklebacks thus infected with the metacestode of Schisto- cephalus gasterostei are commonly found. The adult in this case reaches maturity when eaten by a bird. Man acquires Dibothriocephalus latus, a nearly related form, by eating pike infected with the meta- cestode. In the Cyclophyliidea the resting stage in the first host is the "bladder worm" (or cysticercus). The onchosphere on reaching its resting place becomes hollowed out into a ball filled with fluid. A depression then forms in the wall of the sphere and becomes an inverted scolex. In Taenia serrata, the common tapeworm of the dog, the bladder stage in the rabbit (to which the name Cysticercus pisi- formis was given before the connection with the adult was discovered) has but one head inverted into the cyst. In the bladder-worm stage of Taenia coenurus, which is found in the brain of the sheep and causes the disease known as "gid" or "staggers", many heads are formed and invaginated into the cyst so that multiple infection may CESTODA MEROZOA 229 occur when a sheep is devoured and torn to pieces by dogs or wolves. In Taenia echinococcus ^ the adult of which lives in the alimentary canal of the dog and is remarkable for having but three proglottides, the cysticercus stage is found in domestic animals and also in man in countries where men live in close association with dogs. The cyst stage is very large and the bladder may contain a gallon or more of fluid. Such a cyst, known as a " hydatid ", rapidly proves to be fatal. It is particularly dangerous and difficult to eradicate because the walls of the cyst have the power of budding off asexually daughter cysts. A still further development of asexual budding in the cysti- cercus stage occurs in Staphylocystis, where the onchosphere imbeds itself in the liver and then develops a stalk or stolon which buds off cysts which are detached and fall into the body cavity of the host. Where the cysticercus is swallowed by the final host the head is everted from the bladder, the bladder is digested and proglottides forthwith make their appearance from the neck region of the scolex. So far as is known the production of proglottides continues for the duration of the life of the host. The subdivision of the Cestoda Merozoa depends on the shape of the scolex. There are five divisions, the last of which contains the forms commonly found as adults in the alimentary canal of the Mammalia and is the only group of economic importance. (i) Tetraphyllidea. The four suckers are usually stalked out- growths of the scolex. Parasitic in fish, amphibia and reptiles. Onchosphere enters a copepod and develops into a larva known as a plerocercoid, in which condition it remains until the copepod is eaten, when it develops into the adult. Size moderate usually 20-30 cm. long but occasionally as small as i cm. or as large as. I metre. (ii) Diphyllidea. There are two suckers only and the scolex has a long neck armed with spines. There is only one family and one genus, Echinobothrium, which is found in the spiral intestine of Selachians. The larva, which is of cysticercoid form, is found in the prawn Hippolyte. (iii) Tetrarhynchidea. These have four suckers each provided with a long spiniferous retractile process. The adult is parasitic in the alimentary canal of Elasmobranchs and especially Ganoids. The larva which may be of either the procercoid or cysticercoid type occurs in marine invertebrates of many kinds, fish and occasionally reptiles. (iv) Pseudophyllidea. The scolex has two suckers which may be absent in some forms, there is no clearly marked neck and hooks are usually absent. Occasionally as in Triaenophorus , a common parasite of freshwater fish, the external divisions between the proglottides are indistinct and these are only indicated by the 230 THE INVERTEBRATA regularly placed openings of the uterine birth pores. The majority of these are parasitic as adults in freshwater fishes, but Dibothrio- cephalus latus occurs in man and Bothriotaenia in birds. Archigetes is parasitic as an adult in body of tubilex, an oligochaete worm living in fresh water. The larva is a plerocercoid which in some forms, Caryophyllaceus and Archigetes develops gonads paedogenetically so that there is no adult with proglottides. These paedogenetic forms closely resemble the Cestoda Monozoa in appearance. (v) Cyclophyllidea. The scolex bears four cup-shaped suckers and has a rostellum with a crown of hooks. The Cyclophyllidea comprise the majority of the common tape- worms. Those infesting the gut of mammals all have a scolex closely resembling that of Taenia with four well-defined suckers and a circlet of hooks. Those found in the gut of fish have a more elaborate scolex. The number of proglottides varies considerably, the smallest number (3) is found in Taenia echinococcus ^ while many forms have hundreds of proglottides and are several yards in length. The pro- glottides never drop off before they are mature, as they may do in the other groups and develop generative organs later, consequently the separated proglottides always contain fully developed oncho- spheres. Two interesting forms may be mentioned. Dipylidium cani- num is a tapeworm infesting the alimentary canal of dogs and cats. Each proglottis has a double set of generative organs with two separate generative openings, a feature which gives the animal its name, but which may occur in other forms. The first host is the flea, and puppies and kittens are early infected by catching and eating these insects. The mature proglottis has a double set of male and female generative organs with an opening on either side. Hymenolepis nana is one of the smallest tapeworms. The adult has ten to twenty proglottides and only measures half an inch in length. It occurs in children in certain places, particularly Lisbon and New York, where it is said to be increasing. It is remarkable among tapeworms for being the only one known to go through all its life history in one host. The embryos bore into the intestinal wall where they pass through the cysticercus stage and emerge again into the alimentary canal when adult. The homologies of the various ducts of the genitalia of the Platy- helminthes (Figs. 165, i66) present great difficulties. While one or two, the oviduct and the vas deferens for example, are quite clearly homo- logous throughout, the homologies of others, particularly the accessory organs such as uterus, bursa copulatrix, vagina, are very doubtful. The ** uterus" of the Trematoda is clearly the ductus communis of the Turbellaria greatly elongated and used for egg storage, while the vii. — -- Fig. 165. gen.at. — vas def. Fig. 166. Figs. 165 and 166. Diagram of the arrangement of genital organs and ducts in the Platyhelminthes. i. Rhabdocoelida. 2. Tricladida. 3. Trematoda, Heterocotylea. 4. Cestoda, Dibothriata. b.c. bursa copulatrix (stalked organ); d.c. ductus communis; gen.at. genital atrium; Laur.c. Laurer's canal; ov. ovary; p.s.o. pear-shaped (muscular) organ; pe. penis; rec.sem. receptaculum seminis; sh.gl. shell glands; test, testis; ut. uterus; ut.op. uterine opening to exterior; vag. vagina; vag.op. opening of vagina to ex- terior; vas def. vas deferens; ves.sem. vesicula seminalis; vtt. vitellarium; d* 6f $ op. common opening of genital atrium to exterior. 232 THE INVERTEBRATA vagina of the Cestoda is the same, but the relation of the *' vagina" of the Heterocotylea or the ** uterus" of the Cestoda remains at present obscure. If the vagina of the Cestoda is homologous with the uterus of the Trematoda, the uterus of the Cestoda, which is a single duct, may correspond with the vagina of the Trematoda, which is however a paired structure. The homologies of the ducts in the Trematoda are further complicated by the presence of Laurer's canal, a duct leading out of the ductus communis and opening to the exterior in the Malacocotylea but into the gut in the Heterocotylea. The bursa copulatrix and the muscular pear-shaped organ, which open into the genital atrium in the Turbellaria, are accessory reproductive organs which are probably not represented in the parasitic forms. (See Figs. 165 and 166.) CHAPTER VII THE NEMERTEA ROTIFERA AND GASTROTRICHA PHYLUM NEMERTEA Elongated flattened unsegmented worms with a ciliated ectoderm and an eversible proboscis lying in a sheath on the dorsal side of the alimentary canal, with which it is not connected; no perivisceral body cavity, the spaces between the organs being filled with paren- chyma; aHmentary canal with mouth and anus; excretory system with flame cells; a blood vascular system; gonads simple, repeated; sexes separate; sometimes a larval form {Pilidium). The Nemertea in their general organization resemble the Platy- helminthes very strongly. In certain positive features they have advanced, e.g. in the development of a proboscis independent of the gut, in the presence of a vascular system, and a second opening, the anus, into the alimentary canal, but in the simplicity of the gonads and absence of hermaphroditism the Nemertea are less specialized than the Platyhelminthes. There can be no doubt, however, that the two phyla are very closely connected, although the presence of an anus and a vascular system is an enormous advance. The proboscis (Figs. 167, 168) is the most characteristic organ of the nemerteans. It lies in a cavity (rhynchocoel), completely shut off from the exterior, which has muscular walls (the proboscis sheath), and is attached to the posterior end of the sheath by a retractor muscle which is really the solid end of the piroboscis. The proboscis may be compared with the finger of a glove with a string tied to the inside of the tip ; when the proboscis is at rest the string, i.e. the retractor muscle, keeps it turned inside out within the sheath ; when the muscles of the proboscis sheath contract and press upon the fluid in the rhynchocoel the proboscis is everted, but never completely, because the retractor muscle keeps it from going beyond a certain point. At this point, in the Metanemertini, is a diaphragm cutting off the apical part of the proboscis cavity, and mounted on this is a spike or stylet with reserve stylets in pouches at the side (Fig. 168 C). This part of the cavity probably contains a poisonous fluid which is ejected through a canal in the diaphragm into wounds caused by the stylets. The proboscis in this class of nemerteans is thus a formidable weapon. In other nemerteans, though the stylet is not developed, the proboscis is prehensile and can be first coiled round its prey and then nep. al.c. d.v. ■gen.op. St. intjd. '•?• Pf- P*- ect mt. Fig. 1 68. an. Fig. 167. Fig. 167. Diagrammatic dorsal view of nemertine. From Kukenthal. al.c. alimentary canal; an. anus; brn. cerebral ganglia; c.o. cerebral organ; c.v. connecting and d.v. dorsal vessel; e. eye; gen. op. genital openings; l.v. lateral vessel l.n. lateral nerve; M. mouth; nep excretory system and np.o. one of its pores ; pb. proboscis in the rhynchocoel. Fig. 168. A, Longitudinal vertical section of a metanemertine to show the relation of the various cavities. After Benham. brn. cerebral ganglia; int. intestine ; int.d. coecum ; pb. proboscis ; pb.' solid non-eversible part of the former, attached to the proboscis sheath and acting as a retractor muscle ; ps. proboscis sheath ; re. rhynchocoel ; rh. rhynchodaeum ; std. stomodaeum ; St. stomach. B, Transverse section of a palaeonemertine, passing through a pouch of the intestine on the right and an ovary on the left. After Coe. ect. ectoderm; l.v. lateral blood vessel; l.n. lateral nerve; m.c. circular muscles; m.l. longitudinal muscles; ov. ovary; p.al. pouch of intestine; par. parenchyma. Other letters as above. C, Proboscis of a metanemertine to show the diaphragm and the stylets sy. and sy.' After Bresslau. NEMERTEA 235 retracted to bring it within reach of the mouth. Some forms use the proboscis to aid in burrowing. The part of the proboscis in front of the brain is called the rhynchodaeum. The ectoderm is completely ciliated : there are gland cells amongst the ciliated epithelium; within this are layers of, first, circular, and then longitudinal, muscles. There is a nerve net which in the most primitive nemerteans lies at the base of the ectoderm cells, in others between the circular and longitudinal muscles, and in the most ad- vanced forms within both layers of muscle. While the nervous system is thus extremely primitive there are concentrations of the nerve net to form lateral nerve cords and a pair of cerebral ganglia above the mouth, each cerebral ganglion being divided into a dorsal and ventral lobe and connected by commissures above and below the proboscis sheath. The dorsal lobe is subdivided into an anterior and posterior part: the posterior part is in close relation with an ectodermal pit, the cerebral organ, which is situated in some forms in a lateral slit. As yet, however, the control of the movements of the organism is not dependent on the cerebral ganglia. There are occasionally eyes of simple structure. Inside the muscle layers the body is filled with parenchyma like that of the Platyhelminthes (Fig. 168 B), but in it are one, two or three longitudinal vessels, connected together by transverse vessels with contractile walls, which constitute the vascular system. The blood is generally colourless, but has corpuscles which sometimes contain haemoglobin. The circulation is assisted by the movements of the body. It can hardly be supposed that the blood system, situated so deeply in the body, can be respiratory in function. The alimentary canal is a straight tube, the mouth and anus being nearly or quite terminal. The excretory system is formed by a pair of canals situated laterally, each of which communicates with the exterior by one or several pores and gives off many branches, ending internally in flame cells like those of the Platyhelminthes. In some cases the end organs come into contact with the blood vessels. The generative organs are series of paired sacs alternating with the pouches of the mid gut and these each develop at the time of maturity a short duct to the exterior. Most nemerteans develop directly, but in some a pelagic larva with a remarkable form of metamorphosis is found. This larva is known as the Pilidium (Fig. 169). A conical gastrula with a flattened base is first formed by invagination and it passes into the Pilidium by the following changes. A band of cilia round the base constitutes the prototroch and forms the locomotory organ of the larva ; it is drawn out into two lateral lappets. An apical sense organ is formed by a thickening of the ectoderm. Two cells migrate into the blastocoele and 236 THE INVERTEBRATA break up into a tissue called mesenchyme, which is partly converted into larval musculature and partly remains undifferentiated until needed as raw material for the adult organs. The gut is connected with the exterior by an ectodermal oesophagus, ending in a large mouth on the flattened base between the lappets. Thus a creature appears which has many resemblances to the trochosphere larva to be described later. Inside this larva the young nemertean is produced (Fig. 169 A, B). Five ectodermal plates (imaginal discs) sink below the surface Fig. 169. Pilidium larva. A, Side view of late form enclosing young nemer- tean. After Korschelt and Heider. B, Frontal view of earlier stage showing the imaginal discs. The anterior unpaired invagination is continued to form the proboscis. After Burger, al. alimentary canal; ap.o. apical organ; amn. ectoderm of the amnion; ect. ectoderm of the adult; M. mouth; mesc. mesenchyme of Pilidium ; ns. nervous system ;/)r. prototroch ; rh. rhynchocoel. and each forms the floor of a sac. Eventually these sacs join round the gut and a continuous cavity is formed separating the adult inside from the larval skin (sometimes known as the amnion) which is thus its protecting husk while it develops. The imaginal discs join together and form the secondary or adult ectoderm. The Pilidium continues to swim about with the little nemertean inside it, even when the organs of the latter are developed and cilia cover its surface so that the adult moves freely as if a parasite of the larva. At length it bursts through the tissues of the amnion and the latter sink like a discarded mantle. NEMERTEA AND ROTI FERA 237 The nemerteans are classified as follows : Palaeonemertini. Proboscis without stylets; cerebral ganglia and lateral nerves in the ectoderm or between the two layers of muscles, Carinella. Metanemertini. Proboscis armed with stylets; lateral nerves within all the muscle layers. Tetrastemma, Geonemertes^ Malacobdella. Heteronemertini. Proboscis without stylets; a second layer of longitudinal muscles outside the circular muscles ; lateral nerve cords lie between the two. Linens, Cerebratulns. PHYLUM ROTIFERA Minute animals, unsegmented and non-coelomate, typically with a ciliated trochal disc for locomotion and food collection, a complete ali- mentary canal with anterior mouth and posterior anus, and a muscular pharynx with jaws of a special type ; excretory system with flame cells joining the hind gut to form a cloaca ; no blood system or respiratory organ ; very simple nervous system ; sexes separate, two kinds of eggs, one developing immediately without fertilization and the other, which is fertilized, thick-shelled and developing only after a resting period. This group contains a large number of forms of great interest to the microscopist which are easily obtained from many kinds of fresh water. They are, generally speaking, the smallest of all metazoa. They vary little in structure and present a remarkable similarity to the trochosphere larva. It must be admitted that the Rotifera are on a lower stage of organization than the annelids and molluscs which possess this larva and may even be related to a common ancestor of these phyla. On the other hand, the Rotifera come near to the Platy- helminthes, the Gastrotricha and Nematodes. An elastic external cuticle covers most of the body. Under this is a syncytial ectoderm ; a continuous layer of muscles forming a body wall is absent (as in the Arthropoda), but isolated bands of muscle, chiefly longitudinal, traverse the body (or perivisceral) cavity (Fig. 171). What is the true nature of the body cavity is a question which has never been properly answered. It is a wide space between ectoderm and endoderm, traversed by muscles, and is neither a coelom nor a haemocoele in the narrower sense, but probably only a derivation of the segmentation cavity of the gastrula (the blastocoele), as in the trochosphere larva. But they do possess a body cavity and not a solid parenchyma, and so differ from the Platyhelminthes. Their excretory system is, however, very similar to that of the latter phylum, and in the union of the excretory duct with the gut the rotifers resemble certain specialized trematodes. 238 THE INVERTEBRATA B Fig. 170. Hydatina senta. A, Female, ventral view. Original. B, Male, side view. After Wesenberg Lund. al. rudiment of alimentary canal; la.an. lateral antenna; bl. bladder; brn. brain; cng. cingulum; cl.a. cloacal aperture ; d.an. dorsal antenna ;f.c. flame CG\\\g.gl. gastric gland ; ect. syncytial ectoderm of trochal disc; M. mouth; mc. circular and ml. longitudinal muscle cells ; ms. muscular mastax and trophi ; np. nephridium, intracellular duct represented by double dotted line; ov. ovary; oe. oesophagus;/), penis retracted; ped.gl. pedal gland; pa. papillae with large cilia; st. stomach; t. testis; tro. trochus; vit. vitellarium. ROTIFERA 239 Like the Nematoda they consist of a small number of cells and all the tissues, except the cells of the velum, may lose their cell boundaries and become syncytial. Not only is there a superficial resemblance to heterotrichous ciliates in the Protozoa but the tendency to the acellular condition carries this a step further. Hydatina senta may be taken as a type of the group (Fig. 170). jC.m.c- ecdu. ect.< Fig. 171. A, Side view, diagrammatic, from Shipley and MacBride, and B, tansverse section of a female rotifer. An. anus (cloacal aperture); c.m.c. circular muscle cell; cu. cuticle; D. dorsal; e. eye; ecdu. excretory duct (nephridium) ; ect. ectoderm; int. intestine; l.m. longitudinal muscle; m. muscle; od. oviduct; tc. trochus; V. ventral. Other letters as in Fig. 170. The female is pear-shaped, the posterior end being the stalk. The anterior end is flattened and form^ the trochal disc. This is, in many rotifers, bordered by a double ciliated ring, the velum, the outer part of which (the cingulum) is the original velum and is composed of strong cilia. The inner is called the trochus. Between the two rings, which are thus preoral and postoral respectively, is a ciliated groove in which is situated the mouth. The velum in life gives the impression 240 THE INVERTEBRATA of revolving wheels, the reason for the scientific name of the group. In Hydatina the cingulum forms a complete ring and the trochus is reduced to a double transverse row of cilia; in the groove between them is situated a number of papillae on which are stiff cilia. (In Copeus and other creeping forms there are no trochus and cingulum but cilia cover the trochal region and part of the ventral surface. This is said to be a primitive arrangement.) The posterior end is called the foot and it terminates in a pincer-shaped appendage, on which open glands with a sticky secretion. By means of this apparatus the rotifer can anchor itself in the intervals of its free-swimming life. The dorsal surface of the rotifer is marked out by the position of the cloaca! aperture just in front of the foot ; on this surface immediately behind the velum is a sense organ, the dorsal antenna^ and below it the brain. There are also two lateral antennae ; all three are prominences bearing stiff sense hairs. Elsewhere the body is covered by a thin, smooth, transparent cuticle secreted by the ectoderm. The food, which consists of micro-organisms of various kinds, is swept by means of the ciliary currents of the disc into the mouth and then through the oesophagus into the muscular pharynx or mastax which is provided with chitinous jaws, the trophic which are in con- stant movement and, in Hydatina^ masticate the food as it passes through. This first part of the alimentary canal is ectodermal and constitutes the stomodaeum. Then follows the endodermal stomach, lined with ciliated epithelium, in which digestion takes place. ^ Two gastric glands open into it anteriorly. A narrow intestine leads into the cloaca, into which the excretory system also opens. The latter consists of two lateral ducts , coiled at intervals , consisting of perforated cells placed end to end into which flame cells (vibratile tags) open frequently but irregularly. Anteriorly the ducts communicate by a transverse vessel just behind the disc and posteriorly they open into a pulsating vesicle which expels its contents into the cloaca. It has been calculated that in some species this bladder expels a bulk of fluid equal to that of the animal about every ten minutes. The single ovary is a bulky organ : it is divided into a small gerinar- ium (the ovary proper) and a much larger vitellarium or yolk gland which occupies much of the space between the stomach and the body wall. The ovary is continued into a duct which opens into the cloaca. The female is still the only individual known in many kinds of rotifers. It was not until 1848 that a male rotifer of any kind was described. In only a few species is the male equal in size and organi- zation to the female. In all the rest there is a more or less pronounced sexual dimorphism. In Hydatina (Fig. 170 B) the male has no ^ Digestion is usually extracellular, but in Ascopus and other rotifers it is intracellular. ROTIFERA 241 alimentary canal, but the ciliated disc, musculature and excretory- system are well developed. Usually the male is not only smaller but its ciliated disc and the alimentary canal are very much reduced and the excretory system may be absent. The chief organ is the large testis^ usually filled with ripe spermatozoa, which opens by a median dorsal penis in many cases. Where the penis is absent the tapering hinder end may be inserted in the cloaca of the female. Finally, it may be mentioned that in one large family, the Philodinidae, which in- cludes the genus Rotifer^ no male has ever been found. Two kinds of reproduction occur in the rotifers as in the cladoceran Crustacea, but in this case there are two kinds of females, one of which always reproduces parthenogenetically, the eggs developing to form females (female producers), while the other may reproduce bisexually. In this second type (male producers) there are eggs, often smaller than the female eggs, which develop quickly by parthenogenesis into males. At various seasons after the appearance of these male eggs there are produced by the same individual also other eggs, distinguished by a thicker shell, and these have been fertilized by the spermatozoa of the just hatched males injected through the skin. These *' resting" eggs are fertilized "male eggs" and they only develop after a dormant period into females. The reproduction of a rotifer runs through a cycle in which at first only parthenogenesis occurs but which is terminated by sexual re- production. In rotifers which are typical members of freshwater plankton, the cycles run to a time-table. There are "dicyclical" rotifers like Asplanchna, which have two sexual periods, one in spring and the other in autumn, while other forms like Pedalion are " mono-- cyclical" and have only a sexual period in the autumn passing the winter as resting eggs. In rotifers like Hydatina, which inhabit puddles and ponds, the sexual periods are very frequent and begin soon after the resting eggs have hatched. The resting egg is a stage in which the species can survive when the puddle dries up. Sexual re- production can be brought on in cultures by alteration of the external conditions. Besides the environmental types which have been mentioned above as free-swimming and inhabiting larger and smaller bodies of water, the following rotifers may also be mentioned : Stephanoceros and Floscularia (Fig. 172 bis) are sedentary forms which secrete a protecting gelatinous tube into which they can with- draw rapidly. Melicerta is another sedentary form which produces a tube formed out of mud particles or its own faeces. Callidina and other genera are terrestrial forms which can remain for a great part of the year in a dried-up condition but come to life immediately when moistened by rain. Such forms are found, for 242 THE INVERTEBRATA l.v.m. Fig. 172 bis. Fig. 172. Fig 172. Ventral view of Chaetonotus. From Kukenthal. an. anus; brn. brain- Z.m. longitudinal muscle; l.v.m. lateroventral muscle; M. mouth; mg. mid gut; nep. nephridium; od. oviduct; ov. ovary; ped. foot; ped.gL. pedal gland ; ph. pharynx. Fig 1726/5. A, Floscularia cornuta. Female within its gelatinous tube. From Hudson. i3, F. campanulata Male, ves.sem. vesicula seminalis ;/>. penis. GASTROTRICHA 243 instance, in roof gutters and amongst moss. The group to which these forms belong is called the "bdelloid" or leech-like rotifers, because they not only swim, but progress by a looping method like that of Hydra or a leech. PHYLUM GASTROTRICHA Minute wormlike, unsegmented animals, with certain tracts of the skin ciliated, the cuticle often forming bristles and scales; a non- cellular hypodermis, forming adhesive papillae, longitudinal muscle cells which do not form a continuous sheath; straight alimentary canal consisting of a muscular pharynx like that of the nematodes and a mid gut without diverticule ; a pair of nephridia in freshwater representatives; a nervous system consisting of a cerebral ganglion and two lateral cords ; hermaphrodite individuals in one division of the phylum (Macrodasyoidea) and parthenogenetic females in the other (Chaetonotoidea) ; the single female aperture opening near the anus, and the male aperture when present variable in position. Development direct and cleavage total. These small animals (Fig. 172) are usually elongated and creep or swim by means of their cilia or move in a leech-like manner using their musculature. They feed on minute animals and plants which are sucked in by the pharynx. The Gastrotricha have features in common with the Rotifera, such as the external ciliation, the bifid foot and the excretory system with flame cells, but in the character of the gut they recall the Nematoda. CHAPTER VIII THE NEMATODA, NEMATOMORPHA AND ACANTHOCEPHALA PHYLUM NEMATODA Unsegmented worms, with an elongated body pointed at both ends; ectoderm represented by a thin sheet of non-cellular hypodermis, con- centrated to form two lateral lines and to a less degree dorsal and ventral midlines^ secreting an elastic cuticle, made of protein, not chitin, usually moulted four times in the life of the individual; cilia absent from both external and internal surfaces; a single layer of muscle cells underneath the hypodermis, divided into four quadrants, each muscle cell being elongated in the same direction as the body and composed of a peripheral portion of contractile protoplasm and a larger internal core of unmodified protoplasm which sends a process to a nerve ; the space between the body wall and the gut sometimes filled by a small number of highly vacuolated cells, the vacuoles joining together and simulating a perivisceral cavity ; excretory system consisting of two intracellular tubes running in the lateral lines; nervous system made up of a number of nerve cells rather diff^usely arranged but forming a circumpharyngeal ring and a number of longitudinal cords of which the mid-dorsal and mid-ventral are the most important; sense organs of the simplest type; sexes usually separate, gonads tubular, continuous with ducts, the female organs usually paired, uniting to open to the exterior by a ventral vulva, the male organ single, opening into the hind gut, thus forming a cloaca, in a diverticulum of which lie the copulatory spicules ; spermatozoa rounded and amoeboid, fertilization internal; alimentary canal straight and composed of two ectodermal parts, the suctorial fore gut and the hind gut and an endodermal mid gut without glands or muscles ; segmentation of egg complete and bilateral in type, develop- ment direct, larvae only differing slightly from adult. The nematodes appear to occupy an isolated position, but many of their characters, though more specialized, resemble those of the Platyhelminthes and Rotifera. They are certainly closely related to the Acanthocephala, Gastrotricha, and the Nematomorpha. One of their peculiar features is certainly secondary, namely the absence of cilia. There are in some nematodes cilium-hke processes to the internal border of the endoderm cells ; in one case active movement has been reported. The excretory canals, when the absence of flame cells is NEMATODA 245 taken into account, are seen to resemble those of the Platyhelminthes. Nearly all the other characters may be called primitive. The sim- plicity of organization, the absence of segmentation at all stages and a vascular system, the diffuse nature of the nervous system and the structure of the muscle cells are all signs of a lowly origin. But it is still maintained by some that these features are not primitive but degenerate and that the origin of the phylum is to be sought in the arthropods, probably in the parasitic forms of that group (the degenerate arachnids called linguatulids). If this view is taken it must be supposed that the parasitic nematodes are the most primitive members of the phylum and that some of their descendants became less and less parasitic, until entirely free-living forms came into existence. This would be an extraordinary reversal of evolution for assuming which at present there are no grounds. The view taken in this book is that the free-living nematodes are ancestral to the parasitic forms and that there is no real connection between the arthropods and the nematodes. Not only do the nematodes present no indications of segments or appendages at any point of the life history but also the cuticle is of an entirely different chemical composition in the two phyla, and the loss of cilia most likely a phylogenetically recent phenomenon in the nematodes as in the parasitic platyhelminthes. The anatomy of the nematodes is best known from the study of Ascarts which is one of the largest members of the group and the only one adapted for dissection in class. Full accounts of this form are given elsewhere, but the following points must be emphasized. In Ascarts (Fig. 173) there appears to be a wide space between the muscle layer and the endoderm cells, with no epithelial boundary, walls, but on closer examination it is seen to be occupied by a very small number of greatly vacuolated cells, and what appears to be a continuous cavity is really the confluent vacuoles of adjacent cells, and so the term "intracellular" may be applied to it. This arrange- ment has not been verified in many other nematodes but connective tissue cells can usually be demonstrated in the space. They may be phagocytic; the enormous branched cells of Ascarts (Fig. i75)> lying on the lateral lines, take up in their tiny corpuscle-like divisions such substances as carmine and indigo which are injected into the body. A striking feature of the histology of Ascarts is the presence of greatly enlarged cells. Not only do the body cavity cells show this, but in the excretory system the greater part of the canal is contained in the body of one cell which divides into two limbs each running the whole length of the body on opposite sides. As a simple type of nematode the genus Rhabditis (Fig. 174) will 246 THE INVERTEBRATA be described, as it is seen alive as a transparent object under the microscope. Most species are free-living. They are obtained by allowing small pieces of meat to decay in moist earth. The larvae vs^hich exist in an "encysted" condition in the soil are attracted by the products of decay, and in a few days become sexually mature. m,co. g.c.n. oes. lat.l. cx.c. cut. m.n ^v.n. Fig. 173. Diagrammatic transverse section through Ascaris in the region of the oesophagus, showing the single large cell occupying the space between the body wall and the gut. Original, ait. cuticle ; d.n., v.n. dorsal and ventral nerves ; g.c.n. nucleus of giant cell, cytoplasm dotted, vacuoles {vac.) shown as clear spaces; ex.c. excretory canal; hyp. hypodermis; lat.l. lateral line; m.co. contractile part of muscle cells; in.t. tails of the muscle cells running toward the nerves in the median lines ; oes. oesophagus with three gland cells gl.c. and radiating muscles ni.r. which increase the lumen of the oeso- phagus and cause suction. The number of muscle cells in each quadrant is much greater than in the drawing. Great numbers of adults and young can then be scraped off the surface of the meat in the liquefied matter formed by bacterial decomposition. It will be seen that the animal progresses by alternate contractions of the muscles on each side of the animal, which bend the animal into S-shaped curves and enable it to wriggle slowly through thick hquid or on soil. The cuticle which covers the body is thin, tenacious but elastic. It enables the animal to keep an almost constant round cross- cav. Fig. 174. Rhabditis. Altered from Maupas. A, Mature female. B, Mature male. C, Ventral view of hind end of male, slightly turned to one side so that the vas deferens is seen only to the right of the alimentary canal. D, Side view of hind end of male to show the relations of the cloaca. E, Encysted larva enclosed in the stretched skin {cut.) of the last moult, an. anus ; h.cav. buccal cavity; c.b. copulatory bursa; c.sp. copulatory spicule; cl. cloaca; ex.a. excretory aperture; gl.c. gland cells ;/.^. fore gut; ni.g. mid gut; h.g. hind gut; n.c. nerve collar; ov. ovary; o. egg ready to be fertilized; o. eggs, one just fertilized, the other in the two-cell stage; p.v. pharynx with its valves ; rec.sem. receptaculum seminis ; t. testis ; ut. uterus ; va. vagina ; vd. vas deferens. 248 THE INVERTEBRATA section and length ; in the presence of such a cuticle and the absence of circular muscles the peristaltic movements of a worm like Lumbricus are impossible. A cross-section through Rhahditis shows a similar structure to Ascaris, though the muscle cells are much less numerous (only two to each quadrant) : each cell contains a number of contractile fibrils arranged in a different way to those in the Ascaris cell. The body cavity has not been investigated; that of Ascaris has therefore been described above. The alimentary canal consists first of all of an ectodermal fore gut lined by cuticle in which the following parts can be distinguished: (i) a mouth, surrounded by papillae, opening into a narrow buccal cavity^ with parallel sides, (2) an oesophagus, with muscular walls and a small number of unicellular glands, forming two swellings, the oesophageal bulbs. The posterior of these (the so-called pharynx) exhibits rhythmical pumping movements, caused by the contraction of the radial muscles which enlarge the cavity of the bulb and open the valve formed by the thickened cuticle. In this way the surrounding fluid is drawn into the oesophagus : no solid particles much larger than bacteria can be admitted through the narrow lumen. When the muscles relax and the cavity disappears the fluid is driven on into the midgut. This is composed of a single layer of cells, which internally are naked but externally have a fine cuticle. These are entirely absorptive in function, gland cells being absent. There are no muscles, but the gut contents are circulated by the locomotory movements of the animal. The hind gut which follows is lined with cuticle and opens at the ventrally situated anus. Near the anus is a sphincter muscle, but there are also dilator muscles running from the hind gut to the body wall, and during the periodic contraction of these the gut contents are evacuated. The alimentary canal of the nematodes as thus seen in action represents a type simplified because the animal usually lives on food which has been split up into easily assimilable substances — in this case by bacterial action, in the case of Ascaris by the ferments of the living host — and this is passed with great rapidity through the alimentary canal by the pumping action of the oesophagus. In addition there are easily seen in living Rhabditis the ventral aperture of the excretory canal, not far behind the mouth, and when the animal is compressed under the coverslip the coiled line of the excretory canal ; the only part of the nervous system which can be so seen is the ring round the oesophagus. The genital organs are of the type seen is Ascaris but simpler. In the female there are two tubular gonads bent once on themselves, dis- charging by a single genital aperture, situated about half-way between ^ In some free-living nematodes which are carnivorous (e.g. Mononchus) the buccal cavity is very wide and rotifers and other animals are taken into it. NEMATODA 249 )ex.c. —HI. -oe.t. 'h.g. Fig. 175- Fig. 176. Fig. 175. Dissection of an Ascarid to show position of the branched excretory cells. /./. lateral lines ; ex.c. excretory cells. After Nassonow. Fig. 176. Mermis. Showing the blindly ending oesophagus and the isolated mid gut, the cells of which are full of fat globules, oe.t. end of oesophagus ; f.b. mid gut; h.g. hind gut; sti. stylet. Original. 250 THE INVERTEBRATA the head and the tail. The ovary is a short syncytial tube, the nuclei becoming larger and larger and the centre of more definite and larger aggregations of cytoplasm and yolk nearer the uterus. Finally, there is a single ovum discharged at a time into the oviduct ; as soon as this happens another ripens in its place. To reach the uterus the egg has first to pass through a portion of the oviduct {receptaculum seminis) filled with the amoeboid spermatozoa of the male. Fertilization takes place, a shell is formed and at the same time maturation proceeds. The two uteri join to form the median vagina. In this the fertilized egg develops and the young larva is formed and may hatch within the vagina. The stages of segmentation are seen nowhere with such ease or clearness as in a small transparent nematode of this kind. The male, on the other hand, has only a single gonad. The apical testis is syncytial like the ovary. Nearing the vas deferens a zone may be seen of free spermatocytes and in the vas deferens itself can be seen large numbers of rounded spermatozoa. The genital duct opens into the gut to form a cloaca. This contains a dorsal pocket in which is secreted a chitinous apparatus consisting of two converging rods, the copulatory spicules ^ with a grooved connecting piece to hold the points together. The pocket has a special muscle which protrudes the spicules from the anus (cloacal aperture). To each side of this aperture is a lateral cuticular flange, supported by ribs, which meets its fellow at the root of the drawn-out tail. This acts as a sucker (copulatory bursa) ^ by which the male retains its position on the body of the female until the spicules are thrust through the female aperture and keep the female and male apertures both apposed and open. Then by the con- traction of the muscles of the cloaca the spermatozoa are expelled and passed into the vagina of the female. Here they become amoeboid and travel up the uteri so that they can meet the ova as the latter are discharged. Besides the normal condition in which males and females are pro- duced in equal numbers, many species of Rhabditis occur in which there is a remarkable disparity in numbers of the sexes. For a thou- sand females there may be only ten or twenty males, and they are lethargic in their sexual activities. The females, on the other hand, have developed a curious kind of hermaphroditism. When the gonad first becomes ripe a number of spermatozoa are produced. Afterwards the gonad produces nothing but eggs which are fertilized by the in- dividual's own spermatozoa, and after these are exhausted nothing but sterile eggs are laid. Experiment has proved that in these animals self-fertilization may occur for an immense number of generations without any deterioration of the species. In Rhabditis, as in the majority of nematodes, there are four moults. After the second moult the animal may remain within the loosely NEMATODA 251 fitting skin as a so-called "encysted" larva which possesses, however, the power of movement. The protection of the cast skin and possibly other factors enables this stage in the life history to resist desiccation and to remain in a state of dormant metabolism until some odour of decaying substances attracts the larvae and the opportunity of rapid reproduction is given for a brief period. This third larval period is characteristically the period of wandering in many nematodes, and this is seen in a remarkable manner in the classical life history of Ancylostoma (Fig. 177). These animals hve attached in the adult stage to the mucous membrane of the human small intestine, sometimes in such numbers as to present an aspect comparable to the pile of a carpet. They feed on the intestinal tissues and only accidentally rupture the blood vessels, causing anaemia in the host. The females are fertilized in situ and eggs are laid, which begin to segment before they pass out into the faeces. The rest of the life history may be shown as follows : (i) First larval form (rhabditoid) with a buccal cavity like Rhabditis. This lives in the soil for three days before the first moult, which produces the (2) Second larval form which moults after two days, the skin re- maining as a cyst round this strongyloid larva (3). In this stage the animal becomes negatively geotropic and thigmotropic, ascending through the soil and being specially attracted to the moist skin of human beings. This they penetrate by way of the hair follicles, though occasionally the larva enters the gut by the mouth. In the former event, the minute larva is able to make its way through the skin to lymph spaces and to blood vessels, eventually being swept into the circulation by the vena cavae to the right auricle, thence to the right ventricle and then to the lung. In the pulmonary capillaries this career is ended and the larvae make their way into the alveolar cavities of the lung. They then travel by the bronchi and the trachea to the oesophagus and so to the intestine. Here the animal is freed from the second skin, producing the larva without buccal capsule. The third moult produces the last larval stage towards the fifth to seventh day and this is termed the larva with provisional buccal capsule (4). Finally, about the fifteenth day the fourth moult produces the worm with the definitive buccal capsule (5), and in three to four weeks from hatching the parasite has become sexually mature and is attached to the epithelium of the intestine. - This most important human parasite shows in its earliest stages the structure and the free-living habit of the primitive form Rhabditis, and it is noteworthy that there are many species of the latter genus which have already become parasites. It may, however, be supposed that a less specialized life history is 252 THE INVERTEBRATA Fig. 177. Nematodes parasitic in man. A,B and C,Ancylostoma. After Looss. A, Adult worm attached to epithelium of small intestine of the host, with some of the tissue of the latter sucked into the buccal cavity of the worm. d.g. dorsal gland; la. lancet; oe. oesophagus; v.t. ventral tooth; tis. host tissue, lacerated by the lancets and partly digested ; vil. villi of small intestine. B, Larvae pene- trating the skin of mammal, x. through the horizontal fissures of the epi- dermis ; y. along the hair follicles ; z. larvae which have arrived in the lymph vessels of the subdermis ; ep. epidermis. C, Copulatory bursa of adult male, spread out to show the arrangement of the rays. D. dorsal; V. ventral; sp. copulatory spicule. D, Filaria bancroftt, longitudinal vertical section through a mosquito (Stegomyta) to show wandering of the larvae. After Bahr. a, larvae just swallowed and now in the mid gut (mg.) ; some migrating through the gut wall ; b, larvae developing in the thoracic muscles (th.m.) ; c, larvae which have finished development (8-15 days) migrating in the haemocoele of the head ; d, larvae in the blood space of the labium, which they leave by rupturing the body wall when the mosquito bites; cr. crop; ph. pharynx; pr. proboscis ; sg. salivary glands. NEMATODA 253 that of the species of Oxyuris in which the tgg is swallowed by the host and the remaining stages of development take place in the gut. It is said that several successive generations of the parasite may occur within the same host. On the other hand, the wandering habit of nematodes is a fundamental character and even forms in the first stage of parasitism (facultative) may penetrate host tissues. The life histories of the principal nematode parasites of man and domestic animals are summarized on pp. 254-5. They are arranged in a definite order passing from the simplest type in Haemonchiis to the most specialized life histories in Filaria. Two other classes of nematode parasites merit particular attention. They are, respectively, parasites of plants and insects. Plant parasites. Nematodes are particularly fitted for a parasitic life in plants by reason of their form and activity and their capacity (at the end of the second larval stage) for resisting desiccation and other unfavourable conditions. They are small enough, as larvae, to obtain entrance through the stomata of leaves, and sometimes possess dart-like projections of the buccal lining which enable them to penetrate the cell walls of plants. They feed on cell sap and by their interference with the life of the host plant cause the for- mation of galls, wilting and withering of the leaves, and stunting of the plant. Tylenchus tritici passes through a single generation in the course of the year, and infects wheat. The animal becomes adult when the grain is ripening and a pair, inhabiting a single flower, produce several hundred larvae. Instead of the grain a brown gall is produced, and in this the larvae (after moulting twice) may survive for at least twenty years. If the grain falls to the ground the larvae may remain there over the winter or may escape into the soil. When the corn begins to grow in the spring they enter the tissues of the plant and make their way up the stem to the flower, where they speedily mature. The great interest of this life history lies in the easy adaptation of the parasitic life history to the annual cycle of the wheat plant and the extreme capacity for survival in a dormant and desiccated condition until the right plant host becomes available. Tylenchus devastatrix, on the other hand, may pass through several generations in the year and attacks indiscriminately clover, narcissi bulbs and onions, and many other useful plants. Heterodera (Fig. 178 D) is a parasite of the roots of tomatoes, cucumbers and beets, and is remarkable because the female attaches herself in larval life to a rootlet from which she sucks a continuous flow of sap. She is fertilized by wandering males and grows enormously, becoming lemon-shaped. 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  • 3 o a; g' 4-> 3 CJ 3 <3 ^ 256 THE INVERTEBRATA Insect parasites. Four of these may be mentioned, though other life histories are also of great interest. In Mermis (Fig. 176) a curious reversal of the typical nematode life cycle occurs. The sexual forms are all free-living either in the soil or fresh water. On summer days after showers the sexual forms of Mermis nigrescens exhibit a curious tropism, leaving their haunts two or three feet in the ground and crawling up the stems of plants, but disappearing when the sun grows warm. The eggs are laid in the ground and when the larvae hatch they pierce the skin of insect larvae Fig. 178. Insect and plant parasites. A, Atractonema. Female showing the beginning of the prolapsus of the uterus, which has proceeded in Spherularta, B and C, until it is far larger than the rest of the worm. In C, the body is a minute appendage (bd.) of the prolapsed uterus (ut.pr.) not much longer than one of the greatly enlarged cells of the latter. D, Heterodera ; i and 2, larvae attached to a rootlet with their heads imbedded in its tissues ; 3, the full-grown female (on a smaller scale), removed from the plant. The alimentary canal is shown in black to emphasize that its growth c auses the increase in size of the parasite. 0. ovary; Mi. uterus. A-C, after Leuckart; D, after Strubell. and wander into the body cavity where they nourish themselves by absorbing fluid food through the cuticle. The mid gut has become a solid body, having no connection with the mouth and anus, and in it fat is stored up which serves as raw material for the production of eggs. When the animals become sexually mature they escape into the soil. In Tylenchus dispar (a form which is thus placed in the same genus as the well-known plant parasites) the adult female and innumerable larvae are found in the body cavity of the bark beetle, Ips, during the winter. Allantonema has similar relations to another bark beetle, Hylobius. The female is enormously developed, the uterus and other NEMATODA AND NEMATOMORPHA 257 female organs occupy the whole of the body, the gut having entirely disappeared. In the spring the larvae (having undergone two moults) bore through the walls of the end gut and undergo further develop- ment in the **frass" (faeces of the beetle). The male develops pre- cociously and fertilizes the female which, when it becomes mature, is still of normal proportions. After fertilization the females (only) infect the beetle larvae which by this time have appeared. Entrance is obtained by means of a "dart" exactly like the similar organ in the plant parasites. In the body cavity the female Allantonema grows rapidly, and when metamorphosis occurs and the mature bark beetle seeks another tree to form a new colony, it is full of larvae. Spherularia (Fig. 178 B, C) is a parasite of the humble bee. In the summer the moss and soil near the bee's nest is inhabited by the sexually mature worms, and after fertilization has taken place the female wanders into the body cavity of the insect, as in the preceding life histories. Though the number of cells in the somatic tissues of the bee is said not to increase in number there is an enormous growth in size of the vagina which becomes prolapsed and forms eventually an organ many times the size of the rest of the body, which remains attached for some time but eventually disappears. The parasitized humble bees, after passing the winter in their nests, tend to emerge early. In the spring very often inactive bees may be caught which prove, on dissection, to contain one or more of these enormous sausage-shaped bodies, each of them full of eggs and larvae, which escape through the gut wall and become free-living. Atractonema (Fig. 178 A), a parasite of the Cecidomyidae (p. 509), has a similar life history. PHYLUM NEMATOMORPHA As in Nematoda but lateral line and "excretory" canal absent, nervous system consisting of a dorsal "brain" and a single ventral cord, genital ducts in both sexes opening into the hind gut to form a cloaca, development very characteristic — gastrulation by invagination and a larva with peculiar boring organ which infects insects. In addition it should be mentioned that the alimentary canal is always more or less degenerate and the body cavity may either be occupied by parenchymatous tissue or by reduction of this becomes more or less empty of cells. An example of this group is Gordius robustus with the following life history. The adults are found in brooks from October till May when they copulate. The sperm is not directly introduced into the cloaca, but placed in masses on the body near it. The eggs are laid in the water and the larvae soon hatch. By using the boring organ which they possess they find their way into the body cavity of crickets which 258 THE INVERTEBRATA live near the water. There they remain and grow until the autumn when they leave the host and enter the water again as mature animals. Other forms have similar life histories. Parachordodes first infects chironomid larvae and then these are eaten by the second host, the beetle Calathus in which they grow to maturity. OV.S. ,0V. -in. cu. n.c. al.c. Fig. 179. Transverse section through Parachordodes. al.c. alimentary canal; cu. cuticle; m. muscular layer; n.c. nerve cord; ov. ovary; ov.s. ovarian sinus; par. parenchyma; pa. dorsal sinus. Fig. 180. Larvae of Gordius in the leg of an insect. PHYLUM ACANTHOCEPHALA As in Nematoda, but possessing an eversible proboscis provided with hooks for attachment and glandular organs (the lemnisci)^ cuticle delicate, hypodermis containing a peculiar lacunar system, a layer of AC A NTH O CEP HAL A 259 circular muscles as well as longitudinal, nervous system consisting of a brain and two lateral cords, excretory organs, which when they occur are nephridia with modified flame cells ; body cavity without parenchyma but traversed by a tubular organ, the ligament, containing gonads, eggs developing inside the body until the provisional hooks of cem.gl. Fig. 181. Neoechtnorhynchus. cent. gl. cement gland; g.nu. giant nucleus; /. lemnisci; pr. proboscis; pr.s. proboscis sheath; retr.pr. retractor muscle of proboscis ; t. testis ; v.d. vas deferens, the pharynx are formed, larvae developing further when laid in water and eaten by a crustacean, becoming mature when eaten by a verte- brate after which the animals attach themselves to the wall of the intestine by their proboscis. An example of these is Echinorhynchus proteus, the adult of which lives in ducks and the larvae in Gammarus. CHAPTER IX THE PHYLUM ANNELIDA Segmented worms in which the perivisceral cavity is coelomic ; with a single preoral segment (prostomium) ; with a muscular body wall in which externally the elongated muscle cells are arranged with their longitudinal axes across the width of the worm (circular layer) while internally their axes are parallel to the length of the worm (longitudinal layer) ; with a central nervous system consisting of a pair of preoral ganglia connected by commissures with a pair of ventral cords which usually expand in each segment to form a pair of ganglia from which run nerves to all parts of the seg- ment; with nephridia and coelomo- ducts; and the larva, if present, of the trochosphere type. While the above definition is the only one that can be applied to all the annelids, typical representatives of the phylum can also be described as possessing a definite cuticle and bristles or r/?«e^ae composed of chitin, arranged segmentally, imbedded in and secreted by pits of the ectoderm (Fig. 182). The cuticle is thin and not composed of chitin, thus differing from that of the Arthropoda. r-u . f r a • ^ , 111 Fig. 182. Chaeta 01 Lumbricus in Four classes compose the phylum. ^^^^ ^^j^ ^^^^^^^ f^^^ Stephen- Of these the largest and most typical son. cu. cuticle ; ect. ectoderm ; ch. is that of the Chaetopoda, which are chaeta; cm. circular muscles ; /)r.m. well segmented, have a spacious peri- protractor and n.m. retractor mus- 9 , 11 cles : per. peritoneum : joL. follicle Visceral coelom and a ways possess ^^^y^,^ formative cdl of chaeta chaetae. All these characters are (^^^^ nucleus), primitive. The Archiannelida is a small group characterized by small size, ciliation of skin, loss of external segmentation and often of chaetae. Several members of the group, however, like Saccocirriis, retain chaetae. It is almost certain that the archiannelids are derived from the chaetopods by a process of simplification. The Leeches or Hirudinea are adapted to a specialized mode of life — ectoparasitism — and their whole organization is affected ANNELIDA 261 by it. They retain the segmentation characteristic of the phylum in most of their organs but the coelom is usually much restricted and broken up into a system of small spaces and the chaetae are lost. In one primitive form, Acanthobdella, there are chaetae and a spacious perivisceral coelom in the anterior segments. In all leeches the anterior and posterior suckers and a hermaphrodite reproductive system, closely paralleled in a subdivision of the Chaetopoda, the Oligochaeta, show the speciahzation of the group. The Echiuroidea and Sipunculoidea are two groups of burrowing marine worms in which segmentation has been almost entirely suppressed in the adult but is sometimes shown in the larvae by mesoblastic somites or ganglion rudiments. Chaetae are lost except in a few forms, but a large perivisceral coelom is preserved. Class CHAETOPODA Well-segmented Annelida, with chaetae and a spacious perivisceral coelom, usually divided by intersegmental septa. In a typical chaetopod there is a distinct preoral region or pro- stomium and a postoral body composed of many segments. Each segment owes its distinctness to the development in the larva of a pair of mesoblastic somites which join round the gut, the cavities which develop in them becoming the perivisceral cavity of the adult segment. At the same time the larval ectoderm (epiblast) develops segmentally repeated organs : the ganglia^ swellings in the continuous ventral nerve cords, the nephridia or excretory organs and the chaetae. In the Polychaeta, one of two orders into which the Chaetopoda are divided, the chaetae are borne in groups upon processes known as parapodia, whose projection from the body wall is due to the develop-, ment of special muscles for moving the chaetae. In the other order, the Oligochaeta, there are no parapodia. The chief feature of the nervous organization is that the musculature of all parts of the body is co-ordinated by metamerically repeated intra- and intersegmental re- flexes (Fig. 183). In each segment there is, for example, a correlation of the circular and longitudinal muscles by the segmental nerves which acts so that contraction of one brings about automatically relaxation of the other. Then there are nervous connections between adjacent segments which act so that excitation of a muscle layer in one segment leads to excitation of the same layer in the other segment. By the working together of the inter- and intra-segmental reflexes the normal peristaltic movement of Lumbricus and other chaetopods is brought about. There is also a system of giant fibres, three in number, running along the whole length of the ventral nerve cord. These are responsible for the reactions which require immediate co-ordination of the whole 262 THE INVERTEBRATA body in response to excitation of the higher centres, the supra- and subpharyngeal ganglia. The rapid contraction of the whole of the longitudinal musculature in response to a noxious stimulus is an example of this kind of reaction. On p. 200 it was shown that the primary function of the primitive central nervous system is that of a sensory relay. In the annelids there is added the second great func- tion, that of inhibition. A Nereis, which has had the suprapharyngeal cm. Fig. 183. A, Longitudinal and B, transverse sections of Lumbricus to show the musculature and its innervation. In A, segment 2 shows neurones con- stituting an intrasegmental reflex arc, segments 3 and 4 show those which make up an intersegmental arc. B shows the distribution in the body wall of two segmental nerves and their branches, al.c. alimentary canal; cm. circular muscles ; g.f. giant fibre ; l.m. longitudinal muscles. ganglia removed, moves about ceaselessly, showing that a function of the ganglia in the normal animal is the inhibition of movement. If the supra- and subpharyngeal ganglia are both removed then the animal is permanently quiescent, a condition like that of a polyclad turbellarian when the cerebral ganglia are removed. The coelom is bounded by an epithelial layer, ihQ peritoneum^ which gives rise to i\\& gonads ^ which in polychaets are usually developed in most of the segments, to t)\& yellow cells ^ which play a part in the work of nitrogenous excretion, and to the coelomoducts by which the eggs ANNELIDA 263 and sperm pass from the coelom to the exterior. In most of the poly- chaets the eggs are fertiHzed externally, forming a trochosphere larva, the method of reproduction thus conforming to that of other marine groups. In the terrestrial and freshwater oligochaets (as in leeches) fertilization is internal and the young hatch in a form resembling the parent. There is no doubt that the former mode of development is more primitive. The nephridia are essentially tubes developed from the ectoderm which push their way inwards so that they project into the body cavity. In some polychaets they end blindly — this is the primitive condition. In the majority of chaetopods they have acquired an opening (nephrostome) into the body cavity itself. In some cases there is a partial fusion with a mesodermal element, the coelomoduct, so that a compound tube consisting mainly of ectoderm but partly of mesoderm exists {nephromixium). Nephromixia may take on the functions of coelomoducts where these do not exist independently. All types of tubes are termed here segmental organs. The head and accompanying sense organs may be well developed, for instance, in some of the pelagic Polychaeta where the eyes are re- markably complex. In such cases the brain (prostomial ganglia) may attain a structure almost as complicated as in the higher arthropods. The head processes (tentacles, palps) vary greatly. While they may be very complicated in the Polychaeta, they are frequently absent in burrowing members of that group and invariably so in the Oligo- chaeta. The blood system also varies greatly. In small forms it is absent altogether. Typically it consists of a dorsal vessel in which the blood moves forward, and a ventral vessel in which it moves backward and from which the skin is supplied with venous blood. The whole of the dorsal vessel (Fig. 201) is usually contractile : there may also be vertical segmental contractile vessels which are usually called ''hearts". In some forms, for example Pomatoceros (Fig. 186 C), there are no separate dorsal and ventral vessels but a sinus round the gut : the peri- stalsis of the latter brings about the movements of the blood. While the whole of the skin is sometimes richly supplied with blood vessels and usually performs an important part in the aeration of the blood there are often branched segmented processes which may rightly be called gills (Arenicola (Fig. 189)): the alimentary canal is probably a respiratory organ too. While haemoglobin is often present in the blood, usually in solution, a related pigment, chlorocruorin, which is green, occurs in many tubicolous polychaets. The variable state of the mechanism of respiration is shown by the fact that one species of a genus (the polychaet. Poly cirrus) may possess haemoglobin while another has no respiratory pigment. 264 THE INVERTEBRATA The Chaetopoda are, in this work, divided into the following orders: (i) Polychaeta, (ii) Oligochaeta. To the latter, however, the Hirudinea are very closely related. Order POLYCHAETA Marine Chaetopoda with numerous chaetae arising from special prominences of the body wall called parapodia ; usually with a distinct head which bears a number of appendages; nearly always dioecious, with gonads extending throughout the body and external fertilization ; with a free-swimming larva, the trochosphere. The structure of the Polychaeta is very variable and dependent on the habit of life, both externally (especially the head appendages and parapodia) and internally (especially the segmental organs). The variation in methods of reproduction is also very characteristic. For these reasons an account will first be given of some of the very large number of families into which the Polychaeta are divided, in which a rough oecological grouping is adopted. A summary of the variation in segmental organs and reproductive habits follows at the end. '^Eunicidae. Eunice^ Leodice (the Palolo worm). Nereidae. Nereis. Syllidae. Syllis, Myri- anida. Phyllodocidae. Eulalia, Aster ope. Polynoidae. Aphrodite, . Lepidonotus, Panthalis. Chaetopteridae. Chaeto- pterus. Terebellidae. Terebella, Amphitrite. Serpulidae. Pomatoceros, Filigrana. Sabellidae. Sabella, Spiro- gr aphis. Arenicolidae. Arenicola without jaws. Glyceridae. Glycera with jaws. The errant Polychaeta with unmodified head and armed eversible pharynx (proboscis); fitted for an active life but often living in tubes ; very often greatly modified in structure and physiology at the sexual season. The true tubicolous Polychaeta, much modified for the collection of micro- scopic food ; anterior part of gut not eversible and jaws absent ; inhabiting tubes which they rarely or never leave. The burrowing Polychaeta with re- duced head ; with proboscis. The errant Polychaeta The external structure is known to the elementary student through the type Nereis (Fig. 184). The prostomium bears two kinds of POLYCHAETA 265 filiform, tactile appendages, the tentacles which are dorsal and the palps which are ventral ; there are also one or two pairs of eyes upon it. The anterior part of the gat (pharynx) is eversible and serves for grasping food ; its lining may be chitinized in places to form the jaws and paragnaths oi Nereis or teeth as in Syllis. These are not necessarily ^^^^•^•->^ . pr. ten. ac^ r^ieur. Fig. 184. Nereis. A, Dorsal view of head and ifirst trunk segments with everted pharynx. B, Side view of same with pharynx retracted. C, Para- podium of unmodified type. D, Parapodium of Heteronereis. E, Example of unmodified compound chaeta. F, Oar-shaped compound chaeta of Hetero- nereis. The peristomium is stippled, pr. prostomium; ten. tentacle;^, palp; ten.c. peristomial cirri ; d.c. dorsal cirrus ; v.c. ventral cirrus ; not. notopodium ; neur. neuropodium ; /./. foliaceous outgrowths of parapodia ; ac. aciculum ; ch. chaetae ; ch. oar-shaped chaetae ; j. jaws ; pg. paragnaths. the sign of a carnivorous habit but may be used for cutting up pieces of seaweed or boring in sponges. , The ordinary trunk segment has a double parapodium consisting of a dorsal notopodium and a ventral neuropodium ^ usually with rather different types of chaetae. A dorsal cirrus and a ventral cirrus are nearly always present ; they are filiform structures but may be modi- fied to form pectinate gills [Eunice) or plate-like elytra (Polynoidae). 266 THE INVERTEBRATA From the conical noto- and neuropodia spring a bundle of chaetae ; the chaetal sacs project into the coelom and each bundle is supported by an enlarged and wholly internal chaeta — the aciculum, which also forms the point of origin of the parapodial muscles. The chaeta are of two kinds, simple and compound. The segment (or segments) just behind the mouth, forming the peristomium, is, however, much modified. There are no notopodia or neuropodia (except in occasional species, which retain chaeta-bearing Fig. 185. Errant Polychaeta. Peristomial segments stippled to show extent of cephalization. Anterior end. A, Syllis, single peristomial segment; pharynx retracted in sheath, ap. aperture of pharynx sheath cavity ; M. mouth ; p. palp ; ph. pharynx ; ph.sh. cavity of pharynx sheath ; pro. proventriculus ; t. tooth ; ten. tentacle. B, Eulalia, three peristomial segments and five pairs of ten- tacular cirri, pharynx protruded, covered with papillae. B , Parapodium with leaf-like dorsal and ventral cirri, notopodium only represented by dorsal cirrus neuropodium with compound chaetae. C, Asterope, head with five tentacles and three pairs of tentacular cirri (ten.c.) ; conditions in the head region largely governed by the presence of the enormous eyes. Pharynx protruded. processes as a primitive feature). But the cirri remain as the peri- stomial cirri in pairs consisting of a dorsal and ventral member. In Nereis there are two pairs of peristomial cirri on each side, indicating the fusion of two segments to form the peristomium. In some families (Syllidae) (Fig. 185 A) this is constituted by a single segment, but usually two or more have been pressed forward towards the mouth and modified. This is the first indication of the process of cephali- zation carried much further in the arthropods and vertebrates. The worms in this group used to be definitely classed as the POLYCHAETA 267 Errantia or free-swimming forms, but a great number of them (e.g. the Nereids) do live in tubes which, however, they can leave and reconstruct anew. The most beautiful example of tube building in the Polychaeta is furnished by Panthalis^ a polynoid. In this the chaetal ch.m.x ,m.d.v. nep. n.c. C Fig. 186. Transverse sections through different types of Polychaeta. A, Aphrodite. After Fordham. B, Arenicola, middle region. After Ashworth. C, Pomatoceros, thorax. Original, al. alimentary canal ; cm. coecum of mid gut ; ch.m. matted notopodial chaetae ; cil. ciliated groove ; d.v. and v.v. dorsal and ventral blood vessels; el. elytron; m.c, m.d.v., m.l. circular, dorsoventral and longitudinal muscles; ohl.m. oblique muscles; nep. nephridium; neur. neuro- podium ; not. notopodium ; n.c. nerve cord ; sin. sinus ; th.m. thoracic mem- brane; v.cir. ventral cirrus. pits of the notopodium produce not stiff bristles but plastic threads which are woven by the comb-like ventral chaetae and the shuttle-Hke action of the anterior parapodia into a continuous fabric which forms the lining of the mud-covered tube. Aphrodite^ the sea mouse (Fig. 186 A), is a short, broad form which burrows in mud, and though it 268 THE INVERTEBRATA does not form a separate tube it covers its back with a blanket made from interwoven chaetal threads similarly formed from the noto- podium. Between this blanket and the back is a space into which water is drawn by a pumping action of the dorsal body wall, being filtered through the matted chaetae. In this there are special plate- like modifications of the dorsal cirri — the elytra — round which circulates the water from which they possibly obtain dissolved oxygen. In other polynoids (e.g. Lepidonotus^ which lives under stones but does not burrow) the elytra can have no respiratory function but are probably protective, spreading over the whole or greater part of the back (sometimes bits of sand or shell are attached to special papillae). Not all the dorsal cirri are modified to form elytra: typical filiform cirri are placed on alternate segments. Aphrodite has remarkable segmental coeca of the alimentary canal in which takes place digestion of the fine food particles which pass a sieve at the junction with the intestine. The diagnostic features of Nereis and other genera mentioned in the classification are given below. Nereis (Fig. 184). Two tentacles, two palps; pharynx with two jaws and twelve groups of paragnaths; noto- and neuropodium each double; chaetae all compound; most species have a special sexual form (Heteronereis). Eunice. Five tentacles, two palps; pharyngeal armature well developed; a single peristomial segment; gills in many segments; chaetae simple and compound. Eulalia (Fig. 185 B). Five tentacles, no palps; pharynx very long with soft papillae only ; three peristomial segments ; dorsal and ventral cirri leaf-like; chaetae all compound. Asterope (Fig. 185 C). Similar to Eulalia but a pelagic polychaet with transparent body and enormous eyes of complicated structure. Syllis (Fig. 185 A). Three tentacles, two fused palps; pharynx enclosed in a pharynx sheath with a single conical tooth and a mus- cular proventriculus which functions as a pump; no notopodium. Autolytus (Fig. 194 B). Like Syllis but pharynx long, with a circle of teeth; no ventral cirrus. Myrianida has similar characters. The true tubicolous Polychaeta Here the prostomium has become much smaller and its appendages enormously modified and increased. The peristomium may be pro- duced into a collar which in some forms grows round the prostomium and encloses a funnel-like cavity at the bottom of which lies the mouth. The food consists of small animals or plants or organic debris and it is collected by ciliary mechanisms. In the terebellids (Fig. 187 A), serpulids (Fig. 188) and sabeHids, the appendages of the head, tenf.'x Fig. 187. Tubicolous Polychaeta. Terebellid (Loimia). A, Side view of young form taken from its tube. A', Side view of pelagic larva in its gelatinous case. After D. P. Wilson. Chaetopterus pergamentaceus . Original. B, Side view of worm in tube. Arrows show the direction of the water currents. B', Dorsal view of anterior part to show the ciliated grooves. Original. Arrows show the direction of the food currents, abd. abdomen ; col. peristomial collar; cup, organ for forming foodballs; e. eye; fan., mu. fan. muscles for moving fans; gl.sh. mucous glands; iVf. mouth; neur. neuropodia forming suckers for attachment of worm to tube ; not. notopodia ; not. 4, notopodia with enlarged chaetae in 4th chaetiferous segment; not. 10, food-collecting notopodia; ot. otocyst; tent, tentacle; thor. thorax; up.l. upper lip, the lower lip (l.l.) is a prominent structure to the right of the tentacles. 270 THE INVERTEBRATA which probably correspond to tentacles, are very numerous. Each tentacle has a ciliated groove running from the tip to the mouth and along this minute particles may be seen to travel. In the terebellids these tentacles are extensible and capable of independent movement when separated from the body. In the serpulids and sabellids, they are rather stiff branched structures, which can, however, curl up when withdrawn into the tube ; they sometimes bear eyes and some- times are wonderfully pigmented. Besides the food-collecting tentacles there are gills in the tere- bellids. These are branched processes, usually three pairs, situated just behind the head, full of circulating blood. In the serpuHds and sabellids, there are no special respiratory organs but the whole surface of the body serves for the exchange of gases. In the terebeUids the tubes are composed of a soft cementing substance mixed with mud or a parchment-like material to which adhere sand grains, sponge spicules, foraminifera or fish-bones. It is usually porous (so that change of water can take place through it) and the animal occasionally leaves its shelter; there are at least two openings to the exterior. The tube of the chaetopterids is parchment- like but in the serpulids there is a groundwork of mucin in which carbonate of lime is laid down. In the latter family there is only one opening from which the crown of tentacles emerges but never any more of the body. The tentacles are violently withdrawn in obedience to any such stimulus as touch or change of illumination. In all the types except Chaetopterus the body is divided into two regions, an anterior thorax and a posterior abdomen. The thorax is composed of segments in which the notopodium is a conical structure with capillary chaetae while the neuropodium is a vertical ridge in which are imbedded short-toothed chaetae called uncini^ which only just project from the body wall. It is suggested that the notopodium assists movement up and down the tube while the neuropodia are braced against the tube and maintain the worm in position. In the abdomen the arrangement of the parapodia is different, and in the serpulids and sabellids the uncini become dorsal and the simple chaetae ventral (introversion). In the serpulids (Fig. 188) the peristomium is similar to the other thoracic segments but it is produced into a collar which folds back over the ventral surface and sides and secretes successive hoop-shaped rings which are added to the tube. Other features are the thoracic membrane f^ lateral frill possibly respiratory, and the operculum, a much enlarged and stopper-like branch of a tentacle which exactly closes the mouth of the tube when the animal is retracted. The renewal of water round the body is of the utmost importance in respiration. It is brought about by undulatory movements of the POLYCHAETA 271 abdomen and sometimes by sharp rhythmic contractions and ex- pansions of the body which pump the body in and out of the tube. The great development of the dorsal bands of longitudinal muscle seen in a transverse section of a serpulid (Fig. 186 C) is characteristic of the tubicolous worm. Another typical modification seen in the serpulids and sabellids is the median ciliated groove, which starts from the anus, runs along the ventral surface of the abdomen, turning on to the dorsal surface when the thorax is reached. It serves to conduct the faeces to the mouth of the tube. Fig. 188, 'D[2igv2i.m oi Pomatocer OS triqueter \n Its tube. Original. The aperture of the tube is represented in black : the top and base of the tube are shown by vertical lines {tb.), the sides not represented so that the thorax can be seen within. The collar {col.) is shown by stippling, folded back over the top and sides of the tube ; and the thoracic membrane also by stippling. The collar is transparent showing the prostomium and the lip of the tube beneath. The fact that the tube is composed of successive rings is indicated in the neigh- bourhood of the aperture (ann.). ap.tb. aperture of tube; neur. neuropodia; not. notopodia; op. operculum ; />r. prostomium; fen. tentacle. Chaetopterus (Fig. 187 B) is probably the most modified of all tubicolous worms. It lives in a parchment-like tube which is U- shaped with at least two apertures. There is a peristomial collar as in other tubicolous worms, but the tentacles are a pair of rudimentary processes. A very complicated mechanism exists for obtaining food, which can be observed by taking a live Chaetopterus from its tube and replacing it within a glass tube of the same calibre in an aquarium. The worm fits very loosely in its tube and there is plenty of room for a current of water to sweep through from end to end. Such a current is maintained by the rhythmical oscillation of the fans (fused noto- podia) of the middle region. Food particles contained in the current are entangled in mucus secreted by the dorsal surface of the anterior region, and ciliated currents, working in grooves in the enlarged 272 THE INVERTEBRATA notopodia of the tenth chaetiferous segment, carry these strings of mucus to the cup-shaped organ where they accumulate to form a ball of food which is carried forward in a dorsal groove to the mouth. The burrowing Polychaeta Arenicola marina (Fig. 189) is the type of a burrowing polychaet and it has a rounded cross-section like an earthworm. In its division of the body into regions, the modification of the parapodia, and the internal anatomy it resembles the tubicolous worms. The prostomium is much reduced, however, without any appendages and there is an eversible pharynx, covered with minute papillae, which is the organ for locomotion through the sand as well as for feeding. In general form it thus resembles an earthworm : the chief obvious difference is the presence of gills and parapodia. It is divided into three regions : the anterior, consisting of the peristomium, an achaetous segment, and six segments which have a notopodium with capillary setae and a neuropodial ridge with chaetae resembling uncini (crotchets); the median, the segments of which have gills in addition; and the pos- terior, in which parapodia and chaetae are entirely lost. The body wall consists of the typical circular and longitudinal muscle layers as in Lumbricus, and by their alternating contraction and expansion the peristaltic movements which are characteristic of the earthworm and other burrowing forms are carried out. In Nereis and other surface-living forms progression takes place in two ways, (i) By alternate flexing of the two sides swimming movements are brought about. The longitudinal muscles, which are arranged in four bundles, are much more important than the circular and are capable of rapid contraction. (2) By successive movement of the parapodia crawling movements occur (as in a centipede), the special parapodial muscles coming into action. In tubicolous forms peristalsis occurs, but the longitudinal muscles are even more important than in Nereis for the violent movements of contraction which withdraw the animal into its tube. They form a bulky dorsal mass and resemble the columella muscle of the gasteropod in their action (Fig. 186 C). Arenicola is the most convenient polychaet type for dissection and therefore the following details of internal anatomy are given (Fig. 190). In several prominent features it differs from Lumbricus and also from Nereis or Eunice. The body cavity is spacious, it is not en- croached upon by the longitudinal musculature, and the vertical septa which primitively separate the body cavities of the segments have nearly all disappeared. Only the three anterior septa and an indefinite number of the most posterior are preserved. In the greater part of the body the coelom is thus uninterrupted. In its general development the alimentary canal resembles that of the earthworm. The muscular POLYCHAETA 273 pharynx, however, is not well developed, the oesophagus is a thin- walled tube with no such development as the gizzard of the earthworm "'^'^^ T /?>'•• not.~^ in.r.{ Fig. 189. Arenicola marina. Side view. After Ashworth. ac/i. ist achaetous segment; a.r. anterior region; m.r. median region; not. notopodium; neur. neuropodium; pr. prostomium; p.r. posterior region; per. peristomium; phar. pharynx. and it bears only a single pair of coeca. The intestine is the longest part of the gut, the seat of digestion and absorption, and it is invested by a layer of yellow cells. The blood system, which also contains -s.o. —nephr. yiot.m. .not.m. Fig. 190. General dissection of Arenicola marina. After Ashworth. aff.b.v., eff.b.v. afferent and efferent vessels of gills and body wall; an. auricle, d.v., l.v. dorsal and lateral blood vessels; nephr. nephrostome; not.m. noto- podial muscles; oe.p. oesophageal pouch; sep. septa; s.o. segmental organ; s.i.v. subintestinal blood vessel ; ven. ventricle ; v.n.c. ventral nerve cord ; v.v. ventral blood vessel. The direction of the flow of blood is indicated by arrows. POLYCHAETA 275 haemoglobin in solution in the plasma, differs slightly from that of Lumbricus : there is a single pair of large hearts, each divided into a ventricle and auricle which connect the important lateral intestinal vessels from which the branches supplying the gills are derived with the ventral vessel. The circulation for that region just behind the heart may be ex- pressed as follows : lateral vessels->auricle^ventricle-> ventral vessel -^afferent vessel to body wall and gill->efferent vessel to subintestinal vessel->intestinal plexus->dorsal vessel or lateral vessel. The dorsal vessel does not communicate directly with the heart. The segmental organs are, like the gills, only found in the middle region. They are prominent organs lying beneath the oblique muscles, remarkable for the large size of the nephrostome, the dark secretory bag-like portion, the cells of which contain insoluble excreta, and the small gonad which lies just behind it. In Arenicola as in Lum- bricus the gonads are restricted to a small number of segments, but the reproductive cells are shed into the body cavity at maturity and completely fill it. In Glycera the prostomium is narrow and conical, the tentacles being very small. It possesses a very large proboscis armed with four sharp teeth. The parapodia are reduced in size, and bear compound chaetae and in its internal structure too Glycera comes nearer to the errant worms than does Arenicola. The excretory and reproductive organs of the Polychaeta Now that a survey of the chief types of the Polychaeta has been made a brief description of the segmental organs found in the group will be given. These are tubes, repeated in successive segments, which serve to convey the excretory and generative products from the coelom to the exterior. They are primarily divided into nephridia^ derived from ectoderm, and coelomoducts , formed from mesoderm. The typical nephridium is a closed tube, whose bhnd end projecting into the coelom is fringed with solenocytes, cellular organs which have a very close resemblance to the flame cell of Platyhelminthes and Rotifera. Such "closed" nephridia (protonephridia) are found in the Phyllodocidae, Glyceridae and Alciopidae. But in the majority of the Polychaeta and all Oligochaeta there is another type of "open" tube, which usually serves for the escape of excreta, and this possesses a small funnel or nephrostome. It may, however, take over the function of the coelomoduct and carry sperm or eggs to the exterior. In this case the nephrostome becomes wider and the tube more glandular. The familiar example of the open tube is the nephridium of Lumbricus, which is purely excretory. In this type 276 THE INVERTEBRATA the tube consists of ectoderm : the funnel of the nephrostome in Lum- bricus (and probably of other forms) is derived from a single ecto- dermal cell. The coelomoduct is entirely formed from mesoderm and usually has a wide coelomic funnel easily distinguished from the typical nephrostome. The oviducts and the sperm ducts oi Lumbricus are coelomoducts. In a family of the Polychaeta called the Capitel- lidae there are coelomoducts in most segments of the body serving as gonoducts (Fig. 191 I, D). In the majority of Polychaeta they have co.d- nep. -nmx._ cor. ,nephr. co.d. Fig. 191. Segmental organs of Polychaeta. After Goodrich. I, Transverse section (right half) of body segment showing combinations of nephridia and coelomoducts. A, Hypothetical. B, Phyllodocidaeand Alciopidae. C, Neph- thyidae and Glyceridae. D, Capitellidae. E, Capitellidae. F, Nereidae. co.d. coelomoduct; cor. ciliary organ; nep. "closed" nephridium; nep.o. "open" nephridium; nmx. nephromixium. II, Segmental organ of Vanadis (Alciopidae). nephr. coelomic funnel; sol. solenocytes. however disappeared altogether and their function is otherwise performed. There is in addition a type of organ called a nephromixium which is formed by the union of a nephridium and coelomoduct. In the Alciopidae the separate components of the nephromixium are clearly seen (Fig. 191 II). Here the union is with the closed nephridium but in the Capitellidae and many other polychaet families there are open nephridia and these have often an intimate fusion with the coelomoducts. Thus in one form of the Capitellidae shown in Fig. POLYCHAETA 277 191 I, E the funnel of the coelomoduct has completely fused round the opening of the nephridium. In Nereis, Nephthys and Glycera the functional segmental organ is an open nephridium, but a rudiment of the coelomoduct, which does not open to the exterior, the so-called ciliary organ, occurs in each segment. In the majority of polychaets, however, the coelomo- duct has disappeared altogether. co'.d. nephr. nepi nu. ^~"^} — nep. Fig. 192. Fig. 192. Segmental organ of G/om/)/zoma (Hirudinea). After Oka. Showing mesodermal part with ciliated nephrostome and a single cell of the ectodermal part, with intracellular duct. nu. nucleus. Fig. 193. Development of Megascolides australis (Oligochaeta). At the posterior end the nephridia are single ; traced anteriorly they break up into a number of loops each of which becomes a separate micronephridium (nep.'). al. gut; sep. septa. Other letters as in Fig. 191 for both figures. Then again there may be a great difference between the nephridia in different parts of the same worm. In the serpulids, terebellids and other families there are one to three pairs of long segmental organs situated anteriorly. In most of the segments behind there are short funnels in the body wall which are open nephridia but serve for the escape of the eggs and sperm. There is thus a division of labour between the segmental organs in tubicolous worms: the anterior are specialized for excretion, the posterior are genital ducts. ■<^ Q> .V ^IC/I^ .00 S /v, •>-<, LIBRARY =* X V MASS CN> 278 THE INVERTEBRATA The closed nephridium appears to be the most primitive type of segmental organ and a survival of the time when the coelom had not yet developed. The open nephridium is far commoner in the Chaetopoda with their extensive coelomic cavities. The origin of the coelomoducts is doubtful. They may be thought to have arisen as genital ducts but now the nephridia often serve for the escape of the gametes. The gonads in the polychaets are usually patches of the peritoneal epithelium, repeated in most of the segments, proliferating until a great number of the germ cells have been detached into the body cavity which they almost entirely fill and where they undergo maturation (Fig. 199 A). When ripe they reach the exterior usually through the segmental organs, but occasionally the body wall ruptures and so opens a way of escape. Like so many other marine animals the polychaets thus liberate eggs and sperm freely into the sea, fertilization taking place externally. This habit is associated in many forms with the phenomenon of swarming in which a worm, usually crawling or burrowing on the sea bottom, when sexually mature rises to the surface and swims vigorously, eventually discharging its genital products and sinking to the bottom as suddenly as it rose. In most nereids this occurs irregu- larly through the summer months, but in at least two forms (Leodice viridis, the ** Palolo " of the reefs of the Southern Pacific, and Leodice fucata of the West Indian reefs) the phenomenon (Fig. 194 E) has acquired the strictest periodicity. As the day of the last quarter of the October-November moon dawns the Pacific Palolo breaks oflf the posterior half of its body, already protruding from the mouth of its burrow in the coral rock, and these fragments rise to the surface in such quantities that the water writhes with worms and is later milky with the eggs and sperm discharged. Immediately afterwards the remaining anterior end begins to regenerate the missing portion, but a whole year elapses before the gametes are again ripe — even two days before spawning occurs fertilization cannot be brought about arti- ficially. In the West Indian species the phenomenon is similar but takes place in the third quarter of the June-July moon. In the syllids the phenomena of swarming are vastly more varied. The whole animal may produce germ cells and swarm. Usually how- ever the gonads are confined to the posterior part of the body which is detached as a free-swimming unit ; this often develops a head but never jaws and pharynx. It can live for some time but not feed. In the majority of forms a single bud is produced, but in Autolytus (Fig. 194 B) and Myrianida 2i proliferating region is established at the end of the original body and from this a chain of sexual individuals is budded off, the oldest being situated most posteriorly. The whole chain may be found swimming at the surface, the original worm dragging after Fig. 194. Diagrams of reproduction in the Polychaeta. A-D, Syllidae. E, Eunicidae. A, Syllis with the posterior region forming a reproductive in- dividual. B, Autolytus with a chain of reproductive individuals budded off successively from pr.r. a proliferating region. C, Trypanosyllis gemmipara, longitudinal section through the end of a budding stock showing two kinds of reproductive individual, rs. a single individual which contains al. , the continuation of the alimentary canal {al.) of the stock, and rs. successive rows of individuals without alimentary canal formed from the proliferating cushion, pr.r. D, Syllis ramosa, showing branching of the asexual stock and budding of reproductive individuals, r.s., r.s. , from parapodia of the stock. E, Diagram of the swarming of the Palolo worm, Leodice. a, mature female protruding posterior end from its burrow; b, male — the sexual part of which has just become detached ; c, sexual fragments swimming up to the surface and d, discharging the eggs and sperm. In e they are emptied and sink to the bottom ; /, parent worm regenerating the sexual region. 28o THE INVERTEBRATA it the chain of sexual individuals which one by one detach and lead a short independent existence. In some species of Trypanosyllis (Fig. 194 C) the zone of proliferation is in the form of a cushion of tissue on the ventral surface of the last two segments and this produces not a linear series of buds but successive trans- verse rows, amounting to more than a hundred — the fully formed sexual indi- vidual possesses a head but no vestige of an alimentary canal. The extraordinary branching form, Syllis ramosa (Fig. 194 D), shows remarkable capacity for heteromorphic growth in the production of sterile side branches from the stock and reproductive buds. In the syllid there is usually no noto- podium during asexual life but during the maturation of the gonads the para- podium is reconstructed, a notopodium being formed from which spring bundles of long capillary swimming chaetae, while a corresponding development of new muscles takes place. Even greater is the change in the parapodia of the maturing nereids. The muscles of the asexual period break down and the fragments are digested by leucocytes Fig. 195. A Heteronereis. )^ho- before the new muscles are formed. The tograph of specimen stained parapodium of the sexual form, the in borax carmine and mounted Heteronereis /is produced into memhr^n- in Canada balsam Notice the - .,, J ^ . , r enormously developed eyes, ous frills and contains a new type ot j^^^ peristomial cirri, anterior oar-shaped chaeta (Fig. 184 D, F). The unmodified trunk region, pos- eyes become immensely larger and the terior modified region with animal itself very sensitive to light. The parapodia sloping backwards TT • J • r ^ ui 4-u and darker appearance owmg Heteronereis does in fact resemble those ^^ ^^^^^^^^ of gonads. members of the Phyllodocidae and Alciopidae which have become perma- nently pelagic. The increase in the surface of the parapodia may be useful in swimming and floating: it has without doubt some connection with the increased gas exchange associated with an active life. It is easy to see in the swarming habit an adaptation for securing fertilization of the greatest possible number of eggs. There are remarkable cases in the syllids (Odontosyllis) where the meeting of POLYCHAETA 281 the sexes is facilitated by the exchange of light signals, and in the nereids the discharge of sperm may only be brought about by the influence of a secretion from the swarming female. Discharge of the gametes is nearly always followed by the death of the sexual individual. The fertilized egg gives rise to an unsegmented larva, the trocho- sphere, which is described in the next section. Development of the Polychaeta The cleavage of the egg in the Polychaeta and the Archiannelida, the polyclad Turbellaria, the Nemertea and the Mollusca follows almost exactly the same plan. Division occurs rhythmically, affecting the whole or greater part of the blastomeres at the same time. The first two divisions are equal, producing four cells (Fig. 196, 2) lying in the same plane, which are called A, B,CyD; each cell in its further cleavage resembles the others and gives rise to one of the quadrants of the embryo. D tends to be larger than the others and becomes the dorsal surface of the embryo, while B is ventral, A and C lateral. The next divisions (third, fourth and fifth) are unequal and at right angles to the first two and result in three quartettes of micromeres being divided oflf successively from the macromeres as A, B, C and D are then termed. The region in which the micromeres lie is the upper or animal pole of the embryo, while the macromeres form the vegetative pole. The micromeres are not directly over the macromeres from which they are formed but in one quartette they are all displaced to the right, while in the next they will be displaced to the left of the embryonic radius and the next to the right again. The cleavage is therefore said to be of spiral type and successive cleavage planes are at right angles. At a later period it is replaced by cleavage in which there is no alternation of the kind described above, and the result is that the embryo becomes bilaterally symmetrical. The rest of the description is drawn from the Polychaeta but can be applied with slight modifications to the other groups. The cells of the first three quartettes give rise to the ectoderm of the larva and of the adult. The sixth division, however, results in the separation from the macromeres of a fourth quartette which is com- posed of cells differing notably in size and density from those of the first three. Of the fourth quartette d^ (Fig. 196, 4) alone produces the mesoderm, while the other three, «*, Z>* and c*, reinforce the macromeres to form the endoderm. The mesoderm is, however; only in course of differentiation during larval life and a larval mesoderm or mesenchyme is produced from which particularly the musculature of the trochosphere is fashioned. The mesenchyme is derived from the inward projections of cells of the second and third quartettes. 282 THE INVERTEBRATA Gastrulation (Fig. 196, 7). The amount of yolk in the macromeres determines the character of the cleavage within certain limits and the type of gastrulation. In forms like Polygordius with very little yolk the micromeres and macromeres are nearly the same size and gastru- lation takes place by invagination; in Aremcola, Nereis and nearly all Polychaeta and Mollusca the micromeres are much smaller than the macromeres, and as they divide to form the ectoderm they grow round the massive macromeres and an **epibolic" gastrula is formed. The cells of the fourth (and fifth) quartettes approach each other from the two sides. The mesoblast cell (d^) begins to withdraw from the surface into the blastocoele, and the blastopore, that is the un- covered surface of the macromeres, becomes much smaller and slit-like. Eventually as gastrulation is completed the lips of the blastopore join in the middle, the same cells meeting each other in every case, leaving an anterior opening which becomes the mouth and a posterior, which closes, but in the neighbourhood of which the anus of the trocho- sphere arises later. The blastopore therefore represents the ventral surface of the larva. At the same time the macromeres withdraw into the interior to form a second cavity, the archenteron, bringing with them the cells of the fourth and fifth quartettes («*, ^^, c^\ a^, c^, d^). The somatohlast (d^) breaks up into a large number of cells to form the ventral plate. The change from gastrula to trochosphere (Fig, 197) follows quickly and with little further cell division. The first quartette of micromeres have by this time been diflFerentiated (Fig. 196, 5) into (i) the apical rosette, consisting at first of four small cells and becoming the apical organ of the trochosphere ; (2) the cells of the so-called annelid cross which alternate with those of (i) and form the cerebral ganglia; (3) the prototroch, forming four groups of cells which constitute the preoral ciliated ring of the trochosphere; and (4) the intermediate girdle cells, forming most of the general ectoderm of the part in front of the prototroch, which is called the umbrella. The expansion of the subumbrellar ectoderm, i.e. that behind the prototroch, is due to the proliferation of a single cell in the second quartette of micromeres, d^ (the somatoblast (Fig. 196, 6)). It forms a plate which spreads from its originally dorsal position round the sides, the two wings uniting behind the mouth to form the ventral plate, becoming the ventral body wall. The descendants of this single cell thus make up nearly the whole of the subumbrellar ectoderm. Its sisters a^, b^, c^ give rise to the stomodaeum and are tucked in at the mouth at the close of gastrulation. This marks the completion of the alimentary canal. The young trochosphere now possesses a very thin outer epithelium, thickened in the region of the apical disc and the equatorial ring of cilia, the prototroch, and in the region of the ventral plate, which is POLYCHAETA 283 Fig. 196. Partly after Dawydoff. 1-3, Diagrams of radial and spiral cleavage. 4-7, Development of Nereis, i, Eight-cell stage in a radial type, e.g. Echino- derm. 2, Spiral cleavage, four-cell stage just before cleavage, leading to eight-cell stage, sp. spindle. 3, Macromeres stippled. 4, Diagram of seg- menting egg (Nereis), seen from the animal pole, showing the macromeres, the first three quartettes of micromeres and the mesoblast cell (^*) in the fourth quartette. 5, Later stage, also^rom animal pole, to show the rosette cells (a^^-d^^), the annelid cross (indicated by stippling), the four groups of prototroch cells (horizontal shading and cilia) and the intermediate girdle cells (a^^-d^^). 6, Vertical section through the same stage as 4, along the line XY. 7, Vertical section through a later stage to illustrate gastrulation : cells derived from d^ (cross-hatched) growing over the macromeres, the mesoblast cell withdrawing into the interior. 284 THE INVERTEBRATA the rudiment of a large part of the trunk of the aduh worm. It will form ventral nerve cord, chaetal sacs and the ventral and lateral ecto- derm of the trunk. The larval gut opens by a mouth in the equatorial region and consists of an ectodermal oesophagus (stomodaeum) open- ing into the endodermal stomach and an ectodermal hind gut opening to the exterior by an anus. The cavity between the ectoderm and the gut (blastocoele) is spacious and traversed by the pseudopodia-like processes of the mesenchyme cells, larval muscles and nerves, and also contains the two larval nephridia, each of which is composed of two hollow cells placed end to end, one of which contains a "flame'* of cilia. They are descended from the first quartette of micromeres and sink in from the surface. mesc ^-prt. DCS. an.v.^ Fig. 197. Trochosphere larva of £'M/)owaiw^. Side view. After Shearer, ap.o. apical organ; e. eye; prt. preoral ciliated ring; hk. "head kidney", larval nephridium ; otc. otocyst ; mes. mesodermal band ; an. anus ; an.v. anal vesicle ; bl.c. blastocoele ; mesc. mesenchyme ; oes. oesophagus ; st. stomach. The trochosphere drifts hither and thither in the sea, swimming feebly by the action of the ciha of the prototroch and sometimes also by secondary postoral rings of cilia (e.g. metatroch formed from cells of the third quartette). During this pelagic existence the rudiments of the adult worm continue their development — which is best traced in Polygordius — the apical organ develops into the prostomium of the adult with brain, tentacles and eyes, while the trunk rudiment formed by the proliferation of the ventral plate and the mesoblast cell grows backwards as an ever-lengthening cyHndrical process containing the end gut. In the ectoderm of this is developed ventrally the rudiment of the ventral nervous system, while to the sides of this and internally are the mesodermal strips (derived from the single cell d^), which POLYCHAETA 285 show at once metameric segmentation (Fig. 198), first as pairs of solid blocks, then with cavities, to form the somites. Each of these box-like mesodermal segments has then an inner wall which is applied to the gut (splanchnic mesoderm) and an outer (somatic mesoderm) lying under the ectoderm. The right and left rudiments meet in the middle lane and are only separated by the dorsal and ventral mesenteries which are formed by their apposed walls, while the anterior and posterior borders of each segment are septa. At the same time the adult nephridia develop from ectoderm rudiments and the blood vessels differentiate in the septa and mesenteries. A B Fig. 198. Development of PoZy^orJm^. After Woltereck. A, Trochosphere with rudiment of prostomium and trunk. B, Metamorphosing larva with the prostomium and trunk brought close together by the contraction of the longitudinal muscles and the umbrella of the trochosphere shrivelled and about to be discarded. Three segments only of the trunk are shown, brn. brain ; e. eye ; m.l. longitudinal musculature ; m.l. part of the same which by contraction brings the prostomium and trunk rudiments into contact; M. mouth ; nep. protonephridium with solenocytes ; pre. prostomium ; prt. pro- totroch ; mtr. metatroch ; oe.c. oesophageal commissure ; ten. tentacle. The advanced larva (Fig. 198 A) thus consists of two rudiments of the adult body, separated by the body of the larval trochosphere. They are joined by a pair of longitudinal muscles and of nerves, and in one species of Polygordius metamorphosis of the larva into the adult is brought about by the shrivelling up of the larval tissues and the drawing together and the union of the head and trunk assisted by the contraction of these muscles (Fig. 198 B). The larval mouth re- mains in the adult. After metamorphosis the animal sinks to the bottom and begins its adult life. 286 THE INVERTEBRATA Order OLIGOCHAETA Chaetopoda, nearly all land and freshwater forms, with a compara- tively small number of chaetae, not situated on parapodia, with pro- stomium distinct but usually without appendages ; always hermaph- rodite, the male and female gonads being few in number (one or two pairs), situated in fixed segments of the anterior region, the male always anterior to the female; with special genital ducts (coelomo- ducts) opening by funnels into the coelom, spermathecae , and a clitellum present at sexual maturity ; with reproduction by copulation and cross-fertilization; eggs being laid in a cocoon, developing directly without a larval stage. In addition the pharynx is not eversible and pharyngeal teeth (such as frequently occur in the Polychaeta) are absent, except in one small family, the Branchiobdellidae, which have ectoparasitic habits similar to the leeches and resemble them in some particulars of structure. Though the chaetae are not borne on parapodia they are usually divided into two bundles or groups on each side which roughly correspond to the noto- and neuropodia. They may be classified into hair chaetae which are long and fine (dorsal chaetae of Stylaria) and shorter chaetae which are rod-like (Lumbricus) or needle-like. The point of the needle is single- or double-pronged. There is not, how- ever, the great variety found in the Polychaeta. Certain main features of the reproductive system (Fig. 199) are the salient characters of the group. Its members are, without exception, hermaphrodite, and with a single possible exception cross-fertilization only is possible. The restriction of the gonads to a few segments occurs also in some sabellids among the Polychaeta and in some archiannelids. The sexual cells are shed into the coelom either into the general coelomic cavity as in the Polychaeta or into special parts of it divided off from the rest (seminal vesicles of Lumbricus) where they mature. Spermathecae are usually present to contain the spermatozoa received from another worm in copulation. The clitellum is a special glandular development of the epidermis whose principal function is the secretion of the substance of the cocoon and the albuminoid material which nourishes the embryo. It is a secondary sexual character which is only present in the reproductive season in most Oligochaeta, but the earth- worms [Lumbricus^ Allolobophora) used in zoological laboratories in this country always possess it. Both the clitellum and the cocoon pro- duced by it are found in the Hirudinea. It may also be mentioned that many oligochaets have special copulatory chaetae, sometimes hooked for grasping the other worm or with a sharp point for piercing it. For the purposes of the elementary student it is probably best to recognize that the Oligochaeta contain two well-marked oecological OLIGOCHAETA 287 types, the ''earthworm", a larger burrowing terrestrial form, and the aquatic oligochaet which is much smaller and simpler in structure. It is probable that the former type is the more primitive ; the aquatic oligochaet shows many characters which resemble those of the archi- annelids and are most likely due to a process of simplification. The reasons for the conclusion that the aquatic oligochaets are not the oldest of these groups are given below. The Earthworms These are divided into a number of families of which the most important are the Lumbricidae, containing Lumbricus and Allolobo- phora^ and the Megascolecidae which is the largest of all. The primitive forms in all families resemble Lumbricus in the following characters. There are a large number of segments and each one is furnished with eight chaetae arranged in pairs and all on the ventral side of the worm. A series of dorsal pores is found along the back in the intersegmental grooves. The alimentary canal is cha- racterized by a large muscular pharynx by which the food is sucked in, with many glands, the secretion of which is used in external digestion. The oesophagus in one part of its length gives rise to one or more pairs of diverticula, the cells of which secrete carbonate of lime {oesophageal pouches and glands). At the end of the oesophagus or the beginning of the intestine there is a thick-walled gizzard in which the food is masticated with the aid of the soil particles. The intestine has a dorsal ridge, the typhlosole^ to increase the absorptive surface. The nervous, muscular and circulatory systems exist through- out the earthworms with little variation from the condition in Lumbricus. The reproductive system (Fig. 199 C) consists essentially of two pairs of testes in segments 10 and 11 and one pair of ovaries in segment 13, followed by ducts which open by large funnels just be- hind the gonads and discharge to the exterior in the next segment in the case of the oviduct, and several segments behind in the case of the sperm duct. The testes, at least, are enveloped by sperm sacs (vesiculae seminales) which are outgrowths of the septa, and in the cavity of these the sperm undergo development. In some earthworms there are no sperm sacs and this condition, resembling that in the Polychaeta, is probably the earliest in the group. There are two pairs of spermathecae in the region in front of the testes. In the neighbourhood of the male external aperture there are spermiducal [prostate) glands which do not actually open into the sperm duct. A single pair of segmental organs (open nephridia) is present in each segment. The variations which occur in more specialized members of all families are as follows. The chaetae may increase in number and come A 8pth.}j^ Fig. 199. Reproductive organs of the Chaetopoda. A, Polychaeta (longi- tudinal section of Serpula intestinalis to one side of the middle line). Original. Oligochaeta. B, Naididae, diagrammatic. After Stephenson. C, Lumbricus terrestris, diagrammatic. After Hesse, at. atrium ; cl. clitellum ; ect. ectoderm ; m.c. circular and m.l. longitudinal muscles; o. ovary; od. oviduct; o.s. ovisac; ov. ovum; s.f. funnels of vas deferens; sp.s. sperm sac; s.v. seminal vesicle; spth. spermatheca ; t.s. testis sac ; t. testis ; v.d. vas deferens. D, dorsal and V, ventral. The numbers are those of the segments, the vertical lines are septa. OLIGOCHAETA 289 to be arranged in a complete ring round the body [perichaetine). The dorsal pores may disappear. The oesophagus may lose its calciferous glands and the gizzard may be absent or develop into several. The reproductive organs vary in small but important particulars. There are nearly always two pairs of testes in segments 10 and 11 and one pair of ovaries in segment 13, but the testes may be reduced to a single pair. There are usually two pairs of spermathecae but the number varies and occasionally they are absent altogether. Th^ prostate glands (of unknown function) are nearly always present in earthworms except in the Lumbricidae. The simplest method of copulation in earthworms is that found in Eutyphoeiis, where the end of the sperm duct can be everted to form a penis. This is inserted into the spermathecal apertures and the spermatozoa thus pass directly from one worm to another. It is obvious that the mechanism of copulation is far more complicated in the Lumbricidae. Here the worms come into contact along their ventral surfaces and each becomes enveloped in a mucous sheath. Close adhesion is secured between the clitellum of one worm and the seg- ments 9 and 10 of the other, partly by embracing movements of the clitellum and partly by the chaetae of the same region being thrust far into the body wall of the partner. The sperm passes out of the male aperture and along the seminal groove to the clitellum ; how it enters the spermathecae of the other worm has never been observed. The cocoons are formed some time after copulation. The worm forms a mucous tube as in copulation. The cocoon is then secreted round the clitellum and finally the albuminous fluid which nourishes the embryo is formed between the cocoon and the body wall and the worm frees itself from the cocoon by a series of jerks. All three products, mucus, cocoon substance and albumen, are secreted by the clitellum and each probably by a distinct type of cell. The eggs are sometimes extruded and passed backwards into the cocoon while it is still in position on the clitellum but the spermathecae eject the spermatozoa when the cocoon passes over them. The embryo oi Lumbriciis is illustrated in Fig. 200. The prototroch is absent but the gut and stomodaeum are developed early to absorb the albumen in the cocoon. There are two mesoblast pole cells at the hinder end which bud off the mesodermal strips: there are three ectodermal pole cells on each side, the ventralmost a neuroblast forming half the nerve cord and the two others nephroblasts giving rise to longitudinal rows of cells which divide up to form the nephridia. A primitive kind of nephridium in the Oligochaeta is that de- scribed in Lumbricus, of which there is a pair for each segment, the nephrostome projecting through the septum and opening into the cavity of the segment in front. A great many modifications of this 290 THE INVERTEBRATA arrangement exist especially in the Megascolecidae. Here, in addition to the type already described which is distinguished as a mega- nephridium, there are micronephridia of which enormous numbers may exist in a single segment (2500 in Pheretima). These are small tubes which may or may not open into the coelom by a nephrostome. They may exist in the same segment as a pair of meganephridia. There is ^end. Fig. 200. Embryo of Lumbricus foetidus. After E. B. Wilson. A, Lateral view of an embryo in which the mesoblast is unsegmented. B, Ventral view of the same embryo. C, Longitudinal section of a later embryo a little to one side. D, Transverse section of ventral part of the same embryo along the line XY in C. brn. brain; coe. coelomic cavity of mesoblastic somites; ect. ectoderm; end. endoderm; ent. enteron; M.t. mesoblastic teloblast; npb. nephroblasts ; nrb. neuroblasts ; nep. nephridia ; sep. septa ; std. stomodaeum. good evidence for supposing that an originally single meganephridium has been broken up into a multitude of micronephridia. In the development of the earthworm Megascolides the segmental organs first appear as cords of cells like meganephridia. These are thrown into a series of loops and each loop is separated from the rest as a micronephridium. OLIGOCHAETA 29I Other modifications are those in which the nephridia open into the ahmentary canal instead of to the exterior. They may ho. peptonephridia^ opening into the interior part of the ahmentary canal ; whether they have a digestive function is not known. On the other hand they may unite to form a longitudinal duct (or ducts) which discharges into the hind end of the intestine. Whether there is any physiological meaning for the variations in the segmental organs of the earthworms is entirely unknown. There is a well-developed blood circulation. Blood flowing through the parietal and dorso-intestinal vessels of each segment is collected in the dorsal vessel. It is prevented from returning by an elaborate system of valves (Fig. 201). Waves of peristaltic contraction beginning at the hind end of the dorsal vessel and continued by the "hearts" press it forwards and ventralwards into the ventral vessel which is the main distributing channel. The aquatic Oligochaets As a type of these, Stylaria, belonging to the family Naididae, will be shortly described (Fig. 202). This is a transparent worm rather less than a centimetre long found crawling on water weed. The prostomium bears minute eyes and is produced into a long filiform process. In most of the segments there are two bundles of chaetae on each side, the dorsal consisting of hair chaetae and needle chaetae, while the ventral has only "crotchets" with a double point. The first four segments have no dorsal bundles (incipient cephalization). The alimentary canal is simpler in character than that of Lumhricus, 3. gizzard being absent. The intestine is ciliated and the action of the cilia brings in from the anus a current of water which probably assists respiration. The testes (Fig. 199 B) develop in segment 5 and the ovaries in segment 6, while a pair of spermathecae is found in the testis segment. The sexual cells develop in the seminal vesicle and the ovisac which are unpaired backward pouchings of septa 5/6 and 6/7 respectively. The male ducts open by a funnel on septa 5/6 and discharge into an atrium, which is lined by the cells of the prostate. While sexual individuals are often met with and can be recognized at once by the appearance of the opaque clitellum in segments 5-7, individuals reproducing asexually are much commoner. Chains of worms attached to one another may be found, and the existence of one or more zones of fission, where new segments are being formed and separation of two individuals will take place, is easily observed under the microscope. Stylaria is a delightful object of study. The operation of many of the organs can be easily observed with a low power and the results 292 THE INVERTEBRATA form a useful supplement to work with Lumbricus in understanding oligochaet organization. d.i.v. Fig. 201, Fig. 202. Fig. 201. Dorsal vessel oi Lumbricus to show connections and valves. After Johnston, d.v. dorsal vessel ; vessels leading to dorsal vessel ; d.i.v. from sub- intestinal vessel, p.v. from subneural vessel (parietal vessel); sep. septum; va. valves open with dilation and va. closed with contraction of the dorsal vessel. Fig. 202. Stylariaproboscidea. Original. Dorsal view. /ew. median prostomial process ; cr. crop ; e. eye ; M. mouth ; ph. pharynx ; oe. oesophagus ; int. intestine (stippled); sep. septum. The four anterior segments have hooked ventral chaetae {ch.v.) only, the rest with long dorsal hair chaetae (ch.d.) as well. From the above account it will be seen that Stylaria differs from Lumbricus not only in its small size and transparency but also in the number and appearance of the chaetae — which give it a certain OLIGOCHAETA 293 resemblance to the Polychaeta. The reproductive organs, however, are entirely different from those of the latter group and it is in this system that the real contrast between polychaet and oligochaet lies. The aquatic oligochaets when they are of small size often show re- duction of the vascular system, ciliation of the under surface (in one form, Aeolosoma), and a nervous system of embryonic type. These are characters which may be primitive but, as in the archiannelids, so here, they are probably the results of simplification; it is generally agreed that the replacement of sexual by asexual reproduction is a secondary feature, and the frequency with which it is found in the aquatic Oligochaeta shows them to be, on the whole, specialized types. Fig. 203. Blood circulation in Lumhriculus variegatus. After Haffner. A, Head and anterior region showing dorsal and ventral vessels joined by a network of vessels round the gut. B, Single segment of the middle region with a much closer plexus. C, Posterior end with a continuous sinus round the gut connected at intervals with the dorsal and ventral vessels. An. anus ; bl. blind contractile sac of the dorsal vessel {d.v.) ; M. mouth ; pi. plexus ; sin. sinus; v.v. ventral vessel. Two common genera, Tubifex and Lumhriculus, are larger worms which in their appearance have more resemblance to earthworms. A brief description of them follows. Tubifex. A small red worm with rather numerous chaetae in the dorsal and ventral bundles belonging to various types; without gizzard; testes and ovaries in segments 10 and 11 respectively. It lives in the mud at the bottom of ponds and lakes with its head buried and its tail waving in the water; the latter movements are respiratory. They draw water from upper layers which contain more 294 THE INVERTEBRATA oxygen : when the oxygen content of the water in general falls a greater length of the worm is protruded and its movements become more vigorous. A great deal of detritus passes through its alimentary canal so that Tubifex plays the same sort of part in fresh water that the earthworms play on land. Lumbriculus resembles Tubifex superficially but has only eight chaetae in a segment, placed as in Lumbricus; chaetae double pointed; not often met with in sexual state but reproduces habitually by breaking up into pieces each of which regenerates the missing segments. In this worm the primitive nature of the blood system is well seen (Fig. 203). At the posterior end there is a continuous sinus round the gut, in the middle region this becomes resolved into a dense plexus of capillaries and at the anterior end there is the beginning of a seg- mental arrangement. Class ARCHIANNELIDA Small marine annelids with simplified structure, parapodia and chaetae being usually absent. This group was founded to receive two genera, Polygordius and Protodrilus, which were formerly considered to be primitive forms from which the larger groups of annelids might be derived. From time to time other genera have been included which show some, but not all, of the characters which distinguish the original genera. The series of diagnoses of the best known genera given below starts with Polygordius and works back to forms which come very close to the Chaetopoda. There can be little doubt that the Archiannelida are derived from this latter group by the loss of some of its distinctive features (e.g. parapodia and chaetae), and retention of juvenile characters (ciliation and connectionof nervous system with epidermis). These changes are also found within the limits of the Polychaeta, and if it was not that other characters link up its members the group might well be considered as a family of polychaets. Dinophilus comes late in the series because, though evidently related, it does stand rather apart. It has a superficial resemblance to a small turbellarian enhanced by the great reduction of the coelom. Polygordius (Fig. 198 B) with elongated cylindrical body, head with two tentacles and ciliated pits; without parapodia or chaetae; with segments of the coelom separated by septa with a pair of seg- mental organs opening into each by nephrostomes; with longitudinal muscles in four quadrants, the circular muscles being usually absent; with a reducedvascular system and nerve cords lying in the epidermis ; with a trochosphere larva, Fig. 198 A. ANNELIDA 295 Protodrilus . As in Polygordius but with segmentation marked ex- ternally by ciliated rings and with a longitudinal cihated groove in the middle of the ventral surface ; with a ventral muscular pharyngeal sac ; hermaphrodite. ,ten. Fig. 204. Examples of the Archiannelida. A, Nertlla, dorsal view of female: A', parapodium. B, Saccocirrus, side view of anterior end. C, Histriobdella, dorsal view of male. D, Dinophilus, dorsal view of male. amp. ampulla of ten- tacle {ten.)\ a.f. anterior foot; ch. bundle of chaetae; cl. clasper; cil.b. bands of cilia ; e. eye ; gen.s. genital segment ;]. jaw ; /. eyes ; nep. nephridia ; o. ovary ; od. oviduct ; p. penis ; ph. pharynx ; p.f. posterior foot ; p.p. palp ; rm. rectum ; St. stomach; v.s. vesicula seminalis; f.. testis. A, B, after Goodrich; C, after Shearer; D, after Harmer. A single species, P. chaetifer^ has recently been discovered with four short chaetae in each segment. Saccocirrus (Fig. 204 B). As in ProtodriluSy but with chaetae 296 THE INVERTEBRATA arranged in a single bundle on each side of each segment ; with separate sexes, each with complicated genital apparatus, the females with spermathecae and males with a pair of protrusible penes in each segment behind the oesophagus. Nerilla (Fig. 204 A). As in Protodrilus, but with two bundles of chaetae separated by a single cirrus on each side of each segment; three prostomial tentacles and a pair of palps ; with separate sexes and a reduced number of genital segments (three in male, one in female), three pairs of sperm ducts uniting at a common median genital aperture, and two oviducts with separate genital apertures. Dinophilus (Fig. 204 D) with very short flattened body consisting of only five or six segments, a ciliated ventral surface and ciliated ring in every segment; without septa, dorsal and ventral mesenteries, and a vascular system; with greatly reduced coelom and longitudinal muscles; five pairs of *' closed" nephridia; separate sexes, male with median penis injecting spermatozoa into female through skin, female with eggs of two sizes, the smaller giving rise to males and the larger to females. Histriobdella^ which may be mentioned here (Fig. 204 C), is a parasite of the eggs of the lobster, having no chaetae but two pairs of "feet" by which it executes acrobatic movements. It resembles Dinophilus in its reduced coelom and musculature but has jaws, and from the structure of these it has been claimed that Histriobdella is a much modified polychaet belonging to the family Eunicidae. The value of the Archiannelida to the elementary student of zoology is that they illustrate an evolutionary process which may be called simplification or reduction (but not degeneration), and which is not unlike the changes which parasitic forms have undergone. Class HIRUDINEA Annelida with a somewhat shortened body and small, fixed number of segments, broken up into annuli and without chaetae (except in Acanthohdella) or parapodia ; at the anterior and posterior ends several segments modified to form suckers; coelom very much encroached upon by the growth of mesenchymatous tissue and usually reduced to several longitudinal tubular spaces (sinuses) with transverse com- munications. Hermaphrodite, with clitellum. Embryo develops in- side cocoon. In the typical leeches the constitution of the body is remarkably constant. There is a prostomium and thirty-two body segments; an anterior sucker (in the centre of which is the mouth) is formed from the prostomium and the first two segments, and a posterior from the last seven. Both suckers are directed ventrally. The subpharyngeal ANNELIDA 297 "ganglion" (Fig. 205 B) is composed of four single ganglia fused together and the posterior "ganglion" of seven. Between them lie twenty-one free ganglia, and the number of segments is estimated by summation of all the ganglia. The number of annuli to a segment varies in different forms. Fig. 205. Anterior part of nervous system in A, Lumhricus. After Borradaile. B, Hirudo. After Leydig. The brain in both consists of a single dorsal pair of gangHa belonging to the prostomium. In Lutnbricus the subpharyngeal ganglion (sbp.) and lower part of the circumpharyngeal commissures give off nerves to segments i (peri.) peristomium, 2, 3 and so belong to three segments. In Hirudo the subpharyngeal mass consists of four (or five) pairs of ganglia fused together, e. eyes ; M.c. mouth cavity ; 7. jaws ; pr. prostomium ; ph. pharynx (with network of visceral nerves) ; so. sense organs. / The alimentary canal is highly characteristic and consists of the following parts, (i) A muscular pharynx with unicellular salivary glands. In the Gnathobdellidae, which includes Hirudo, there are three chitinous plates or jaws. In the Rhynchobdellidae (Fig. 206), there is a protrusible ^ro^o^m surrounded by 2i proboscis sheath. (2) A short oesophagus follows, leading into (3) the mid gut (crop) which is 298 THE INVERTEBRATA often provided with lateral coeca, varying in number, and is used for storing up the blood or other juices of the host. This is kept from coagulating by the ferment (anticoagulin) contained in the salivary secretion [Hirudo). In the mid gut a very slow digestion takes place, the blood appearing almost unchanged even after several months. (4) An intestine, which is also endodermal, and has, in Hirudo, a pair of diverticula. (5) A very short ectodermal rectum discharging by the anus, which is dorsal to the posterior sucker. The body wall consists of a single layer of ectodermal cells between which blood capillaries penetrate, a dermis with pigment cells and blood vessels, and an outer circular and inner longitudinal layer of muscles. The muscle fibres have a characteristic structure, consisting rm Fig. 206, Glossiphonia as example of the Rhynchobdellidae. Dorsal view. an. anus ; cr. crop (black) ; int. intestine (stippled) ; oe. oesophagus ; pb. pro- boscis; ps. proboscis sheath; rh. rhynchodaeum ; rm. rectum; sa.gl. salivary glands. of a cortex of striated contractile substance and a medulla of un- modified protoplasm. Inside the musculature are masses of mesen- chymatous tissue : in the Gnathobdellidae this is pigmented and forms the botryoidal tissue, the cells of which are arranged end to end and contain intracellular capillaries filled with a red fluid. The mesenchyme almost completely occupies the space which is the perivisceral cavity in the earthworm. There are, however, longi- tudinal canals, constituting the sinus system, and these represent the remnants of the coelomic spaces ; there are always dorsal and ventral and often (e.g. Glossiphonia, Fig. 207 B) two lateral sinuses, and there are numerous transverse canals in each segment. Into this reduced coelom the nephrostomes open and the gonads are found in it. The HIRUDINEA 299 blood system consists of two contractile lateral vessels (and in the Rhynchobdellidae of dorsal and ventral vessels running inside the corresponding coelomic spaces). These vessels all communicate with m.c. ect. t. n.s. v.s n.c. Fig. 207. Transverse sections of Hirudinea to show the progressive restriction of the coelom. A, Acanthobdella, B, Glosstphonia, C, Hirudo. In A the coelom {coe.) is continuous but encroached upon by growth of parenchyma (stippled). In B it is broken up into a system of sinuses, d.s. dorsal; v.s. ventral; h.s. hyperdermal sinus ; l.s. lateral and i.s. a network of intermediate sinuses. In C the sinuses (outlined in black) are reduced in size, and there is no inter- mediate network, n.s. the nephrostomial sinuses, branches of the ventral sinus, contain the testes {t)\ botryoidal tissue {h.t.) is present; ch. chaetae; cm. coecum; cr. crop; d.v. dorsal, l.v. lateral, v.v. ventral blood vessel; ect. ecto- derm; gl. glands; ni.c. circular, m.l. longitudinal muscles; nep. nephridium; n.c. nerve cord ; oe. oesophagus ; per. peritoneum ; s.o. sense organs. one another. They also communicate with the sinuses of the coelom and with the capillaries of the botryoidal tissue, as has been shown by careful injection. This astonishing condition is unique, but a parallel may be drawn with the vertebrate in which the lymphatic 300 THE INVERTEBRATA system communicates both with the coelom and the blood system. The pecuhar functions of the lymphatic system are not shared by the botryoidal vessels which have no particular connection with the The nervous system is of the usual annelidan type but characterized by the fusion of ganglia anteriorly (Fig. 205) and posteriorly. There are segmental sense organs in the form of papillae, and on the head some of these are modified to form eyes and the so-called "cup- shaped organs". The nephridia consist of two tubes, one ending in a nephrostome, the other with an external aperture ; their lumina do not communicate (Fig. 192); the nephrostomes open into a branch of the ventral or the lateral sinus. The testes^ of which there are often several pairs (nine in Hirudo), and the single pair of ovaries are also present as closed vesicles in the sinuses and are derived from the coelomic epithelium, but in distinction from the rest of the annelids they are continuous with their ducts. The separation of the genital part of the coelom from the rest, begun in the Oligochaeta, here becomes complete. The testes discharge into a common vas deferens on each side; the two vasa unite anteriorly to form a median penis. Similarly the two oviducts join and the eggs pass through a single albumen gland and vagina to the exterior. The spermatozoa, united in bundles, are deposited on the body of another leech and appear to make their way through the skin to the ovaries where fertilization occurs. The eggs are laid in cocoons, the case of which is formed by clitellar glands in the same way as in Lumbricus. The Hirudinea may be divided as follows : AcANTHOBDELLiDAE, a family intermediate between the Oligochaeta and the Hirudinea, containing the single genus Acanthobdella. Rhynchobdellidae, marine and freshwater forms, with colourless blood, protrusible proboscis and without jaws. Gnathobdellidae, freshwater and terrestrial forms, with red blood without a protrusible proboscis but usually with jaws. Family Acanthobdellidae. Acanthobdella (Fig. 207 A), a parasite of salmon, is a hnk with the Oligochaeta. In it the specialized hirudinean characters are only partly developed. There is no anterior sucker but a well-developed posterior sucker formed from four segments. The total number of segments is twenty-nine compared with thirty-two in the rest of the group. There are dorsal and ventral pairs of chaetae in the first five body segments and the coelomic body cavity is a continuous perivisceral space, in- terrupted only by segmental septa as in the Oligochaeta. It is, however, restricted by the growth of mesenchyme in the body wall and split ANNELIDA 30I up into a dorsal and ventral part in the clitellar region. The so-called testes (really vesiculae seminales) are tubes running through several segments, filled with developing spermatozoa and their epithelial wall is continuous with that of the perivisceral coelom, another primitive feature. The vasa deferentia, moreover, open into the testes by typical sperm funnels. It is interesting to find that in the Branchiobdellidae, a family of the Oligochaeta, parasitic on crayfish, there is the same sort of leech- like structure: a posterior sucker, annulated segments, absence of chaetae and presence of jaws. But the condition of the coelom, nephridia and generative organs is so like that of the Oligochaeta that the family must remain in that group. Family Rhynchobdellidae. Pontohdella, parasitic on elasmobranch fishes. Glossiphonia (Fig. 206), a freshwater leech feeding on molluscs like Limnaea and Planorbis and on the larvae of Chironomus ; body ovate and flattened ; hind gut with four pairs of lateral coeca ; eggs laid in the spring, the young when hatched attaching themselves to the ventral surface of the body of the mother. Family Gnathobdellidae. Hirudo, the medicinal leech, at one time a common British species but now extinct ; jaws armed with sharp teeth. Haemopis^ the horseleech, common in streams and ponds, which it leaves to deposit its cocoons and in pursuit of prey ; jaws armed with blunt teeth, which cannot pierce the human skin; a single pair of coeca in the mid gut. This leech is carnivorous, devouring earthworms, aquatic larvae of. insects, tadpoles and small fish. The land leeches of the tropics, of which Haemadipsa may serve as an example, live in forests and swamps and, mounted on leaves and branches, wait until a suitable mam- malian prey presents itself. The following classes, the Echiuroidea and the Sipunculoidea, were formerly classed together as the Gephyrea. There is, however, good reason for separating them in spite of their general similarity, which is possibly due to the fact that they are both composed of burrowing animals and have lost their segmentation. Class ECHIUROIDEA Annelids which show few signs of segmentation, with a spacious coelomic cavity, a well-developed prostomium, a terminal anus, a single pair of ventral chaetae, sometimes several pairs of segmental organs, and in Echiurus a trochosphere larva in the nervous system of 302 THE INVERTEBRATA which there appear to be as many as fifteen pairs of ganglionic swelHngs (Fig. 208). Echiurus, with a spoon-shaped prostomium, two pairs of segmental organs and a trochosphere larva. Bonellia (Fig. 209 A, B) with a prostomial proboscis bifurcated at m. mes. Fig. 208. Echiuriis. Ventral view of larva to show segmentation of posterior end. After Baltzer. a. anus ; ch. chaeta-forming cell ; com. neural commissure ; l.m. longitudinal muscle; m. mouth; rnes. mesoderm ; /)w. larval nephridium with solenocytes ; vn. ventral nerve cord composed of many neuromeres. the end, capable of enormous elongation and extremely mobile; a single segmental organ (brown tube) ; the female is the typical in- dividual and the males are reduced to small ciliated organisms, like a turbellarian, which live in the segmental organ of the female. It is now known that larvae of Bonellia carry the potentialities of both sexes. If they develop independently they become females. If ANNELIDA 303 m.retr. Fig. 209. Bonellia viridis. A, Female. B, Male from nephridium of female. After Spengler. C, Sipunculus. From Shipley and MacBride. a. frilled membrane surrounding the mouth ; al. alimentary canal ; al. degenerate alimentary canal of male; an. anus; an.v. anal vesicle; ht. brown tube (nephridium); ch. position of chaetae; cil.gr. ciliated groove; an. coecum of gut; d.v. dorsal blood vessel; e. cut ends of intestine; g. anal glands; m.retr. retractor muscle of anterior end ; M. mouth; n.c. nerve cord; nephr. nephro- stome; oe. oesophagus; o. ovary; op. o male reproductive aperture; pr. greatly enlarged prostomium; sp. spermatozoa. 304 THE INVERTEBRATA they should come into contact with the body of the adult female, she exercises (probably through the action of some specific secretion) a largely repressive effect on further development, but a male gonad is formed. Thalassema. The British representative, T. neptuni, has a single pair of segmental organs. In two Japanese species, T. taenioides and T. misakiensis, these have been greatly multiplied so that in the former there are 200 pairs rather irregularly arranged. From a consideration of these forms it appears that the multiplication of the segmental organs is a secondary phenomenon. Class SIPUNCULOIDEA Annelids with a spacious uninterrupted coelomic cavity and few signs of segmentation: without prostomium in adult; chaetae always ab- sent, anterior part of body invaginable into posterior part ; anus dorsal and anterior ; with a single pair of segmental organs (brown tubes) ; in Phascolosoma a trochosphere with three pairs of mesoblastic somites which soon disappear. Sipunculus (Fig. 209 C) and Phascolosoma are British genera. CHAPTER X THE PHYLUM ARTHROPODA Bilaterally symmetrical, segmented Metazoa; with, on some or all of the somites, paired limbs, of which at least one pair function as jaws; a chitinous cuticle, which usually is stout but at intervals upon the trunk and limbs flexible so as to provide joints; a nervous system upon the same plan as that of the Annelida ; the coelom in the adult much reduced and replaced as a perivisceral space by enlargement of the haemocoele ; without true nephridia, but with one or more pairs of coelomoducts as gonoducts and often as excretory organs; and (except in Peripatus) without cilia in any part of the body. The Arthropoda have much in common with the Annelida, and must be regarded as derived from the same stock as the Polychaeta in that phylum. The key to most of their peculiar features is an in- crease in the thickness of the cuticle. This brings with it the necessity for joints ; and the stout, jointed limbs can now be adapted for various purposes to which those of polychaetes were not convertible. Always at least one pair of them become jaws ; with this is usually associated the specialization for sensory functions of one or two pairs which have come to stand in front of the mouth, and thus the process of cephaliza- tion, begun in the polychaetes, proceeds further here. Other limbs commonly become legs. In order to move the complex of hard pieces constituted by the jointed cuticle, the continuous muscular layer of the body wall of an annelid has become converted into a system of separate muscles ; with this, and with the fact that turgescence of the body wall is no longer a factor in locomotion, is perhaps connected the replacement of the perivisceral coelom by a haemocoelic space. The loss of the nephridia which in annelids lie in the coelom is prob- ably due to the reduction of that cavity. An interesting feature of difference between the Arthropoda and Annelida is the absence from the former phylum of the chetae, imbedded in and secreted by pits of the skin, which characterize the annelids; though bristles, formed as hollow outgrowths of the cuticle, are common on arthropods. This difference, too, may be connected with the difference in the stoutness of the cuticle. Lastly, it is perhaps that thick covering, hindering the loss of water by evaporation from the surface of the body and pro- viding the skeleton which the lack of support from the medium necessitates, which has enabled arthropods very successfully to invade the dry land. Like those of all other phyla, their earliest known members, the trilobites, were aquatic. Of their surviving groups. SOMITES AND LIMBS Somite Onychophora Arachnida Scorpionida Trilobita I...* Preantennae Embryonic ? 2... Jaws Chelicerae Antennae 3-.- Oral Papillae Pedipalpi ist biram. limbs 4... I St pair of legs I St pair of legs 2nd 5..- 2nd „ 2nd 3rd 6... 3rd 4th 7... 4th 5th 8... Embryonicft 9... CO Genital operc. ? cJ 10... Pectines II... 0 I St Lung books 12... *3 2nd 13... C3 to 3rd CO 1 14... 15... 'u a 4th No limbs <4-l u 16... 0 I St som. Metasoma a 17. . 8... 19... c4 ■M 0 2nd 3rd 4th at G J2 20... CO 5th J3 u 21... 22... CO 0 C3 23... 24... 25... 26... C CO 1 Cfl 27... 28... 29... Last pair of legs Postseg-\ mental Embryonic Telson Telson region * Eyes and frontal organs belong to a presegmental region which may have median ** If the superlinguae be maxillules (see p. 463), the limbs behind them stand on t Terga fused in Scolopendra, free in Lithobius. ff Chilaria in Limulus. § This somite appears to have no limbs, because the limbs of the 8th and 9th somites cJ indicates the position of the male opening, ? that of the female. OF ARTHROPODA Crustacea Malacostraca Insecta Chilopoda (Scolopendra) Diplopoda (Julidae) Embryonic Embryonic Embryonic ? Antennules Antennae Antennae Antennae Antennae Embryonic Embryonic Embryonic Mandibles Mandibles Mandibles Mandibles Maxillules (ist) Maxillae** ist Maxillae Embryonic Maxillae Labium (2nd Maxillae) 2nd Maxillae Maxillae (ist) Maxillipeds ist pair of legs Maxillipeds ^ x Collum 2nd Thoracic limb 2nd ,, ist pair of legs j ist pair of legs 3rcl 3rd 2nd ,, 2nd „ ?c?§ 4th ist Abd. som. 3rd „ 3rd pair of legs 5th 2nd ,, 4th 4th „ 1 6th „ ? 3rd 5th 5th „ J 7th 4th 6th bo 8th 5th „ 7th ist Abd. limb 6th 8th cts n 2nd „ 7th 9th 3rd 8th „ V loth ,, 4th 9th „ (styles) 0 nth 5th loth ,, som. i2th ■bM 6th nth „ (cerci) 13th °^ •• 14th 15th i6th 17th i8th 19th 20th -^ .id ^ 0 'o ^ 0 ^ 0 0 N C rrt 2ISt ,, X ^ •• Genital limbs ? c^ Limbless somite Telson Embryonic Telson Telson mesoblast of its own, and may bear various ganglia which enter into the procerebrum. somites 6, 7, etc. X Lithohius has 15 pairs of legs. have each moved forward one somite. 3o8 THE INVERTEBRATA only one, the Crustacea, remain predominantly of that habit. No other invertebrate phylum has so large a proportion of terrestrial members. A more detailed survey of the organization which we have now out- lined, necessitates a brief exposition of the principal groups into which the phylum falls. One small section stands apart from the rest. The Onychophora have a thin cuticle, without joints; a continuous mus- cular body wall; eyes (p. 310) of annelid type; only one pair of jaws, which moreover are constructed on a different principle from those of other arthropods, biting with the tip and not with the base of the limb ; and a long series of coelomoducts, of which the pair that are the oviducts are ciliated. Only in this group, too, does the first somite bear a pair of limbs : in all others that somite is an evanescent, em- bryonic structure without external representation in the adult. In all these respects the Onychophora show a lower degree of develop- ment of the peculiar features of arthropods than the rest of the phylum. The remaining groups of the phylum fall into two sharply different sections, the crustacean-insect-myriapod section and the arachnid section. In the first of these sections, the first pair of limbs (those of the second somite) are antennae, the succeeding pair, if present, are also antennae, the third pair are mandibles, and behind these limbs are one or more pairs of additional jaws (maxillae). In the crustaceans and insects there is commonly a pair of compound eyes of a complex type peculiar to these animals. The trilobites belong to this section, but their appendages, behind the first pair are undifferentiated. In the arachnid section none of the limbs have the form of antennae or mandibles, the first pair (chelicerae) being usually chelate, the second chelate, palp-like, or leg-like, and the third to sixth pairs leg-like, though often some of the postcheliceral limbs possess biting processes (gnathobases) on the first joint. The members of this section never possess true compound eyes of the crustacean-insect type. The Crustacea differ from the Insecta and Myriapoda in possessing a second pair of antennae, and nearly always in being truly aquatic. The Insecta differ from the Myriapoda in possessing only three pairs of legs, and usually in the possession of wings. The series of somites which, with small pre- and postsegmental regions, constitutes the body of an arthropod is marked out, by differences in width, fusions of somites, or features of the limbs, into divisions known as tagmata. In the Onychophora, Crustacea, In- secta, and Myriapoda, the foremost tagma is a short division, known as the head, which carries the antennae and mouth parts, and the rest of the body, known as the trunk, is often divided into two sections called thorax and abdomen. In the Arachnida, the foremost tagma is the prosoma ("cephalothorax"), and carries legs as well as the limbs ARTHROPODA 309 used in feeding, while the divisions, if any, of the hinder part of the body {opisthosoma or "abdomen") are known as the mesosoma and metasoma. It is important that the student should recognize that each of these divisions varies in size, and that consequently none of them comprises in all arthropods the same somites, so that, for instance, the thorax of an insect is a quite different entity from that of a cray- fish. The most significant variation is that of the head, which, as the organization of its possessor becomes higher, increases in size, taking in behind somites whose appendages become jaws, while, by altera- tion in the position of the mouth, it adds others, whose limbs become antennae, to its preoral sensory complex. Thus, while the head of the Onychophora comprises only the first three somites, and only the first of these is preoral, in the Crustacea there are in the true head six somites (including the embryonic first somite), of which three are preoral, and thoracic somites, whose limbs (maxillipeds) function as jaws, are often united with the head. The paired limbs of arthropods present an enormous variety of form, and attempts have been made to reduce them to a common type. Some of the evidence suggests an archetype with a nine-segmented axis bearing on the median side of the first segment a biting process (gnathobase) and on a more distal segment an outer branch (exopo- dite) ; but there are difficulties in the way of assuming this in all cases, and the problem is still far from solution. The arthropod cuticle has a thin, impermeable, non-chitinous external layer (epicuticle) and a thick, elastic, permeable, lamellar inner layer (endoctiticle),\sLrge\y composed of chitin ^, the outer lamellae usually hardened, often by salts of lime. From time to time during the growth of the animal, the hard outer layers of the cuticle are separated by solution of the inner layers by an enzyme, ruptured, and shed in a moult or ecdysis. A new cuticle which has formed under it then expands to accommodate the body. The nervous system of arthropods contains, in typical instances, on two longitudinal ventral cords and in a dorsal brain, a pair of ganglia for each somite, but where the somites are fused there is often a fusion of their ganglia, and where they bear no limbs their ganglia may be absent. The brain is a complex structure composed of the ganglia of the somites which have become preoral (though in a few Crustacea the antennal ganglion remains postoral), of paired ganglia for certain primitively preoral presegmental &ense organs (eyes, frontal organs), and sometimes also of a median anterior element {archicerebrum, in the strict sense). The ganglia of the first somite are known as the protocerebrum ; with the ganglia anterior to them they constitute the procerebrum {archicerebrum of Lankester). The ganglia of the second ^ Chitin is an amino-polysaccharide which resists most solvent agents. 310 THE INVERTEBRATA somite are the deutocerebrum or mesocerebrum ; those of the third somite are the tritocerebrum or metacerebrum. The identity of some of these gangha may be lost, even in development. Concerning the functions of the central nervous system something is said on p. 448. The eyes of the Onychophora are a pair of simple, closed vesicles, each with its hinder wall thickened and pigmented and its cavity oc- cupied by a lens secreted by the wall. The eyes of all other arthropods (Fig. 211) consist of one or more units each of which is in essence a cup, or a vertical bundle, of cells, over which the cuticle of the body forms a lens. The cells which compose the bottom of each cup are (except in the median eye of the Crustacea) arranged in a sheaf or sheaves called retinulae; in the midst of each retinula is a vertical rod, known as the rhabdom, secreted by the cells of the sheaf in vertical sections which, when they are distinct, are known as rhabdomeres. Each bundle-unit has one such retinula. Sometimes in the cups the retinulae are surrounded by cells which bear on their free ends short rods of the same nature as the rhabdomeres. The retinula cells contain pigment and there is a ring of strongly pigmented cells around the cup. The eye units occur {a) as single cups each with several retinulae (ocelli of insects. Fig. 211 C"), {b) as groups of similar cups placed contiguously (eyes of myriapods), {c) as eyes composed of a number of small cups, each with a single retinula, united together (lateral eyes of Limulus), {d) as true compound eyes (Fig. 212) composed of a number of bundles of cells, each bundle {ommatidium) complex in structure and containing two or more refractive bodies, but each probably representing a narrowed and deepened cup. Compound eyes of this type are found in crustaceans and insects. They vary much in detail, but essentially the structure of an ommatidium is as follows (Fig. 211 D). At its outer end is a transparent portion of the general cuticle of the body, usually thickened to form for the ommatidium a biconvex lens. Under this lie the epidermal cells which secrete it (corneagen cells): the lens is one of the facets of the eye. Under the corneagen cells comes a bundle of two to five vitrellae or crystal cells, grouped around a refractive body, the crystalline cone, which they have secreted. The vitrellae taper inwards and their apex is clasped by a second bundle of cells, four to eight in number, which together form the retinula. Like the vitrellae the retinular cells secrete in the axis of the ommatidium a refractive body. This is the rhabdom, and is made up of rhabdomeres, one for each of the cells. Each retinular cell passes at its base into a nerve fibre which pierces the basement membrane of the eye and enters the optic ganglia. Around each ommatidium, separating it from its neighbours, there are usually pigmented cells, known as iris cells. The eyes of arachnids, other than the lateral eyes of Limulus, simulate the ocelli of insects, but are Opt.hs opt.n.^ ^ tr.com. Fig. 2IO. A plan of the nervous system of Chirocephahis. ob.z, second ab- dominal somite ; an.' antennulary nerve ; ati/' antennary nerve ; an."co?n. com- missure for fibres which unite antennary ganglia ; brti. brain ; fr. nerve to frontal organ; ga. ganglion of ventral cord; tn.e. nerves to median eye; md. mandibular nerve; 7nx.' maxillulary nerve; 7nx." maxillary nerve; oe. oeso- phagus; oe.com. circumoesophageal commissure; opt.l. optic lobes; opt.n. optic nerve; th.\, first thoracic somite; th.iz, nerve of last thoracic somite; tr.com. transverse commissure of ventral cords. 312 THE INVERTEBRATA thought, from details of their structure, to have been formed by the degeneration of compound eyes resembling the lateral eyes of Limulus. The median eye of the Crustacea (Fig. 226) is composed of Fig. 211. Diagrams of a series of eyes of arthropoda. A, Hypothetical start- ing point of the series. B, Cells have sunk in to form a retinula. The units of the lateral eyes of Limulus are substantially in this condition. C, C", Cells from the sides have closed in over the retinula. C, Hypothetical stage in the evolution of an ommatidium from a cup with a single retinula. C", Actual condition of many ocelli of insects, etc. : the cup has several retinulae. D, An ommatidium. b.me. basement membrane of retinular layer; c.c. central cell; cgn. comeagen cells; en. crystalline cone; cu. cuticle; Is. lens; n. nerve fibre; pig. pigmented cells which form a ring in the outer part of the ocellus; pig.' outer iris cells; pig." inner iris cells; rd. "visual rods"; ret. retinular cells; rh. rhabdona; vit. vitrellae; vit.hu. vitreous humour. three cups, which may (some copepods) separate widely. The paired eyes probably do not, as has been suggested, represent a pair of ap- pendages. The foremost, or preantennal, somite, to which they would in that case belong, possesses, in Peripatus and as a rudiment in ARTHROPODA 313 embryonic stages of centipedes and certain insects, an appendage which co-exists with the eye. In most compound eyes, the pigment, both in retinular and in pig- ment cells, flows to and fro, being in dim light retracted towards the inner or outer ends of the cells so as to leave the sides of the om- matidia exposed, and in bright light extending so as to separate the ommatidia completely. In many diurnal insects it is permanently in the latter position. Vision takes place in two ways according to the situation of the pigment. When the latter is extended, in each omma- B n.ji. f>opUga. -w. opt.n. Fig. 212. The eye of Astaciis. A, The left eye. B, A portion of the cornea removed, to show the facets. C, A longitudinal section of the eye. w. muscles which move the eye; n.fi. nerve fibres; oynm. ommatidia; opt.ga. optic ganglion; opt.n. optic nerve. tidium there falls on the retinula a narrow pencil of almost parallel rays. There is then mosaic vision, an apposition image, composed of as many points of light as there are ommatidia, being formed on the whole retinal layer. When the pigment is retracted, each ommatidium throws a complete image of the greater part of the field of vision, and the images together form a superposition image, falling in such a way that their corresponding parts are superposed. Superposition images are less sharp than apposition images, but are formed with less loss of light. Compound eyes are especially adapted for perceiving the move- ments of objects, owing to the way in which such movements affect a series of ommatidia in succession. 314 THE INVERTEBRATA The alimentary canal of the Arthropoda possesses at its mouth and anus involutions of ectoderm, lined by cuticle, which are respec- tively the stomodaeum or fore gut, and proctodaeum or hind gut. These may be short, but in the higher Crustacea and Insecta form a considerable part, and sometimes nearly the whole, of the canal. The cuticular lining of fore and hind gut is shed at moulting. The lining of the fore gut sometimes provides teeth for triturating or bristles for straining the food. Digestion is extracellular, save in certain acarina. The respiratio7i of aquatic arthropods, other than those which are but little modified from terrestrial ancestors, is sometimes, if the animal be small, efi'ected only through the general integument of the body, but usually takes place by means oi gills (branchiae). These are nearly always external processes, known as epipodites, which stand on the bases of the limbs, and are often branched or folded. Among terrestrial arthropods, some of the Arachnida possess luftg books, which are generally held to have arisen by the enclosure of gill books, such as those on the limbs of Limulus, each within a cavity of the ventral side of the body. The remainder of the terrestrial Arthropoda breathe by means of tracheae, which are tubular involutions of the ectoderm and cuticle which convey air to the tissues. In some arachnids tracheae are present as well as lung books. Usually tracheae are branched, and strengthened by a spiral thickening of their chitinous lining. The study of the phylogeny of the Arthropoda leads to the conclusion that a tracheal system has arisen independently in the Onychophora, the Arachnida, and the Insecta and Myriapoda. Among the Crustacea, tufts of tubes which resemble tracheae are found in the abdominal appendages of woodlice. The vascular system is an '* open " one. That is, be the arteries long or short, they end by discharging their blood not into capillaries in the tissues from which veins conduct it to the heart, but into peri- visceral cavities, known as sinuses, which bathe various organs. From these sinuses the blood collects into a pericardial sinus ("pericar- dium"), part of the haemocoelic system, which surrounds the heart. The latter is a longitudinal dorsal vessel, perforated by ostia by which it receives its blood from the pericardial sinus. Among the conse- quences of the structure of the vascular system are a low blood pres- sure and liability to severe bleeding from wounds. The latter danger is met, especially in the Crustacea, by very rapid clotting of the blood. Haemoglobin is present in the plasma of certain of the lower crustace- ans and a few insects, haemocyanin in Limulus, scorpions, and some spiders. The coelom appears in the embryo as the cavities of a series of mesoderm segments (" mesoblastic somites", Fig. 352). It never assumes a perivisceral function, and in the adult is represented ARTHROPODA 315 only by the cavities of the gonads and of certain excretory organs and occasional vestiges elsewhere. The excretory organs of arthropods are of very various kinds. True nephridia appear never to be present. Coelomoducts are present in a number of cases, though in the absence of perivisceral coelom they end internally each in a small coelomic vesicle or "end sac". These are found in the Onychophora in a long series of segmental pairs. In Crustacea there is either a pair of coelomoducts on the third (an- tennal) somite or a pair on the somite of the maxillae, or, rarely, both mdm ect. Fig. 213. Early stages in the development of Astacus. After Morin and Reichenbach. A-C Cleavage, D gastrulation. a. anterior end of embryo; blp. blastopore; ect. ectoderm; end. endoderm; mdm. mesoderm; nu, nuclei; pt. posterior end of embryo; v, yolk; yp, "yolk pyramids", due to the transitory appearance of divisions of the yolk corresponding to the superficial cells. these pairs are present. In various crustaceans other glands, some ectodermal, some mesodermal, appear to have an excretory function, and sometimes replace both pairs of coelomoducts, which become vestigial. In arachnids, coelomoducts open on one or two of the pairs of legs. They are known as coxal glands, but are not homologous with the glands to which that name is applied in certain crustaceans. Mal- pighian tubules are tubular glands which open into the alimentary canal near the junction of mid and hind gut in the Arachnida, Insecta, and Myriapoda. In archnids they, are of endodermal origin, but in insects and myriapods they are part of the ectodermal hind gut. It is interesting that the subphyla differ in the nature of their nitro- genous excreta. In the Crustacea these are principally ammonia compounds and amines, in the Insecta they are urates, in the Arach- nida guanin. 3l6 THE INVERTEBRATA Nearly all the muscular tissue of arthropods is composed of striped fibres, but in Peripatus only the fibres of the jaw muscles are striped, and among the higher groups certain exceptions to the rule are known (some visceral muscles, etc.). The gonads are always, owing to the reduction of the coelom, directly continuous with their ducts, which are probably coelomo- ducts. These have no constant position of opening in the phylum. In the Crustacea they nearly always open at the hinder end of the thorax. In the Arachnida their opening is similarly near the middle of the body. In the Onychophora, Insecta, and centipedes they open near the hinder end, but in the remaining groups of the Myriapoda their opening is not far behind the head. Typical features of the embryonic development are shown in Figs. 213, 316, and 352. The ova are generally yolky, and their cleavage is typically of the kind knownas"centrolecithal",in which (Fig. 213) the products of division of the nucleus come to lie in a layer of protoplasm upon the surface of a mass of yolk which thus occupies the position of a blastocoele. The mode of gastrulation varies from invagination (Fig. 213 D) to obscure processes of immigration and delamination. The formation of the mesoblast as a pair of ventral bands (Fig. 316), proliferated in primitive cases from behind, has already been mentioned (p. 130). As in annelids (p. 285), the meso- blast bands segment, and in most cases the segments (" mesoblastic somites") develop coelomic cavities (Fig. 352). The haemocoele arises by separation of the germ layers. The heart is formed by the dorsal ends of the mesoblast segments approximating. The nerve cords are proliferated from the ventral ectoderm (Fig. 352 A). In spite of the yolky eggs, there is a great variety of larval stages, though direct development is also frequent. The series of somites, which in the adult is often obscured by the loss, obsolescence, or fusion of some of its members, is usually more distinct in the embryo or larva, where the presence of a somite which it is difficult or im- possible to recognize at a later stage is frequently indicated by one or more of three criteria: a pair of segments of mesoblast (mesoblastic somites), a pair of segmental ganglia, and a pair of limbs or limb rudiments. CHAPTER XI THE SUBPHYLA ONYCHOPHORA AND TRILOBITA The two groups of animals with which this chapter deals both present in an apparently primitive condition features which are characteristic of the phylum Arthropoda. One at least of them existed in the Palaeo- zoic period. For these reasons, each of them has been regarded as giving indications concerning the ancestry of the Arthropoda. Where- as, however, the Trilobita are related rather closely to the Crustacea and more distantly to the other subphyla, the Onychophora are, as has been stated above, widely divergent from the rest of the Arthropoda. Some authorities, indeed, prefer to treat this group as an independent phylum. It must at least be regarded as repre- senting a branch which parted at a very early date from the main arthropod stock. The trilobites are indisputable arthropods, on the line of descent which gave rise to the Crustacea and perhaps to other subphyla. SUBPHYLUM ONYCHOPHORA Tracheate Arthropoda with a thin, soft cuticle and a body wall con- sisting of layers of circular and longitudinal muscles ; head not marked off from the body, consisting of three segments, one preoral, bearing preantennae, and two postoral, bearing jaws and oral papillae respec- tively, also with eyes which are simple vesicles ; the remaining segments all alike, the number varying according to the species, each bearing a pair of parapodia-like limbs which end in claws and contain a pair of excretory tubules ; stigmata of the tracheal system scattered irregularly over the body; cilia present in genital organs; development direct. The animals which constitute this very important class are few in number and uniform in structure, all being placed in the genus Peri- patus divided into many subgenera (Fig. 214). They are distributed discontinuously over the warmer parts of the world and occur in very retired positions which are permanently damp as, for instance, be- neath the bark of dead trees and under stones. They have a superficial resemblance to other crawling animals which are found in the same places, like myriapods, slugs and earthworms, and until their anatomy was well known v/ere classed, by different investigators, with all three of these. Certain of the characters of Peripatus such as the feebly de- veloped sense organs, the simple structure of the jaws and feet and the soft skin may be linked with the environment in which they lurk 3l8 THE INVERTEBRATA away from light and enemies. Yet it can hardly be doubted that the Onychophora are a division of the Arthropoda which has preserved more primitive features of an ancestral race than any other living Fig. 214. Peripatus capensis, x very slightly. From Sedgwick. forms, terrestrial or aquatic. Such features are in all probability the thin cuticle, the muscular body wall, the annelid-like eye, the small number of head segments, the complete series of segmental excretory organs, the presence of cilia and possibly also the parapodia-hke limbs. The thinness of the cuticle is responsible for the absence of external segmentation (save for the repetition of the appendages). The head (Fig. 215) bears three pairs of appendages which are none of them Fig. 215. Peripatus capensis, S- Ventral view of anterior end. ant. pre- antenna; o.p. oral papilla; 7. jaw; i, first trunk appendage. After Sedgwick. very highly developed. While elsewhere in the arthropods the first segment is present in the embryo but disappears in the adult, here it persists and bears a pair of appendages which may be caW^dpreantennae (to distinguish them from antennae). They are rather long and very mobile, but not retractile like the tentacles of the slug. The next seg- ment bears the jaws, which are not unlike enlarged claws of the trunk appendages and so bite with the tip and not the side. They are more- over tucked within the oral cavity. But they are borne on muscular papillae arising in the embryo and must without doubt be regarded as appendages. ONYCHOPHORA 319 The trunk appendages are short and conical, hollow, bearing at their distal ends spinose pads and a retractile terminal 'foot" with two recurved claws. The adult body cavity is haemocoelic but the embryonic coelom is well developed. In the development of Peripatus just after the gastrula stage the blastopore becomes elongated, the anterior part ']m V !\§jf,a D pr.s Fig. 216. A, Transverse section through Peripatus. e.g. crural gland; e.m. circular muscles ; d.l.m. dorsal longitudinal muscles ; g. genital organ ; h. heart ; ha. haemocoele; int. intestine containing^, peritrophic membrane; n. excre- tory tubule; n.c. nerve cord; ob.m. oblique muscles; pf. pericardial floor; s.g. slime gland; s.l.g. salivary gland; v.l.ni. ventral longitudinal muscle. B, An excretory organ of Peripatus. bl. bladder ; c.d. ciliated part of duct ; d. duct ; e.s. coelomic sac ; ex. external aperture. C, Part of the ventral and lateral body wall of P. capensis to show irregular distribution of tracheae {tr.). D, Ventral view of embryo of P. capensis to show six pairs of mesoblastic somites. pr.s. primitive streak; and blastopore closed in the middle to form mouth (M.) and anus* {an.). A and B, after Manton; C, after Moseley ; D, after Balfour. giving rise to the mouth, the posterior to the anus, while the median part closes (Fig. 216). Behind the blastopore is a primitive streak which forms the paired mesoblastic somites. The anterior pair move in front of the mouth and help to provide the mesoderm of the tentacular segment. None of the rest become preoral. In all seg- ments the somites early acquire a cavity, the coelom, and later divide into two. Of these the ventral part migrates into the appendage as this is formed, and eventually becomes part of the segmental excretory 320 THE INVERTEBRATA organ. The other part approaches its fellow in a mid-dorsal position to form the heart lying between them (cf. Fig. 352 A) and while in the anterior region it mostly disappears, those of the posterior segments fuse longitudinally to form two tubes which become the gonads (Fig. 217). At the same time the gaps between the organs become filled with blood. A dorsal part of the haemocoele so formed is marked off by a partition as the pericardium. This contains the heart, a long tube with a pair of ostia in nearly every segment. There are, however, no other blood vessels, so that the condition of the circulatory system is by no means so advanced as in the higher Crustacea and the more primitive arachnids. The possession of the perivisceral haemocoele almost diagnoses the group as arthropods, but it was the discovery of the tracheae which led to the inclusion of Pen]^«^w^ in that phylum. The stigmata are scattered over the surface of the body most thickly on the sides and ventral sur- face, several occurring in each segment. Each stigma leads into a pit, penetrating the muscle of the body wall, from which arise bundles of minute air-containing tubes which end in the various organs of the body (Fig. 216 C). It can hardly be doubted that these tracheae are definitely arthropodan in type : their most significant difference from those of other forms is in their non-segmental character. Their irregular distribution is only possible because they originate as pits in soft skin; when once a cuticular exoskeleton has been established tracheae can only be excavated in the joints between segments. Probably then the Onychophora have never had a more definite cuticle than they possess at present; if they had, tracheae have been acquired since it was lost. The alimentary canal consists of short ectodermal fore gut and hind gut and a very long endodermal mid gut, lined by a peritrophic mem- brane (p. 434) which is thrown off periodically. The fore gut consists of a buccal cavity into which open the large salivary glands and a mus- cular suctorial pharynx. The mid gut possesses no separate glands. The excretory tubules (Fig. 216 B) are composed of a distal terminal bladder, a coiled secretory canal and a ciliated duct which opens into a much reduced coelomic vesicle. The bladder and probably the whole of the canal are formed from ectoderm, the rest from meso- derm. It can perhaps be said then that the tubule is a modified coelomoduct which has attained its present condition by the tucking- in of ectoderm at its external opening. The tubules form a complete series, but some of them have been converted into uses other than excretion. Thus the tubules corresponding to the oral papillae form the salivary glands and are much larger and more complex than in other segments. The anal glands and the gonoducts themselves have ONYCHOPHORA 321 the same origin. Only the tubules corresponding to the jaws and the first three trunk appendages disappear. The sexes are separate in Peripatus and the gonads paired, but the ducts unite to form a median passage opening just before the anus. In the male the filiform spermatozoa are bound up in spermatophores in the upper part of the vas deferens ; the lower part is muscular and ejaculatory in function. Fertilization is internal. The ovaries are embraced by a funnel, the receptaculum ovorum, which communi- Fig. 217. Diagram of transverse sections through, embr^'^os of Peripatus capensis to show the haemocoele and the coelom in the following stages: A, before the haemocoele has appeared; B, when the somite has divided into dorsal and ventral parts ; C, when these parts have separated and the heart is formed ; D, at time of hatching. After Sedgwick, i , alimentary canal ; 2, coelom (cavity of mesoblastic somite, dividing into 2 cavity of gonad and 2, coelomic part of excretory tubule ; in C and D the tubule is shown divided into 2 , coelomic sac and 2 , canal); 3, haemocoele, 3 , heart. cates with an oval receptaculum seminis. The eggs are fertilized then at the proximal end of the oviduct : they vary in size according to the species. In the larger, development takes place at the expense of the yolk and the secretions of the uterine wall; but the embryos from smaller eggs become attached to the uterine wall and a placenta is formed. Cilia have been described in parts of the genital tract. There are other derivatives of the ectoderm, the crural glands (Fig. 216 A, e.g.), found on all the legs except the first and consisting 322 THE INVERTEBRATA of a simple sac ; and a single pair of slime glands discharging on the oral papillae, made up of a much branched secretory part and a large reservoir. The slime can be shot out and entangle an enemy. It is never used in obtaining food. S op. Ofl. Fig. 218. Peripatus capensis, S, dissected to show the internal organs, x 2. After Balfour, an. anus ; ant. preantenna ; brn. supraoesophageal gangHa ; c.oes. circumoesophageal commissure; e.g. enlarged crural gland of last pair of legs ; m.g. mid gut ; o.p. oral papilla ; ph. pharynx ; sal.gl. salivary gland ; sl.gl. slime gland; v.n.c. ventral nerve cord; i, 2, 10, appendages of the trunk segments; 4, excretory tubule of the fourth segment; > >> >> 3^ 4 17] 0 0 12 s ^ in w 0 ^ £ o i >> antenna liking leg tomach " rd ; e. eye phthalmi k. kidne; 5 " w O O n ^ !? :: u . Ji e; an first erve op.a deme lied. ^ ^ GO C — C! 57 IS »-i 1) CO .—1 antei ela;/ isoph vent c art o o S' ^ ^ *. ride, an.' ing; cA. ch age; oe. oe lar; v.n.cd. ral thoraci deferens ; shown bu « a ^2 "^ rrt y and ; genit minal om. n( ; v.th. §.2 hiple male abdo: on; c orta) ^ " c5 ^ « V4 t4H « a (u c - = 8.S-g -r! IT ti rom t' enna ; ifiedf nt. int al ab( CO Clc ed f] f ant mod ve ; i: dors CO (U ■•-; o ., o • c c lis, d ; 5r. s ppen< rvical erv ; CO ^ o si "> "S-C =^ 2j ^ 'CUS fliivi M. mou bdomina on ; cv.g. sternal ; • --a ^-> CO CO . _2 .. rt i: . tj a; ,. 4J a o ... C CO ^ S ^ ^ ^' s^ be ti-^ "'^'-^ (U (U 'u., §"§ S-^ CO V ' CRUSTACEA 351 some related genera there is a remarkable system of closed blood vessels without a heart. The blood IS a pale fluid, which bears leucocytes except in ostracods and most copepods. It contains in the Malacostraca the copper- containing respiratory pigment haemocyanin (p. 133). In various entomostraca, notably in Lernanthropus , just mentioned, haemo- globin has been found. r.ov. Fig. 234. A, Male reproductive organs of Astacus fluviatilis. From Howes. r.t. right anterior lobe of testis ; med.t. median posterior lobe of testis ; vas de. vas deferens ; op. external opening of vas deferens ; leg, right fourth ambula- tory leg on which the vas deferens opens. B, Female reproductive organs of Astacus fluviatilis. From Howes, r.od. right oviduct :'the left oviduct is shown partly opened ; r.ov. right lobe of ovary ; l.ov. left lobe of ovary with the upper half removed to show the ovarian cavity, which is the remains of the coelom and into which the ripe ova drop; op. external opening of oviduct; leg, right second ambulatory leg on which the oviduct opens. As is usual with animals that are free and active, the sexes are separate in the great majority of the Crustacea, though the Cirripedia, which are sessile, certain of the parasitic Isopoda, and a few excep- tional species in other groups, are hermaphrodite. Parthenogenesis takes place in many of the BranchioJDoda and Ostracoda, and in these it is often only at more or less fixed intervals that sexual reproduction occurs. The male is usually smaller than the female and in some parasites is minute and attached to her body. He has often clasping- organs for holding his partner, and these may be formed from almost 352 THE INVERTEBRATA any of the appendages. He may also possess organs for the transfer- ence of sperm: these may be modified appendages or protrusible terminal portions of the vasa deferentia. The gonads of both sexes (Fig. 234) are hollow organs from which ducts lead directly to the exterior. Primarily there is one gonad on each side, but they often unite more or less completely above the alimentary canal. The ducts usually open near the middle of the body, though the male openings of cirripedia and some cladocera are almost terminal and the female opening of cirripedia is on the first thoracic somite. Save in the Cirri- pedia, the Malacostraca, and some of the Cladocera, the ducts of the two sexes open upon the same somite. The spermatozoa are very varied in form and often of complex structure ; usually, but not always, they are immobile. They are trans- ferred to the female, often in packets (spermatophores) . The ova have usually much yolk, and meroblastic, centrolecithal cleavage (Fig, Fig. 235. A ventral view of the first Nauplius of Cyclops. After Dietrich. an.' antennule; an." antenna ;^n. gnathobase; Ibr. labrum; md. mandible. 213 A-C), but sometimes are less yolky and undergo total cleavage. Gastrulation may be by invagination (Fig. 213 D) or by immigration. Occasionally the eggs are set free at laying, but in the great majority of cases they are retained for a time by the mother, either in some kind of brood pouch or adhering in some way to her body or appendages. Development is not infrequently direct, but in most cases involves a larval stage or stages. Typically, the crustacean hatches as a Nauplius larva (Fig. 235), a minute creature, egg-shaped with the broad end in front, unseg- mented, but provided with three pairs of appendages — the antennules, which are uniramous, and the antennae and mandibles, which are biramous and should each bear a gnathobasic process or spine directed towards the mouth, though those of the mandibles are often not developed at first. The antennal ganglia are as yet postoral (see p. 340). The median eye is the only organ of vision. A pair of frontal organs (p. 342) are present as papillae or filaments. There is a large CRUSTACEA 353 lab rum. Fore, mid and hind guts can be recognized in the alimentary- canal. Antennal glands may be present. This larva is found in some members of every class of the Crustacea, though among the Mala- costraca only certain primitive genera possess it, and in the Ostracoda it is modified by having already at hatching a precociously developed bivalved carapace. In every class, however, it is also often passed over, and becomes an embryonic stage within the egg membrane or in a brood pouch, the animal hatching at a later stage, such as the Meta- nauplius and Zoaea mentioned below, or even almost as an adult. In the Branchiopoda and Ostracoda the Nauplius is transformed gradually into the adult, adding somite after somite in order from before backwards by budding in front of the telson, much as somites are added to the trochosphere in the development of annelids, while by degrees the other features of the adult develop. The early stages of this process, which possess more somites than the Nauplius, but have not yet the adult form, are known as Metanauplii. The carapace is often foreshadowed quite early by a dorsal shield, which later grows out behind and at the sides to assume the form which it has in the adult, and the appendages, at first mere buds, gradually take on their final shapes. In most cases, however, the process just described is modified. {a) It makes a sudden great advance at one moult. In the Cirripedia the late Nauplius passes with a leap to the so-called Cypris larva, which has many of the features of the adult : a similar leap takes the copepod Metanauplius to the first " Cyclops'' stage (p. 373) and those of Malacostraca to the Zoaea. {b) Certain structures may be pre- cociously developed. In those of the Malacostraca which have Nauplii, the Metanauplius is followed by stages, known as Zoaeae, in which the abdomen is well developed, while the thorax, though it already possesses in front a few pairs of biramous appendages, is still rudimentary in its hinder part. In these larvae also the last pair of abdominal limbs usually appears, or comes to functional develop- ment, before the others. Zoaeae, however, most often are not preceded by a free Nauplius but appear as the first free stage (Fig. 291 A), (c) Temporary retrogression of certain organs takes place during the development of some of the Malacostraca: this affects some of the thoracic limbs in certain Stomatopoda and the prawn Sergestes, abdominal swimmerets and the antennule in the prawn Penaeus. Class BRANCHIOPODA Free Crustacea with compound eyes ; usually a carapace ; the mandi- bular palp very rarely present and then as a minute vestige; and at least four, usually more, pairs of trunk limbs, which are in most cases broad, lobed, and fringed on the inner edge with bristles. 354 THE INVERTEBRATA The Branchiopoda are, on the whole, the most primitive class of the Crustacea. This is seen in the varying and usually large number of their somites, the usually small amount of differentiation in the series of limbs on the trunk, the vascular system of the lower members of the group (p. 348), and the nervous system of all (p. 340). Their mouth parts, on the other hand, are small and simple in structure, a condition in which they are not primitive but exhibit reduction. Nearly all of them are, like sundry other archaic animals, of freshwater habitat, and their characteristic mode of feeding is the taking, by means of setae upon their trunk limbs, of particles of detritus or plankton from suspension in the water. The primary divisions of the class have been mentioned on p. 327. The most conspicuous differences between them are in the cara- pace, the compound eyes, the antennae, the trunk limbs, and the telson. The carapace is very variously developed. In the Anostraca it is not present. The Notostraca have it as a broad, shallow cover over the back. In the normal Cladocera ("Calyptomera") it bends down at the sides to enclose the trunk as a shell, which forms a brood pouch over the back. In the two groups of aberrant Cladocera which (though they are probably not closely related) are together known as "Gym- nomera" this shell has shrunken to a dorsal brood pouch leaving the trunk partly or wholly uncovered. In the Conchostraca it forms in the same way as in the Cladocera a shell, but here the head is usually enclosed as well as the trunk, and there is a distinct dorsal hinge of thin cuticle separating two valves which can be closed by an adductor muscle situated in the maxillulary somite. Usually the carapace leaves the trunk free within it, but in the Cladocera it fuses with two — in Leptodora (p. 368) with all — of the thoracic somites. The antennae, which in the Nauplius are biramous and natatory, retain this condition in the adult of those forms (Diplostraca) in which the enclosing carapace has deprived the trunk limbs of the swimming function, and also in the extinct Lepidocaris (Lipostraca). In the recent Anostraca the antennae are stout but uniramous and not natatory; in the male they are adapted to clasping the female. In the Notostraca, which apply the head to the ground in feeding, they are reduced to uniramous vestiges. The trunk limbs (except in the aberrant Cladocera which constitute the Gymnomera) are phyllopodia (p. 336) which bear on the median side endites furnished with feathered bristles and on the outer side, besides the exopodite or flabellum, a thin-walled branchia and often also one or two proepipodites. With these appendages the Anostraca and Notostraca swim, and all members of the class breathe and gather food. Beating rhythmically forward and backward with a movement BRANCHIOPODA 355 which each pair starts a little earlier than the pair in front of it, they cause, bya pumping action which shall be described presently(p. 358), a flow of water into the median gully whose sides are formed by the two rows of limbs, thence outwards into the spaces between each limb and its neighbours in front and behind, and then backwards. This current brings with it the particles which serve for food, bathes the branchiae, and causes, in the Anostraca and Notostraca, forward movement of the body. As the water passes outwards, the food particles are, by the bristles on the endites, strained off and retained in the median gully. The apparatus varies in detail with the nature of the food. In the Notostraca, which feed mainly by stirring up, with the tips of their thoracic limbs, detritus on the bottom and then filtering it, the bristles on the endites are not adapted to straining out fine particles (which therefore escape with the outgoing current) but detain coarser particles. This is perhaps the primitive mode of feeding of the Branchiopoda, and may even be inherited from the Trilobites. In the Anostraca and Diplostraca there is a special ap- paratus for filtering off fine particles. This consists of a close set row of long, finely feathered setae, placed on the edge of the endites and so disposed as to cover the opening from the median gully to the space between the limb and its neighbour behind. Members of these orders which derive part or all of their food from detritus have various kinds of apparatus, composed of bristles, for removing the coarse particles and passing them backwards to be either swept away with the out- going stream or broken up for food by the hinder members of the series of limbs. Finally, the material gathered is passed forwards to the mouth in a median "food groove" along the belly by a current whose causation is a matter of dispute. The feeding apparatus whose principles have just been described differs greatly in detail in different branchiopods, and reaches its highest complication in the tribe of cladocera known as Anomopoda, to which the common water-flea Daphnia belongs. Examples of it are described more fully below. The Gymnomera have slender, mobile, jointed trunk limbs with which they manipulate the relatively large organisms which serve them for food. The telson is in the Anostraca subcylindrical, with the caudal rami as elongate plates or styles ; in the Notostraca it has the rami long and many-jointed, and is in Lepidurus produced backwards on the dorsal side as a plate. In the typical Diplostraca it is flexed ventrally and produced backwards laterally into a pair of strong, curved, toothed claws, and can be brought forward ventrally to clear the gully between the limbs. In the Gymnomera it has re-straightened. The compound eyes are in the Anostraca stalked (in Lepidocaris they appear to have been absent). In the remainder of the class they are 356 THE INVERTEBRATA sessile and covered by an invagination of the outer cuticle, which forms a shallow chamber over them. Artemia salina (p. 359) and a few marine cladocera are the only members of the class whose habitat is not in fresh water. Throughout the group, thick-shelled eggs capable of resisting drought or freezing are produced by sexual reproduction. Often there is also parthenogenesis, the eggs of which are usually thinner shelled than those that are sexually produced (see p. 367). The name Phyllopoda, which is applied sometimes to the whole class and sometimes to its members exclusive of the Cladocera, is on account of this ambiguity best not employed in systematic nomenclature. Order ANOSTRACA Branchiopoda without carapace ; with stalked eyes ; with antennae of a fair size but not biramous ; with the trunk limbs numerous and all alike; and with the caudal rami unjointed, and flat or subcylindrical. We may take as an example of this group, Chirocephalus diaphanus (Fig. 236), one of its two British representatives. This creature turns up from time to time in temporary pools of water in various districts. It is about half an inch in length, transparent, and almost colourless, save for the reddened tips of most of the appendages and of the ab- domen, the black eyes, and often a green mass of algae in the gut. It is incessantly in motion, swimming on its back. Its delicate appear- ance, and the iridescent gleaming of the bristles on its appendages as they are moved have earned it the name of the fairy shrimp. The body is long, subcylindrical, and enlarged anteriorly to form the head^ upon which the mandibular groove (p. 332) is conspicuous. The head has in front a median eye and a neck organ (p. 342), and bears at the sides : {a) the large, stalked compound eyes; (b) the antennules, slender, un- jointed, and ending in a tuft of sense-hairs ; (c) the stout antennae^ tri- angular in the female but in the male (Fig. 237) elongate, two-jointed, and carrying on the inside at the base a complicated, lobed "frontal appendage " which comes into play when the limb is used for clasping the female ; {d) the mandibles, whose bases are prominent at the sides of the head, while the remaining part of each of them is directed to- wards the mouth as a process with a blunt, roughened end. Below, the head bears {a) the large labrum which is directed backwards under the mouth ; {b) the paragnatha, a pair of small, hairy lobes behind the mouth; [c) the maxillules, a pair ot small triangular plates fringed by long bristles ; {d) the maxillae, which are microscopic vestiges, each bearing three spines. Behind the head come eleven thoracic somites which bear each a pair of phyllopodia. Fig. 238 shows that these possess all the typical BRANCHIOPODA 357 features of such limbs but are remarkable for the distal position of the exopodite and for the very long basal endite, which may be simply the gnathobase (p. 337) but probably represents also the th.M h. a I. md. Fig. 236. A female of Chirocephalus diaphanus. The animal is seen from the right-hand side in the morphological position : normally it swims upside down. ab. I, ab.y, first and seventh abdominal somites; al. alimentary canal; an/ an- tennule; an." antenna; e. compound eye; e.' median eye; egg p. egg pouch; h. heart; Ibr. labrum; Ir. liver; 7nd. base of mandible; nk.on. neck organ; ov. ovary; rayn. ramus of caudal fork; tel. telson; th. 11, eleventh thoracic limb; th. 12, twelfth thoracic somite. nk.on. >an: Fig. 237. Fig. 238. Fig, 237. A front view of the head of a male Chirocephalus. an.' antennule; an." antenna; e. compound eye; e.' median eyQ\ fr.ap. frontal appendage; nk.on. neck organ. Fig. 238. A thoracic limb of Chirocephalus, mounted flat. br. branchia; bri. bristles which strain out the food; ep. epipodite; ex. exopodite ; ^6. flabel- \\xra\pr.ep. proepipodites ; 1-7, endites. second endite. The fringe of long bristles on the median border is, in life, directed backwards, roughly at right angles to the main plane of the limb. The twelfth thoracic somite, upon which are 358 THE INVERTEBRATA the genital openings, is fused ventrally with the first abdominal. In the male, it bears a pair of ventrolateral processes in each of which is the terminal portion of a vas deferens, with a protrusible penis which probably represents an appendage. In the female there is here a median, ventral, projecting egg pouch, which, like the penes, is held to represent a pair of limbs. The abdomen consists of seven simple, limbless somites and a telson which bears a pair of caudal rami as narrow, pointed plates, fringed with bristles. The alimentary canal begins with a short, vertical fore gut, or oesophagus. This leads to a mid gut which continues as far as the telson, where it is succeeded by the hind gut or rectum. The mid gut is somewhat wider in the head, where it is known as the stomachy than in the trunk, where it is called the intestine. From the stomach pro- ceeds a pair of sacculated diverticula ("liver"). The /oo^ consists partly of coarse detritus gathered by the trunk limbs from the bottom of the pool, and partly of small organic particles, especially unicellular algae, which are strained off from the water by the trunk limbs in the following manner (Figs. 239, 240). The space which exists between each limb and that behind it is enlarged at the forward stroke, which finishes with the limbs vertical, and narrowed at the back stroke, which ends with them roughly horizontal, lying against the body. During the forward stroke the enlarging of this space exerts a suction. The proepipodites, exopodite, and large distal endite are drawn back by the suction and pressed back by the resistance of the water, till they reach the limb behind and so convert the space just mentioned into a chamber which is closed except on the median side, where it is separated only by the backwardly directed bristle fringe from the median gully between the limbs of the right and left sides. From this gully, therefore, water is drawn into the chambers at the sides as they enlarge, particles which it contains being strained off by the bristles and remaining in the gully. The latter is of course replenished by the entrance of water from the ventral side. During the back stroke, the chambers, as they become smaller and the pressure of the water in them rises, open owing to this pressure lifting the structures which had closed them ; and the water they contain is driven out and back- ward in two ventrolateral streams, the animal being driven forwards. Thus the same movement of the limbs serves both for the gathering of food and for swimming. The particles which are retained in the median gully are drawn dorsalwards because the suction of the side chambers is greatest where they enlarge most, at the bases of the limbs, and so get into a median food groove of the ventral surface. There they are carried forward to the mouth by a minor stream, which is said to be caused by the escape forwards at the bases of the limbs of some of the water contained in the lateral chambers at a certain ANOSTRACA 359 phase of the movement. The food is agglutinated by a sticky secretion produced by glands in the labrum, and pushed by the maxillules between the mandibles, which pound it and pass it into the mouth. The organs of excretion are a pair of maxillary glands (p. 345), situated in the hinder part of the head and the first thoracic somite. They are wholly of mesodermal origin. The nervous system (Fig. 210) and the vascular system have been described above (pp. 340 and 348). ^ht gonads are a pair of tubes lying one on each side of the alimentary canal in the abdomen, and are continuous in front each with a short duct. The vasa deferentia lead to the penes, the oviducts to a median Fig. 239. A diagrammatic view of a Chirocephalus swimming on its back. The arrows show the direction of the currents set up by the action of the thoracic limbs, the dotted line the course of the gathered particles in the food groove. Fig. 240. Thoracic limbs of Chirocephalus seen from the median side in two phases of their action. A, The forward stroke : water is being drawn through the fringe of bristles into the space between the limbs, which is enlarging. B, The backward stroke: water is being driven backwards out of the space between the limbs, which is contracting. uterus in the egg pouch. The eggs are enclosed in stout shells and will remain alive in dry mud for many months. The larva at hatching is a late Nauplius in which, though there are no appendages behind the mandibles, the trunk is already distinct from the head. Artemia salina^ the other British species of anostracan, occurs in various parts of Europe in salt lakes and marshes and in pans in which brine is being concentrated. It can endure a very high con- centration of salt, and some of its minor features change with the degree of the concentration, so that it has been described under different specific names. It differs from Chirocephalus in having only six abdominal somites and in the form of the antennae of the male. 360 THE INVERTEBRATA Lepidocaris (Suborder Lipostraca), a minute, blind, freshwater form from the Middle Devonian, was closely related to the Anostraca which survive (Euanostraca), but differed from them in the following, among other respects. It had biramous antennae which recall those of the Cladocera; a clasping organ on the maxillule of the male, instead of on the antenna; and the trunk limbs without branchiae and differentiated into two sets — the first three pairs adapted for gathering food, with gnathobase and with the last endite directed inwards and the exo- podite lateral, and the remaining pairs adapted for swimming, with the last endite and the exopodite directed distally side by side at the end of the limb. Order NOTOSTRACA Branchiopoda with a carapace in the form of a broad shield above the trunk ; the compound eyes sessile and close together ; the antennules and antennae much reduced ; the trunk limbs numerous, the first two .fi nid. Fig. 241. A ventral view of the head region of Lepidurus glacialis. From Caiman, a.' antennule; a." antenna; gn. gnathobase; L. labrum (turned forwards); /. paragnathum; md. mandible; mx.' maxillule; mx." maxilla. pairs of them differing considerably from the rest ; and slender, multi- articulate caudal rami. This order contains only the genera Apus and Lepidurus^ which differ in but minor features. Apus cancriformis (Fig. 242) is British, but is now very rarely found in these islands. The head is broad and depressed, fiat below and arched above, and forms with the carapace a horseshoe-shaped structure, which bears the eyes above and the small antennules and antennae beneath, at some distance from the sharp front edge. There is a dorsal organ, which is not used for fixa- tion, but no nuchal sense organ. From under the carapace the hinder part of the trunk projects backwards, ending in two long, jointed caudal rami. The genital opening is on the i ith of the trunk somites. Each of these bears a pair of limbs until the 13th (second of the ab- domen) is reached, after which there are two to five pairs to a somite br. Jib. car. th.^^ c Fig. 242. Apus caficr if omits. A, Dorsal view. car. carapace; d.on. dorsal organ ; e. compound eye ; e.' median eye ; ram. ramus of caudal fork ; sh.gl. shell gland (maxillary gland) seen through the carapace ; th.i, processes of the first thoracic limb. B, Ventral view. ab. abdominal limbs; ab.' limbless somites of the abdomen; an.' antennule; car. carapace; Ibr. labrum; ?nd. mandible; mx.' maxillule; mx." maxilla ;^^n. paragnathum; ram. ramus of caudal fork; th.i, first thoracic limb; th. 10, tenth thoracic limb; th.ii, egg pouch on eleventh thoracic limb. C, Side view with the left half of the carapace cut away. br. branchia;^6. flabellum. Other letters as above. 362 THE INVERTEBRATA as far as the 28th somite. Five limbless somites separate this from the telson. The first thoracic limb is a modified phyllopodium, with the endites slender and many-jointed, very long in Apus though shorter in Lepidurus (Fig. 241). The second thoracic limb is less modified in the same direction, the endites being shorter and unjointed. The re- maining trunk limbs (Fig. 224) are normal phyllopodia: they decrease in size from before backwards, and those of the thorax have the endites well chitinized and mobile. Feeding is most often upon detritus (see p. 355), the flat underside of the head being applied to the bottom during the process, but the animals also devour the dead or living bodies of organisms, clasping them with their strong thoracic limbs and rasping fragments from them with the endites. The Notostraca swim well, but can also crawl with their thoracic limbs or clamber with the anterior pairs. The limbs of the genital somite are in the female modified for carrying eggs, the flabellum fitting as a lid over a cup formed by the distal part of the axis. Pvlales are rare, reproduction being normally by parthenogenesis. Order DIPLOSTRACA Branchiopoda with a compressed carapace which usually encloses the trunk and its limbs ; the compound eyes sessile and apposed or fused ; the antennae large and biramous ; four to twenty-seven pairs of trunk limbs, often considerably diflPerentiated ; and the telson usually ending in a pair of curved claws. Suborder CONCHOSTRACA Diplostraca with 10-27 pairs of trunk limbs; the carapace provided with adductor muscle in the maxillulary somite and with hinge, not fused with thoracic somites, and usually enclosing the head; and nearly always a Nauplius larva. No member of this order is British. The animals haunt the bottom and are mainly or exclusively detritus feeders, dealing differently with fine and coarse particles (P-355)-. Estheria (Fig. 243) is a common European genus. A thoracic limb of a related but exotic form is shown in Fig. 222 C. Suborder CLADOCERA Diplostraca with 4-6 pairs of trunk limbs ; the carapace without hinge or adductor muscle, fused with two or more thoracic somites, and not covering the head ; and without Nauplius larva (save in Leptodora). The members of this suborder are the water fleas. They fall into four tribes. Of these, the first, known as Ctenopoda, show affinities BRANCHIOPODA 363 with the lower Branchiopoda in that their trunk limbs, of which there are six pairs, are all alike and all strain food from the water, the gnathobase projects, and the heart is elongate. The shell is well developed and covers the trunk limbs. Sida, which may be taken add ram. ThA Fig. 243. Estheria ohliqua. From Caiman, after Sars. A, Shell of female, from the left side. B, Male seen from the side after removal of left valve of shell, add. adductor muscle; aw.' antennule; an." antenna; md. mandible; ram. caudal ramus; Th. i, first thoracic limb. among weeds in pools in various, parts of Britain, is one of the Ctenopoda. Penilia, one of the few marine cladocera, is another. The second tribe of the Cladocera, known as Anomopoda, contains most of the genera of the suborder. Its members retain a well- developed shell, but the trunk limbs, of which there are often only- five, and sometimes only four, pairs, are highly differentiated for 364 THE INVERTEBRATA various parts of the process of feeding, only some of them doing the actual filtering off of the food particles. The gnathobases of the filter- ing limbs do not project but are enlarged to bear most of the filter fringe. The heart is a short sac in the first two trunk somites. Daphnia and Simocephalus ^ common British forms, found swim- ming in ponds and ditches, are examples of this tribe. Simocephalus (Fig. 244) differs from Daphnia in possessing a cervical groove (p. 332), and in lacking a median dorsal spine which in Daphnia stands on the hinder edge of the carapace. The following description applies to both genera. The head is bent downwards, so that the median eye and the small antennules are ventral to the antennae. A large, sessile com- pound eye, formed by the fusion of a pair, stands in front. Above it is a nuchal sense organ. Of the rami of the antennae one has four joints and the other three, and both bear long, feathered setae. The mouth parts are much like those of Chirocephalus (p. 356). The seg- mentation of the trunk is obscure. The first two somites are fused with the head, as is shown by the position of their appendages. Behind these are three fairly distinct limb-bearing somites (so that there are in all five pairs of trunk limbs), and then three that are limbless and hardly distinguishable and a telson, which is compressed and produced on each side of the anus into a toothed plate, bearing terminally a spine that may represent a furcal ramus. The third free somite is longer than the others and bears its limbs in the hinder part, which suggests that it is the fifth of the six pairs of Sida which is missing here. The limbless region is commonly known as the "abdomen". Two strong dorsal processes on it close the brood chamber behind. The structure of the trunk limbs is shown in Fig. 245. Together they form a food-gathering mechanism which is very efficient be- cause, instead of all working in the same way as those of the Anostraca, they are diflterentiated in adaptation to diff^erent parts of the task. The third and fourth pairs form a pumping and straining apparatus (Fig. 246) which in principle is the same as those formed by the limbs of Chirocephalus, but has for side walls the carapace, against which the proepipodites play, and is closed behind by a barrier formed by the fifth pair. The broad exopodites of the third and fourth pairs open and close the ventral side of the apparatus as they flap to and fro under the pressure of the water. The long, feathered bristles of the first and of the distal part of the second pair guard the ventral opening of the median gully and keep too large particles from being drawn into it. The complex set of bristles upon the large endite or "gnatho- base" (which corresponds both to the first and to the second endite of the ideal series) in this limb play some part — exactly what is dis- puted— in bringing the food to the mouth. Glands in the lab rum produce a sticky secretion as in Chirocephalus. CLADOCERA 365 - -mx: vas de Fig. 244. A, Side view of male Shnocephalus sima. Highly magnified. From Shipley and MacBride. aii.' antennules"; an." antennae; t. testis; vas de. vas deferens ; hep. hepatic diverticulum ; h. heart ; sh.gl. shell gland ; mes. mid gut ; nk.on. neck organ. B, Side view of female Shnocephalus sima, magnified to the same extent as A. From Cunnington. an.' antennules; an." antennae; md. mandibles; mx.' maxillules; Th.i-Th.5, thoracic limbs; hep. hepatic diverticulum; h. heart; ov. ovary; bdp. brood pouch; sh.gl. shell gland; brn. brain; md.g. mid gut; 7ik.on. neck organ. pnep. D Fig. 245. Thoracic limbs of Daphnia pulex. After Lilljeborg. A, First. B, Second. C, Third. D, Fifth Hmb. ap. apical lobe of endopodite; br. branchia; en. endopodite, indistinctly divided, on the 3rd and 4th limbs, into three joints which are not shown; ex. exopodite ; /r. fringe of bristles which strains out the food : normally this fringe stands vertical to the plane of the limb (see Fig. 246, bri.), but it has been mounted flat for drawing; the part of the limb upon which it stands probably corresponds to the gnathobase and two succeeding endites; "gn." "gnathobase"; /)r.e/). proepipodite. CLADOCERA 367 The alimentary canal resembles that of Chirocephalus (p. 358), but the coeca are unb ranched. The food on being swallowed passes direct to the middle part of the mesenteron, where it is digested, and then forwards to the anterior region and the coeca, where the digested products are absorbed and the indigestible residue sent backwards to be formed into faecal pellets in the hinder part of the mid gut. The maxillary gland lies in the carapace. The gonads are simple, elongated sacs lying in the trunk and con- tinuous with their ducts, which open in the male on the telson, in the female dorsally behind the last limb. The eggs are yolky. They are of two kinds, "summer" eggs which have relatively little yolk and de- p.ch. me.gy. -th.3 Fig. 246. A diagram of a transverse section through the thorax of Daphnia. After Storch. bri. bristles of the fringes which strain out the food ; bri.' bristles of the second pair of thoracic limbs which guard the opening of the median gully; car. carapace; d. dorsal surface of the thorax; fd.gr. food groove; me.gy. median gully or filter chamber ; p.ch. chambers between the limbs : the enlargement and contraction of these chambers by the movements of the limbs set up a pumping action by which water is caused to flow through the bristle fringes from the median gully; pr.ep. proepipodites, playing upon the carapace and closing the pumping chambers at their outer sides; 1/1.2-4, sections through the thoracic limbs, which being directed backwards are cut transversely: each limb underlies that behind it. velop rapidly by parthenogenesis in the brood pouch of the mother, and "winter" eggs with much yolk which need fertilization and develop slowly. The winter eggs are fertilized in the brood pouch, but then the cuticle of the carapace, which has thickened, is thrown off as a case — the ephippium — in which they are contained. They go through the early stages of segmentation within a short time, but after this a period of quiescence sets in, during which they may be. dried or frozen without injury. Sexual reproduction takes place at certain times only, normally twice a year. After the winter eggs develop in spring, there are for some half-dozen generations no males, and re- production proceeds by parthenogenesis. Then, about May, a genera- 368 THE INVERTEBRATA tion appears in which males are present. In this sexual and asexual reproduction go on side by side. The same thing occurs again in autumn or at other times when, in unfavourable circumstances, such as cold or starvation, males appear. It is interesting to note that, since parthenogenesis is never suspended by all the females, there is nothing to show that a sexual phase in the life cycle is necessary. The normal cladocerans which compose the tribes Ctenopoda and Anomopoda are often united under the name Calyptomera in contrast to the remaining two tribes, which are known as Gymnomera. These are aberrant forms whose food consists of planktonic organisms relatively much larger than the particles upon which Daphnia feeds. Their carapace has shrunk till it forms only the brood pouch and leaves free the comparatively slender, prehensile trunk limbs with which the food is handled, and their eyes are prominent and adapted to sighting moving objects. They are often bizarre in form. Polyphemus, a British freshwater genus, is an example* of the Tribe Onychopoda. It has a long telson, but the head and "abdomen" are not elongate and the carapace does not fuse with the hinder part of the "thorax". The trunk limbs have gnathobases. In Evadne and Podon, marine members of the tribe, the telson is not elongate. Leptodora (Fig. 247), the only member of the Tribe Haplopoda, is a pelagic inhabitant of certain fresh waters in Britain and elsewhere. The body is long and slender owing to elongation of the head and of the " abdomen", in which the segmentation is distinct. The fore part of the trunk bears six pairs of slender, jointed, uniramous limbs, without gnathobases. The carapace has fused with all the somites of this region and projects behind it as a brood pouch. The winter tgg gives rise to a Nauplius, the only instance of a larva in the Cladocera. Class OSTRACODA Free Crustacea, with or without compound eyes; with a bivalve carapace and an adductor muscle; a mandibular palp, usually bi- ramous; and not more than two recognizable pairs of trunk limbs, these not being phyllopodia. The small crustaceans which compose this class differ little in the general form of the body but show very great variety in that of their appendages. All their cephalic limbs are well developed and complex; the trunk limbs are uniramous and one or both pairs may be lost. The adductor is in the maxillulary somite. There is often a gastric mill and usually a pair or more of hepatic coeca : the latter and the gonads may (Cypris) extend into the shell valves. Both antennal and maxillary glands are present, both have ectodermal ducts, and both CRUSTACEA 369 are without opening in the adult. Other glands may be excretory. The NaupliuSy if present, has a bivalve shell. There are among the ostracods freshwater and marine, pelagic and bottom-living forms. Parthenogenesis is common among them, and in some males have never been found. fh.1 -ram. Fig. 247. A female of Leptodora kindti. After Lilljeborg. an.' antennule; an." antenna; car. carapace; mdg. mid gut; ov. ovary; ram. ramus of caudal fork; tel. telson; th.i, first thoracic limb; trk.g, ninth trunk somite. Cypris (Fig. 248) is a common British freshwater genus. It swims well, by means of its antennae, but i& not pelagic. It is omnivorous, feeding on algae, small animals, detritus, etc., and taking its food in various ways. Large objects are pushed into the shell by the antennae or pulled in by the mandibles, finer particles drawn in by the action of the epipodites of the maxillules (whose fan of setae is conspicuous in the figure), gathered by long bristles on the palps of the mandibles, 370 THE INVERTEBRATA and passed towards the mouth by the endites and endopodites of the maxillules, assisted by the gnathobase of the maxillae. The first trunk limb is used in crawling, and the second in cleaning. Cypris lacks the compound eyes and the heart, which are found in some other members of the class — for instance in the marine Cypridina, which is also characterized by a large antennal exopodite, turned outwards in a notch of the shell for rowing. an.— an: jTh.Q ram. Fig. 248. Lateral view of Cypris. After Zenker, an/ antennules; an." an- tennae; md. mandibles; mx.' ist maxillae; mx." 2nd maxillae; Th.i, Th.2, thoracic limbs ; ram. ramus of caudal fork ; e. eye. Class COPEPODA Free or parasitic Crustacea, without compound eyes or carapace; with biramous or uniramous palp, or with none, on the mandible; and typically with six pairs of trunk limbs, of which the first is always and the sixth often uniramous, the rest biramous, and none are situated behind the genital aperture (i.e. on the abdomen). The form of the body varies greatly in the members of this class, from the pear-shaped or club-shaped free-swimming genera to the distorted, unsegmented, and sometimes even limbless adults of some of the parasites. In all cases in which the segmentation is complete the number of somites is the same — sixteen, including a preantennu- lary somite but not the telson — throughout the group, but the actual tagmata, which do not conform to the limits of the head, thorax, and abdomen, are not uniform in all members of the class. We shall take as an example of the group the little freshwater crustacean Cyclops (Fig. 249) which, though it is not one of the most primitive members of the Copepoda, is well segmented and can be obtained everywhere in ponds and ditches. The shape of this animal is that of a slender pear with a stalk. The front part of the pear is unsegmented; this is a compound head or "cephalothorax", com- posed of the true head and the first two thoracic somites : beneath, in front, it bears a blunt projection, the rostrum. The rest of the broad CRUSTACEA part of the body contains three somites, the third to fifth of the thorax. The cephalothorax and these free thoracic somites are produced at an'. Fig. 249. Cyclops. A, Dorsal view of female. Partly after Hartog. An. position of anus ; aw.' antennule; «/z." antenna; e.^. egg sacs; e. eye; g.som. compound somite, consisting of the last thoracic (bearing the genital opening) and the first abdominal; od. oviduct; ov. ovary; ram. ramus of caudal fork; 5-^^ spermatheca or pouch for receiving the spermatozoa of the male ; ut. uterus: i.e. pouch of the oviduct into which the eggs pass before being shed. B, Ventral view of male. ab. abdomen; an/ antennule; an." antenna; cop. copula; e. eye; Ibr. labrum; ynd. mandible; mx.' maxillule; w^c." maxilla; 7nxp. maxilliped; pgn. paragnathum; rant, ramus of caudal fork; tel. telson; th. 2, th. 6, thoracic limbs. the sides into low pleural folds. The stalk begins with a short somite which is united to, but distinguishable from, that which succeeds it. The next somite bears the genital openings and is therefore, on the 372 THE INVERTEBRATA convention we have adopted (p. 333), the last somite of the true thorax, but is usually reckoned as the first of the abdomen ; in the female it is fused with the somite which succeeds it. Two free ab- dominal somites and a telson, which bears two styliform, setose caudal rami, complete the body. The somites of the thorax bear limbs, which will be described presently. The limbs of the somite of the genital opening are present in the female only, and in her are re- duced to the condition of small valves over the openings of the oviducts. The abdominal somites are without limbs in either sex. It will be seen that the actual tagmata of Cyclops are not the head, thorax, and abdomen, how- ever the limit between thorax and abdomen be fixed, but are a cephalothorax of eight somites (including the preantennulary), a mid-body (sometimes, but unsuitably, named the " metasome ") of three somites, and a hind body or **urosome" of five somites and the telson. On the head, the median eye is well de- veloped. The antennules are long, uniramous, provided with sensory hairs, divided into seventeen segments, and in the male bent as hooks to hold the female. The antennae are shorter, slender, uniramous, and four-jointed. The mandibles (Fig. 250, md.) have a toothed blade (gnathobase) projecting towards the mouth and a papilla, bearing a tuft of bristles, which represents the palp. The maxillules have a large gnathobase and small endopodite and exopodite. The maxillae are uniramous. The maxillipeds (first pair of thoracic limbs) Fig 250. Mouth parts of f ^ f , . 1 • 1 Cyclops, r rom bedgwick, are also uniramous; they stand immediately ^f^^j. ciaus. en. endopo- internal to the maxillae. The 2nd to ^th dite; ex. exopodite; md. thoracic limbs, of which the 2nd stands on the mandible ;mx/maxillule; head, are biramous, with broad, flat, spiny J^'^-" maxilla ;wj£:;).maxil- rami (Fig. 249 B). The protopodites of each pair are united by a transverse plate or "copula" so that they move together in swimming. The thoracic appendages of the 6th pair are small and uniramous. The swimming of Cyclops is of two kinds — a slow propulsion by the antennae and antennules, and a swifter progression brought about by the use of the swimming limbs (2nd to 5th pairs) of the thorax. In 7nxp COPEPODA 373 the more primitive, pelagic copepods (Calanus, etc.) which have biramous antennae and biramous palps on the mandibles, the an- tennules do not take part in swimming. Such copepods feed by an automatic straining of particles from the water, though their ap- paratus for this purpose (see below) is very different from that of the Branchiopoda. Cyclops, on the other hand, in a manner of which the details are not understood, seizes its food particles from time to time. The alimentary canal is of much the same nature as that of Chiro- cephalus but without mid gut diverticula. It possesses well-developed extrinsic muscles, of which those that run from its anterior region to the adjoining body wall produce rhythmical displacements of the canal and so cause a movement of the blood, while the dilators of the rectum draw in water which is believed to subserve respiration. Special organs for circulation and respiration are wanting in Cyclops, though other copepods have a saccular heart. Maxillary glands are present — probably entirely mesodermal. The ventral cords of the nervous system are concentrated into a single ganglionic mass. The gonads are single median structures which lie above the gut in the first two thoracic somites. The ducts are paired. In the female a large, branched uterus adjoins the ovary on each side, communicating with the lateral opening on the urosome by an oviduct which at its termina- tion receives a duct from the spermatheca. The latter is median, in the same segment as the oviducal openings, with a median entrance of its own. The male transfers his spermatozoa to the female in a spermatophore. The eggs when laid are cemented into a packet {(^gg "sac") which hangs from the opening of the oviduct, and are thus carried until they hatch. The possession of a pair of such packets gives a characteristic appearance to the females of Cyclops, as to those of many other copepods. In some genera, however, there is a single median packet, and in a very few the eggs are laid into the water. The larva hatches as a typical Nauplius (Fig. 235). This is suc- ceeded by several Metanauplius stages, and then suddenly at a moult takes on the first Cyclops stage, which has the general form of the adult but lacks appendages behind the 3rd pair of swimming limbs and also the somites of the urosome. In five successive Cyclops stages the missing somites appear, the tale of limbs being meanwhile completed. Calanus, which is marine and pelagic in all parts of the world, often occurring in enormous shoals which ^re an important item of food for fishes and whales, is in several respects more primitive than Cyclops, having the antennae and mandibular palps (Fig. 222 D) biramous, well-developed and biramous limbs on the 6th thoracic somite, and only one postcephalic somite in the cephalothorax. The 6th thoracic somite is included in the mid-body, not in the urosome. The primi- 374 THE INVERTEBRATA tive custom of feeding by the automatic straining of food particles from the water is retained : the feeding current eddies from the swim- ming current which the antennae, mandibles, and maxillae set up, and is strained through a fringe of bristles on the maxillae (Fig. 251). The parasitic habit has been adopted by members of very different families of copepods, and to very various degrees even by members of a single family. Every stage may be found between normal, free- living forms and the most degenerate parasites. Parasitic forms often have a suctorial proboscis, which is formed by the upper and lower lips enclosing mandibles adapted to piercing. Such a proboscis is not necessarily accompanied by a high degree of degeneration. The life histories of parasites are often complicated, and may involve remark- able changes of habit. Degenerate forms usually reach one of the ex- Fig. 251. The maxilla of Calanus. ex. small prominence which perhaps re- presents the exopodite; i and 2, endites representing the first two segments; 9, terminal segment. Cyclops Stages and may pass through them all before they begin to degrade. Often the male is less degenerate than the female: he may be free-swimming while she is sedentary, or may be much smaller and cling to her body. It is only possible here to mention a few of the numerous genera of these interesting parasites. Notodelphys, commensal in the pharynx of ascidians, is clumsy bodied, and has a large dorsal egg pouch on the 5th and 6th thoracic somites, but can swim and is sometimes captured outside the host. Monstrilla has a very remarkable life history. The adults of both sexes are free-swimming, as are the newly-hatched Nauplii^ but the intermediate stages are parasitic in various polychaets, where they absorb nourishment by means of a pair of long, flexible processes which represent the antennae. In this stage they lay up a food supply for the entire life cycle, throughout which the animals are without functional mouth parts or alimentary canal. COPEPODA 375 Chondracanthus (Fig. 252), which infests the gills of various marine fishes, has in the adult stage a large female, whose body is produced into irregular, paired lobes and her appendages degenerate, though the mouth has not a proboscis but is flanked by the three pairs of minute, sickle-shaped jaws. The males are small, retain more of the Fig. 252. Fig. 252. Chondracanthus gibb OS us . After Claus, A, Female. B, Male, more highly magnified, al. alimientary canal; an.' antennule; an." antenna; e. eye; e.s. egg " sac " ; nixp. maxilliped ; t. testis ; th. 2 and 3, thoracic limbs ; vas d. vas deferens; cJ, males attached to females. Fig. 253. Stages in the life history of Lernaea. A, Metanauplius. B, First Cyclops stage. C, "Pupa". D, Sexual stage: coition. E, Ripe female. an.' antennule; an." antenna ; ^z a;, secretion of a gland by which fixation is effected ; hd.pr. processes of the head of the female which are imbedded in the tissues of the host; mxp. maxilliped; sip. siphon; th.z, second thoracic limb; ram. ramus of caudal fork. copepod organization than the female, and cling by hook-like antennae to her body. Caligus, ectoparasitic, mainly in the gill chambers of fishes, is clumsily built and has a suctorial proboscis, but retains the power of swimming. Its sexes do not differ greatly. Lernaea (Fig. 253) hatches as a Nauplius and at the first Cyclops 376 THE INVERTEBRATA Stage becomes parasitic on the gills of a flat fish, deriving nourish- ment from its host by means of suctorial mouth parts. Here it passes into a "pupal" stage in which the power of movement is lost and retrogressive changes have taken place. Presently it regains the power of swimming and leaves the host in an adult copepod stage. In this stage impregnation takes place. The male develops no further, but the female attaches herself to the gills of a fish of the cod family, where by a great development of the genital somite she becomes con- verted into a vermiform parasite, anchored into the host by processes that grow out from her head, and retaining only the now relatively minute appendages of the thorax. In Herpyllobius, parasitic on annelids, the female is reduced to a mere sac, drawing nourishment from the host by rootlets and bearing minute males which are also sac-like. Xenocoeloma, also parasitic on annelids, is represented in the host's body only by the gonads, which are hermaphrodite, and some muscles, enclosed in a cylindrical outgrowth of the host's epithelium which forms a body wall for the vestiges of the parasite and contains a gut-like prolongation of the host's coelom. Class BRANCHIURA Crustacea, temporarily parasitic on fishes; which possess compound eyes ; a suctorial mouth ; carapace-like lateral expansions of the head which are fused to the sides of the first thoracic somite ; an unseg- mented, limbless, bilobed abdomen with a minute caudal furca; and four pairs of thoracic limbs, which are biramous, with usually a proximal extension of the exopodite. The members of this group in many respects superficially resemble the Copepoda, with which they are generally placed, but differ from that class in certain important features, notably in the possession of compound eyes, the lateral head-lobes, the opening of the genital ducts between the fourth pair of thoracic limbs, and the phyllopod- like proximal overhang of some of the thoracic exopodites (Fig, 254 B). The carp-lice, as the Branchiura are called, are found both on fresh- water and marine fishes. They are good swimmers. The females deposit their eggs on stones and other objects. The larvae differ little from the adult. Argulus (Fig. 254), the principal genus, has a pair of suckers on the maxillae and a poison spine in front of the proboscis. A.foliaceus is common on freshwater fishes in Britain and the Continent. Class CIRRIPEDIA Fixed and for the most part hermaphrodite Crustacea ; without com- pound eyes in the adult ; with a carapace (except in rare instances) as CRUSTACEA 377 a mantle which encloses the trunk; with usually a mandibular palp, which is never biramous; and typically with six pairs of biramous thoracic limbs. The great majority of the Cirripedia are extremely unlike the rest of the subphyllum, and would not be recognized as crustaceans at all by the layman. The familiar members of the class are the ordinary barnacles (Thoracica). Besides these, however, it contains several groups of related organisms, of which the parasitic barnacles (Rhizo- cephala) are the best known. The Ascothoracica link the class to other crustaceans. ,an . sip.- — ^rrrrz^ Fig. 254. Arguliis. A, A ventral view of a female of A. americanus. From Caiman, after Wilson. B, The second left swimming limb of A. foliaceus. After Hansen. An. position of anus; an.' antennule; an." antenna; e. paired eye; ex. exopodite; mx.' maxillule; mx." maxilla; ram. ramus of caudal furca; in some species the rami stand immediately on each side of the anus; sip. siphon, or suctorial proboscis; sp. poison spine. Order THORACICA Cirripedia with an alimentary canal ; six pairs of biramous thoracic limbs; no abdominal somites; and permanent attachment by the preoral region. We shall take as an example of this group the common goose barnacle, Lepas (Figs. 255, 257 A), found all the world over on float- ing objects in the sea. It hangs by a stalk or peduncle which, as we shall see, represents the foremost part of the head, greatly elongated 378 THE INVERTEBRATA but Still bearing at its far end the vestiges of the antennules, imbedded in a cement by which it is held fast. The glands which produce the cement are contained in the peduncle, and open on the antennules. tgm. an! Fig- 255. A view of Lepas anatifera, cut open longitudinally to show the disposition of the organs. From Leuckart and Nitsche, partly after Claus. stk. stalk; cna. carina; tgm. tergum; scu. scutum; an.' antennule; md. man- dible with "palp" in front; mx.' ist maxilla; mx." 2nd maxilla; Th. the six pairs of biramous thoracic limbs; Ihr. labrum; M. mouth; oe. oesophagus; Ir. "liver" coeca; st. stomach; An. anus; ov. ovary; od. oviduct; t. testes; ves.sem. vesicula seminalis; p. penis; cent, cement gland and duct; add. ad- ductor scutorum muscle, which closes the carapace; mtl.ca. mantle cavity, i.e. the space intervening between the carapace and the body. The rest of the body is known as the capitulum, and is completely enclosed in the carapace or mantle, a fleshy structure strengthened by five calcified plates — a median dorsal carina, and on each side two known as the scutum and tergum. The scuta are anterior to the terga, that is, nearer to the peduncle. The mantle cavity opens by a long slit LEPAS 379 on the ventral side. Within the mantle cavity lies the body, turned over on its back with the appendages upwards (or downwards, as the animal hangs) and connected with the peduncle and mantle only at the extreme anterior end, where there is a preoral adductor muscle by which the sides {valves) of the mantle can be drawn together and so the opening closed. The antennae, which should be somewhere in this region, are absent. The prominent mouth is overhung by a large labrum. At its sides stand the mandibles, which have a flat, toothed process towards the mouth and a large, uniramous, foliaceous palp^ and the maxillules, simple structures with a fringe of strong bristles on the notched median edge. A pair of simple, hairy lobes, united by a median fold, which shut in the mouth and its appendages from be- hind, represent the maxillae. The six pairs of thoracic limbs or cirri have each two long, many-jointed, hairy rami, curled towards the mouth. They are successively longer from before backwards. A couple of filamentous epipodites ("gills") stand on the protopodite of the first pair. Behind the cirri stands a long median ventral penis ^ and behind this again is the anus, with a pair of vestigial caudal rami. The animal feeds by thrusting out the cirri through the mantle opening and withdrawing them with a grasping motion, whereby particles are gathered from the water by the setae upon the limbs. If it be molested the motion ceases and the valves are drawn to. The alimentary canal has an oesophagus (stomodaeum) directed forwards from the mouth to the long wide stomach which bears several coeca around its commencement and tapers behind into an intestine. Com- plicated maxillary glands open on the maxillae. There is no heart or system of blood vessels. The nervous system has a suboesophageal ganglion, and a separate ganglion for each pair of cirri behind the first. Lepas is hermaphrodite. The ovaries lie in the peduncle and the oviducts open on the bases of the first pair of thoracic limbs, much further forwards than is usual in Crustacea. The testes are branched tubes which lie at the sides of the alimentary canal and in the basal parts of the cirri. Each vas deferens enlarges into a vesicula seminalis whose duct joins that of its fellow in the penis. Impregnation takes place by the penis depositing a mass of spermatozoa on either side of the mantle cavity of a neighbouring individual, near the opening of the oviduct. It is possible that isolated individuals may be self- fertilized. The ova undergo their early development within the mantle cavity of the mother attached in a flat mass, the ovarian lamella, by a glutinous secretion manufactured by the terminal enlargement of the oviduct, to a fold of the mantle which projects on each side from near the junction with the body and is known as an ovigerous frenum. The young are set free as Nauplii, characterized, as are those of nearly all cirripedes, by a pair of lateral frontal horns , on each of which 380 THE INVERTEBRATA opens a unicellular gland (see Fig. 258). These are processes of a dorsal shield which in later stages acquires other spines. After several moults the larva suddenly passes into the so-called Cypris stage. It is now enclosed in a bivalve shell with an adductor muscle, and possesses a pair of compound eyes. The antennules of this stage possess near their ends a disc on which opens the cement gland. The antennae have disappeared. There are six pairs of biramous thoracic limbs and a small abdomen of four somites. The Cypris larva becomes fixed by the discs on its antennules, and its body rotates within the shell, so M ' I I th. M. e! e. an'. scu. na. B Ih. e'. CC' al. y- an'. Fig. 256. Diagrams of three stages in the metamorphosis of Lepas. From Korschelt and Heider. A, The Cypris stage. B, The attached larva (pupa). C, The young Lepas. ab. abdomen; al. aHmentary canal; an.' antennule; car. cuticle of carapace of larva, not yet shed; cna. carina; e. compound eye; e.' median eye; M. mouth; scu. scutum; tgm. tergum; th. thoracic limbs; X. origin of carapace fold ; y. a ventral fold of the head. that the ventral surface is directed backwards (Fig. 256 A, B). Now the shell and body are rotated upwards on the antennae so that the adult position is assumed (Fig. 256 C); meanwhile the shell plates appear, the preoral region elongates to form the peduncle, and the abdomen disappears. Scalpellum (Fig. 257 C, D) attaches itself to fixed objects, usually in deep waters. It diff^ers from Lepas in possessing a number of ad- ditional plates on the capitulum, and scales of a similar nature on the peduncle. It is more remarkable in possessing what are known as complemental males . A few species of the genus are composed entirely THORACICA 381 of hermaphrodites as Lepas is. In most, however, some individuals are without female organs. These individuals are always smaller than those which possess ovaries, and live within, or at the opening of, the mantle cavity of the latter. In some species they almost perfectly scu. ^'tgm. --cna. Fig. 257. Cirripedia Thoracica. A, Lepas anatifera. B, Balamus. C, Scal- pellum vulgare. D, Male of the same, enlarged. A— C, after Darwin; D, after G. Smith, cna. carina ; cnl. carinolateral ; e. vestige of eye ; la. lateral ; op. open- ing of mantle cavity; rst. rostrum; rstl. rostrolateral ; scu. scutum; stk. pe- duncle ; t. testis ; tgm. tergum ; S, dwarf males. resemble these in organization, but usually they are more or less de- generate, being sometimes even without an alimentary canal. As a rule the more degenerate live within the mantle cavity of the partner, the less degenerate on its mantle edge. In certain species, which have 382 THE INVERTEBRATA very degenerate males, the large individuals are without testes, so that the sexes are separate. The function of the complemental males is probably the effecting of cross-fertilization, for the species which possess them are of solitary habit. The phenomenon perhaps arose from the settling of young hermaphrodite individuals on the stalk of old ones, which is common in stalked barnacles. Balanus (Fig. 257 B), the common acorn barnacle, differs from Lepas in the lack of a stalk, and in having an outer wall of skeletal plates homologous with some of the extra pieces on the capitulum of Scalpellum . Order ACROTHORACICA Cirripedia of separate sexes ; with an alimentary canal ; fewer than six pairs of thoracic limbs; and no abdominal somites; permanently sessile on the preoral region, in which the antennules are absent and the cement glands much reduced. These are minute creatures whose females live in hollows which they excavate in the shells of molluscs, while the males are degenerate and have the same relation to the female as have those of the species of Scalpellum in which the sexes are separate. Alcippe, British, lives in the columella of whelks, etc. Order APODA Hermaphrodite Cirripedia; without mantle, thoracic limbs or anus; whose body is divided by constrictions into rings. Proteolepas, the only known member of the order, is a small, maggot-like animal found by Darwin in the mantle cavity of the stalked barnacle Alepas. The antennules, by which it is attached, and the mouth parts, are those of a cirripede. Since the mouth is terminal, at least some of the more anterior of the eleven rings cannot represent somites. Order RHIZOCEPHALA Cirripedia which are parasitic, almOst exclusively on decapod Crus- tacea ; have at no time an alimentary canal ; and in the adult neither appendages nor segmentation; make attachment in the larva by an antennule; and are in the adult fastened to the host by a stalk from which roots proceed into the host's tissues. Sacculina (Figs. 258-261), parasitic on crabs, is the best known example of this group. Its life history is a very remarkable one. It starts life as a Nauplius (Fig. 258 A), with the characteristic frontal horns of cirripede Nauplii but without mouth or alimentary canal. The Cypris larva (Fig. 258 B) clings to a seta of a crab by one of its CIRRIPEDIA 383 antennules. The whole trunk, with its muscles and appendages, is now thrown off and a new cuticle formed under the old one, with a dart-like organ which is thrust through the antennule and the thin cuticle at the base of the seta of the crab into the body of the latter. Through the dart the remnant of the larva, a mass of undifferentiated cells surrounded by a layer of ectoderm, passes into the host's body cavity. Carried by the blood it becomes attached to the under side of the intestine (Fig. 259). There rootlets begin to grow out from it and eventually permeate the body of the crab to the extremities of the Fig. 258. Larval stages of Sacculina. From G. Smith. A, Nauplius, B, Cypris. A.i, antennule; A. 2, antenna; Ab. abdomen; E. undifferentiated cells; F. frontal horn with gland cells; GL gland cells; Md. mandible; Ten. frontal tentacles (frontal organs) ; Tn. tendon. limbs. Meanwhile a knob also grows from the mass; forms within itself a mantle cavity surrounding an internal "visceral mass" which contains the rudiments of genital organs and a ganglion ; presses upon the ventral integument of the abdomen of the host, whose cuticle is thus hindered from forming at that spot ; and consequently at the next moult of the crab comes to project freely under the abdomen, where it may be found in the adult condition. The phenomenon known as parasitic castration is exhibited by crabs attacked by Sacculina. The moult at which the parasite becomes external produces a change in the secondary sexual characters in the 384 THE INVERTEBRATA new cuticle. The male crabs have a much broader abdomen, reduced copulatory styles (these may disappear altogether), and abdominal svvimmerets (which carry the eggs in the female, and are absent in the normal male). There is, in short, a marked tendency to the female type. In the female crabs there is also a change, but this is held to be not towards the male but towards the juvenile type. The gonads dis- appear, but cases have been observed in which the parasite has been killed and months afterwards what was probably an originally male crab has regenerated a hermaphrodite gonad. Parasitic castration is d.i — A B Fig. 259. Stages in the development of Sacculina upon the mid gut of a crab. From G. Smith. A, Early stage. B, Later stage, b. swelhng caused by the body of the Sacculina; c.t. central tumour upon which the body arises; dd.y d.s. inferior (posterior) and superior (anterior) diverticula of the gut of the host; n. "nucleus" or rudiment of the body of the Sacculina; op. opening of a cavity in the central tumour, the "perisomatic cavity", from which the definitive body eventually protrudes (not the mantle opening); rt. roots; X. final position of the parasite. the most evident expression of a remarkable and at present ill-under- stood interference by the parasite with the general metabolism of its host. Thompsonia (Fig. 262), parasitic on crabs, hermit crabs, etc., is an extraordinary case of extreme reduction by parasitism, in which an arthropod is degraded to the level of a fungus. The rootlets of the parasite are widely diffused through the host. Their branches in the CIRRIPEDIA 385 limbs give off sacs which become external at a moult of the host. These sacs contain neither ganglion, generative ducts, nor testes, but only a number of ova in a space of doubtful nature. When they are ripe the ova have become (probably by parthenogenesis) Cypris larvae, which are set free by the formation of an opening. There is no parasitic castration of the host. ,CS^MU Uh. slk. , Sac. Fig. 260. Fig. 260. A specimen of the shore crab (Carcinus) bearing a Sacculina. op. mantle opening; Sac. Sacculina; stk. stalk. Fig. 261. A vertical section of Sacculina at right angles to the plane of greatest breadth. From Caiman, at. atrium of oviduct; ^a. ganglion;^/, col- leteric gland opening into atrium; o. eggs in mantle cavity; op. opening of mantle cavity; ov. ovary; rt. roots; stk. stalk; t. testis. Order ASCOTHORACICA Parasitic cirripedia, which have an alimentary canal from which diverticula extend into the mantle ; six pairs of thoracic appendages ; and a segmented or unsegmented abdomen ; and are not attached by the preoral region. These animals are parasitic and often imbedded in the tissues of their hosts. They are an early branch^of the cirripede stock which has retained the abdomen, in some cases well segmented and provided with movable caudal rami, and has not the characteristic mode of fixation by the antennules, or frontal horns in the NaupUus. Laura (Fig. 264), imbedded in the tissues of the antipatharian 386 THE INVERTEBRATA Gerardia, has the mantle in the form of a very spacious sac with a narrow opening. Its abdomen has two somites and a telson, Syiiagoga (Fig. 263), external parasite on Antipathes, has a bivalve mantle, from which usually protrudes the long abdomen of four somites and a telson. It is possible that this is an immature stage of an animal which is more retrograde when it is adult. Fig. 262. Fig. 263. Fig. 262. An abdominal limb of the prawn Synalpheus infested by Tho?np- sonia, x 120. From Potts, bl. blind branch of root system which after further development will become an external sac; en. endopodite of limb of host; ex. exopodite of the same; mtl. mantle of sac; stk. stalk; vm. visceral mass, occupied entirely by the ovary. Fig. 263. Synagoga mira. After Norman, ab.i, first abdominal somite; ait.' antennule ; car. mantle (carapace) ; M. mouth ; ram. ramus of caudal fork ; tel. telson ; th. thoracic limbs. Class MALACOSTRACA Crustacea with compound eyes, which in typical members of the group are stalked ; typically a carapace which covers the thorax ; the mandibular palp, if present, uniramous ; a thorax of eight somites and abdomen of six (rarely seven), all (except the 7th abdominal) bearing appendages; and a complex proventriculus. The Malacostraca contain a very large number of species, which exhibit great diversity. Nevertheless they are capable of reference to a common type in respect of more features than the members of any CRUSTACEA 387 Other group, though the Copepoda approach them in this. The ideal malacostracan has twenty somites, including the preantennulary and excluding the telson. Of these, six belong to the head (p. 332), eight constitute the thorax, and six the abdomen. This number is only departed from in the Leptostraca, which have an additional somite at the end of the abdomen. (In the embryos of Mysidacea such an additional somite is present, but in the adult it has fused with that which precedes it.) The female openings are always on the 6th thoracic somite, and the male on the 8th. A carapace encloses the thorax at sk.hst. Fig. 264. Laura gerardiae. After Lacaze-Duthiers. A, The animal intact, attached to the skeleton of its host, after removal of the soft tissues of the latter. B, A view obtained by opening the mantle along the dorsal side. a. anterior end; Ir. liver, branching in mantle; mtl. mantle; ov. ovary; sk.hst. skeleton of the host. the sides. The median eye is vestigial in the adult, and the compound eyes stalked. The antennules are biramous, as they are in no crus- tacean of any other group. The antennae have a scale-like exopodite by extending which the animal keeps its body level in the water. The mandibles have uniramous palps and the part which projects towards the mouth is cleft into "incisor" and "molar" processes. The maxil- lules have two endites (on the first and third joints) and the maxillae four, grouped in twos. The thoracic limbs have a cylindrical, five- jointed endopodite (p. 336), used when the animal has occasion to walk or to grasp large particles of food, a natatory exopodite, and two 388 THE INVERTEBRATA respiratory epipodites. The abdominal appendages are biramous; those of the first five pairs (pleopods) slender and fringed and used in swimming, those of the last pair (uropods) broad, turned backward, and forming with the telson a tail-fan, used in rapid backward move- ment. There are no caudal rami. (The Leptostraca are the only members of the class which possess these rami in the adult.) Food is chiefly collected as particles in a stream which is set up by the action of the maxillae and which passes forwards through a filtering fringe of bristles upon the median margins of those appendages. This type is said to possess the caridoid fades . It is adapted prim- arily to swimming and is best exhibited in the small, prawn-like, pelagic forms, formerly classed together as Schizopoda but now dis- tributed, as the orders Mysidacea (Fig. 265) and Euphausiacea, to the Fig. 265. A female oi Mysis relicta. After Sars. bd.p, brood pouch; md.gr. mandibular groove ; sta. statocyst. two main subclasses of the Malacostraca (see below). Departures from it are many and important, and most of its features have dis- appeared more than once independently. Thus the carapace, the inner ramus of the antennule, the scale of the antenna, the mandibular palp, exopodites of thoracic limbs, etc., have been lost in various branches of the malacostracan tree. Only the number of the somites and the size of the tagmata are constant, save in the case of the Lepto- straca already mentioned and in certain parasitic isopods. Departure from the caridoid facies is associated with the abandonment of the swimming habit for crawling or burrowing, and when that happens the animal ceases to gather food by filtration and adopts other modes of feeding, for which its limbs, and particularly the thoracic endo- podites, become variously modified — as, for instance, by the de- velopment of chelae. MALACOSTRACA 389 An exceptionally large number of members of this class have direct development. Of those which possess larvae only a few {Euphau- siacea, a few of the Decapoda) hatch in the Nauplius stage. A special characteristic of the larval development of the Malacostraca is the occurrence of a zoaeal stage (p. 353), in which the carapace and tag- mata are present, the abdomen is better developed than the hinder part of the thorax, and the animal swims by biramous maxillipeds. In crabs, hermit crabs, and some related families the Zoaea is succeeded by a Metazoaea, which differs from it in having uni- ramous rudiments of thoracic limbs behind the maxillipeds. In other forms with larval development there is at this stage a prawn- au'. -l'ih.2 thA- ab.G + t. th.8 ab.i)+(i+t. Fig. 266. Malacostracan larvae. A, Zoaea of Porcellana. B, Schizopod of the lobster, C, Phyllosoma of Palinurus. D, Young Erichthus of a stomatopod. ab. abdomen; an/ antennule; en. endopodite; ex. exopodite; t. telson; th. thorax. The numerals indicate the somites or theirappendages. like Schizopod larva ('' Mysis'' stage), with biramous limbs on all the thoracic somites, which is not always preceded by a Zoaea. The Malacostraca fall into two large groups and three smaller ones. Of the latter, the Leptostraca retain, in the hinder end of the abdomen, a primitive condition, which has been lost in the other groups. The Stomatopoda (Hoplocarida) stand alone in possessing two free pseudo- somites in the anterior part of the head, certain peculiarities of the thoracic limbs, and peculiar gills oathe abdominal appendages. The Syncarida unite certain features which are characteristic of other groups. The large groups Peracarida and Eucarida contain most of the members of the class. The former of these two divisions is character- ized by possessing a brood pouch, formed by plates (oostegites) upon the thoracic limbs, in which the young undergo a direct development. 390 THE INVERTEBRATA and by the freedom of some or all of the thoracic somites from the carapace. The Eucarida do not possess a brood pouch and usually have larval stages, their heart is a short chamber in the thorax, and their carapace fuses with the dorsal side of each thoracic somite. Independently in each of these two groups the caridoid facies has been lost to various degrees, so that the members of each can be roughly arranged in a series which, starting with prawn-like "schizo- pods", ends in the Peracarida with the woodlice and in the Eucarida with the crabs. Subclass LEPTOSTRACA Malacostraca with a large carapace provided with an adductor muscle and not fused with any of the thoracic somites; stalked eyes; the Fig. 267. A female of Nebalia bipes. From Caiman, after Claus. a.' an- tennule; a." antenna; ah} and ab.^ first and sixth abdominal limbs; add. ad- ductor muscle of carapace; /. ramus of caudal furca; p. palp (endopodite) of maxillule; r. rostrum; t. telson; i, 7, first and seventh abdominal somites. thoracic limbs all alike, without oostegites, biramous, and usually foliaceous; seven abdominal somites, of which the last bears no ap- pendages; and caudal rami on the telson. Nebalia (Fig. 267) is the commonest and typical genus of this group. A^. bipes, the British species, may be found between tide- marks, under stones, especially in spots which are foul with organic remains. Nebalia has a rostrum, which is jointed to the head. The antennae have no scale, while the antennules are unique in possessing one. The carapace has an adductor in the region of the maxilla and encloses the four anterior abdominal somites. The thorax is short. MALACOSTRACA 391 Its limbs (Fig. 222 E) are flat. Their endopodite is narrow and possesses five indistinct joints. Sometimes the long basipodite is divided and its distal region added to the endopodite as a preischium (p. 336). The exopodite is broad and there is a very large epipodite, which serves as a gill. (The related Paranebalia, however, has a slender exopodite with a flagellum, and a small epipo- dite.) The first four pairs of abdominal limbs are large and biramous, the fifth and sixth small and uniramous. The alimentary canal possesses a proventriculus of relatively simple type, several pairs of simple mid gut coeca, and an unpaired posterior dorsal coecum. The heart is long, reaching from the head to the 4th abdominal somite. The nervous system is of primitive type (p. 340). The excretory organs have been alluded to on pp. 346, 348. The 2imTsi2\ feeds by straining particles from the water by means of an elaborate arrangement of setae of different kinds on the thoracic limbs, the necessary currents being set up by a pumping action of the same limbs. These work upon a principle similar to that employed by the Branchiopoda, the exopodites and epipodites acting as valves for pumping chambers between the limbs, but it is the backward stroke that enlarges the chambers, and they are closed by the forward flap- ping of their valves. Development is direct, the embryos being carried between the thoracic limbs of the mother, held in by the long setae on the limbs, but not glued to them like the eggs of the crayfish. Subclass HOPLOCARIDA (STOMATOPODA) Malacostraca with a shallow carapace which is fused with three thoracic somites and leaves four uncovered ; two free pseudosomites on the head ; stalked eyes ; the first five thoracic limbs subchelate and Fig. 268. A male Squilla fna?itis. From Caiman, a.' antennule; a." antenna; p. penis; sc. scale (exopodite) of antenna ; th.\ thr, th}, first, second, and last thoracic limbs. the last three biramous ; no oostegites ; a large abdomen whose first five pairs of limbs bear gills on the exopodites, while the sixth forms with the telson a tail fan; and a large, branched "liver". 392 THE INVERTEBRATA The 2nd thoracic limb bears a large, raptorial subchela. The ali- mefitary canal has a rather simple proventriculus and a large branched "liver"; the latter and the gonads extend along the large abdomen. In the nervous system eight pairs of ganglia are fused as the sub- oesophageal ganglion. The heart is very long, reaching from the head to the fifth abdominal somite. The excretory organs are maxillary glands. The larvae are pelagic and of the same general type as the Zoaea but with a peculiar facies of their own (Fig. 266 D). The members of the subclass are all marine, and for the most part live in burrows. Squilla (Fig. 268) occurs in British waters. Subclass SYNCARIDA Malacostraca without carapace; with eyes stalked, sessile or absent; most of the thoracic limbs provided with exopodites and none of them chelate or subchelate ; no oostegites ; a tail fan ; and simple coeca on the mid gut, A small group of freshwater malacostracans with a combination of features which forbids their inclusion in either of the other subclasses. In typical genera, they possess most of the features of the caridoid VIII n cgr Fig. 269. Anaspides tasmaniae, x 3. From Woodward, cgr. mandibular or "anterior cervical" groove; ii, viii, second and eighth thoracic somites; I, 6, first and sixth abdominal somites. facies except the carapace ; and the relatively slight differentiation of thorax from abdomen is a primitive character possessed by no other member of the class. Anaspides (Figs. 223, 269), from pools at 4000 feet in Tasmania, is a normal member of the group. Bathynella, from subterranean waters in Central Europe and Eng- land, small, degenerate, and eyeless, has various limbs reduced or absent and the first thoracic segment free. MALACOSTRACA 393 Subclass PERACARIDA Malacostraca whose carapace, if present, does not fuse with more than four thoracic somites ; whose eyes may be stalked or sessile ; and which possess oostegites; a more or less elongate heart; and a few simple coeca on the mid gut. A large subclass, containing several orders, which range from the prawn-like Mysidacea, in which the caridoid facies (pp. 387, 388) is practically intact, to the Isopoda and Amphipoda (slaters and sand- hoppers) in which the carapace is lost and other features are greatly modified. The important common characters which all these orders possess are the presence of oostegites and the retention of the young, which are directly developed, in a brood pouch formed by those organs. Certain peculiarities, however, of the mandibles, which bear behind the incisor process a movable structure known as the lacinia viobilis, of the thoracic limbs (p. 336), etc., are also possessed in common by the Peracarida. Order MYSIDACEA Peracarida with a carapace which covers most or all of the thoracic somites; the eyes (when present), stalked; the scale of the antenna well developed ; exopodites on most or all of the thoracic limbs, of which one or two pairs are maxillipeds; and a well-formed tail fan. Small, usually pelagic crustaceans, most of which are marine, though a few occur as "relicts" or immigrants in fresh waters. They are mostly carnivorous, but take vegetable matter in the course of feeding. Small food particles are obtained in a current set up by the maxillae (p . 388) and when there are no gills also by a whirling action of the thoracic exopodites, and are strained off by the maxillae: large food masses are seized by the endopodites of the thoracic limbs. My sis (Figs. 265, 270), British, possesses a statocyst on the endo- podite of each uropod, but has not the branched gills (thoracic epipodites) which are found in some of the Mysidacea (Lophogas- tridae). Its respiration takes place through the thin lining of the carapace, under which a current is drawn from over the back by the action of the epipodites of the maxillipeds (first pair of thoracic limbs). Order CUMACEA Peracarida with a carapace which covers only three or four thoracic somites but is on each side inflated into a branchial chamber and pro- duced in front of the head to lodge the expanded end of the exopodite of the first thoracic limb; eyes (when present) sessile; no exopodite on the antenna or endopodite on the maxilla ; three pairs of maxilli- 394 THE INVERTEBRATA t-car. bri.- th.lfed. Fig. 271. Fig. 270. Fig. 270. Maxilla of Mysis. bri. bristles used in straining out the food; en. endopodite; ex. exopodite; 1-6, segments. Fig. 271. Part of a transverse section through the hinder region of the head of Hemimysis. After Cannon and Manton. hri. bristles of the fringes on the maxillae by which food particles are strained out ; car. the edge of the cara- pace; hd. head; mx.' base of maxillule; mx." section of maxilla; th. 1, section of first thoracic limb; th. i, ed. section of endite of first thoracic limb. The arrows show the direction of the currents. Note that the outgoing water from the food current joins that of the respiratory current, which comes down from under the carapace. car. an'. Th.4 Th.6 Fig. 272, Female Diastylis stygia. After Sars. ab. abdomen; ab.6, appendage of the sixth somite of the abdomen; an.' first antenna; car. carapace; gill, gill borne on first maxilliped and seen through the carapace; Th. free part of thorax; Th.4-Th.8, fourth to eighth thoracic limbs. The male has pleopods and long antennae. PERACARIDA 395 peds; a large epipodite, bearing a gill, on the ist thoracic limb and natatory exopodites on some of the others; and slender uropods, which do not form a tail fan. Small, marine organisms which live in mud or sand and are highly specialized, especially in their respiratory mechanism, for that habitat. The first thoracic exopodites form a valved exhalant siphon with the carapace lobes which lodge them. Diastylis (Fig. 272) is a British genus. Order TANAIDACEA Peracarida with a very small carapace, covering only two thoracic somites, with which it fuses; eyes (if present) on short, immovable stalks; a small scale, or none, on the antenna; thoracic exopodites absent or vestigial, a branchial epipodite on the maxilliped; and slender uropods, which do not form a tail fan. Small, marine crustaceans, usually inhabiting burrows or tubes, which are in an intermediate condition between the Cumacea and Isopoda in respect of the loss of the caridoid facies. Apseudes (Fig. 273 A), and Tanais, which differs from it in having short, uniramous antennules and liropods and no antennal scale, and lives in a mass of fibres it secretes, are British genera. Order ISOPODA Peracarida without carapace ; with sessile eyes ; the body usually de- pressed; the antennal exopodite absent or minute, the thoracic limbs without exopodites, the first pair modified as maxillipeds, the re- mainder usually alike ; the pleopods modified for respiration, and the uropods usually not forming a tail fan. (Any of these features may be absent in the adults of parasitic forms.) The Isopoda are a large group and exhibit much variety. We will study as an example Ligia, the shore slater (Fig. 274), found just above tidemarks in Britain and most parts of the world. This creature has a depressed, oval body, the cephalothorax, formed by fusion of the ist thoracic somite with the true head, lying in a notch on the anterior edge of the 2nd somite of the thorax. Two large, sessile com- pound eyes take up the sides of the head. The abdomen continues the outline of the thorax, and its 6th SQmite is fused with the telson. The anteiiniiles , which are usually short in isopods, are here minute. The antennae are of a good length, which is due to the elongation of the two joints which precede the flagellum. The mandibles, unlike those of most isopods, lack the palp, but otherwise they are complicated, having between the incisor and molar processes a row of spines and 396 THE INVERTEBRATA an: ah.Q + t. Fig. 273. Malacostraca. A, Apseudes ; B, Cyamus ; C, Phrojiima 9 ; D, Leucifer S. ab. abdomen; an.' antennule; an." antenna; the antennae of Cyamus are minute and those of Phronima $ reduced to a tubercle containing the green gland; br. gill; e.' , e." the two sections of the eye oi Phronima; t. telson; th. thorax. The numerals indicate somites or their appendages. MALACOSTRACA 397 the movable structure known as the lacinia mobilis (Fig. 275 A, la. mo.) which is characteristic of the Peracarida. The maxillules and maxillae are less well developed than those of most isopods. The maxillipeds are broad and close the mouth region from behind. The rest of the thoracic limbs are uniramous and leg-like. Their coxopodites are fused with the body, so that the brood pouch plates (oostegites) of the female, which are epipodites of the legs, seem to arise from the sterna. The first five pairs of abdominal limbs are broad, with plate-like, re- spiratory endopodite and exopodite. The endopodite of the second pair of the male is produced into a copulatory style. The uropods have slender, styliform rami. The alimentary canal has an elaborate pro- ventriculus, adapted, not to chew the food, but to press the juices Fig. 274. Dorsal and ventral views of Ligia oceanica. From the Cambridge Natural History. out of it and to strain off solid particles from them ; and there are three pairs of mid gut coeca. The heart lies in the hinder part of the thorax and in the abdomen, where blood returns from the respiratory limbs to the pericardium. The nervous system has a concentration of ganglia in the abdomen as well as one for the mouth parts. The gonads are paired, and the testes bear three follicles, characteristic of the Isopoda (see Fig. 276 A). The young when set free from the brood pouch re- semble the adult but lack the last pair of legs. Ligia is omnivorous, but chiefly eats Fucus. It gnaws with its mandibles, feeding hurriedly at low tide. Armadillidium, the common woodlouse, is more completely ter- restrial in its habits than Ligia. Its antennae and uropods are short and thus permit the body to roll up into a ball in the familiar manner. 398 THE INVERTEBRATA The air tubes on the abdominal limbs have been alluded to on P-348. Asellus (Fig. 276 A), the hog slater, is a common freshwater crus- tacean. It differs from Ligia, among other ways, in having all the abdominal somites fused, a flagellum on the antennule, a palp on the mandible, and free coxopodites on the legs. Idotea, common among weeds, etc., on the British coast, differs from Ligia in having the last four abdominal somites fused with the telson and the uropods turned inwards as valves to cover the pleopods. cp.- Fig. 275. Limbs of Li^g/a. A, Mandible. B, Maxillule. C, Maxilla. D, Max- illiped. E, Third abdominal limb. cp. coxopodite; en. endopodite; ex. exo- podite; mc. incisor process; la.mo. lacinia mobilis; niol. molar process; pr. protopodite; spi. spine row. i & 2 first two joints, fused; i', 3', endites. Many of the Isopoda are parasitic. Among these there is found every grade from well-organized temporary parasites to some which are as adults mere sacs of eggs. Aega (Fig. 276 B), a fish louse, has the ordinary isopod form, though heavily built, and with piercing mouth parts and some of the legs hooked. Its broad uropods form a tail fan. Bopyrus (Fig. 277 A), in the gill chamber of prawns, with dwarf males, is more degenerate but still recognizable as an isopod. Cryptoniscus (Fig. 277 B), a " hyperparasite " on members of the Rhizocephala and a protandrous hermaphrodite, is extremely degenerate. Many of these parasites produce parasitic castration (see p. 383). ISOPODA 399 Th.Q Fig. 276. A, Asellus aquaticus. Male viewed from above. From Leuckart and Nitsche. an.' antennule; an." antenna; ah. 6, the last pair of abdominal limbs ; t. testes with their efferent canals : the nervous system is shown in black ; Th. z-Th. 8, thoracic limbs. B, Aega psora. B', Maxillule of the same. All after Sars. 400 THE INVERTEBRATA mzp. ...thA. .00. op.bd. Fig. 277. A, Bopyrus fougerouxi: a female in ventral view. From the Cam- bridge Natural History, after Bonnier, mxp. maxiiliped; th.4, fourth thoracic Hmb (third leg); 00. oostegite; (^, male attached to female. B, Cryptoniscus paguri: ripe female stage in ventral view. After Fraisse. M. mouth; op.bd. line along which brood pouch will open ; rsp. one of two openings through which a respiratory current passes to and from brood pouch. Order AMPHIPODA Peracarida without carapace ; with sessile eyes ; the body usually com- pressed; no antennal exopodite; the thoracic limbs without exo- podites, the first pair modified as maxillipeds, the remainder of more than one form, the second and third usually prehensile; the pleopods when fully developed divided into two sets, the first three pairs with multiarticulate rami, the last two resembling the uropods, which do not form a tail fan. We will take as an example of this order Gammarus (Figs. 278-280), of which closely related species occur in Britain in fresh waters and between tidemarks in the sea. The body of this animal is compressed and elongated, with the ist thoracic somite fused to the head and no sharp distinction between the thorax and abdomen, which are of nearly equal length. At the sides of the head are pleural plates. The pleura of the thorax are short; but large, hinged coxal plates on the legs take their place. All the segments of the abdomen are free. The telson is deeply cleft. The antennules have two fiagella; the uniramous PERACARIDA 401 antennae are much like those oi Ligia. The mandibles have the same parts as those of Ligia ^ with a palp. The maxillules, maxillae, and maxillipeds are shown in Fig. 279. The maxillipeds are united by the fusion of their coxopodites. The first two pairs of legs are subchelate, the third and fourth pairs are turned forwards and help the subchelae in feeding, the last three pairs are turned backwards and used when the animal crawls on its side. The first three pairs of abdominal limbs med.dxm cpx, ^{' Th. j^^^^:^^=^^> \-T/u7 p.ca. Fig. 278. Gammariis neglectus. Female bearing eggs seen in profile. From Leuckart and Nitsche, after G. O. Sars. cpx. cephalothorax; Th. free thoracic somites; ab. the six abdominal somites; an.' antennule; an." antenna; md. mandible; mx.' maxillule; mx." maxilla; ?nxpd. maxilliped; Th.2~ Th.8, thoracic limbs; ab.i-ab.2, three anterior abdominal limbs for swim- ming; ab.4-ab.6, three posterior abdominal limbs for jumping; h. heart with six pairs of ostia; or. ovary; /lep. hepatic caecum; p.ca. posterior caeca of the alimentary canal; med.d.cni. median dorsal caecum; al. alimentary canal; n.sy. nervous system ; o. ova in egg pouch, formed from oostegites on the coxae of the second, third and fourth thoracic limbs; tel. telson (cleft). are used in swimming and to direct water towards the gills, the last three pairs are used together to kick the ground in jumping. Simple gills (epipodites) are found on the coxopodites of the legs, and ooste- gites on those of the third to fifth pairs in the female (Fig. 280). The alimentary canal has a single-chambered but complex proventriculus, two pairs of *' hepatic" coeca, and a pair of coeca at the hinder end of the mid gut which have been supposed to be excretory. The prin- cipal organs of excretion are antenna! glands. The heart extends from 402 THE INVERTEBRATA the 7th to the ist thoracic somite. The young are born with all their legs. The females with young are carried by males. After they have inc., la.vio.- spi.- — ''"/4 mol. A hr. Fig. 279. Fig. 280. Fig. 279. Mouth parts of Gammarus. A, Mandible. B, Maxillule. C, Maxilla. D, Maxillipeds. en. endopodite; inc. incisor process; la.?fio. lacinia mobilis; mol. molar process;/)//), palp; spi. spine row; 1-3, segments of limb. Fig. 280. A diagram of a transverse section through the thorax of Ga?nmartis. br. branchia; bp. basipodite; cp. coxopodite; cp.' coxal plate; g. gonad; h. heart; hep. "hepatic" coeca; int. intestine; n. nerve cord; 00. oostegite. th.3. Fig. 281. Caprella gra?tdimana. From the Cambridge Natural History, after P. Mayer, ab. abdomen; br. gills; th.2, th.H, thoracic somites. parted with the young they moult and are immediately re-impreg- nated. When the cuticle has set they are liberated. Caprella (Fig. 281), slender-bodied and living upon seaweeds, hydroids, etc., has two thoracic somites in the cephalothorax, no legs MALACOSTRACA 403 on the 4th and 5th thoracic somites, all the remaining legs subchelate, and the abdomen reduced to a minute stump. Cyamus, the whale louse (Fig. 273 B), is a Caprella with a short, wide body, adapted to its habit and habitat. Phronhna (Fig. 273 C), marine and pelagic, often inhabiting pelagic tunicates, jellyfish, etc., is transparent and has a large head with immense eyes. Subclass EUCARIDA Malacostraca with a carapace which is fused with all the thoracic somites; stalked eyes; no oostegites; a short heart situated in the thorax; and a large, branched "liver". The differences between the two orders which compose this sub- class are not great. The small, prawn-like Euphausiacea are not far from the lower genera of the true prawns, members of the Decapoda. Order EUPHAUSIACEA Eucarida in which the exopodite of the maxilla is small ; none of the thoracic limbs are maxillipeds; there is a single series of gills, and these stand upon the coxopodites of thoracic limbs; and there is no statocyst. Fig. 282. Nyctiphanes norzoegica. Slightly magnified. From Watase. The black dots indicate the phosphorescent organs. The Euphausiacea are marine and pelagic, and at times form an important part of the food of whales. Like many pelagic animals they possess (in nearly all species), phosphorescent organs, which in this case are complex and situated on various parts of the body. They are filter feeders. Most (perhaps not all) are hatched as Nauplii, and subsequently pass through stages of the Zoaea type. Nyctiphanes (Fig. 282) is a British example of the group. 404 THE INVERTEBRATA Order DECAPODA Eucarida in which the exopodite (scaphognathite) of the maxilla is large; three pairs of thoracic limbs are more or less modified as maxillipeds, and five are " legs " ; there is usually more than one series of gills, of which some (podobranchiae) stand upon the coxopodites of thoracic limbs, others {arthrobranchiae) upon the joint-membranes at the bases of the limbs, and others (pleurobranchiae) upon the sides of the thorax; and a statocyst is usually present in the proximal joint of each antennule. The Decapoda owe their name to the condition of the hinder five pairs of thoracic limbs, which are adapted for locomotion, typically by walking but sometimes by swimming. Often, however, as in the crayfish, one of these pairs bears large chelae and is incapable of the locomotory function: others may also be incapacitated for it, as, for instance, the two small hinder pairs of the hermit crabs (Fig. 292) . Only in some of the lower genera is there any vestige of the exopodite upon these five pairs. This order contains the most highly organized crustaceans. Among its members there is great diversity in the habit of body and in the form of the appendages, but two principal types can be observed. In the first or macrurous type the caridoid facies is in the main retained, the body is long and subcylindrical or somewhat compressed, the abdomen is long and ends in a tail fan, the appendages are usually slender, and any of the legs may be chelate. An example of this type, the common crayfish, Astacus (Figs. 283 ; 212, 213, 222 G, 224, 226, 227, 231, 233, 234, 286), is described in most textbooks of elementary zoology. The second or brachyurous type — which is not confined to the Brachyura sensu stricto but occurs independently in various members of certain groups, known collectively as the Anomura, that are intermediate between the macrurous divisions of the order and the Brachyura — has the cephalothorax greatly expanded laterally and more or less depressed, while the abdomen is reduced and folded underneath the cephalothorax. In it the appendages are as a rule shorter and stouter than in macrurous forms, and only the first pair of legs has a true chela. The suborders Penaeidea (primitive prawns), Caridea (prawns and shrimps), Astacura (crayfishes and lobsters), and Palimira (craw- fishes and bear-crabs) are macrurous. They are for the most part swimmers, though some of them, as the Astacura and Palinura, do more walking than swimming. The suborders Anomura and Brachyura are walkers, though some of the crabs have their own ways of swim- ming by means of flattened legs. The Brachyura proper are dis- tinguished from other brachyurous forms by the occurrence in nearly DECAPODA 405 M '-' 4o6 THE INVERTEBRATA BRACHYURA 407 all the latter of well-formed uropods, which the true crabs do not possess, and by a fusion of the edge of the carapace with the epistome, a sternal plate which lies in front of the mouth (see p. 410). As an example of the Brachyura we shall describe Carcinus maenaSy the common shore crab of Britain (Figs. 284, 285, 287-291). The depression to which is due the difference in shape between the cephalothorax of this typical crab and that of a crayfish or prawn has brought it about that in a transverse section (Fig. 285) the carapace cb.sp. en^ £p< Fig. 285. Fig. 286. Fig. 285. Diagram of a transverse section through a branchial chamber of Carcinus maenas. From Borradaile. a.l.e. anterolateral edge; cp. coxopodite; eb.sp. epibranchial space; ep.i, mastigobranch (distal part of epipodite) of the first maxilliped; ep.^, mastigobranch of the third maxilliped; hy. hypo- branchial space ; i.r. inner layer of branchiostegal fold ; o.r. outer layer of the same; proc. process of flank of thorax, which meets branchiostegite and separates two of the openings above the legs into the chamber. Fig. 286. The left third maxilliped of Astacus. cp.set. coxopoditic setae; en. endopodite; ep. epipodite; ex. exopodite. has at the sides (where, as the branchiostegite, it covers the gills), not an arched profile but runs out almost horizontally and is then bent in, at an angle {a.l.e.) which is more acute in the anterior part of the body than in the hinder part, to end against the flank above the coxo- podites of the legs. At the angle, the branchiostegite, viewed from above, describes the lateral part of the outline of the body. That out- line begins between the eyes, where in the crayfish the rostrum stands, with the/row^, a low, three-toothed lobe. On each side of this is the orbity an excavation of the surface of the head for the reception of the 4o8 THE INVERTEBRATA eye. From the orbit the notched anterolateral edge curves outwards and backwards as a crest on the branchiostegite, forming with its fellow and the front a semicircle. From each end of the semicircle a slightly concave posterolateral edge carries the outline slanting inwards to the short, transverse /)0^/mor edge of the carapace. To return to the transverse section : the thin inner layer of the fold which makes the branchiostegite is not so much drawn out as the stout outer layer, so that a considerable space is left between them. In the hinder region the two layers are not very widely separated, and there are in this space only blood channels and connective tissue, but anteriorly branches of the liver and gonad intrude there. The edge of the branchiostegite fits close against the flank of the thorax and the exopodites of the maxillipeds, leaving however the following openings: (i) small slits, one above each leg, (2) a large opening in front of the coxopodite of the chela, (3) a still larger opening in front of the mouth. These openings lead to and from the gill chamber. In th; flattening of the body, the lateral wall of the thorax has come to face in great part upwards, so that the gills instead of being directed vertically from their attachments, are directed more or less hori- zontally inwards over the convex, mound-like inner wall of the gill chamber. The gills are of the kind known as phyllobranchiae. That is, the axis of each, instead of bearing filaments as in the gills of the cray- fish {trichobranchiae), has on either side a row of plates, set close like the leaves of a book. The podobranchiae stand out from the base of an epipodite, which bears also a slender process known as a mastigo- branchia. In the crayfish the gill lies along this and is fused with it (Fig. 286). The first maxilliped has a mastigobranchia without a podobranchia. The gill series of Carcinus is shown in the following table : Mxpd I Mxpd II Mxpd III Leg I (Cheli- ped) Leg II Leg III "rf LegV Total Podobranchiae Anterior Arthrobranchiae Posterior Arthrobranchiae Pleurobranchiae Mastigobranchiae (I) I I (I) I I I (I) I I 2 I I I 2 3 2 2 (3) Total (I) 2 + (l) 3+(i) I — 9 + (3) The mastigobranchiae lie in the gill chamber, that of the first maxilliped in the epibranchial space above (external to) the gills and CARCINUS 409 those of the second and third maxillipeds in the hypobranchial space below the gills. Their function is the cleaning of the gills. In front, the gill chamber narrows to an exhalant passage^ which contains the scaphognathite and leads to the large anterior opening. The scaphognathite, working to and fro, drives water out of this opening and so draws in a current through the other apertures. The opening in front of the chela can be closed by a flange on the coxopo- dite of the third maxilliped, and so the current can be regulated. The pb.mp^s pb.mp.2 plb.i ar.ch. Fig. 287. A dorsal view of the organs in the left branchial chamber of Car- cinus maenas. From Borradaile. ab. abdomen; ar.ch. arthrobranchs of the cheliped; ar.mp.z, arthrobranch of the second maxilliped; ar.m^.3, arthro- branchs of the third maxilliped; ep. i, base of the mastigobranch of the first maxilliped; i.r. inner layer of the branchiostegal fold, reflected; pb.mp.2, pb.mp.2, podobranchs of the first and second maxillipeds; pel. pericardial lobe, a thin fold of the body wall, of undetermined function; plb.i, plb.2, first and second pleurobranchs ; p.hy. posterior opening of the hypobranchial chamber; scl. sclerite which keeps open the entrance to the exhalant passage; scp. scaphognathite. water which enters this opening is prevented from taking a short cut to the exhalant passage by a large expansion of the base of the mastigo- branch of the first maxilliped, which directs it under the gills. The current from the openings over the legs also passes under the gills. All the water then passes upwards through the gills into the epi- branchial space above them and so to the exhalant passage. Thus the gills are thoroughly bathed. Owing to the width of the body the sterna are more easily dis- tinguished than in the crayfish. Those of the maxillulary to second 410 THE INVERTEBRATA maxillipedal somites are fused into a triangular mass. In front of the mouth the plate known as the epistorne represents the mandibular and antennal sterna. From this a ridge extends to the median rostral tooth, separating two sockets in which stand the antennules. A down- ward process from the front, abutting on the basal joint of the antenna, separates each of these sockets from the orbit of its side. The two- jointed eyestalk arises close to the median line and passes through a gap between the frontal process and the antennal base to enlarge within the orbit. The abdomen is reduced to a flap, turned forwards and closely ap- plied to the sterna of the thorax. Its ventral (upper) cuticle is thin. It is broader in the female than in the male, in which its 3rd to 5th somites are fused. Two small knobs on the 5th thoracic sternum, fitting into sockets on the 6th somite of the abdomen, lock the two together as by a press button. The antcnjmles have short flagella and can fold back into the sockets mentioned above. The antennae also have a short flagellum. They have no exopodite (scale) and their coxopodite is represented by a small operculum over the opening of the antennary ("green") gland. The month parts are shown in Fig. 288 . In the mandibles, the biting edge (incisor process) is toothless and the molar process reduced to a low mound behind the biting edge. The palp is stout and the first two of its three joints are united. The maxillules and maxillae have the usual endites well developed. The scaphognathite of the maxilla is shaped to fit the exhalant passage of the gill chamber. The maxillipeds have epipodites produced into long, narrow mastigobranchs, fringed with bristles which brush the gills. The flagella of their exopodites are turned inwards and the endopodite of the first of them is expanded at the end and helps to border the exhalant opening for the respiratory current. The third pair are broad and enclose the mouth area from below. The legs lack an exopodite and have the usual joints (p. 336) in the stout endopodite, but the basipodite and ischiopodite are united. The first leg has a strong chela: concerning the physiology of its muscles something is said on pp. 138, 142. The others differ from those of the crayfish chiefly in that none of them are chelate. The animal, as is well known, walks sideways with them. Abdominal limbs are present in the female only on the 2nd to 5th somites. On a short, one-jointed protopodite they bear two long, equal, simple rami, covered with setae to which, as in other decapods, the eggs are attached by a covering secreted by dermal glands. In the male, the abdomen bears limbs only on its first two somites, and they are uniramous and adapted for transferring the sperm, the endopodite of the second working as a piston in a tube formed by that of the first. In feeding the food is seized by the chelae, which place it between DECAPODA 415 Of the various examples of the order which are mentioned below, all except Leucifei\ Birgus, and Gecarcinus occur in British waters. The most aberrant member of the Decapoda is the minute, pelagic Leucifer (Fig. 273 D), which has a very slender, macrurous body with an extremely elongate head, long eyestalks, no limbs on the last two thoracic somites, no chelae, and no gills. Like the normally built prawn Penaeus and the rest of the group (Penaeidea) to which both belong, Leucifer starts life as a Nauplius. Leander, the common prawn, one of the Caridea, is macrurous like the crayfish, but built for swimming rather than walking, with phyllobranchiae, and with chelae only on the first two pairs of legs. Crangon, the shrimp, is related to Leander but has a broader and flatter body, a very small rostrum, and the first leg subchelate. Nephrops, the Norway lobster, one of the Astacura, differs from the crayfish in minor points, among others in having the podobranchs free from the mastigobranchs. Homarus, the lobster, differs from Nephrops in size, form of chelae, etc. Palinurus, the crawfish or spiny lobster, one of the Palinura, differs from the crayfishes and lobsters in having a small spine in place of the rostrum, no antennal scale (exopodite), and no chela on any leg. It has a peculiar broad, thin schizopod larva, the Phyllosoma (Fig. 266 C). Eupagurus (Fig. 292), the hermit crab, one of the Anomura, lives in the empty shells of gastropod molluscs. It has a large, soft abdomen, containing the liver and gonads, twisted to fit into the shell, and without appendages on the right side, save for the uropods, of which both pairs are present, roughened, and serve to hold on the shell. The first three pairs of legs are as in a crab, the last two small and chelate. Birgus, the robber crab, is a hermit crab which has grown too large to use the shells of molluscs, and has accordingly re-developed ab- dominal terga. It lives on land in the Indopacific region, and is adapted to aerial respiration by the presence of vascular tufts on the lining of the gill chambers. Its Zoaeae are marine. Lithodes, the stone crab (Fig. 293), is by origin a hermit crab, but has lost the habit of living in shells and so thoroughly taken on the build of the true crabs that only some asymmetry of the abdomen and a few other minor points of structure betray its ancestry, even the uropods being absent. Galathea, the plated lobster, another of the Anomura, is lobster- like but has the abdomen bent under the thorax, and the last leg small and slender and folded into the gill chamber. Porcella?ia, the china crab, related to Galathea^ has a form of body Fig. 292. Eiipagurus bernhardus, S. ab. 3, third abdominal limb; tel. telson; th.S, last thoracic limb; sc. scale of antenna. Fig. 293. Lithodes maia, ?, in ventral view. From the Cambridge Natural History. ab.-T,, lateral plates of the third abdominal somite; 06.5, left lateral plate of the fifth abdominal somite; mar. marginal plate; Te.t, telson and sixth abdominal somite, fused ; th. 8, brush-like last thoracic limb. DECAPODA 417 resembling that of the true crabs, but possesses uropods. Fig. 266 A shows its remarkable zoaea. Cancer, the edible crab, is a member of the Brachyura, nearly re- lated to Carcinus but more heavily built, without the slight powers of swimming possessed by the latter, and differing in other small points. Gecarcinus, containing land crabs of the tropics, differs from Car- cinus and Cancer in the shape of the third maxillipeds, which gape, the sternal position of the male opening, and the highly vascular lining of its swollen gill chambers. Its Zoaeae are marine. Maia, the spiny spider crab, is narrow in front, with bifid rostrum and feeble chelae, and a habit of decking itself with seaweed for concealment. CHAPTER XIII THE SUBPHYLUM MYRIAPODA Land-living tracheate arthropods, usually elongated, with numerous leg-bearing segments; a distinct head with a single pair of antennae, a palpless mandible and at least one pair of maxillae ; tracheal system with segmentally repeated stigmata, tracheae usually anastomosing; eyes, if present, clumps of ocelli; mid gut without special digestive glands, hind gut with Malpighian tubules; young hatching at a stage resembling the adult but possessing fewer than the adult complement of segments. It has long been recognized that the group Myriapoda as defined above contains two chief divisions which are here treated as classes, one of which, the Chilopoda, is more closely related to the Insecta than the other, the Diplopoda. It is, however, convenient to retain the group, though the similarity of the chief members is probably more superficial than natural. Classification. Chilopoda (Opisthogoneata), centipedes; Diplo- poda, millipedes (with two smaller classes, the Symphyla and the Pauropoda, these form the Progoneata, all distinguished by having the genital opening near the anterior end of the body). Class CHILOPODA Carnivorous arthropods with the genital opening situated at the hind end of the body (opisthogoneate) ; body segments all similar (at least in the more primitive members of the division), body usually flattened dorsoventrally ; ocelli present, head bears also antennae and three pairs of jaws (mandibles and two pairs of maxillae) ; the ist body segment bears a pair of poison claws; the rest, each a single pair of ambulatory limbs, except the last two, which are legless ; blood system consists of a dorsal heart and a ventral vessel connected by an anterior pair of aortic arches ; tracheae typically branch and anastomose and have a spiral lining; gonads dorsal to gut. The type used for the study of this division is the centipede, Lithobius (Fig. 294), which is found under bark and stones, and is a much more active creature than the millipede, lulus, which is found in the same situation. The chitinous exoskeleton is flexible and is moulted frequently. The body is flattened dorsoventrally and the legs in each pair are widely separated. The head consists of six segments all represented by coelomic sacs in the embryo which disappear in the MYRIAPODA 419 aca^ ac Jv.n.c ... ue.se. Fig. 294. Fig. 295- Fig. 294. Lithohius forficatus. Dorsal view of whole animal, i, antenna; 2, maxilliped; 4, pair of walking legs. Fig 295. Lithohius forficatus, S- Dissected to show internal organs, ant anfenna; amb. walking legs; ac.gl. accessory glands; a/, alimentary canal; mt. Malpighian tubules; p.cl. poison claw; sal.gl. salivary gland, t. testis, ve.se. vesicula seminalis ; v.n.c. ventral ner\^e cord. Both from Shipley and MacBride. 420 THE INVERTEBRATA adult, including a preoral and (between the antennae and the mandibles) an intercalary. All segments except the first are originally postoral but in development the mouth moves back and comes to lie between the mandibles. The number of head segments is the same as in the embryo insect and the crustacean, and a remarkable homology may be observed between the chilopod and insect head appendages. Thus hd.c- /-. / / ant. Abr. ^md. .^- mx- hd.c. t.mxp.- /..-./Y^^Qx^ mxp. I Fig. 296. Ltthobiusforficatiis. Original. Ventral view of head and two succeed- ing segments in a specimen boiled in potash and mounted in Canada balsam. On the right of the observer the maxilliped, and the sternum belonging to it, is lightly stippled : on the left the maxilla is more coarsely stippled, ant. antenna ; amb. I , base of the first ambulatory appendage ; hd.c. head capsule ; Ibr. labrum ; md. mandible ; mx. maxilla ; ?nx.' first maxilla ; mxp. maxilliped (poison claw) ; t.mxp. tergum of the maxilliped. the antennae are jointed mobile appendages varying in length; the mandibles are toothed plates without palps, the ist maxillae consist of a basal portion bearing inner and outer lobes, while the 2nd maxillae are usually fused together to form a sort of labium and possess a palp-like jointed structure (Fig. 296). The difference between the mouth parts of an orthopteran insect and a chilopod lies in the reduction in size of the two pairs of maxillae in the latter, which CHILOPODA 421 cox. is possibly connected with the great development of the first pair of trunk appendages as maxillipeds, which are four-jointed, the distal joint being a sharp claw perforated by the opening of the poison gland, while the proximal joint is enlarged and meets its fellow in the middle line to form an additional lower lip. The body segments in Lithobiiis number eighteen. Of these, the ist carries the poison claws (maxillipeds), the 17th the genital opening and usually a pair of modified appen- dages, the gonopods, and the last (telson), which is greatly reduced in size and not seen in Fig. 297, the anus, while the 2nd to i6th have each a pair of seven-jointed walking legs. Each segment has a broad tergum and sternum and between them a soft pleural region with a few small chitinous sclerites and the stig- mata. In Lithobius and the group of chilopods to which it belongs, the terga are alternately long and short (Fig. 294). Only the segments which have long sterna have stigmata, but all have walking legs. In other centi- pedes, e.g. Scolopendra (see Table, genital segment, pp. 306-7), the terga are equal gonopods. throughout. The alimentary canal consists of a short fore gut into which open two or three pairs of salivary glands, a very long mid gut without any associated glands, and a short hind gut into which open a pair of Malpighian tubules. The vascular system is better developed than in insects. The heart runs the whole length of the body and possesses in each segment not only a pair of ostia but also lateral arteries. It ends anteriorly in a cephalic artery and a pair of arteries which run round the gut and join to form a supraneural vessel. The arteries branch and open into haemocoelic spaces. There is a pericardium and below it a horizontal membrane, perforated and provided with alary muscles as in insects. In the respiratory system the tracheae branch and anastomose and possess a spiral thickening, but in the remarkable form Scutigera the stigmata are unpaired and dorsomedian in position and the tracheae are unb ranched and simple in structure. The reproductive organs (Fig. 295) consist of an unpaired ovary or testis, with a duct which divides into two and passes round the end gut to open by the median genital opening. There are two pairs of --'g.pod. Fig. 297. Lithobius forficatus, ?. Hind end, ventral side. 16, last segment bearing ambulatory legs ; cox. coxopodite of last leg; 17, pre- hearing g.pod. 422 THE INVERTEBRATA accessory glands and in the male two vesiculae seminales. Spermato- phores are formed but it is doubtful whether copulation occurs. Lithobius lays its eggs singly and buries them in the earth. The young are hatched with seven pairs of legs. The nervous system comprises a cerebral ganglion supplying the antennae and the eyes, a suboesophageal ganglion giving branches to the other head appendages and the maxillipeds, and a ventral chain with a pair of ganglia in each leg-bearing segment. Class DIPLOPODA Arthropods with the genital opening situated on the 3rd segment behind the head (progoneate) ; trunk segments arranged in an anterior region {thorax) of four single segments and a posterior region [abdomen) of double segments, each with two pairs of legs; body usually cylin- drical; skeleton strengthened by a calcareous deposit; ocelli present, head bears also short club-shaped antennae, mandibles and a single pair of maxillae; vascular system well developed as in Chilopoda; tracheae arise in tufts from tracheal pouches, do not anastomose; — 1 Fig. 298. lulus terrestris, sometimes called the "wire-worm", x about 3^, From Koch, i, antennae; 2, eyes; 3, legs; 4, pores for the escape of the excretion of the stink glands. gonads ventral to gut ; young hatch usually in a stage with three pairs of legs and development takes place gradually. Though the head of the adult millipede appears to have fewer segments than that of the Chilopoda a study of the embryo shows that there are really the same number. An intercalary segment exists between the antennal and mandibular segments and behind the mandibles a pair of rudimentary appendages appear but soon vanish. These are the first maxillae: the second maxillae (labium) persist in the adult. lulus is one of the commonest genera of millipedes. It is vege- tarian. It has an elongated body, consisting of a large number of segments (up to seventy), which can be rolled into a ball. The head (Fig. 299 A) carries a pair of short antennae with seven joints. The labrum is continuous with the front of the head and is a toothed plate; the mandibles, which have no palp, bear a movable tooth and DIPLOPODA 423 a ridged and toothed plate; behind them is an organ known as the gnathochilarium (Fig. 299 C), which, in structure and position, recalls Fig. 299. lulus terrestris. A, Side view of anterior end. an. antenna; ah. abdomen ; col. collum (ist thoracic segment) ; e.e. clump of ocelli ; gn. gnatho- chilarium ; g.op. genital opening (on basal joint of 2nd pair of legs) ; hd. head ; Ibr. labrum ; 77id. mandible ; th. thorax. B, A segment detached from the rest. st77i. sternum; tg77i. tergum. C, Gnathochilarium seen from inner side. (The parts — basilar plate — which it is suggested may belong to the segment of the collum are stippled.) c.b. central body; hyp. hypostoma; LI. lamellae linguae; merit, mentum; p.m. promentum; p.p. palps; st. stipes. D, Diagram of four rings seen in side view (above) when the animal is stretched out straight ; (below) when- it is coiled in a spiral. The dorsal part of the ring is clearly seen to be longer than the ventral. A, B and C, original ; D, after Kukenthal. the labium of insects and, like it, is formed by the junction of paired appendages, the principal part of it by the appendages of the labial 424 THE INVERTEBRATA segment. Also a postlabial segment contributes to it forming the basilar plate. The tergite of this segment, however, forms what is ap- parently the first segment after the head. This is known as the collum\ there are no stigmata and no separate appendages, though the first pair of legs appears to belong to it they are those of the second segment. The next three have a single pair of ambulatory legs apiece, a pair of ganglia and a pair of stigmata, and in the embryo a pair of coelomic sacs. These four segments maybe said to constitute the thorax, though, as related above, the first takes part in the formation of a head structure. The genital openings are situated in the basal joint of the second pair of legs, which appear to arise from the second segment, but really belong to the third. Behind this is the abdomen consisting of an indefinite number of double segments (up to a hundred in lulus). The exoskeleton of a body segment consists of a tergum and two sterna. In the double segment of lulus (Fig. 299 B) the sclerites of two segments are fused together to make a continuous ring. The sterna carry two pairs of stigmata and legs. In the embryo there are two pairs of coelomic sacs ; there are two ostia in the heart and two pairs of ganglia. In lulus the sterna are much shorter than the terga and also much narrower so that the legs come oif very close together ; also the terga are narrower in front so that they can be telescoped into the terga in front. The diagram here given (Fig. 299 D) shows that this relation occurs when the diplopod body is straightened out ; when the animal rolls up the adjacent rings are completely disengaged. The stigmata are elongated slits, which can be closed by a valve, and they communicate with a tracheal pocket from which spring two thick bunches of unbranched tracheae. These are of two sorts; one long and slender, the other shorter and thicker with a spiral lining. In other millipedes {Glomeris) the tracheae may become much longer and branch but they never anastomose. The circulatory system is in a stage of development higher than that of the insects. The alimentary canal bears a pair of long Malpighian tubules arising from the hind gut. The legs consist of the same elements as in the insect, but the tarsus is divided into three joints, the last of which carries a claw. In the male the first leg is modified for copulation and in the 7th segment there is an auxiliary copulatory apparatus, consisting of processes used for transferring sperm into the vagina of the female. These processes may occur together with legs and so are not homologous with them. There are no similar organs in the female. The generative glands are unpaired with ducts opening on the 3rd body segment. The eggs are yolked and are laid after copulation in a nest made of hard earth. The mother keeps watch over them before hatching. CHAPTER XIV THE SUBPHYLUM INSECTA (HEXAPODA) Tracheate Arthropoda in which the body is divided into three distinct regions, the head, thorax and abdomen. The head consists of six segments and there is a single pair of antennae ; the thorax consists of three segments with three pairs of legs and usually two pairs of wings; the abdomen has typically eleven segments and does not possess ambulatory appendages; genital apertures situated near the anus (Fig. 300). th. an Fig. 300. Lateral view of a grasshopper to show external segmentation typical of insects. From Metcalf and Flint, ab. abdomen ; an. antenna ; ex. coxa; e. eye;fe. femur; hd. head; ///. thorax; ti. tibia; to. trochanter; ts. tarsus. The head is enclosed by an exoskeleton which consists of several plates or sclerites, both paired and unpaired, united together, having no clear relation to the segmentation of the head. The segments are indicated by the paired appendages, the ganglia of the nervous system (neuromeres) and the coelomic sacs which can be demonstrated in sections of the embryo but which disappear later. Thus the six seg- ments are indicated by the evidence which follows : Segment Preantennal Antennal Intercalary Mandibular Maxillary Labial Neuromere Protocerebrum Deutocerebrum Tritocerebrum Mandibular ganglion Maxillary ganglion Labial ganglion Appendage Antenna Embryonic Mandible Maxilla Labium 426 THE INVERTEBRATA In the embryos of most generalized insects only, are coelomic sacs present in all head segments. In addition to compound eyes there are simple eyes or ocelli, of two kinds. Lateral ocelli are usually the sole type of eye in larval insects and represent the larval counterparts of the compound eyes which function in the adult. Dorsal ocelli on the vertex of the head of adult insects are structures distinct from the lateral and co-exist with compound eyes. The ocellus consists of a single cornea, a trans- parent area of cuticle which usually forms a lens-like body, the cells which secrete it, and the visual cells arranged in groups, the retinulae, having in the centre the optic rod or rhabdome. The compound eyes (as described more fully in the section on the Arthropoda) possess a cornea which is divided into a number of facets; corresponding to each facet is a group of visual cells, the ommatidium. The current theory of mosaic vision states that each ommatidium, isolated from its neighbours by a coat of pigment, conveys to the retinula at its base only such rays of light as travel parallel to the axis of the ommatidium. The total impression is that of a mosaic composed of as many separate pictures as there are ommatidia, every picture different from its neighbours, but all combining to form a single "coherent" picture. The compound eye has probably the advantage that it can detect movements of the smallest amplitude. It gives, however, only a vague idea of the details of objects, for there is no focussing apparatus and only objects very close to the eye can be perceived clearly. In some insects the eye is divided into two parts : a dorsal with coarse facets which probably only serves to detect variation in illumination, and a ventral with finer facets which gives fairly definite images of objects. Possibly in some insects the first function in night vision the second by day. It must also be mentioned that experiments show that many insects can distinguish colours. The development of flower colour and pattern is generally supposed to have taken place simultaneously with that of the aesthetic senses of insects. The antennae are a pair of appendages consisting usually of many joints. They are sometimes filiform but may show complicated varia- tions in structure. In all cases they carry sense hairs, particularly those which serve an olfactory function ; it is well known that in some insects the removal of the antennae or coating them with paraffin wax destroys the olfactory sense, but this is not always the case. The mouth is bordered dorsally by the labrum, a median plate or sclerite which is underlain by the membranous roof of the mouth — the epipharynx. The first two pairs of mouth parts, mandibles and first maxillae, lie at the sides of the mouth, while the second maxillae, invariably fused together, bound the mouth posteriorly, and are known as the INSECTA 427 labium. Such a fusion characterizes the maxillipeds of certain Crus- tacea. The primitive and generaHzed condition is, undoubtedly, that which is found in insects which feed on soHd food, e.g. the Cockroach or MachiHs. The mandible which is rarely jointed, represents the toothed basal segment of an, originally, jointed limb, and corre- sponds in form and function, to that of Crustacea, but never possesses a palp (Fig. 301). Each first maxilla is articulated to the head by a Fig. 301. Mouth parts of Machilis (Petrobius) maritimiis. After Imms. I, Mandible. 2, Maxilla. 3, Hypopharynx (li.) and superlinguae {sL). 4, Labium, c. cardo; g. galea; gl. glossa; /. lacinia; Ip. labium; m. post mentum ; 7nx.p. maxillary palp ; pf. palpifer ; pg. paraglossa ; p?n. prementum ; pgr. palpiger; s. stipes. basal segment, the cardo. The succeeding segment, the stipes, carries an outer palp bearing sclerite, and distally, bears two lobes, inner spiny lacinia, and outer hood-like galea. In the labium, the basal plates corresponding to cardo and stipes of the two sides, are fused to form the suhmentum and post mentum. The more distal prementum bears a palp at either side and a number of lobes, typically four, between 428 THE INVERTEBRATA them, known collectively as the ligula. Where there are four as in the Cockroach, the two median glossae are defined from the outer para- glossae. Each of these lobe? is further divided into two in Machilis (Fig. 301). Arising from the floor of the mouth is a small sclerite, the hypopharynx, which bears the salivary aperture. A pair of sclerites, superlinguae, are normally fused to the sides of the hypopharynx. Fig. 302 indicates the similarity between the insectan and crus- tacean mouth parts. Such an attempt at a comparison is only possible with the more generalized mouth appendages of the Insecta. With the evolution of different feeding habits, the structure of the mouth parts, just described, has been departed from in a variety of ways. Comparative and embryological study, however, clearly reveals a uniformity of plan throughout, and the student must realize that the modifications to be met with in bugs, butterflies, bees and flies are Fig. 302. To show the resemblance between the insectan maxilla and labium and the biramous limb of the Crustacea. From Imms, after Hansen. A, Bi- ramous crustacean appendage. B, Insectan maxilla. C, Maxillipeds of a Gammarid crustacean. D, Insectan labium, end., end.i endites; en. endo- podite; ep. epipodite; ex. exopodite; sni. submentum. all referable to the basal plan as exemplified in the mouth parts of Blatta or Machilis. The thorax is separated from the head by a flexible neck region usually containing cervical sclerites, which, however, have not any segmental value. It consists of three segments — the prothorax, which carries a pair of legs but no wings, the mesothorax and the metathorax, which each bear a pair of legs and, typically, wings. The legs are made up of five main segments, the coxa and trochanter (both of which are small), the femur and tibia (which form the greater part of the limb), and the tarsus (which is usually further subdivided into a number of joints, and ends in a pair of claws with a bifid cushion between them called the pulvillus). Of the many adaptations exhibited by the legs of insects the jumping type found in grasshoppers, the digging type in the mole-cricket Grylloialpa, the swimming type in the water beetles like INSECTA 429 Dytiscus, the prehensile type in the fore legs of the praying insect Mantis may be mentioned, in addition to the ordinary running type as seen in a cockroach. Modifications for the production or reception of sound as in the Orthoptera and for the collection of food (the combs and pollen baskets of bees) are also familiar. The wines of an insect are thin folds of the skin flattened in a horizontal plane, arising from the region between the tergum and pleuron. A section of a wing bud shows two layers of hypodermis, the cells of which are greatly elongated (Fig. 320). Into the blood space between the layers grow tracheae, and when in a later stage the two layers of hypodermis come together and the basement mem- branes meet and fuse, spaces are left round the tracheae which form the future longitudinal wing veins. These spaces contain blood and sometimes a nerve fibre during development. The cuticle round the veins is much thicker than in the general wing membrane, so that the veins are actually a strengthening framework for the wing. The number and arrangement of the veins is highly characteristic of the difi^erent groups. Though the majority of insects possess wings there are important orders which are wingless. Some such as those to which the fleas and lice belong are secondarily so, because of their parasitic habit. Others, however, constituting the large division Apterygota, are primitively wingless, and these, both on morpho- logical and palaeontological evidence, must be regarded as the most ancient types known. Among many orders of insects, there has developed a tendency for the two pairs of wings to act as one. This is accomplished by various devices which couple the fore and hind wings together, on each side. In the scorpion flies, e.g. Panorpa, bristles project back from the posterior or jugal lobe of the fore wing to overlie the anterior border of the hind wing. Corresponding bristles to these constituting the frenulum, project forwards from the anterior border of the hind wing, and overlie the posterior border of the fore wing. In most Lepidop- tera, frenular bristles of the hind wing are held in position by a group of curved setae forming a retinaculum on the fore wing. The two wings of a side, in Hymenoptera are coupled by a row of hooks — the hamuli — on the anterior border of the hind wing, engaging in a fold of the posterior border of the fore wing. In other orders, we find one pair of wings diverted to uses other than flight, the latter operation, then, being dependent on one pair of wings. The fore wings, for instance, of Orthoptera and of Dermap- tera, are protective to the more delicate folding flight wings, behind them. The elytra, or fore wings of beetles, are similarly protective, and are held passively extended while the second pair of wings propel the animal through the air. In the males of Strepsiptera, the anterior, 430 THE INVERTEBRATA and in the male Coccid bugs and all Diptera, the posterior wings are minute structures, flight being performed by the remaining pair, which are normally developed. Thus, either by linking two pairs of wings together, or by dispensing with one pair, flight is commonly brought about by one functional unit on each side of the body. The variations in form, consistency, and size of the wings are briefly dealt with under the different orders. Simple up-and-down movements of the wings are sufficient to account for the elementary phenomena of insect flight. In moving through the air the anterior margin remains rigid but the rest of the membrane yields to the air pressure; so that when the wing moves downward it is bent upwards (cambered) ; as the wing moves upward the membranous part is bent downwards, therefore, by becoming deflected the wing encounters a certain amount of pressure from be- hind which is sufficient to propel it. The faster the wings vibrate the more they are cambered, the greater the lateral pressure and the faster the flight. Smaller insects have as a rule a greater rate of wing beat. Thus a butterfly may make only 9 strokes a second while a bee makes 190 and a housefly 330. The wing muscles of insects thus con- tract immensely faster than those of any other animals. It is inter- esting to note that the intracellular respiratory pigment, cytochrome, occurs in high concentration in them. To bring about wing movement direct muscles attached to the wing base and others called indirect inserted on the body wall are employed. The extent to which direct and indirect muscles are present varies. In the Odonata a direct musculature is strongly developed, the muscles being attached to the intucked wing base. In the specialized orders Lepidoptera, Diptera and Hymenoptera, indirect muscle action is responsible for most of the movement and those muscles attached directly to the wing base serve for folding the wing to a position of rest as well as for flight purposes. Fig. 303 represents diagrammatically the condition in the winged aphides. The thorax is a box whose roof is capable of being arched and flattened by longitudinal and dorsoventral muscles respectively. Since the wing base has two points of attachment, (i) to the pleural plate, and (ii) to the edge of the tergum, the wing operates as a lever of the second order. The arching of the tergum raises the wing base and depresses the wing, while a flattening of the tergum depresses the wing base and raises the wing. The abdomen consists of a series of segments less dift'erentiated than those of the head and thorax. The number is eleven, as seen to be present in the embryo insect (with the addition of a transient telson) and in primitive groups (Thysanura and Odonata). In other groups, the nth segment is represented by the podical plates which INSECTA 431 bear the cerci anales (as for instance in the cockroach). In specialized insects the apparent number of abdominal segments may be greatly reduced . In insect embryos rudiments of appendages are borne on each of the abdominal segments, but these rudiments disappear in the adult except in the Apterygota. Only those which become the cerci anales in the nth segment are frequently retained. In the 8th and 9th t.irM- V .IV. a l.rti. P .w.a Fig. 303. To illustrate the mechanism of wing movement in an Aphid. Wing depression: A, left side view of mesothorax; B, transverse section. Wing elevation: C, left side view of mesothorax; D, transverse section. dv.tn. dorsoventral muscles; l.m. longitudinal muscles; p.zv.a. pleural wing attachment; t.w.a. tergal wing attachment. Effective muscles shown by- dotted lines in A and C. After Weber. segments in the female and the 9th segment in the male there are paired structures known as gonapophyses which perform various reproductive functions (oviposition in the female, copulation in the male). It is highly probable that these are modified appendages. The alimentary canal (Fig. 304) varies greatly in length; in many larvae it is no longer than the animal itself, but in certain types of insects like the Homoptera, which feed on plant juices, it is much coiled and may be several times the length of its possessor. It consists 432 THE INVERTEBRATA of an ectodermal stomodaeum or fore gut, an endodermal mid gut and an ectodermal proctodaeum or hind gut. The fore gut consists of (a) the buccal cavity succeeded by (b) the pharynx, which may be muscular and form a pumping organ (Fig. 328 A), (c) the oesophagus, which has a posterior dilatation, the crop. This functions as a food reservoir and may have a diverticulum enormously developed in sucking insects to store the liquid food. Lastly there is (d) the pro- mi. Fig. 304. General view of internal organs of Apis ?nellifica as seen from above ; musculature and tracheal system not shown. From Carpenter, an. antenna; bn. brain; co. colon; cr. crop; e. eye; ga. ganglion; ?ng. mid gut; 7nt. Mal- pighian tubule; oe. oesophagus ; r?n. rectum; sa.gl. salivary glands (three types are shown); pv. proventriculus ; il. ileum. ventriculus or gizzard, most typically developed in insects which eat hard food as in the Orthoptera. The chitinous lining of the fore gut is here greatly thickened and the sphincter muscles in this region control the passage of food between fore gut and mid gut. Into the buccal cavity discharge the salivary glands (Fig. 304), which may as in the cockroach have a very similar function to those of the mammal, in producing enzymes for the digestion of carbohydrates. In other INSECTA 433 insects, however, they are specialized in ways which are mentioned later. Such glands are usually associated with the labium; in some insects, however, mandibular and maxillary glands are found. The mid gut (Fig. 305) is lined by a layer of cells frequently all similar, which perform almost the whole task of digestion and ab- Fig- 305. A, Longitudinal section of wall of oesophagus of a termite. B, Longitudinal section of mid gut of termite in secretory phase. C, Trans- verse section of mid gut of Blatta in resting phase. After Imms. bm. base- ment membrane; c, chitinous intima; cm. circular muscle; cr. crypt; ep. cellular layer ; e. enteric epithelium ; h. striated hem ; l.m. longitudinal muscles ; nc. group of regenerative cells ; pm. peritoneal membrane. sorption of all classes of foodstuffs. While secreting, the cells break down and their contents are discharged into the gut cavity. In the absorptive phase the border of the cells has a striated appearance. The same cell may be capable of both absorption and secretion, but the epithelium as a whole often passes through rapid cycles which necessi- tate the constant supply of fresh cells. These are found (Fig. 305) in 434 THE INVERTEBRATA the troughs of folds or bottoms of pits into which the mid gut epithelium is thrown. In many insects the surface is increased by the formation of long diverticula, the pyloric caeca, the cells of which are not in any way diiferent from the rest of the epithelium. These vary greatly in number. Though the mid gut epithelium has not an internal chitinous lining there is a curious chitinous tube, free in its cavity, the peritrophic membrane, which is, however, secreted by special cells of the pro- ventricular region (which may be ectodermal). Its function and place in digestion are not understood. In certain cases, however, the mid gut is differentiated into func- tional regions. The first part of the mid gut of the tsetse fly, for instance, is concerned with water absorption which reduces the meal of blood to a viscid mass. Digestion of the food takes place in a region behind this, and in the lowest region of the mid gut absorption is effected. These functional regions are histologically distinct. In the cockroach in which no such histo-physiological distinction exists between the several parts of the mid gut, it appears that much di- gestion takes place in the crop to which place the enzymes from other parts may have to pass to meet the food before its further passage backwards. The so-called gizzard has been shown in this case to act not only as a triturating organ, but as a complicated sphincter guarding against the passage of any but the finest particles from the crop to the mid gut. After digestion has proceeded in the crop as the result of salivary and other secretory activity, the food passes through the gizzard, there to be triturated ; and so on to the mid gut to meet the enzymes produced by the walls of this part of the gut. Resorption of the digested food takes place in the mid gut as well as in the hind gut. The hind gut begins where the Malpighian tubules enter the alimentary canal and is usually divided into a small intestine or ileum, a large intestine or colon, in both of which the chitinous lining is sometimes folded and produced into spines, and a short globular rectum. In most insects rectal glands in the form of thickened patches of epithelium occur .These have been shown to absorb water from the faeces and therefore play a most important part in water conservation. Though the digestive enzymes of insects in the main belong to the same classes as those of mammals there are many significant differ- ences. An omnivorous insect like the cockroach produces all the classes of enzymes except that represented by pepsin which is peculiar to vertebrates. Then also the enzymes of insects appear to work in a rather more acid medium than do the enzymes of mammals. Finally the specialization in feeding habits in insects is responsible for the absence of enzymes which are not wanted and either the acquisition of enzymes not generally found in the Animal Kingdom or the formation of a symbiotic partnership. INSECTA 435 Thus when we compare the cockroach with such forms as the tsetse fly (Glossina) and the blow-fly Calliphora^ we find these latter deficient in certain enzyme classes, the former in carbohydrases, the latter in tryptases and peptidase. The evolution of the habit of feeding on blood (which consists so largely of proteins) involves the loss of the enzymes which digest carbohydrates and fats. Similarly the blow-fly which exists on a diet in which carbohydrates are predominant has to a certain extent lost its proteolytic and lipolytic enzymes. This principle has an even wider application. In the leaf-mining caterpillars of the Lepidoptera, certain species are restricted to the upper and others to the lower parenchymatous layer of the leaf. If an egg of one species is accidentally deposited in the wrong layer of the leaf, death of the larva ensues owing to its inability to digest the proteins of that layer. Thus each species, it is said, has enzymes which are specialized in the narrowest degree for digestion not only for the proteins of a single plant but for those of a particular part of that plant (all others being unsuitable). Sucking forms, like Aphis, explore diff^erent regions of the plant tissue and it may perhaps be inferred that they have a wider range of enzymes than the leaf-miners. Most interesting of all is the relation of phytophagous insects to cellulose, which is incapable of digestion by any vertebrate. Only a few wood-boring beetle larvae [Cerambycidae) have been shown to possess an enzyme which digests cellulose. The great majority of insects do not possess a cellulase and as all plant cell contents are contained within cellulose envelopes, it is clear that digestion can only follow when either protoplasm is released by mechanical injury of the cell-wall or the enzymes are able to penetrate the cell-wall and act upon the contained protoplasm. In lepidopterous caterpillars, which digest vegetable protoplasm with much greater success than do mammals, the latter explanation has been shown to be true. The insects which live on wood (excluding the Cei^ambycidae) can be divided into two classes: (i) those, like bark beetles, which feed on fungi, growing in their tunnels, and (2) those which harbour sym- biotic organisms in special parts of their alimentary canal. In the latter class maybe mentioned the wood-boring larvae of certain crane- flies and of death-watch beetles (e.g. Xestobium). In these cases the supposed symbiotic organism is the yeast, Saccharomyces. How it assists in the assimilation of wood is not known. On the other hand, those of the termites which eat wood in normal life always contain the flagellates belonging to Trichonympha and other genera (p. 68) living free in the intestine. The absolute dependence of certain termites on the flagellate is shown by the fact that when the flagellate fauna is removed (which can be done without harming the termite by heating to 40°) the termites will starve although they continue to 43^ THE INVERTEBRATA eat their usual diet. Since the trichonymphids are known to digest wood inside their own bodies, it is probably only indirectly that the termites benefit from the wood, the flagellates being their immediate source of food. Termites will live on a diet of cellulose (e.g. cotton wool) but not when the last traces of nitrogenous material have been removed. The majority of so-called saprophagous insects are really phyto- phagous, in that they feed on yeasts, and micro-organisms effecting the decomposition of the decaying matter. The house-fly is probably such a case. Blow-fly larvae feeding on decaying meat do, however, employ proteolytic enzymes, and to this extent are truly saprophagous, as is also the dung beetle Geotrupes. The flesh-fly Luctlia, though saprophagous in this way, still requires the microflora of the decaying food to complete a diet suitable for full development, these organisms supplying the vitamines necessary for growth. The great range of environments occupied by insects as a whole is largely an expression of their diverse feeding habits, and few materials have escaped their attentions. In addition to the foods mentioned above, may be noted keratin, which undergoes fermentative digestion in the larval gut of the clothes-moth Tmea biselliella. Silk can be utilized as the sole diet of the museum beetle Anthrenus museoruniy the amino-acids in this case supplanting both fats and carbohydrates. The saliva of various insects shows great variety according to their habits; thus the larva of the tiger-beetle (Ctcindela), the flesh-eating larvae of flies, e.g. Sarcophaga, and the aquatic larva of Corethra, pour their saliva, which contains a proteolytic enzyme, on their food and suck up the products of digestion {external digestion). Bees, with their reliance on pollen and honey as food, have four different kinds of salivary glands. These probably serve different purposes such as to invert sugars, to ensure preservation of food by adding formic acid, and to predigest pollen in the manufacture of " bee bread" on which the young are fed. The proportion of carbohydrate to fat and protein in the food after the early stages of feeding, determines whether a larval bee shall become a queen (fertile female) or a worker (sterile female). The former is fed throughout on a richer protein diet pre- pared from pharyngeal glands while the latter has its diet changed to pollen and nectar containing a higher carbohydrate content. In wood-boring larvae the secretion of a mandibular gland softens the wood and thus assists mastication, while, in caterpiUars, silk pro- duction is the main function of labial glands. The principal excretory organs are the Malpighian tubules, opening into the anterior end of the hind gut, and therefore are just as much ectodermal structures as the nephridia of annelids. The proof of their function is the presence of crystals, which can be identified micro- INSECTA 437 chemically as uric acid, inside the cells and in the lumen of the tubule. A mass, mainly of uric acid, is found in the hind gut of pupating in- sects, having been deposited there by the tubules. But in addition nitrogenous end products are found in the nephrocytes (cells found commonly associated with the fat body and the pericardium), the fat body and the hypodermis in quantities which increase with age, and in the hollow wing scales of certain butterflies, e.g. Pieridae, so that it appears that the mechanism of the Malpighian tubules for ridding the body of the insect of nitrogenous excreta is by no means efficient. Of non-nitrogenous excretory products may be mentioned the carbonates of calcium, potassium and magnesium. Calcium car- bonate may be excreted in the integument, but in many cases it is eliminated by the Malpighian tubules, either gradually, e.g. Droso- phila, or expelled en masse by way of the blood and the hypodermis during pupation, e.g. Ascidia, the celery fly. In this latter example the recrystallization of the compound on the inner wall of the puparium (see p. 509) may serve to strengthen the weakness of the latter. To the majority of insects the matter of water conservation is of considerable importance. It is interesting, therefore, to find that in a blood-sucking bug (Rhodnius) the proximal part of the Malpighian system is concerned with the withdrawal of water from the lumen of the tubules, and its return to the body cavity. In this insect, the distal parts of the tubes secrete into their lumen potassium and sodium urate, water and base returning to the blood through the walls of the proximal parts of the tubes (Fig. 306). A circulation of water and base exists, therefore, within the system, similar to that obtaining in the vertebrate kidney. The circulatory system. There is, first, a heart, primitively con- sisting of thirteen chambers, each corresponding to a segment, with a pair of ostia guarded by valves precluding outflow, at the base of each chamber. The blood is driven forward in these by muscular action of the heart wall, and passes into an anterior aorta which opens into the general body cavity in the head region. The haemocoelic body cavity is very spacious and the blood bathes all the organs. There is a dorsal horizontal diaphragm perforated by many holes, which separ- ates off the pericardium in which the heart lies, and attached to this are paired alary muscles, the outer ends of which are inserted in the terga (Fig. 307). By their contraction the passage of blood from the body cavity into the pericardium and heart is facilitated. Though the circulatory system is usually simple, accessory vessels are known, which direct blood backwards along the nerve cord and upwards towards the pericardium in the metathorax (in the moth Protoparce) 438 THE INVERTEBRATA Fig. 306. Diagram of a longitudinal section through the gut of Rhodnius prolixus to show the entrance of Malpighian tubules. Amp. ampulla in wall of hind gut, the long cells of which project into the rectum and with- draw water from the excretory matter ; Mg. mid gut ; Mt.D. distal part of Malpighian tubule at which place water and excretory matter enter the tube; Mt.P. proximal part of Malpighian tubule from which water is returned to the body cavity, leaving the excretory material in a granular state ; Rg. rectal gland in wall of rectum. Modified after Wigglesworth. Fig- 307- Transverse section through dorsal part of the abdomen of Apis mellifica to show attachment of heart to the body wall and to the diaphragm by the alary muscles (al.m.). After Snodgrass. (The insertion of the alary muscles in the tergum is not shown.) dg. diaphragm ;/.6. fat body; h. heart; mg. mid gut; mt. Malpighian tubule; tra. trachea. INSECTA 439 Further, in certain insects accessory hearts are present which assist in the circulation through special regions (in the thorax of the beetle Dytiscus and in the bases of the legs of Aphids where they propel blood through the wings and legs of these forms respectively). The much reduced system is on the whole greatly in contrast with the complex arrangements of the decapod Crustacea and of such Arachnids as Limulus and the Scorpions where the respiratory pigment haemo- cyanin renders the blood of the greatest importance in respiration. The part played by blood in respiration introduces a topic which can only adequately be considered with the tracheal system next to be described. In anticipation of that account it may suffice to note the following points. The walls of the tracheae are freely permeable to gases and there must therefore occur an exchange of gases between the blood and the air in the air in the tracheae. In some insects the walls of air sacs within the tracheal system become intucked so as to form "inverted tracheae" through which blood circulates, thus giving rise to an organ which may act as a veritable lung, e.g. Sphinx and Crabro. Though these facts suggest a special oxygen-carrying function for the blood, it appears that its oxygen capacity is no greater than can be accounted for by physical solution. Haemocyanin does not occur and to this fact must be put down the rather vestigial nature of the blood system in insects. Haemoglobin occurs in a few, e.g. the larva of the midge Chironomus, the male apparatus of the water bug Macrocorixa and in certain tracheal cells of the horse-fly Gastro- philus. This pigment may be derived from intracellular cytochrome and its occurrence be of the nature of a chemical accident of little functional significance. On the other hand it may serve, as it appears to do in Chironomus, as a means of enabling the animal to utilize oxygen when this occurs only at low tensions in the surrounding medium. The occurrence of chlorophyll invariably owes its origin to the food plant. Of the several kinds of blood cells which exist, per- haps those which play an important part in the histolysis of larval tissues during the pupation of holometabolous insects, e.g. the blow- fly Calliphora, are of most interest. Associated with the blood are the following cellular tissues, the fat body, the nephrocytes, the oenocytes, the corpora allata, and in various beetles, the photogenic organs. The fat body consists of closely adherent cells, in the vacuoles of which products of digestion are stored up. Fats, albuminoids and glycogen occur in this way. In addition are found urates showing that this organ serves for excretion. Oenocytes occur as bunches of large cells close to the spiracles in the abdomen and develop as hypodermal invaginations in these places. The corpora allata arise similarly in the mandibular segment and subsequently come to lie above the oesophagus behind 440 THE INVERTEBRATA the brain. These small compact glands are probably hormonic in function though the precise nature of them (as of oenocytes) awaits elucidation. 1 Photogenic organs, found in the glow-worm larva and the female beetle Lampyris, possess a rich supply of tracheae and produce light by the oxidation of luciferin by the enzyme luciferase. In the insects the tracheal system characteristic of terrestrial Arthro- poda attains its most complete development. The ectodermal tubes of the system form a network of which every part is in communication a.s. Fig. 308. Tracheal system of the locust, Dissosteira Carolina. Modified from Vinal. A, Side view. B, Dorsal view, the lower half to show air sacs, the upper half to show tracheal supply to the alimentary canal, a.c. alimentary canal; a.s. air sacs; l.t. longitudinal trunk; sp. spiracles. with every other part. Typically it communicates with the exterior by two pairs of openings called stigmata or spiracles on the thorax and eight pairs on the abdomen (Fig. 308). The main branches leading from the stigmata not only divide into finer capillaries leading to the adjacent organs but communicate by means of lateral trunks with each other. The capillaries or tracheoles never end blindly in the blood but always in the cells of the body, whether muscular or glandular or connective tissue, so that normally the oxygen is conveyed directly ^ It has recently been shown in Rhodnius that they act as ductless glands controlling metamorphosis. INSECTA 441 to the latter without the intervention of the blood. These end tubes, as may be seen in Fig. 309, are of the smallest calibre and their lumen is intracellular. The chitinous lining, which in the main tracheae is strengthened, forming the spiral threads which prevent collapse of the tubes, in the tracheoles is thinned down so much that gaseous diffusion can take place easily between the cell fluid and the lumen of the tube. The system is further elaborated to secure regular circulation of air in the main passages. Thus the stigmata are oval slits which can be closed and opened in various ways — usually by valves operated by Fig- 309. Tracheal end cell and tracheoles from silk gland of caterpillar, Phalcra bucephala. From Imms, after Holmgren, c. tracheoles ; e. end cell ; t. trachea. special muscles. Respiratory movements can easily be observed in such insects as wasps and grasshoppers. They are effected by the alternate contraction of the abdomen in its vertical axis by tergo- sternal muscles and recovery to the original form usually by the elasticity of the abdominal sclerites. Abdominal contraction with open spiracles results in expiration, but if the spiracles are closed the air already in the system will be forced into the finer capillaries where the oxygen pressure is thus increased. In some Orthoptera it has been found that certain stigmata are normally inspiratory and others expiratory. Thus, in various grass- 442 THE INVERTEBRATA hoppers (Fig. 308), the first four pairs are open at inspiration and closed in the expiratory phase, while the last six pairs are open in the expiratory phase and closed at inspiration. It follows that an air circulation through the main trunks is set up, aiding considerably in the diffusion of gas through the whole system. Air sacs (as mentioned above) in the form of thin-walled diverticula of the main tracheae occur in many insects (Fig. 308), particularly those, such as bees, migratory locusts and house-flies, with the power to fly for prolonged periods. These also assist considerably in the circulation of air through the tracheal system owing to the ease with which they can be compressed. Thus to assist respiration in typical insects a neuro-muscular mechanism has been evolved which ensures some control of the ventilation of the tracheal system. Spiracular closing mechanisms and compressible air sacs are important in this process. But though a circulation of air certainly does take place in some, there are forms, such as lepidopterous larvae, which exhibit no respiratory movements and so, it may be inferred, possess no means of ventilating the air tubes. Forces of diffusion have been shown to be adequate to supply oxygen to the tissues of such examples as have no ventilating mechan- ism. These same forces will also explain the transfer of oxygen from the wider to the narrower air-containing tracheae. With regard to the ultimate problem concerning the way in which the air reaches the cell, modern theory on insect respiration assumes that the blind ends of the tracheolar tubes are bounded by a membrane which is im- permeable to lactic acid and such metabolites. Each tracheole con- tains a variable amount of fluid, the height of the column of which is determined in a state of equilibrium by hydrostatic pressure and capillarity on the one hand and by forces of osmotic pressure in the tissue fluids and of atmospheric pressure on the other. If now the osmotic pressure of the tissue fluids, for any reason, increases, water will then be absorbed from the tracheole tubes and the column of air will be made to extend more deeply into the tissue. It has been shown that muscular activity of insects is associated with such withdrawal of water from the tracheoles. The evidence points to the conclusion that the change from glycogen to lactic acid which accompanies muscle contraction would provide the necessary osmotic changes to withdraw water from the tube and so bring the column of air to the tissues when and where their need is greatest (Fig. 310). From the air column, thus brought deeply into the tissues, the oxygen must diffuse into the surrounding tissue fluids. The control of respiratory movements by nerve centres is of in- terest. Though each nerve ganglion of the ventral chain serves as a centre for the respiratory movements of its own segment, there are INSECTA 443 certain regions of the nervous system which exercise a controlHng influence over the respiratory activity of the insect as a whole. One example will serve to illustrate this point. The nymph of the dragon- fly Libellula pumps water for respiratory purposes into its rectum. In the natural state it responds to changes in oxygen content of water Fig. 310. Diagrams to illustrate the theory of tracheal respiration. A, Trache- ole ending in resting muscle; B, In active muscle, i, trachea; 2, tracheole cell; 3, parts of tracheoles containing air; 4, parts of tracheoles containing liquid; 5, muscle. After Wigglesworth. quite readily, by increasing the rate of its respiratory movements, when there is oxygen lack ; reducing such movements in water satur- ated with oxygen. When however the prothoracic ganglion is de- stroyed, respiratory movements continue evenly, without reference to the oxygen tension of the water. 444 THE INVERTEBRATA There are thus primary respiratory centres, each responsible for movement in its own segment, and specially localised secondary centres, which can influence those movements in accordance with the demands for oxygen. The site of the secondary centre varies in different animals, but never appears to lie in the head. Just as secondary centres respond to oxygen lack, so have they been shown to respond to the influence of carbon dioxide. Though the above remarks would apply to the majority of insects, there are many stages of reduction in the group, culminating in the wingless Collembola, many of which have no tracheae at all, gaseous exchange taking place through the skin. Aquatic insects fall into two physiological groups. The first is distinguished by direct breathing, at least one pair of functional Fig. 311. Pupa of Anopheles maculipennis. After Nuttall and Shipley. /. respiratory funnel. Spiracles being retained. In the water beetle Dytiscus the abdominal spiracles communicate with a supply of air under the elytra which is renewed when the beetle comes to the surface : in the larva of the mosquito the spiracles are open to the air while the animal is sus- pended from the surface film (Fig. 312). The second group includes the early stages of the Odonata, Plecoptera, Ephemeroptera and Trichoptera. These have no func- tional spiracles but breathe by means of tracheal gills — expansions of the body wall through whose thin walls respiratory exchange between the animal and the water is effected according to the laws of diflf"usion (Fig. 333). They are usually external but in certain dragonfly nymphs {Aeschna and Libellula) the rectal wall is raised into such gills and respiration is effected by pumping water in and out through the anus. INSECTA 445 Fig. 312. Larva of Anopheles maculipennis After Nuttall and Shipley- r feeding brush; c. antenna; d. maxilla; .. thorax;/, spiracles, g. palmate hairs for suspending from surface film; /. anal gills. 446 THE INVERTEBRATA Certain larvae show an even more complete adaptation to life in water in that though they possess a tracheal system this is entirely closed from the exterior and in their early stages it is filled with fluid. Such forms respire of necessity by a process of simple diffusion through the general integument, e.g. Chironomus and Simulium. Reproduction. The sexes of insects are separate, leery a purchasiy a remarkable exception, being the only known self-fertilizing her- maphrodite in the class. The usual method of reproduction is by de- position of yolky eggs following copulation. The egg, except in many parasitic Hymenoptera, is richly supplied with yolk and invested with a vitelline membrane and further protected by a hard shell or chorion. The chorion exhibits different degrees of external sculpture and it is perforated at some point or points to allow of sperm penetration. The spermatozoa, which are of the filiform type, may be transmitted to the female in the form of a spermatophore. Though insects are on the whole prolific creatures capable of producing large numbers of eggs, a few cases are met with where females only lay a few eggs in the course of their life. Thus, in the viviparous tsetse flies, a single egg is passed to the uterus about every nine or ten days. The larva is there nourished by special "milk" glands till it is fully fed when it is passed out for immediate pupation. Viviparity and reduced tgg production are here obviously associated with one another. In a large number of cases reproduction is effected without the intervention of the male. This phenomenon of parthenogenesis is best seen in the aphides or plant lice where several generations resulting in the pro- duction of parthenogenetic females are passed through. The racial advantage accruing from this greatly increased reproductive capacity is obvious. Parthenogenesis is in certain cases, e.g. among the family Cecido- myidae of the order Diptera, found to occur in larval forms. In Miastor, a form living in decaying wood and bark, reproduction in this manner (paedogenesis) occurs for the greater part of the year. These larvae contain prematurely-developed ovaries from which as many as thirty larvae may grow. In summer, larvae occur which are morpho- logically different from the paedogenetic forms. These summer larvae pupate and the small midge-like flies which emerge lay four or five large eggs; from these a further series of paedogenetic larvae arises. Among a few of the parasitic Hymenoptera, e.g. some Chalcididae, the phenomenon oi polyembryony has been observed. This consists in the development of more than one embryo from a single Qgg. In Copidosoma gelechiae, which parasitizes a caterpillar living on the goldenrod Solidago, a hundred or more embryos may result from the deposition of a single egg. INSECTA 447 Organs of reproduction (Fig. 313). In the male the testes are usually small paired organs lying more or less freely in the body cavity. The extent to which they are divided into follicles, and the form of follicle, vary in different orders. Thus, in the Diptera, each testis is unifoUi- cular, while in the Orthoptera a multifollicular condition prevails. Each follicle is divided into Sigermarium or formative zone, a zone of growth and maturation, and a zone in which spermatids are trans- formed into spermatozoa. In multifollicular testes the connection between each follicle and the main duct is known as the vas efferens Fig. 313. Diagram of reproductive organs of A, a male, and B, a female honey bee. C, Longitudinal section of an ovariole of Dytisciis marginalis. A and B after Comstock. ac.gl. accessory gland; be. bursa copulatrix; cgl. colleterial gland; ed. ejaculatory duct;/, follicle cells; ge. germarium; ov. ovary; od. ovi- duct ; o. ovum ; sc. spermatheca ; t. testis (multifollicular) ; vd. vas deferens ; V. vagina; ve.se. seminal vesicles. and each testis leads to the median ejaculatory duct by a vas deferens which is swollen at some point to form a seminal vesicle. The ejacu- latory duct opens between the 9th and loth abdominal sterna in association with the external genital plates (gonapophyses) of copu- latory significance. Accessory glands of various kinds and little understood function are usually found associated with the genital ducts. The female organs (Fig. 313) consist of ovaries, oviducts, sperma- thecae, colleterial glands and a bursa copulatrix. 448 THE INVERTEBRATA Each ovary consists of a number of ovarioles, corresponding to the testicular follicles of the male. Reduction of the ovary to a single ovariole occurs in such insects as Glossina^ the tsetse fly, where the minimal number of eggs is produced. Each ovariole (Fig. 3 13) is tubular and contains zones corresponding to those met with in the follicle of the testis. In addition to the de- veloping ova, nutritive cells are found in association with the latter. Such cells are concerned with the transference of yolk to the growing ova and they or other cells may entirely encircle the ova, round which they secrete the chorion or outer egg shell. The ovarioles forming an ovary are connected together anteriorly in the body cavity by their peritoneal coverings, known at this point as terminal filaments, and these are attached either to the body wall or the pericardial diaphragm, thereby maintaining the ovary in position. The oviducts leading from the ovaries unite in the middle line to form a common duct which widens to form the vagina immediately before reaching the exterior on or between the 8th, 9th and loth abdominal sterna. Colleterial glands providing fluid for the formation of an ootheca (a case surrounding the eggs), or a sticky secretion for fastening eggs to surfaces, usually open into the vagina. The pouch for the reception of spermatozoa is the spermatheca. It is an ectodermal invagination, lined by chitin and provided with a muscular coat. The spermatheca opens into the vagina or into the bursa copulatrix, this being an in- vagination of the body wall around the genital aperture adapted for receiving the intromittent organ of the male. The fiervous system of insects (Fig. 314) consists of a dorsal brain and a ventral double chain of ganglia connected by longitudinal and transverse commissures. The anterior three pairs of ganglia of the ventral chain are always fused to form the suboesophageal ganglion^ the nerves from which supply the mouth parts. The suboesophageal ganglion is united by paraoesophageal connectives to the brain. The brain consists of three pairs of closely fused ganglia which supply the eyes, antennae and labrum respectively (see p. 425). In addition to this is the sympathetic system (Fig. 314 B, C) which supplies the muscles of the alimentary canal and of the spiracles. In the insects, and indeed the arthropods in general, there has been a great advance over the stage of nervous organization in the annelids. The complex nature of the appendages and the necessity of co-ordi- nating groups of these for locomotion, feeding and so on, has led to the association of special parts of the nervous system with these functions. We will call each such part a "functional unit". Each functional unit is to some extent self-regulating and is not dependent INSECTA 449 for its autonomous action on the higher centres. For example a de- capitated wasp can still walk and if a limb be removed from one side, compensating movements of the remaining five legs enable the animal to walk in a straight line. But the working together of the functional units concerned, into different reactions, is controlled by the brain, ocn. Fig. 314. Nervous system of a grasshopper. After Uvarov. A, Ventral chain. B, Brain and associated nerves. C, Optical section through head. Mtrn. antennary nerve and ganglion; dc. deutocerebrum ; ocn. ocellar nerve; op.ga. optic ganglion; pc. protocerebrum ; sug. suboesophageal ganglion; syg. sympathetic ganglia ; tc. tritocerebrum. and the inhibitory character of that control is shown when the ganglia are removed. A "decerebrate" bee will try to fly, walk, feed and polish its abdomen all at the same ^ime. This is because no inhibi- tion is being exercised on the functional units, which themselves remain intact in spite of the removal of the higher centres. Sense organs. There can be no doubt that insects perceive stimuli similar to those causing sensations in ourselves. They are sensitive 45° THE INVERTEBRATA to the waves of light and sound, to changes of temperature, to chemi- cal stimuli by contact or at a distance, e.g. as in the sensations of taste and smell, and to tactile impressions. The sensory equipment is complicated, and the solution of the functional problem which many of its parts present is not made easier by the fact that though the principle of the reaction may be the same as in ourselves, insects often react to stimuli of an amplitude which is beyond our receptive capacity. For instance, they do react to pitches of sound which the human organ cannot detect, and though they do not appreciate the full spectrum in colour vision, they can perceive ultra-violet rays. No matter what the sense organ may be, the fundamental element is the sensilla. In the case of a simple sensory hair {trichoid sensilla) the following elements are present: a trichogenous cell which gives rise to the seta ; a hair membrane cell which produces the fine mem- brane at which the seta is articulated to the body wall, and a bipolar nerve cell which lies within the trichogenous cell (Fig. 315). Such sensillae are generally tactile, though in certain cases olfactory, gusta- tory and heat-perceiving functions have been shown to rest in them. Olfactory sensillae commonly occur on the antennae. These are generally placoid (with plate-like cuticle covering the sense cell) or coeloconic (where the covering plate is thin and sunk in a depression below the surface) (Fig. 315). But though the antennae are usually olfactory in function, this sense is also located elsewhere, since re- moval of the antennae does not entirely inhibit olfactory sensation. The power of insects to diffuse scents from special glands is well known. These serve for defence, or to attract the sexes to each other, and their prevalence, and wide distribution throughout the class, postulate the existence of an olfactory sense. In moths the faculty possessed by males of discovering the exact position of unpaired females is of so astonishing a character that many observers have dis- believed the olfactory explanation, and resorted to theories of etheric wave-transmission. The production of a volatile chemical is clear, however, in those cases where male moths have assembled at an empty box in which a female had been recently housed. It is com- paratively simple to demonstrate the existence of a taste sense in insects. Preferences for sugar to other substances in solution can readily be shown in a feeding butterfly. To find however, for example as in Pyrameis, the red-admiral butterfly, that the taste organs lie in the feet, is perhaps sufficient reason for using the term chemo-tactile ^ for a sense which has no exact parallel in our own experience. Taste organs occur also in the mouth, and on the palps of the mouth-parts. Many insects, such as grasshoppers and cicadas, are provided with sound receptors known as tympanal organs, with which are incor- porated chordotonal sensillae. Each of the latter consists of a sense INSECTA 451 cell, to one end of which is attached a nerve fibre. To the other end is connected a rod or scolopale which ends in an apical thickening or is free to vibrate in the fluid protoplasm of an enveloping cell. The whole structure is attached to the hypodermis by covering cells at one end and by a ligament at the other (Fig. 315 D). Fig. 315. Insect sensillae. A, Trichoid sensilla. B, Placoid sensilla. C, Coelo- conic sensilla. Tc. trichogenous cell; He. hair-membrane cell; Nc. nerve cell. D, Chordotonal sensilla. Cc. cap cell to scolopale ; Ec. enveloping cell ; Ek. end knob of scolopale; H. hypodermis; L. ligament of attachment; Nc. nerve cell; S. scolopale and V. its fluid filled vacuole. A, modified from Eltringham after Snodgrass. B, modified from Imms after Hess. C and D from Imms. Scolopale sensillae of this type may or may not be associated with a tympanum or ear drum. When they are, as in cicadas and grass- hoppers, there is clear evidence of response to sound waves set up by sound-producing organs possessed by themselves. 452 THE INVERTEBRATA In the numerous cases in which no tympanum capable of respond- ing to sound waves exists, a precise function is not clearly indicated. According to some, they may act as rhythmometers, i.e. co-ordinators of the rhythmical movements of the insect's body. A more probable function is that of perceiving vibratory stimuli from without. The organs of vision have been dealt with in Chapter x, and it is perhaps enough to mention that the ommatidia of which the com- pound eye is built up, are specialized sensillae of hypodermal origin, essentially similar to those already mentioned. Embryology . Though Arthropod eggs vary in the amount of yolk contained within them they are for the most part yolky and are ceiitrolecithal in type (p. 316). To this feature must be ascribed those distorting influences which make Arthropod development so different from that of other invertebrates. Among insects it is only in the primitive Apterygota and in many parasitic Hymenoptera that are found small, comparatively yolkless eggs which undergo total cleavage. But though these may represent the primitive condition, they cannot be taken as typical of modern insects. The typical 3^olky egg is provided with a vitelline membrane and a stout chorionic shell which is commonly sculptured. After fertiliza- tion, incomplete cleavage sets in, a process involving only the suc- cessive mitoses of nuclei. In this early stage, therefore, the egg is a syncytium of very yolky cytoplasm in which lie the cleavage nuclei. These wander to the peripheral cytoplasm, there to form an outer cellular layer or blastoderm (Fig. 316 A and B). In this latter, occurs a thickening, thus separating embryonic from extra-embryonic blastoderm and in its relation to the yolk the embryo now resembles an inverted chick embryo, but, as might be expected, its method of differentiation is highly different. Gastrulation proceeds as follows. From the middle line of this embryo certain cells pass inwards towards the yolk by invagination, by proliferation or by their overgrowth by cells of the germ band lateral to them. This enclosed cell mass is mesoderm (together with endoderm in certain cases). The plate left outside constitutes the ectoderm (Fig. 316 C and D). In such cases where endoderm is not included in the enclosed mass as above, this layer rises from growth centres, anterior and posterior, at the places where the stomodaeum and proctodaeum will appear or already have differentiated. The result in any of these cases is a three-layered embryo relegated to the ventral side of the egg, i.e. beneath the yolk. It consists of a layer of outer ectoderm, within which is the mesoderm from which segmental somites develop. Against the yolk lies the endoderm destined to form the mid gut. The mesoblastic somites give rise on their upper borders A' Ect. C Ect EM. D' End. H f Fig. 316. To illustrate the main features of insect embryology. A, B,C, Sagittal sections. A', B', C, Transverse sections through corresponding stages. D, Sagittal section of embryo with germ layers present. C", Transverse section through C-stage embryo in mouth region. D', Transverse section through D-stage embryo. E, Transverse section through older embryo with haemocoele between endoderm and ectoderm (in which nerve cord is developing). The mesoblastic somite on each side has given rise to the gutter-like heart rudiment, to the fat-body and to the muscles of body wall and gut. Embryonic membranes, the amnion A and the serosa 5 cover the embryonic rudiment. Bl. blastoderm; Ect. ectoderm; End. endoderm; Exe. extra-embryonic blastoderm; F. fat-body; G.B. germ band; H. haemo- coel; ///. heart rudiment; M. mesoderm; Mc. muscles; Pr. proctodaeum; St. stomodaeum ; Y. yolk. After Eastham. 454 THE INVERTEBRATA to the heart rudiments, and on their outer and inner borders to the muscles of body wall and gut respectively. The lower border of each somite breaks down to form fat-body. In so doing the coelomic cavity disappears, and, minute as it always was, becomes continuous with those spaces arising by separation of the germ layers from each other, viz. the haemocoele. This latter as in all Arthropods consti- tutes the main body cavity (Fig. 316 E). Metamorphosis. Insects, like all other arthropods, attain their maximum size by undergoing a succession of moults or ecdyses. The number of moults which an insect passes through is fairly constant for the species, and the form assumed by the animal between any two ecdyses is termed an instar. The animal's existence is thereby made up of a succession of instars, the final one being the adult. In the simplest and most generalized insects the several instars are very similar to one another and only differ from their appropriate adults in the absence of wings and the incomplete development of the re- productive system. Where the adult is primitively wingless, as in silver fish and springtails (Fig. 323), the change from young to adult is so slight as to be ignored, and metamorphosis, involving only a development of the reproductive system, is conveniently regarded as being absent. The insect orders falling in this category are grouped under the heading Ametabola. In winged insects, however, the winged adult is in sharp contrast to the wingless young stage. Such forms are said to undergo a meta- morphosis (Fig. 341). The degree of metamorphosis varies consider- ably, irrespective of wings, in winged insects according as the young stages resemble their adults or not. A growth stage of a cockroach, for instance, possesses the general appearance of the adult. On the other hand the young stage of a housefly is a grub and has no re- semblance to the final instar with its wings, elaborate body form and mouth parts (Fig. 349). Metabolous insects, those passing through a distinct metamor- phosis, are therefore further divided into two subclasses, (i) the Heterometabola, e.g. the cockroach, and (ii) the Holometabola, e.g. the fly. A classification of insects based on degree of metamorphosis is therefore possible and such a basis for classification is used in all modern systems. The orders composing the Heterometabola are the Orthoptera, Dermaptera, Hemiptera, Isoptera, Embioptera, Psocoptera, Ano- plura, Thysanoptera, Plecoptera, Ephemeroptera, Odonata, Mallo- phaga, the last three orders being sometimes classed as Hemimetabola owing to the young stages being aquatic and distinguished from the adults by the possession of features adapting them to life in water. The young stages of all the Heterometabola, however, strongly re- INSECTA 455 semble their adults in body form, type of mouth parts, and the possession of compound eyes, and are known as nymphs (Fig. 317). Fig. 317. Metamorphosis of a capsid bug {Plesiocaris vagicollis). After Petherbridge and Hussain. 1-4, nymphal instars; 5, imago. The orders composing the Holometabola are the Neuroptera, Mecoptera, Trichoptera, Lepidoptera, Coleoptera, Strepsiptera, 456 THE INVERTEBRATA Hymenoptera, Diptera, and Aphaniptera. The young stages of these are known as larvae and differ from their aduUs in body form, mouth parts, and the absence of compound eyes. So great is the difference between the larva and the aduh that an instar known as the pupa has been specialized to bridge the gulf (Fig. 341). This stage, one of apparent rest, is actually one of great physiological and developmental activity, and it is here that many larval tissues, e.g. the muscles and the alimentary canal, are broken down by phagocytic or other action and the new adult tissue is built up from many growth centres, generally known as imaginal discs. A less obvious prepupal instar is also present, enabling the change from larva to pupa to be effected. It may reasonably be assumed that metamorphosis of the Holo- metabola has arisen through larval and adult specialization going on concurrently but in opposite directions, and it is not surprising to find among the orders composing this group, as for instance in many Coleoptera, larvae which are rather nymph-like in that they are well chitinized and possess well-developed legs, and mouth parts re- sembling those of the adults (Fig. 318 A). The forms of larvae vary considerably and indicate to a great extent the degree of metamorphosis passed through. A campodeiform larva (Fig. 3 18 A) is one strongly resembling certain members of the ametabolous Thysanura and possesses well-developed legs, biting mouth parts, antennae and cerci, e.g. many Coleoptera. An eruci- form larva (Fig. 318B) is fleshy and thin-skinned, its legs are often in the form of supporting struts rather than organs of active loco- motion, and there are no cerci. Further, prolegs are often found on the abdomen, e.g. caterpillars of Lepidoptera and sawflies (Fig. 344). A grub (Fig. 318 C) is an apodous larva which in other respects re- sembles the cruciform type, e.g. certain Diptera, Coleoptera and Hymenoptera. Pupal modifications are also found; thus the exarate type, cha- racteristic of the Hymenoptera, Mecoptera, Neuroptera, is that in which the cases, in which the adult appendages lie, are free of any attachment to the body (Fig. 341). In obtect pupae (Fig. 338), wing and leg cases are fused to the body wall, e.g. most Lepidoptera and Diptera. In the most specialized Diptera the last larval skin is re- tained as a barrel-shaped puparium over the pupa within. Such pro- tected pupae are called coarctate (Fig. 349). In the Heterometabola the development of adult form is a gradual process and the appendages, including mouth parts, antennae and legs, grow directly into those of the adult. Wings in such forms develop gradually as external dorsolateral extensions of the meso- and metathoracic body wall (Fig. 317). All the Heterometabola have such a wing development and therefore the alternative name Ex- opterygota is often given to the group. INSECTA 457 Larvae of the Holometabola on the other hand possess, for the most part, mouth parts having a form and mode of working different from that of their aduks, their legs are reduced in size and complexity or even absent, and they show no sign of external wing growth. It is in the pupal stage that adult appendages appear for the first time on the surface. Though the many forms of larvae may be regarded as adaptive modifications of a primitive type (for example the eruciform larva as Fig. 318. Types of coleopterous larvae. A, Campodeiform larva of Ptero- stichus, Caraiiidae (original). B, Eruciform larva of Melolontha, Scarabaeidae (original). C, Legless larva oi Phyllobius urticae, Curculionidae. After Rymer Roberts. an adaptation to a sedentary life among abundant food) their origin may be explained by reference to embryology. In the development of insects a germ band lies ventrM to the yolk and this undergoes development from before backwards progressively, into segments which bear limbs. At an early stage (Fig. 3 19 A), the cephalo-thoracic segments and appendages may be present while the abdomen is as yet unsegmented. A little later (Fig. 319 B), the abdomen becomes segmented and later still (Fig. 319 C), on these segments embryonic 458 THE INVERTEBRATA legs occur. Finally these abdominal embryonic legs disappear and the insect may then hatch with thoracic legs only (Fig. 319 E). These Fig. 319. To illustrate the principles of Berlese's theory. A, B and C are 3-> 3*- and 4-day embryos oi Hylatoma berberidis in the Protopod oligomero, Protopod polymero and Polypod stages of development. D is the Oligopod stage of Melolontha. Corresponding with these are the early larval forms, E, oi Platygaster (protopod), F and G of Figites anthomyiarum (protopod poly- mero and polypod respectively) and H, of Sitaris (campodeiform or oligopod). A, B, C and D, after Graber; E, after Kulagin; F and G, modified after James; H, after Korschelt and Heider. A, antenna; A i, first abdominal leg; Md. mandible; Mz, second maxilla; Mi +2, first and second maxillae ; T3, metathoracic leg. Stages are known as the Protopod oligomero (imperfectly segmented abdomen), Protopod polymero (segmented abdomen), Polypod (with INSECTA 459 abdominal legs), and Oligopod (thoracic legs only) stages respectively. It is noteworthy that among larvae there are forms resembling these different stages. Thus the first stage larva of Platygaster is in a Proto- pod condition (Fig. 3 19 E). The first stage larva of Phaenoserphus is in a Polypod state. The first two stages in the larval life history of the Cynipid Figites resemble Protopod and Polypod embryonic stages respectively (Fig. 319 F, G). The Campodeiform larvae of Carabid beetles, of certain Trichoptera, and of the Neuroptera, resemble the final Oligopod stage of embryonic development (Fig. 319 H). Because of facts of this nature, it has been suggested by Berlese in a theory which now carries his name, that the moment of eclosion from the ^gg> perhaps decided by the amount of yolk, is one of significance in determining the form of the larva. Thus in the Holometabola, the stage in which hatching occurs corresponds with one or other of the embryonic phases alluded to. Some insects hatch as veritable em- bryos, i.e. as protopod, others as polypod, or oligopod larvae. A fourth larval form, the apodous grub of Diptera, of many Hymenop- tera and of some Coleoptera may be derived by degeneration from the preceding oligopod stage. The theory further maintains that in Heterometabolous insects, the above stages, with the exception of the Apodous, are always passed through in the egg, and emergence from the egg in these insects occurs as a nymph which has thus reached in embryonic life a higher stage of differentiation than any larva. The natural corollary of this theory is that certain if not all of the nymphal stages of the Heterometabola correspond to the prepupal and pupal instars of the Holometabola. The development of adult appendages in the larva is only one of the many aspects of metamorphosis. The wings which suddenly appear in the pupa of the butterfly grow gradually through each of the five larval instars, but instead of growing externally as in the Heterometabola (Exopterygota) they arise as outgrowths from the bottom of intuckings of the body wall. In other words an accom- modating fold of the body wall forming a sac, opening at the surface by a minute pore, hides the growing wing bud within it and this is the main difference between endopterygote and exopterygote develop- ment. At pupation the sac carrying the wing disc or bud at its base be- comes straightened out by contraction of its walls and the wing bud is thereby brought to view. Similar limb buds are to be found for the adult legs and mouth parts, which always grow in association with the corresponding larval organs. Such buds are known collectively as imaginal discs and their existence characterizes all endopterygote insects (Fig. 320). 460 THE INVERTEBRATA Fossil record. Though the insects form an undoubted natural group — all its members being referable to some generalized form, possess- ing among other things mouth parts similar to those of the cockroach, ivr.- Fig. 320. The internal development of a wing in the larva of the butterfly Pieris rapae as seen in transverse sections. A, Instar i. B, Instar 2. C, In- star 3. D and E, Instar 5. ch. chitin; hy. hypodermis; m./. middle lamella; p.m. peripodal membrane ; tch. tracheoles within the veins ; tel. tracheole cells ; tra. trachea; v. vein; wr. wing rudiment. efficient for chewing solid food, an ii-segmented abdomen, a 3-seg- mented thorax and a 6-segmented head, and two pairs of membranous wings carrying parallel longitudinal veins with a reticulum of cross veins between them — the orders are clearly defined. They are not INSECTA 461 easily linked together by intermediate forms and the story of evolution within the subphylum consists rather of disjointed sentences than a continuous theme. The two divisions already mentioned, however, the Exopterygota and Endopterygota, are natural groups which we may for convenience call the "generalized" and the "specialized" respectively. The former have for the most part biting mouth parts (the Hemiptera forming an important exception), while the latter have their mouth parts modified in many remarkable ways enabling them to tap sources of food forbidden to the others, such as the in- ternal fluids of plants and animals and the deeply hidden nectar of modern flowering plants. Moreover, the life cycle in these two divi- sions is very difl^erent, the exopterygote (hemimetabolous) insects having a gradual metamorphosis with external wing growth and the endopterygote (holometabolous) forms having a complex meta- morphosis with internal wing growth and a pupal stage intercalated in the life history to bridge the gulf between dissimilar larvae and adults. From a morphological study alone one is driven to the conclusion that the insects with biting mouth parts and simple metamorphosis are the most primitive — i.e. more nearly resembling the ancestral forms than the Endopterygota. It is of great interest therefore to find that the palaeontological record, though discontinuous, supports the conclusions drawn from comparative anatomical investigations. The first records of insects are to be found in rocks of the Devonian period. Here they consist of remains which, though fragmentary, suggest that wingless insects similar to our present-day Apterygota abounded then. If they were as soft-bodied as those we know to-day the poverty of the record can well be understood and it is fairly certain that thysanuroid insects similar to the silver fish Lepisma existed, throughout the Devonian age. There is abundant evidence, however, that winged insects existed in the Carboniferous period. There were insects with prominent meso- and metathoracic wings, with lateral wing-like expansions on the prothorax, and shorter pleural processes on the abdomen. The order Palaeodictyoptera in which such forms have been placed has given rise to much speculation as to the origin of wings, one idea being that wings are hyper-developments, on the appropriate segments, of lateral processes which occurred on all segments behind the head. In rocks of the same period have been found forms so similar to our modern cockroaches that it is xlifficult not to place them in the same family, mouth parts and wing venation being almost identical in the ancient and modern types. Since such forms have existed from the Carboniferous till to-day the student making his first essay into the intricacies of entomology by dissecting the cockroach should keep 462 THE INVERTEBRATA in mind that he is dealing with a very ancient type — a real aristocrat among insect species ! In both the Ephemeroptera and Odonata we find many generalized characters — in the mouth parts and the reticulate wing venation — and these orders had their origin in the Permian, when forms assigned to the two orders Protephemeroptera and Protodonata abounded. Even as early as this, these orders had taken to a nymphal aquatic existence. In the Permian rocks we find primitive dragonflies, stone- flies and Hemiptera of which the Heteroptera with their character- istic half-horny anterior wings appear to be the more recent develop- ment. Up to this stage none of the important endopterygote orders had made their appearance. The mandibulate Mecoptera form an order which is more general- ized in structure among the Endopterygota, and Permian Mecoptera from Kansas and New South Wales have been discovered which have wing features that link up five of the important higher orders, the Diptera, Trichoptera, Lepidoptera, Neuroptera and Mecoptera. The highly specialized Hymenoptera make their first definite ap- pearance in the sawfly form in the Jurassic, but remains from the Permian have been described as Protohymenoptera. These had two pairs of wings of equal size without coupling apparatus and a venation of a generalized hymenopteran type. Hymenoptera of the specialized kinds — the bees, wasps, ants — are found first in the Tertiary period. In the same way we find nemato- ceran Diptera (craneflies, etc.), in the Upper Lias, but not till the Tertiary age do we find forms more nearly resembling our highly organized blowflies, etc. Little can be said here of the Lepidoptera except that they occur in the Tertiary period. The Coleoptera are far older geologically than the Diptera, Lepi- doptera and Hymenoptera. Already there were water beetles, weevils and the leaf-eating chrysomelids in the Triassic, and recognizable beetle remains, though scarce, have been extracted from the Upper Permian. This is not without interest, since the Coleoptera as we know them to-day possess, particularly in their mouth parts, a number of features which place them in the generalized category. Now if we consider the order of events hinted at in the above brief account, it will be seen that though the ancestors of the Hymenoptera, Diptera and Lepidoptera may have existed in the Permian, the latter age with the Carboniferous was essentially one of insects with in- complete metamorphosis and with no feeding mechanism for dealing with flowering plants. It has been suggested that the change from the perpetual warmth and humidity of the Carboniferous to the transitional epoch of the Pernio- Carboniferous with its glacial con- INSECTA 463 editions may have accounted for the onset of metamorphosis, the pupal stage being evolved for the purpose of surviving cold periods while in a quiescent state. The most interesting fact, however, is that the main evolution of our specialized bees, flies and butterflies coincided in point of time with the evolution of the flowering plants to which by their manner of feeding they are now on the whole so inseparably bound. Class APTERYGOTA Primitively wingless insects carrying on the abdomen a varying number of paired appendages other than the external genitalia and cerci. Metamorphosis slight or absent. Order THYSANURA (Bristle-tails) Biting mouth parts (Fig. 301); antennae many-jointed; compound eyes present; abdomen of eleven segments, some or all of which bear styliform appendages which probably represent the coxites of limbs no longer present; anal cerci usually jointed, rarely (e.g. Japyx) in the form of forceps. Lepisma saccharina (Fig. 321), the common "silver fish" which inhabits dwellings of man, and Machilis (Petrobius) maritimuSy found above high-tide mark along the sea shore and estuaries, are common examples. In Machilis (Figs. 322, 301) interesting features are pre- sented by the well-developed superlinguae and the jointed mandibles both of which are primitive characters. The superlinguae in Machilis are paired structures attached to the hypopharynx and possess inner and outer lobes and a palp-like process. This superficial resemblance to maxillae gave considerable weight to the view that an additional head segment was involved. Embryological evidence in support of this conclusion is of a doubtful nature, and the most acceptable view to take is that the superlinguae are processes attached to the hypo- pharynx and perhaps homologous with the paragnaths of Crustacea. Order COLLEMBOLA (Springtails) Small wingless insects with biting mouth parts deeply withdrawn into the head ; compound eyes absent ; 6-segmented abdomen which often carries three pairs of highly modified appendages serving the purposes of adhesion and jumping; a tracheal system is commonly absent and there are no Malpighian tubules; metamorphosis absent. Four-jointed antennae, ocelli and postantennal sensory organs are characteristic features of the head. 464 THE INVERTEBRATA There are no tarsi on the legs, claws being borne by the tibiae. The I St abdominal segment carries a ventral tube which is moistened by a glandular secretion from behind the labium poured down a ventral Fig. 321. Lepisma saccharina. Fig. From Imms, after Lubbock. 322. Machilis (Petrobius) maritimus. From Imms, after Lubbock. groove running along the middle of the thorax. This ventral tube, re- garded as adhesive, is formed by the fusion of the embryonic append- ages of this segment. On the ventral side of the 3rd segment, the INSECTA 465 nearly complete fusion of a pair of appendages has resulted in the formation of the hamula, which engages the furcula prior to leaping. The latter is a forked structure representing a pair of limbs of the 4th segment (Fig. 323). By contraction of the extensor muscles of the furcula the latter is pulled down out of contact with the hamula and the animal is propelled forwards into the air. The absence of tracheae is a secondary feature due to the small size of the animals rendering surface respiration sufficient for their mode of life. Collembola have a wide distribution. They are found along the sea shore between tidemarks and submerged by each tide, e.g. Anurida maritima. Common aquatic forms are denizens of fresh waters, e.g. Podura aquatica. They have been reported to be so 1^^: TTlO Fig. 323. A, Axelsonia (Collembola). B, Hamula of Tomoceros showing c. basal piece, and r. its rami. From Imms, after Carpenter and Folsom. p. ocular pigmented area; v. ventral tube; h. hamula; m., d. and mc. caudal furcula. abundant in Arctic zones as almost to cover the snow, and in Europe sometimes to be present in such large numbers that the progress of railway trains is impeded owing to their having prevented the wheels from gripping the rails. Order PROTURA Minute insects without wings, eyes or antennae ; with piercing mouth parts deeply inserted in the head capsule ; with abdomen of twelve segments, the first three of which bear papillae. This is a small group of doubtful affinities. Its members are found in decaying organic matter. The fact that on hatching the abdomen is 9-segmented and that subsequent moults bring about the full number of segments is regarded by some authorities as sufficient ground for their inclusion in a class distinct from the Insecta. An example is Acerentomum doderoi of Europe. 466 THE INVERTEBRATA Class PTERYGOTA Subclass EXOPTERYGOTA Order ORTHOPTERA Insects with generalized biting mouth parts; ligula 4-lobed, con- sisting of inner paired glossae and outer paraglossae ; fore wings rather narrow and somewhat hardened {tegmina) ; hind wings membranous ; abdomen usually with jointed cerci of short or moderate length; ovipositor generally present. This order comprises terrestrial insects of large size which have great powers of running and jumping. There are many flightless species in all the families (cf. the female of Blatta orientalis). The main structural features are exemplified by Periplaneta, the cockroach. Its generalized character is shown by the character of the mouth parts, the nervous system (six abdominal ganglia), the circulatory system (heart with thirteen chambers, three in the thorax and ten in the abdomen), and the obvious ten segments of the abdomen. The order is divided into two sub -orders, the Cursoria in which the legs are of approximately equal size and the Saltatoria in which the last pair of legs are modified for jumping (Fig. 324). The former consists of the Blattidae (cockroaches) which are swift-running, omnivorous forms, usually tropical in their distribution, the Mantidae (praying insects), which are carnivorous, with modified raptorial fore legs, and the Phasmidae (stick and leaf insects), some of which are immensely elongated and attenuated to resemble sticks or twigs, while others have laminar expansions of the skin that give the animal a re- semblance to leaves, which is closer in the female than in the male. The female phasmid at any rate is almost motionless, and the habit of feigning death is commonly developed in the family. All these cha- racters help to protect the female from observation in the plants which it frequents and of which it eats voraciously. In the Saltatoria there are the Acridiidae (locusts and short-horned grasshoppers), the Locustidae (long-horned grasshoppers), and the Gryllidae (crickets). The latter include a form remarkably adapted for a burrowing life, namely Gryllotalpa. Nearly all these insects are vegetarian, and in the Acridiidae, while the species commonly live a solitary existence and are harmless, under certain conditions a form with a gregarious and migratory instinct develops in countless num- bers which invade cultivated districts causing incalculable harm. Thus in the case of Locusta migratoria, when environmental con- ditions favour an increase in numbers, there is an inevitable trend towards the production of swarming migrants, i.e. the gregarious INSECTA 4^7 phase. The subsequent decline in numbers leads to the production of solitary non-migrants, i.e. the solitary phase. The two phases differ morphologically, biologically and in distribution so distmctly as to have been regarded as distinct species. Between them are transient individuals which form a series with no fixed characters, Fig. 324. Pachytylus migratorius. A grasshopper. Natural size. From Shipley and MacBride. merging imperceptibly into the gregarious phase at one end and into the solitary phase at the other. . A very characteristic feature of the Saltatoria is the possession ot stridulating organs. In one type, exhibited by the cricket Gryllus, a file on one of the anterior wings is rubbed over a scraper on the other. 468 THE INVERTEBRATA In another type, e.g. Locusta, a row of pegs on the hind limb is rubbed against a thickened area of the fore wing. Where there are organs for producing sound, there are also organs for perceiving it. These are tympana^ chitinous ear drums, which can be set in vibration and then affect special auditory sense organs. The auditory organs may be found on the front tibiae or on the ist abdominal segment. The posterior wings of the Saltatoria possess many parallel longitudinal veins with a network developed between these by numerous cross veins. They fold in a fan-like manner, a line of folding, the anal suture, separating a prominent posterior "anal" area of the wing from the main part of the wing in front. Besides the fully winged forms, like locusts, there are found in the several families all stages of wing re- duction to mere scales as in certain stick insects, or to their complete absence as in Grylloblatta. Fig. 325. Forficula auricularia. Male. From Imms, after Chopard. Order DERMAPTERA Insects with biting mouth parts ; ligula two-lobed ; fore wings modi- fied to form short leathery tegmina; cerci unjointed, always modified into forceps; metamorphosis slight. The common earwig, Forficula auricularia (Fig. 325) is the best example of this small but definite order. It comprises a number of small, usually nocturnal insects, omnivorous in diet. The female deposits the eggs in the soil, remains with them until they hatch, and even protects them afterwards. The hind wings have a characteristic venation and fold up along transverse as well as longitudinal furrows. INSECTA 469 thus contrasting with the Orthoptera. When unfolded, the wing presents the appearance of a half wheel, the " spokes " radiating back- wards from the anterior border, which is greatly strengthened. The large posterior membranous portion corresponds to the anal wing area of Orthoptera, that part corresponding to the anterior area of the latter order having been greatly strengthened by the coalescence of a number of longitudinal veins. The forceps are organs of defence and offence. In Labidura they are used for seizing the small animals on which this form lives. Order ISOPTERA (Termites or White ants) Social and polymorphic insects with biting mouth parts ; four-lobed ligula; wings very similar, elongate and membranous, capable of being broken off along a line at the base ; cerci short ; metamorphosis slight. The animals of this order abound everywhere in the tropics. Like the true ants they have types of individuals (castes), specialized for the purpose of reproduction, labour and defence (Fig. 326). The termite community usually contains a dealated royal pair^ the king and queen, who are the founders of the colony, and also supple- mentary reproductory individuals of two kinds: {a) winged, which normally serve for the formation of new colonies, and {b) wingless, which become capable of reproduction if occasion demands. There is usually a vast number of sterile wingless individuals belonging to two castes, the workers and soldiers. The termite nests may be merely series of burrows in trees, dry timber or in the ground, or they may be huge mounds made of earth cemented together with the saliva of the termites. Those living in the ground excavate the soil of the tropics, turning it over and enriching it just as earthworms do in temperate regions. Their food consists chiefly of wood and other vegetable matter and many species are extremely harmful, e.g. Neotermes, which damages structural timbers, and Calotermes militarise which bores into and does much harm to tea plants in Ceylon. The winged sexual forms in several colonies usually swarm at the same time so enabling intercrossing between members of different colonies to take place, and of the countless numbers a few individuals escape the attacks of birds and ojther animals and alight and cast their wings. A single pair forms a new colony first of all by making a small burrow, the nuptial chamber. The first formed young are mostly workers, and having themselves been tended to maturity by their parents take over the nursing of the young. The queen becomes 470 THE INVERTEBRATA enormous and helpless and is fed by the workers ; she lays eggs at an incredible rate, up to a million eggs a year, it is said. It is now known that digestion and growth of wood-eating termites can only go on when there is a protozoan fauna of trichonymphids (p. 67) and other flagellates in the hind gut. The fragments of wood Fig. 326. Hamitermes silvestri Hill. Tropical Australia. After Tillyard. A, Neoteinic queen. B, Winged male. C, Worker. D, Soldier. E, Nymph. are ingested by the Protozoa and converted into sugars, being largely stored up in the form of glycogen. The flagellates seem to form the main food source to the termites, the wood having been already digested within them. Termites may forage by night for plant food and the genus Termes also cultivates in its nest fundus gardens . The fungus which grows on INSECTA 471 a bed of chewed vegetable matter serves as the food for the royal pair and the nymphs. The workers and soldiers differ from the sexual individuals, not only in their sterility, but also in having more powerful mandibles. In the soldiers the head can produce a protective secretion and the mandibles are greatly specialized for defence (Fig. 326). Both these castes consist of males and females, though secondary sexual cha- racters are not very marked. If, as is stated, slight caste differences are already apparent in the newly hatched young, caste-formation cannot be a matter of nutrition. Order PLECOPTERA (Stoneflies) This is a small order of mandibulate insects with a heterometabolous metamorphosis. Though in possession of two pairs of well-developed wings, they are weak fliers and do not move far from their aquatic breeding grounds. Prominent, elongate antennae and cerci are cha- racteristic features, as also are the three-jointed tarsi. According to some authorities the wing venation represents a primitive type. Much variation in venation is, however, found in the order. The nymphs are always aquatic, for the most part inhabiting swift- flowing streams with stony beds. They possess the antennal and cereal features of the adult and breathe by means of gill tufts in various positions. In some cases gill vestiges are found on adults though these are not aquatic. Like most aquatic insects, they have a wide distri- bution, the most generalized families being found in southern, the most specialized in northern, regions. Perla maxima is a common species found in European streams. Order EMBIOPTERA Small insects with elongated and flattened bodies ; two pairs of similar wings with reduced venation; females apterous; cerci two-jointed, generally asymmetrical in male; metamorphosis absent in female, slight in male. These insects are widely distributed in the warmer parts of the world. Many are gregarious, living in tunnels formed of silk produced by tarsal glands, e.g. Embia major from the Himalayas. Order PSOCOPTERA (Booklice) Small insects, either winged or wingless; with biting mouth parts; thoracic segments distinct ; wings with reduced venation from which cross veins are largely absent; metamorphosis slight. These insects are to be found on bark and leaves of trees and feed on lichens and dry vegetable matter. The eggs are laid on the bark 472 THE INVERTEBRATA or leaves and covered by a protecting sheath of silk by the female, e.g. Peripsocus phaepterus. Atropus pulsatoria, the booklouse, is found in damp dark rooms and feeds on the paste of book bindings, wallpaper, etc. Order O DON AT A (Dragonflies) Predaceous insects with biting mouth parts; two similar pairs of wings with characteristic reticulate venation; prominent eyes and small antennae ; elongated abdomen with accessory male genitalia on the 2nd and 3rd sterna; metamorphosis heterometabolous ; nymphs aquatic, possessing a modified labium known as the mask. The members of this order are large insects, and in the Carboni- ferous period genera existed which had a wing expanse of two feet. They are strong and rapid fliers, catching their food in the form of small insects, on the wing. The forwardly directed legs play an im- portant part in catching the prey and holding it w^hile it is masticated by the mouth parts. The thorax has a peculiar obliquity of form, the pleural sclerites being directed downwards and forwards at each side with the result that the leg bases are carried forwards towards the mouth and the wing bases backwards. The wings (Fig. 327) have a complex venation of a reticular nature, characteristic features being a stigma or chitinous thickening of the wing membrane near the apex, a nodus or prominent cross vein at right angles to the first two longitudinal veins, and a complex of veins near the wing base known as the triangle, Fig. 327. There is no coupling apparatus. All the mouth appendages are strongly toothed, maxillae and labium assisting the mandibles more efficiently in mastication than in most insects with biting mouth parts. Though the male pore is on segment 9 of the abdomen, the copu- latory apparatus is found in the sternal region of segments 2 and 3. Before copulation, spermatozoa are transferred to this apparatus. The male then grasps the female in the region of the prothorax by means of his posterior abdominal claspers. While in flight in this tandem position the female turns her abdomen down and forwards and receives sperm from the accessory copulatory apparatus of the male. Dragonfly eggs are laid in water or on water weeds. The nymphs breathe by means of tracheal gills and are of two kinds: (i) those with external gills in the positions of cerci anales and caudal fWaments—Zygoptera, (ii) those with gills on the walls of the rectum — Anisoptera. In the latter case water is pumped in and out through the anus, and this action may be made use of in locomotion — the sudden expulsion of water causing a rapid forward movement on the Fig. 327. The emergence of the dragonfly Aeschna cyanea. After Latter. 474 THE INVERTEBRATA part of the nymph. The nymphs are, however, on the whole slow- moving creatures, lurking well camouflaged among water weeds while in wait for their prey. The main diff^erence between the mouth parts of the nymph and imago concerns the labium. In the adult this has normal proportions, but in the nymph the mentum and sub- mentum are elongated and capable of being shot out rapidly from the folded resting position, so impaling the prey, e.g. a tadpole, on the labial hooks. Order HEMIPTERA or RHYNCHOTA (Bugs) Mouth parts for piercing and sucking ; palps absent ; labium forming an incomplete jointed tube which receives dorsally two pairs of slender stylets (maxillae and mandibles) ; wings usually two pairs, the anterior harder than the posterior; metamorphosis gradual. The existence of this large order of insects has largely been de- pendent on the store of easily obtainable food which exists in the sap of flowering plants and the mouth parts form an efficient mechanism for obtaining this. There are, however, families like the Reduviidae and Cimicidae (bed bugs) and the various water bugs (e.g. Nepa^ water scorpion, and Notonecta, back-swimmer) which feed on animal juices. On either count they are of immense economic importance, not only for the damage which the loss of sap and blood causes to the host organism, but also because they open the way for bacterial in- fection and carry the agent of such diseases as "mosaic disease" among cultivated plants and trypanosomiasis among mammals. The antennae are usually short. The labium projects from the head as a rostrum which is jointed, and dorsally grooved to carry the stylets (Fig. 328). At its base the groove does not exist but the lab rum roofs over an enclosed space. The stylets are modified mandibles and maxillae which are withdrawn at their base into divergent pockets in the head, but converge and interlock as they pass into the space between the labrum and labium and into the groove of the latter, in which they fit tightly; where the inner pair of stylets (the maxillae) meet together there are left two narrow channels, of which the dorsal serves for the inward passage of the food juices and the ventral for the outward flow of the saliva (Fig. 328). At rest the rostrum is bent beneath the body, and when the insect feeds it is extended forward and the stylets projected to penetrate the host tissues (Fig. 328). In some plant-feeding species the stylets are immensely long and very slender and it is difficult to explain the mechanism by which they are forced into the tissues as far as the vascular bundles, but the mechanical insertion of the stylets is greatly assisted by a solvent action of the saliva which appears to loosen the plant cells from one another and to allow the stylets to pass between. In Aphis rumicis INSECTA 475 the phloem cells of the plant are eventually pierced and their contents sucked out. The pumping action is performed by the muscles of the pharynx. This order comprises a large number of families which in the following scheme of classification are arranged in two suborders, the Heteroptera and the Homoptera. The Heteroptera have wings which are horny distally, but membranous apically (Fig. 331). The pro- Fig. 328. Mouth parts of the Hemiptera. A, Sagittal section through head of Graphosoma italiciim. After Weber. B and C, Diagrams of mouth parts and adjacent region of the head. C is a transverse section across B at the point X X. After Imms. Ibyn. labium; Ibr. labrum; nid. mandible; mx. maxilla; ph. pharynx; ph.p. muscles of pharyngeal pump; sty. stylets. boscis is terminal and free. In the Homoptera the fore wings have a homogeneous texture and are ^often membranous. The head is ventrally flexed so as to bring the base of the proboscis into contact with the anterior coxae (Fig. 329). There are two tribes of insects within the Heteroptera, {a) those which are aquatic and whose antennae are obscure, the Cryptocerata^ and {b) mostly terrestrial forms with conspicuous antennae, the Gym- 476 THE INVERTEBRATA nocerata. The former are noteworthy for their numerous adaptations to aquatic life. They commonly lay their eggs in the tissues of sub- merged plants. Many, e.g. the water boatman, Corixa, and back- swimmer, Notonecta, have powerful legs fringed with hairs which, by the simultaneous movement as members of pairs, propel the animals through the water as oars do a boat. They breathe air at the surface film, making use either of a terminal abdominal tube (Nepa) or of unwettable hairs between which air is trapped to enable the animal to breathe during its periods of complete immersion (Notonecta). Among the Gymnocerata may be mentioned the bed bug, Cimex, an ectoparasitic insect, with vestigial wings, flattened body and prominent claws. It inhabits human dwellings, and its retiring habits coupled with its power to fast for long periods make it a difficult creature to eradicate when once it is established. The shield bugs {Pentatomidae) are phytophagous. The mesothoracic tergum is greatly enlarged to extend at least as far over the abdomen as the junction between the horny and membranous parts of the wing when these are at rest. The red bugs (Pyrrhocoridae) are also phytophagous. Certain species, e.g. of Dysdercus, are known as "stainers" from their habit of feeding on cotton-bolls into which they inject a micro-organism responsible for the appearance of a red stain on the fibre. The Capsidae are almost exclusively phytophagous, some of their members being very serious pests of our English orchard trees and shrubs. Plesiocoris, until recent times restricted to such trees as willow, now attacks black currant bushes, apple trees, etc. An exception to this phytophagous habit is found in Cyrrtohinus mundulus which sucks the eggs of the sugar cane hopper, Saccharicida, so eflPectively controlling this pest in Hawaii. In the family Reduviidae are many forms which transmit trypanosomiasis, in the tropics, e.g. Rhodius prolixus. The extent to which the head flexure has brought the point of emergence of the rostrum into the thoraco-sternal region forms the basis for the separation of the Homoptera into two tribes. The least modified from the heteropterous condition in this respect are the Auchenorhyncha (Fig. 329 B). These are all active animals and though the rostrum is close to the thorax it clearly arises from the head. Here belong the cicadas, frog hoppers, tree hoppers, and leaf hoppers. Cicada septendecim is an example with a life cycle which may last as long as seventeen years. Eggs are deposited in holes in the twigs of trees. From here the newly hatched nymphs fall to the ground, into which they burrow to feed on the tree roots. A stage resembling the pupa of holometabolous insects is passed through before final emergence. The second tribe is known as the Steniorhyncha. In these forms the rostrum appears to arise from between the fore limbs. The antennae INSECTA 477 are well developed and do not possess a terminal spine (arista) — a feature characteristic of the first series. To this group belong the Fig. 329. Lateral views of proboscides of Rhynchota to illustrate the difference between the Heteropterous condition (A), and the Homopterous condition (B). A, Deraecoris fasciolns, modified after Knight. B, Zammara tympimnm (Cicadidae). m. niesothorax; iv. wing. scale insects, Coccidae. Females of these are wingless, often scale- like, and devoid of legs. The winged males have atrophied mouth 47^ THE INVERTEBRATA parts and the second pair of wings are reduced to short clawed structures. Well-known examples are Pseudococcus the mealy-bug, Tachardia lacca the lac-insect of commerce and Aspidiotus perniciosus the San Jose scale-insect of citrus trees. Fig- 330- Types of Rhynchota. A, Macrotrista angularis (Homoptera, Cica- didae). B, Aphis rumicis (apterous viviparous female). C, Winged viviparous female of same. B and C, after Davidson. The plantlice (Aphididae), Fig. 330, notable for their wide dis- tribution and for their prolific reproduction, have transparent wings and a two-jointed tarsus, that of the Coccidae being one-jointed. Wax-secreting cornicles are borne dorsally in the abdomen. In the last family the reproductive phenomena are of immense scientific importance. A comparatively simple life cycle is that of Aphis rumicis. The winter is passed on the spindle tree Euonymus as eggs laid in the autumn after the fertilization of females. In spring INSECTA 479 these eggs hatch, giving wingless parthenogenetic females which produce young viviparously. A variable number of these partheno- genetic generations is passed through in the summer and then winged parthenogenetic females occur which migrate to another host (the bean or other plants), and there reproduce, giving rise to generations of parthenogenetic females which eventually produce winged females which migrate back again to the primary host, the spindle tree Euony- Fig. 331. External anatomy of Leptocoris trivittatus with wings spread on one side. After Essig. an. antenna; he. hemielytron. mus. This generation gives rise to oviparous females which copulate with winged males, migrants from the secondary host plant. In other forms, such as Phylloxera vastatrix, the notorious pest of vineyards, the life history is immensely complicated and involves migrations between root and stem of the host plant. The reproductive capacity of these insects is most remarkable and is fortunately offset by the number of enemies which they possess. 480 THE INVERTEBRATA The following summary will assist in the understanding of the life cycle of Aphis rumicis : Fertilized eggs laid in autumn I Viviparous parthenogenetic females I Euonymus Winged migrant parthenogenetic females ^ . . . Wingless parthenogenetic viviparous females I !- Vicia {aha Winged viviparous females (autumn) J I Winged males x Wingless oviparous females] \ \ Euonymus Eggs laid in autumn J The cyclical reproductive phenomena in aphides as just described raise important problems relating to the intrinsic differences between sexual and parthenogenetic individuals, and to the environmental conditions governing the occurrence of these phases in any life cycle. Fertilized eggs produce only strictly parthenogenetic females. These multiply by diploid parthenogenesis, i.e. the eggs retain the full complement of chromosomes and are not capable of fertilization. Eventually come individuals capable of bearing sexual forms, sexu- parae. The sexual forms arising from these produce germ cells undergoing normal reduction and which are therefore haploid. It follows then that fertilisation will restore diploid partheno- genesis. Sexual differences are indicated in the chromosomes; the female of Aphis saliceti possessing six, of which two are sex chromo- somes; the male only five, one only being a sex chromosome. Sexual reproduction leads however only to the production of parthenogenetic females and not to males and females in equal numbers, as might be expected. This appears to be due to the fact that in the maturation of sperms, those with only two chromosomes die. Fertilization there- fore is always between sperms and ova each with three chromosomes, of which in each case two are normal chromosomes {autosomes) and one is a sex chromosome X. The capacity of females with six chromosomes to produce male offspring with only five is due to the fact that in the maturation of male-producing parthenogenetic eggs, reduction in the number of chromosomes only affects the sex {X) chromosomes, one remaining in the tgg^ the other going to the polar body. In this way a parthenogenetic female with six chromosomes, i.e. 4 + XJ\r, gives rise to males with only five, i.e. 4 + X. INSECTA 481 A complete analysis of the environmental conditions governing the onset of sexual phases after a period of parthenogenetic reproduction is yet to be made. Food, temperature and light seem to be important, and of these a reduction of the last mentioned factor seems to be associated with the production of sexual winged individuals. Though the order contains insects for the most part harmful to man and his property, a few are useful in that they yield the dyestuffs Kermes (females of Kermes ilicis) and Cochineal {Dactylopius coccus) ^ and the resin stick-lac {Tachardia lacca). The usually harmful plant- sucking habit is being put to good use in Queensland where the coccid bug, Dactylopius tomentosus, is employed against the prickly pear cactus with considerable success. Order EPHEMEROPTERA (Mayflies) Vestigial mouth parts reduced from the biting type; wings mem- branous with a reticulate vena- tion ; the hinder pair small ; caudal filament and cerci very long (Fig. 332). The nymphs are aquatic and an active winged stage known as the subimago occurs before the last moult yields the adult. The eggs are laid in water, either scattered over the surface or attached to stones, etc., by the female, which enters the water for the purpose. The nymphs at first possess no gills but subsequent instars bear on the abdomen movable tracheal gills (Fig. 333), which may be branched or lamellate, exposed or protected in a branchial chamber. The body form^varies with the habits. Thus inhabitants of fast- flowing streams have flattened bodies with legs provided with strong clinging claws, e.g. Ecdyo- nurus. Those which live in clear still water have a stream-lined form for rapid movement, e.g. Chloeofi, while burrowing types have fossorial legs, e.g. Ephemera, and are often provided with protective gill opercula, e.g. Caenis. The Fig. 332. Ephemera vidgata. From Imms. 482 THE INVERTEBRATA mouth parts are of the biting type, and the two-jointed mandibles and well-developed superlinguae are features of importance. The nymphs are essentially herbivorous. Nymphal life is usually of long duration : Fig- 333 Nymphal instars of Heptagenia. After Imms. A, Third instar. B, Seventh instar. C, Eighth instar. a, a\ b and c, gills belonging to these instars respectively; w. wing rudiment. as many as twenty-three instars may occur. In order to emerge, the fully fed nymph creeps out of the water on to a plant stem. A moult gives rise to the winged subimago stage. This flies away and after a period which varies, according to the species, from a few minutes to about twenty-four hours, a final moult yields the adult which enjoys, INSECTA 483 as the name of the order implies, a similarly short life. In the adult the mouth parts are vestigial, no feeding is done, and the alimentary canal, full of air, serves no longer for digestion. Economically these insects are of importance in so far as they con- stitute a proportion of the food of freshwater fishes, the adults being caught by fish during their nuptial dance, and the nymphs being de- voured by bottom-feeding fish. Order MALLOPHAGA (Biting lice) These insects are ectoparasites of birds (less frequently of mammals). Their reduced eyes, flattened form and tarsal claws are features corre- lated with this mode of life. Unlike the Anoplura they have no piercing mechanism and devour with biting mouth parts small particles of feathers, hair, or other cuticular matter. The common hen louse, Menopon pallidum (Fig. 334), may be taken as an example. The head is semicircular in form and articulates with a prothorax which is freely movable on the rest of the body, a tagma formed by the fusion of the meso- and metathorax with the abdomen. The mouth is placed ventrally on the head and surrounded by biting mandibles and less prominent ist and 2nd maxillae. Eggs are laid separately on feathers or hairs and the life cycle is completed in about a month — the young instars resembling the adult in form and habit. The various families of biting lice are strictly confined to particular groups of birds, indicating that evolution of the parasites has pro- ceeded concurrently with that of their bird hosts. Order ANOPLURA (Sucking lice) Ectoparasites of mammals, with mouth parts adapted for piercing the skin and sucking the blood of their hosts. The eyes are ill-developed or absent. The single-jointed tarsus carries a large curved claw ad- mirably adapted for clinging to the host. The thoracic segments are fused, and a flattened abdomen of nine segments possesses large pleural areas allowing the body to swell on feeding. The minute mouth parts are accommodated at their bases in a stylet sac which is a diverticulum ventral to the pharynx. There are two stylets of which the dorsal is a paired structure, the halves of which maintain contact with each other Idistally to form a half-tube which is completed by the ventral stylet. This also consists of two elements. Between the dorsal and ventral stylets lies the salivary duct which appears to be a modification of the hypopharynx. The stylet complex can be sufficiently everted so as to make contact with the skin. Into 484 THE INVERTEBRATA the wound is poured the salivary fluid, and the mouth funnel is thrust in to enable the blood to be sucked up by the pharyngeal pump. Embryological evidence tells us that the ist maxillae unite to form the dorsal stylet, the ventral being formed by the labium. A pair of mandibles also develops but these remain in a rudimentary and un- chitinized condition. Pediculus humanus, the body louse (Fig. 335), is associated with the spread of many diseases, such as typhus and relapsing fever. The disease known as trench fever, prevalent in all war areas during the Great War, has also been shown to be transmitted by this insect. Eggs are laid attached to hairs of the body or clothing, and the three instars passed through before attainment of the mature state closely resemble the adult. msth mtfli. Fig. 334- Fig. 335. Fig. 334. Hen louse, Menopon pallidum. Dorsal view, showing biting mandibles by transparency, an. antenna; md. mandible; mxp. maxillary pd\ip',pth. prothorax; msth. mesothorax; mtth. metathorax. Fig. 335. Body louse, Pediculus humanus. After Imms. The louse has been found to lay about ten eggs daily, depositing in all about three hundred. Temperature plays a big part in con- trolling the development of these animals. Under average conditions, the life cycle is completed in about three or four weeks. INSECTA 485 Order THYSANOPTERA (Thrips) Minute insects with asymmetrical piercing mouth parts; prothorax large and free ; tarsus two- or three-jointed with terminal protrusible vesicle; two pairs of similar wings, provided with a fringe of pro- minent long hairs, veins few or absent; metamorphosis slight, including an incipient pupal instar. These insects are for the most part plant feeders, a few being carnivorous. They are regarded as serious pests in that they rob the plant of sap. They also often cause malformations and in some cases inhibit the development of fruit. Parthenogenesis is of frequent occurrence. In the case of the pea thrips, Kakothrips robustus, the eggs are inserted in the stamen sheath of the flower and the nymphs emerging feed on the young fruit, inhibiting its growth. Later they feed on the soft tissues of pea pods, causing scar-like markings. The nymphs leave the plant and bury themselves deeply in the ground, where they remain till the following spring, when they pupate. Common thrips of importance are Taeniothrips inconsequens of pears and Anapho- thrips striatus of grasses and cereals. Subclass ENDOPTERYGOTA Order NEUROPTERA (Alder flies, lacewings, antlions) Rather soft-bodied insects with biting mouth parts; two similar pairs of membranous wings held in a roof- like manner over the body when at rest. The wings have a primitive type of venation, a distinguishing feature being the ladder-like arrangement of veins along the anterior border. The abdomen is without cerci. The larvae are invariably carnivorous — campo- deiform, with biting or suctorial mouth parts. Aquatic larvae usually possess abdominal gills. The alder fly, Sialis, may be taken as an example with an aquatic larva. In June and July the adults fly rather sluggishly in the neighbourhood of water. They lay eggs in clusters on grass blades and leaves overhanging water, and the larvae on hatching Fig. 336. Larva of Sialis lutaria. From Inims, after Lestage. 486 THE INVERTEBRATA fall into the water. In this larva (Fig. 336), more than in any other, the paired segmented tracheal gills on the abdomen show a great resem- blance to paired limbs. Pupation takes place in the moist earth near the water's edge. The larva of Sialis differs from those of the majority of Neuroptera in that its mouth parts are of the biting type, whereas in antlion larvae and the larvae of lacewings, etc., the mouth parts are adapted for piercing the skin and sucking the juices of animal prey. For this purpose, the points of the mandibles and maxillae are used for piercing, and the mandibles, being grooved, form with the closely fitting maxilla a tube up which the fluid is drawn. The carnivorous habit of neuropterous larvae plays an important part in insect pest control, for example, larvae of lace wing flies feed largely on aphides. Order MECOPTERA (Scorpion flies) A small order of insects distinguished by their vertically directed and elongated head capsule carrying the biting mouth parts at its end; two pairs of similar wings with a simple venation in which a number of cross veins divide the whole area into a number of nearly equal rhomboidal cells. The male genitalia are prominent and the terminal segments of the abdomen carry them in a dorsally curved position in the manner of the scorpion's tail. The cruciform larvae are caterpillar-like and may possess prolegs on all segments of the abdomen. This feature, together with the presence of a large number of ocelli on the head (there may be twenty or more on each side), readily distinguishes these larvae from those of the Lepidoptera. Panorpa communis, the common English scorpion fly, lays eggs in crevices in the soil and the larvae hatching from these feed on decay- ing organic matter. Pupation occurs in an earthen cell and the life cycle is an annual one. Much information is still wanting on the life histories of the members of this order. Order TRICHOPTERA (Caddis flies) Medium-sized insects with bodies and wings well clothed with hairs ; mandibles vestigial or absent; maxillary and labial palps well de- veloped; two pairs of membranous wings, with few cross veins and held in a roof-like manner when at rest. The cruciform larvae are aquatic and usually live in cases formed of such material as particles of wood, sand, small shells, etc. A pair of hooked prolegs on the last abdominal segment which assists in adhering to the case is a characteristic feature. The eggs are laid in or near water and the larvae quickly cover INSECTA 487 themselves with some foreign substance (Fig. 337), building a form of tube from the wide end of which the head projects. Respiration is effected by tracheal gills generally found on the abdomen, water currents being passed through the tubular case by the undulatory movements of the body. The larvae may be herbivorous or carnivor- ous. Pupation usually takes place within the case after the openings to the case have been closed by silk. The pupa is provided with large mandibles by means of which it releases itself before the emergence of the adult. The free pupa swims to the water's edge by means of its mesothoracic legs and shortly afterwards the adult emerges. Common caddis flies are Phryganea, Limnophilus and Rhyacophila. Fig. 337. A, B, C, D, Cases of Trichoptera. A, Hydroptila maclachlani. B, Odontocerum. C, Phryganea. D, Hydropsyche, pupal case. E, Halesus. guttatipennis. After Imms. Order LEPIDOPTERA (Butterflies and moths) Mouth parts of the imago usually represented only by a sucking pro- boscis formed by the maxillae; two pairs of membranous wings, clothed with flattened scales, as also is the body; metamorphosis complete; larvae cruciform with masticating mouth parts, with three pairs of legs on the thorax and often five pairs of prolegs on the abdomen; pupae obtect, either enclosed in a cocoon or an earthen case, or free. The imagines live on the nectar of flowers, and to absorb this a highly specialized proboscis has been formed from the greatly elon- gated galeae of the maxillae, each being grooved along its inner face and locked to its neighbour (Fig. 338). The laciniae are atrophied and the maxillary palp is usually much reduced. The mandibles are 488 THE INVERTEBRATA nearly always absent and the labium is represented by a transverse plate and a pair of three-jointed palps. Each half of the proboscis is a tube in itself into which passes blood from the head, and also a trachea and a nerve. Across the cavity of this tube there pass a number of diagonal muscles, the contraction of which causes the whole organ to roll up into its characteristic position beneath the head and thorax (Fig. 339). How the proboscis is ex- tended is not fully understood; in all probability, blood pressure plays an important part. The length of the proboscis in many cases corresponds to the depth of the corolla of the flower which the species frequents, and in the Sphingidae (hawkmoths) is greater than that of the body. Sometimes Fig* 338. A, Tryphaetia pronuba, with venation and frenulum (Jr.); ^ con- dition on right. Original. B, Obtect pupa of Platyhedra gossypiella. After Metcalf and Flint. the organ is reduced or absent and the animal does not then feed in the adult state at all. The beginnings of the proboscis can be traced in primitive forms. In the Micropterygidae there are biting mandibles and maxillae of the type usually found in insects which masticate their food: in Micropteryx there is no proboscis, the animal feeding on pollen; in Eriocrania the mandibles are non-dentate, the laciniae are lost and the galeae form a short proboscis. The characteristic feature of the wings is the clothing of scales (Fig. 338). These latter are formed by enlarged hypodermal cells, and their main function appears to be the presentation of colour due either to striation of the surface causing interference colours, or in lesser degree to the pigment they contain (like the uric acid of the Pieridae). There also occur "scent scales" which may have a sexual INSECTA 489 significance. Several methods of wing coupling have been developed independently in the order. In addition to the type already referred to on p. 429 and consisting oi frenulum and retinaculum^ there is the further method met with in the ghost moths in which a jugal lobe from the fore wing engages the anterior border of the hind wing. In other forms there is neither frenulum nor jugum and the wings are Fig- 339- Head and proboscis of a moth. A, Front view. B, Side view. After Metcalf and Flint. C, Transverse section of proboscis. After Eltringham. dp. clypeus; drn. diagonal muscles; e. eye; ep. epipharynx; gal. galea; Ibr. labrum; l.h. locking hooks; Ip. labial palp; md. mandible; 7nxp. ma.xillary palp; n. nerve; tra. trachea. coupled by a considerable overlap of the two wings of a side, e.g. the butterflies {Papilionina). In the females of certain Lepidoptera the wings are totally lost and the animals are confined to the food plant on which they spend their larval life. The male is attracted to the female, under these circum- stances, by scent. Lepidopterous larvae (Fig. 344 A-C) have three thoracic and ten abdominal segments with nine pairs of spiracles situated on the pro- 49^ THE INVERTEBRATA thorax and first eight abdominal segments. The mandibles are typi- cally strong and dentate; the maxillae are stumpy and consist of a cardo, stipes and single maxillary lobe with a two- or three-jointed palp : the labium has a large mentum, a prementum bearing a median spinneret and small two-jointed palps. The thorax bears three pairs of legs, and the abdomen five pairs of prolegs on segments 3-6 and 10. Such prolegs are different from the typical insect limbs, being conical and retractile with hooks on the apex (Fig. 344 C). In many families there are less than five pairs of prolegs, and in Micropteryx there are eight pairs. These larvae feed almost exclusively on flowering plants (excep- tions being the Lycaenid caterpillars which are carnivorous, feeding on aphides or entering ants' nests and devouring the larvae). Their digestive enzymes are modified for dealing with plant tissues. The pupa, which is disclosed after the last larval moult, is usually protected by a cocoon previously prepared by the larva. In the case of Tortrix moths the cocoon is largely composed of leaves drawn to- gether by silk strands. In others, e.g. the silkworm moth, Bombyx mori, it is composed of silk and from it the silk of commerce is pre- pared. Agglutinated wood particles form a hard cocoon in the puss moth, Dicranura. In Pieris, the pupa is naked and attached to the substratum by the hooked caudal extremity, the cremaster^ and by a delicate girdle of silk about its middle. In the most primitive forms (e.g. Micropterygidae) the pupae are free, their segments are free to move and the appendages are not fused to the body. Obtect pupae, in which only few segments are movable and the appendages are fused to the sides of the body, are most common, e.g. Platyhedra (Fig. 338 B). Free or incompletely free pupae often emerge from the cocoon before the emergence of the adult. Lepidoptera are almost invariably harmful in the larval stage, few plants being free from their attacks, and some of the world's most serious insect pests, such as the cotton bollworm, Platyhedra gossy- piella^ and the gypsy moth, Porthetria dispar, are included in this order. The order is divided into two suborders. In suborder I, Homo- neura, the fore and hind wings have venations which are almost identical. To this primitive feature may be added that of the included family Micropterygidae whose mouth parts are mandibulate and the structure of whose maxillae and labium are easily comparable with those of the cockroach. The ghost moths or swifts (Heptalidae) are also included in this suborder. These nocturnal insects have vestigial mouth parts and short antennae. Their jugate type of wing coupling has already been described. In certain species^t.g. Hepialus humuli, the female searches INSECTA 491 for the male prior to mating. The larvae live in the ground and are white and hairless. The second suborder, Heteroneura, is more specialized in that the venation of the hind wing has undergone reduction and so presents a venational form very different from that of the Homoneura. Here are included the vast majority of moths and all butterflies. Since the families are distinguished largely on venational characters no attempt will be made to deal with them in a classificatory scheme. Among the numerous families of this suborder may be mentioned the Tineid moths — small species still retaining maxillary palpi and possessing narrow fringed wings, with a frenular bristle on the hind wing for coupling purposes. Tinea hiselliella is one of the clothes moths whose larvae can live on the keratin of woollen goods. The goat moths (Cossidae) are large moths without maxillary palps and with a frenular coupling apparatus. These are nocturnal, and lay their eggs on trees. Their larvae tunnel in timber, e.g. Cossus. Ephestia the flour moth and Plodia the meal moth are most im- portant as pests of stored products, while Chilo is a form whose larva bores into the shoots of the sugar cane in India. Galleria the wax moth inhabits beehives in most parts of the world, having become arti- ficially distributed. These belong to the family Pyralidae. Hawk moths (Sphingidae) are large stoutly-built moths whose fore wings are much larger than the hind ones. A further feature is the obliquity of the outer margin of the wings. The proboscis is long and the antennae, which are thick, end in a hooked tip. The larvae have ten prolegs and usually bear an upturned spine or process on the back of the last segment. Of slender build are the geometer moths {Geometridae). They are weak in flight and a coupling mechanism is not always present on the wings. Some species, e.g. Cheimatobia the winter moth, are wingless as females. The family gets its name from the fact that in most of the larvae, prolegs are borne by segments six and ten of the abdomen only. Such larvae, in consequence, walk by looping the body, bringing the hind segments near to the thoracic and so appear to be measuring distances along the surface walked upon. The owl moths or Noctuidae are the most dominant family of the order. They usually fly at night and to this fact is related their sombre colouring which assimilates the insects to their surroundings when resting during the day. The larvae, are almost hairless, and in such forms as pupate in the ground the pupa is naked. Tryphaena pronuba (Fig. 338) is a common species whose larvae devour roots. The larvae of nearly related species known as cut worms and army worms rank among the worst insect pests of North America. In the above mentioned forms, collectively known as moths, the 492 THE INVERTEBRATA antennae taper to a point and the frenular coupling apparatus is common. The remainder forming the superfamily Papilionina may be grouped for convenience as butterflies, whose antennae are clubbed and on whose wings there does not occur a frenulum. Here are found the Whites, e.g. Pieris, the larvae of many of which are restricted to a cruciferous diet, and the Blues and Coppers in which the metallic colouring on the wings and the larvae tapering towards both extremities are distinguishing features. There are also the Swallow-tails, e.g. Papilio, in which the hind wings are commonly extended into tail-like prolongations. Finally may be mentioned the skippers, so-called because of their erratic darting flight quite distinct from the sustained flights of other forms. Order COLEOPTERA (Beetles) Biting mouth parts ; fore wdngs modified to form horny elytra which meet along the mid-dorsal line; hind wings membranous — folded beneath the elytra — often reduced or absent; prothorax large and mobile; mesothorax much reduced; metamorphosis complete, larvae (see p. 457) campodeiform or cruciform or, more rarely, apodous. In the larvae the head is well developed (Fig. 318) and the mouth parts are of the biting type, resembling those of the adults. The most primitive larvae are those of the campodeiform type (found for instance among the Cicindelidae (tiger beetles), Carahidae (ground beetles) and the Staphylinidae (rove beetles)). They are very active in movement and often predaceous, with well-developed antennae and mouth parts, and chitinized exoskeleton. In the eruciform type (Fig. 318 B), found among plant-eating forms like the lamellicorn beetles, the legs are shorter, and the animal much less active in its search for food, the body bulkier and cylindrical. Finally there is the apodous type which is found in the Curculionidae (the weevils), in which not only are the thoracic legs lost but the antennae and mouth parts are reduced (Fig. 318 C). The apodous and eruciform larvae usually live inside the soft tissues of plants or beneath the soil attached to roots. The relation which these larval forms bear to one another is indicated by the larval stages passed through in the life history of the oil beetle, Meloe, the larvae of which are parasitic on solitary bees of the genus Andrena. The first instar is known as the triiingulin. This is an active campodeiform larva which attaches itself to its host after searching actively for it. The second instar which is enclosed with an abundance of honey in the cell of the bee is intermediate in form between the campodeiform and the eruciform types, legs being present, but very small. The third stage is a legless maggot. From this series it may be inferred that the form of larva in Coleoptera is related to the ease or difficulty with which food is obtained. INSECTA 493 In such a large order of insects it is to be expected that all manner of habit and food will be found. Beetles occur in large numbers in water, soil, and plant tissues. Circumscribed environments like dung, rotting vegetation, wood and fungi are never without prominent coleopteran associations. A large number, such as many coccinellids (lady birds), carabids, e.g. Carabus violaceus, and staphylinids, e.g. Ocypus olens, are carnivorous and to this extent useful insects. On the other hand, among the phytophagous forms are to be found some of the most serious agricultural pests, the boll weevil, Anthonomus grandis, causing so much damage to the cotton crop in America that it has Fig. 340. External anatomy of Calosoma semilaeve, with left elytron and wing extended. After Essig. aw. antenna; e/. elytron; />. palp; 5^. spiracles. been seriously proposed to cease growing cotton for a period of time in order to eradicate this pest. A large number cause considerable damage to timber, probably the most notable being Xestobiiim riifo- villosum, the death-watch beetle, destructive to structural timber. The order falls into tw^o suborders, the Adephaga and the Polyphaga. The Adephaga are distinguished by filiform antennae, a five- jointed tarsus and a larva of the campodeiform type, with a tarsus bearing two claws. To this group belong those families including the large water beetle Dytiscus, the ground beetles Carabus and Calosoma (Fig. 340), the tiger beetle Cicindela, and the aquatic whirligig beetles Gyrinus. 494 THE INVERTEBRATA The second suborder, the Polyphaga, includes a large number of families grouped into several superfamilies the members of which show much variation. There is a tendency towards reduction in the number of tarsal joints from five to three, and though some forms possess filiform antennae, clavate (clubbed), geniculate (elbowed), Fig. 341. The hornet, Vespa crabro. A, Larva. B, Pupa. C, Adult S' Fig. 342. Three types of Coleoptera. A, Ocypus olens (Staphilinidae). B, Scarahaeus Thomsoni (Scarabaeidae). C, Corymbites cupreus (Elateridae). and lamellate (segments extended to form a "book" of closely arranged leaves or lamellae) antennae occur, as in the Coccinellidae, Curculionidae and Scarabeeidae respectively. Larvae vary from the INSECTA 495 campodeiform to the legless grub, but where a tarsus is present it invariably carries only one claw. The Staphylinidae range from carnivorous to phytophagous forms, and, as adults, are characterized by the short elytra which leave the abdomen exposed. The larvae are campodeiform, closely resembling those of ground beetles, e.g. Ocypus (Fig. 342 A). Meloidae or oil beetles also have short elytra but these being wider at the base than is the prothorax are readily distinguished from members of the staphilinid group. The interesting changes undergone by their larvae during metamorphosis have already been mentioned. The Chrysomelidae or leaf beetles are exclusively phytophagous. Their bodies are rounded and smooth, and are often high-coloured with a metallic lustre. Antennae of these beetles are filiform and relatively short (e.g. Phyllotreta^ the flea beetle). Weevils belonging to the family Curculionidae are easily dis- tinguished by their greatly extended head, forming a rostrum at the end of which mouth parts are borne. Anthonomus grandis the cotton boll weevil of America, and Ceuthorrhynchus the turnip gall weevil, are typical examples. The larvae are apodous. The chafer beetles {Scarabceidae), Fig. 342 B, have lamellate antennae. Their legs are often fossorial and bear four-jointed tarsi. Characteristic of these is the fat bodied cruciform larva, almost incapable of movement, and which feeds on roots, e.g. Melolontha (Fig. 318). Aphodius is a dung beetle whose larva develops in the faecal matter of farm animals. The family Coccinellidae (ladybirds) is of extreme importance, its members being carnivorous in young and adult stages, aphids and scale insects figuring very largely in their diet. The beetle is smooth and rounded, with head concealed beneath the prothorax. The four- jointed tarsus appears to possess only three joints, owing to the small concealed third joint, e.g. Coccinella of Europe. Novius cardinalis is a classical example of a predatory insect being used in the biological control of the scale-insect, Icerya purchasi, of citrus trees. Order HYMENOPTERA (Bees, wasps, ants, sawflies, etc.) Mouth parts adapted primarily for biting and often secondarily for sucking as well; two pairs of membranous wings coupled together by booklets fitting into a groove, hind wings smaller; ist segment of the abdomen fused to the thorax, and a constriction behind this segment commonly found; an ovipositor always present, modified for piercing, sawing, or stinging; metamorphosis holometabolous ; larvae generally legless, more rarely cruciform, with thoracic and abdominal legs; pupae exarate, protected generally by a cocoon. 49^ THE INVERTEBRATA This order is remarkable for the great specialization of structure exhibited by its members ; for the varying degrees to which social life has developed, and for the highly evolved condition which parasitism has reached. Specialization of structure is evidenced in the mouth parts of the an. Fig- 343 • Head and extended mouth parts of the honey bee, Apis mellifica. After Cheshire, an. antenna; gal. galea; gs. glossa; Ibr. labrum; Ip. labial palp; md. mandible; mxp. maxillary palp; oc. ocellus; pg. paraglossa. Hymenoptera. The biting mouth parts of the phytophagous and carnivorous sawflies closely resemble those of the cockroach. In the wasps, e.g. Vespa, which are predaceous, the mouth parts are adapted for licking as well as for biting. The maxillary laciniae are reduced but the galeae are enlarged into broad setose membranous lobes which absorb juices. A correspondingly large bilobed glossa occurs on the labium. INSECTA 497 The next important line of evolution is that concerned with the development of a mechanism for obtaining juices from deeply placed nectaries of flowers. For this purpose, e.g. in Apis^ the honey bee (Fig. 343), a complicated tubular proboscis is formed. The glossae of the labium have become fused and elongated, the paraglossae remaining small. The labial palps enclose the fused glossae (median ^%{{+^C Fig. 344. Caterpillar of Lepidoptera, A, B, C, and of Hymenoptera, D, E. A, Larva of Tryphaena pronuba. B, Its head capsule. C, An abdominal leg. D, Larva of apple sawfly, Hoplocampa testndinea. E, Head capsule of latter. an. antenna; dp. clypeus; /r. frons; lb. labium; Ibr. labrum; ind. mandible; mx. maxilla; oc. ocellus; v. vertex. lobes of the labium), they being concave on their inner surfaces. Outside these the large hood-like galeae of the maxillae form an additional enclosing jacket. The glossa is grooved along its dorsal surface and fluid passes up this by capillarity, assisted by movements of the proboscis. It is finally pumped up by pharyngeal action, the labial palps and maxil- 498 THE INVERTEBRATA lary glossae undoubtedly playing an important part in maintaining a complete tube. The mandibles are now no longer biting organs but tools used for manipulating material such as pollen and wax. Such a feeding mechanism is the climax in an evolutionary process which has involved in succession the fusion of the glossa lobes, as in the sawtlies, the lengthening of the basal joints of the labium and maxilla as in ColleteSy and finally the elongation of the glossa, e.g. Apis and Bombus. The highly complex social life found in the bees, ants and wasps, in which caste development is a feature of prime importance, is fore- shadowed in the interesting behaviour of solitary wasps and bees. The supply of food to the larva by progressive feeding , instead of mass provisioning, appears to enable the parent to become acquainted with its offspring, and this establishment of family life may be regarded as the forerunner of the complex social state of higher forms .^ A second important feature in the development of social life has been the phenomenon of trophallaxis . Among wasps, for instance, the worker taking food to a grub receives in turn a drop of saliva from the grub. This is eagerly looked for by the workers, and it is suggested that it is the mutual exchange of food between young and adult which engenders in the adult an interest in the welfare of the colony. A third important feature in social development has been the exploita- tion of a particular form of food material which can be obtained in large quantities, e.g. pollen and honey. The phenomenon of parasitism (Fig. 345) is highly developed in the Hymenoptera; Ichneumons, Chalcids and Proctotrypids being almost entirely parasitic. Almost all orders of insects are affected by the activities of these very important insects, Qgg, larval, pupa, and adult stages being parasitized. From the foregoing it will be seen that some of the most important insects are included in this order. The sawflies are important as agricultural pests. Flower- visiting bees are of great value in the pollination of flowers. Carnivorous wasps do good by devouring other insect pests such as aphides, while to a large extent the parasitic Hymenoptera are useful in checking the depredations of phyto- phagous insects. Two main types of larvae are found in this order, the legged larva of the sawflies (Fig. 344 D) and the legless form of bees, wasps and ants (Fig. 341 A). The sawfly larva has a superficial resemblance to the lepidopterous caterpillar, but is distinguished by its single pair of ^ In English species of the wasp Odynerus the egg is laid in a cell and sufficient caterpillars stored to serve as food for the whole of the larval life (mass provisioning). Certain African species of this genus supply their growing larvae from day to day with fresh caterpillars (progressive feeding). INSECTA 499 Fig. 345. A and B, Exarate pupae of Phaenoserphus viator. C, Pupae of same projecting from empty skin of host, the ground beetle larva, Pterostichns. After Eastham. s. spiracle; t. invagination to form tentorium. 500 THE INVERTEBRATA ocelli and the absence of crotchets or spines on the abdominal legs. The prolegs of the abdomen occur on different segments in the two forms under consideration as reference to Fig. 344 clearly shows. The order, falls naturally into two suborders, the Symphyta and the Apocrita. Suborder I, the Symphyta, includes those species with the most generalised form, both as adults and as larvae. None of them show the highly specialised habits and instincts which characterize most of the remaining suborder, and with few exceptions they are phyto- phagous. The first abdominal segment is not perfectly fused to the metathorax nor is the fusion accompanied by the constricted waist so characteristic of the remaining Hymenoptera (Fig. 346 D). The ovi- positor is used in oviposition as a saw or drill for piercing plant tissues. The trochanter is two-jointed. Larvae are cruciform (Fig. 344 D) and in addition to thoracic legs, certain of the abdominal segments often carry prolegs devoid of distal spines or crotchets. To this group belong the wood-wasps, the ovipositors of which are used as a drill for perforating growing timber, in which the eggs are laid. The six-legged, strong-headed larva bores through the wood (in the case of Sirex gigas, this stage lasts as long as two years), pupation occurring near the surface of the affected timber, from which the adult bites its way out. The sawflies (Fig. 346 D), with saw-like ovi- positors, are most important as agricultural pests, and are distinguished from the wood-wasps by their softer bodies, their smaller size, and by the presence of two apical spurs on the anterior tibiae, e.g. Nematus ribesii the gooseberry sawfly. The second suborder, the Apocrita, includes all the remaining Hymenoptera. The second abdominal segment is invariably con- stricted to form a narrow waist or petiole, the first segment being firmly amalgamated with the thorax (Fig. 346). Larvae are apodous when full grown. Ichneumon flies (Fig. 346 A) are distinguished by their slender curved antennae, and by the stigma on the wing. The ovipositor is long and issues far forwards beneath the abdomen. The larvae of Lepidoptera and of sawflies are their commonest hosts. Rhyssa parasitizes the larvae of Sirex. Cynipid flies have similarly slender antennae, but by the absence of the stigma on the wing, and by their reduced venation are easily distinguished from the foregoing. Many of these are plant gall- formers, e.g. Neuroterus responsible for oak galls, and Rhodites for the pin-cushion galls of roses. Others, e.g. Eucoila, are parasitic on fly larvae. Chalcid wasps (Fig. 346 B) also have a venation of the wing which is so reduced as to present no closed cells. The antennae are geniculate INSECTA 501 or elbowed. Though most of these small wasps are parasites, e.g. of lepidopterous and dipterous larvae, and of homopterous nymphs, a Fig, 346. Types of Hymenoptera. A, Cryptus ohscuriis (Ichneumonoidea) ; B, Bruchophagus funebris (Chalcididae), after Howard. C, Polistes aurifer (Vespidae), after Essig. D, Pamphilus sp. (Tenthredinidae), original. E, Monomorium minimum (Formicidae), after Essig. few feed as larvae on plant tissues such as Harmolita which produces galls on grasses. 5^2 THE INVERTEBRATA In ichneumons, chalcids and cynipids the ovipositor issues far for- wards beneath the abdomen, and these insects differ in this feature from the Proctotrypidae in which the ovipositor is terminal. Dipterous larvae are often parasitized by these insects, as are also the eggs of Orthop- tera and Hemiptera. Many hyper-parasites, i.e. parasites of other parasites, occur in this family. Phaenoserphus is parasitic on carabid beetle larvae (Fig. 345), and Inostemma is an egg-parasite of dipterous gall midges. Whereas parasitism is a character, largely though not wholly, common to the foregoing families, the ants, wasps and bees next to be considered show a tendency, in varying degrees, towards the development of the social habit. The ants (Formicotdea) are social, polymorphic insects in which two segments are involved in the formation of the abdominal petiole. Further, this petiole is always characterized by the possession of one or two nodes (Fig. 346 E). The females are endowed with a well- developed sting, the modified ovipositor. Polymorphism reaches its highest degree of complexity in this group, as many as twenty-nine morphologically different castes having been recognized. Some of these are pathological phases due to infection by parasites, e.g. Nematode worms or other Hymenoptera. In such colonies as produce winged forms of both sexes, mating takes place during a nuptial flight in which several colonies in one neighbourhood indulge at the same time. This ensures intercrossing between individuals from different colonies. The females then cast off their wings and start colonies in the ground, each one for itself. The workers are sterile females, whose power to lay eggs in certain circumstances may return. For instance, when a colony loses a queen several workers may, under the influence of suitable diet, take her place. In addition to the environmental complexity which a social existence involves, the lives of ants are further complicated by association with other organisms. Some, e.g. certain myrmecine ants, have adopted an agricultural habit, living on fungi which they specially cultivate. Others gather seeds from which they destroy the radicle to prevent germination, special chambers or granaries in the nest being constructed for their storage. The pastoral habit characterizes others, a symbiotic relation being set up with such insects, e.g. Aphides, as exude fluids which are palatable to the ants. In addition to associations of this kind there are numerous others of an indifferent or little understood nature, but which may range from the symbiotic to the parasitic. Finally may be mentioned the slave- makers; Formica sanguinea, for instance, captures from the colonies of F. fusca, pupae which on emergence serve as slaves in the colony which has adopted them. The wasps of the super-family Vespoidea are both social and INSECTA 503 solitary in habit. In these, the abdominal petiole is smooth (Fig. 346 C) and, in species with a worker caste, this is always winged. The pro- thoracic tergum extends back towards the wing base. Among solitary species may be mentioned Odynerus which stores with caterpillars its nest in which its larvae are developing. Pompilid wasps are ex- clusively predatory on spiders. Certain forms have adopted the "Cuckoo" habit, laying their eggs in the nests prepared and pro- visioned by other species. Thus the ruby wasp Chrysis usurps the nest of Odynerus. Mutilla behaves similarly towards many solitary bees and wasps and has been bred out from the puparia of the tsetse fly. Social wasps, e.g. Vespa, live in nests commonly constructed of paper obtained in the form of wood pulp by these insect architects. The larvae living in closely arranged cells on horizontal combs are fed on insect food gathered by the workers. In early summer, our common social wasps are useful in the control of such insects as plant lice, etc. Later in the season, however, their liking for sweet fruits may make them a nuisance both in the garden and in the home. In autumn the colony perishes, fertilized females being the only sur- vivors. Vespa germanica and Vespa vulgaris are common English wasps. Vespa crabro is the Hornet (Fig. 341). Closely resembling these are those wasps belonging to the super- family Sphecoidea, the distinctive feature of which is the possession of a prothoracic tergum which does not extend back as far as the wing bases. These are all solitary predaceous forms, which sting their prey and so paralyse it before placing it in the larval cells which have been previously prepared, e.g. Sphex. A tendency towards the social habit is exhibited by Bembex which leaves its larval cells open and so can provision its young from day to day on small flies. The super-family Apoidea includes the social and solitary bees. Distinctive of bees are the dilated hind tarsi and the plumose hairs of the head and body to which pollen adheres. Inner metatarsal spines of the posterior legs comb the body hairs free of pollen, this being then transferred to the outer upturned spines (pollen basket) of the hind tibia of the opposite side. These legs are further adapted by possession of special spines for the manipulation of wax plates when being removed from the abdomen. The median glossa is also characteristic and in certain solitary forms, e.g. Anthophora and all the social bees, e.g. Apis and Bombus, is greatly elongated along with the parts other than the mandibles^ for gathering nectar from deep- seated nectaries of flowers. Larvae are fed exclusively on pollen, nectar and salivary fluids. Megachile, the leaf cutter, is a solitary bee which makes cells of neatly cut leaf fragments. Each cell containing an egg is stored with honey and pollen. Such cells are commonly made in the walls of houses, the mortar being removed for this purpose. 504 THE INVERTEBRATA Andrena constructs burrows in the ground and, though solitary, is usually found in groups of individuals occupying a common terrain which may include a "village " of several hundred nests. Nomada has adopted the ** cuckoo" habit. Bombus enjoys a social existence similar to that of Vespa in that only impregnated females survive the winter. The colony of the Honey bee Apis mellifica has more permanence, only the males dying off in the autumn to leave the rest of the colony to hibernate. The nest is of wax, an exudation from abdominal glands of the worker (sterile female), and a material known as propolis of vegetable origin serves to fasten parts of the nest together and to render the whole weatherproof. The workers are graded according to age into nurses^ who see to the welfare of the larvae by incorporating salivary juices with their food, ventilators who by wing-fanning set up currents in the nest or hive to reduce the temperature and to evaporate the honey, scavengers or cleaners^ 2ind foragers who collect pollen and nectar. The changes from nursery work to house work and to field work are necessitated by changes in glandular capacity as age increases. Though the density of the population of the colony determines to some extent when a queen with a number of workers will depart from the hive as a swarm, it appears that this event is also dependent on the relative proportions of the above age-groups among the working caste .^ Order DIPTERA (Flies) Insects with a single pair of functional wings, the hind pair repre- sented by stumps (halteres) (Fig. 347); mouth parts suctorial and sometimes piercing or biting, usually elongated to form a proboscis ; prothorax and metathorax small and fused with the large mesothorax ; metamorphosis complete, larvae cruciform and always apodous, the head frequently being reduced and retracted; pupa either free or enclosed in the hardened larval skin (puparium). This is a very large and highly specialized order of insects. The imagines are mostly diurnal species, feeding on the nectar of flowers, but a number are predaceous, living on other insects (e.g. the robber flies), while some, e.g.Tachinids, are parasites. A further development which takes place in several families is the acquisition of blood- ^ In Bees, Wasps and Ants, haploid parthenogenesis results in the pro- duction of males. A fertilized (diploid) female has control over the fertiliza- tion of eggs which she lays. If an egg is fertilized by sperm from the spermatheca a female (diploid) offspring develops. If not, a male offspring (haploid) develops. Whether the young female produced in the former case becomes a worker (sterile) or a queen (capable of fertilization) depends on nutrition. Contrast this with diploid parthenogenesis in Aphids (p. 480). INSECTA 505 Fig- 347- Anopheles maculipennis, ?. After Nuttall and Shipley. 506 THE INVERTEBRATA sucking habits. The representatives of this oecological class are of great importance because they harbour and transmit pathogenic organisms, causing such diseases as malaria, sleeping sickness, elephantiasis, yellow fever and some cattle fevers. The several kinds of mouth parts which have been developed in the Diptera have departed widely from the primitive biting type. There is always a proboscis formed principally by the elongated labium, ending in a pair of lobes, the labella. This labium serves as a support and guide to the remaining mouth parts which are enclosed within it (Fig. 348). The most complete system is to be found in the gadflies, e.g. Tahanus and Chrysops. Within the groove of the labium are to be found a pair of mandibles and a pair of maxillae, sword-like piercing organs by means of which the wound through the skin of mammals is made. Into the wound so formed is inserted a tube composed of the eptpharynx, an elongated chitinization of the roof of the mouth to which the lab rum is fused, and the hypopharynx, a corresponding elongation of the mouth floor. The blood passes into this tube, being drawn up by the pharyngeal pump within the head. The hypopharynx carries a duct down which the salivary fluid is passed. Besides this, the proboscis of a gadfly can be used for taking up fluids exposed at surfaces. Such exposed fluid is drawn into small channels, the pseudo- tracheae, which converge to a central point on the underside of the labellar lobes. There it meets the distal end of the epi-hypopharyngeal tube, up which it passes. The mouth parts of the female mosquito (Fig. 348 A) in principle diff"er from those described above only in the absence of a pseudo- tracheal membrane on the labellar lobes and the more slender and elongated labium. Mandibles are absent in the males, maxillae being represented only by palps in this sex. The housefly Musca (Fig. 348 D, E, F) has lost all piercing mechanism, mandibles being absent, maxillae only being represented by the palps, and the mouth parts consist of a folding labium with highly developed pseudotracheal membrane on the labellar lobes and prominent epi-hypopharyngeal tube. Musca feeds largely on fluid matter but in the presence of soluble solid food, e.g. sugar, solution is effected by regurgitating alimentary fluid on it. By means of small chitinous teeth situated round the point to which the pseudotracheae converge, surfaces of solids can be scraped so enabling enzymes in the regurgitated fluids to act rapidly. The tsetse fly, Glossina (Fig. 348 B), also possesses no mandibles and only the palps of the maxillae. It does, however, feed on mam- malian blood after piercing the skin. In this form the whole labium is rigidly chitinized; the labellar lobes, from which all traces of pseudotracheae have disappeared, are small and provided with *■ mx. Fig. 348. Types of mouth parts of the Diptera. A, Culex pipiens, ?. B, Glossina suhmorsitans. C, Transverse section through proboscis of Culex. D, Transverse section through proboscis of a muscid fly. E, Proboscis of a muscid fly, extended and with left labellar lobe removed. F, Proboscis of a muscid fly, half folded, an. antenna ; e. eye ;/.c. food channel ; hyp. hypopharynx ; Ibm. labium; Ibl. labellum; Ibr.ep. labrum-epipharynx ; md. mandible; mx. maxilla; mxp. maxillary palp; ph. pharynx; ph.p. pharyngeal pump; pstra. pseudotracheae ; sd. salivary duct. A-D, after Patton and Cragg; E, F, original. 508 THE INVERTEBRATA chitinous teeth which make the wound. Thus a second kind of blood sucking mechanism has been evolved from a form like Musca, which only possessed the faculty of sucking fluid from surfaces. The larvae of Diptera are among the most specialized in the Insect Kingdom. Legs have been entirely lost, and the head and spiracular system have undergone varying degrees of reduction. Thus the most generalized larvae are at the same time eucephalous, i.e. with complete head capsule, and peripneustic ^ i.e. with lateral spiracles on the ab- n ■\ f t"" ■'"1 ■"■ f V-- B Pig. 2^f). Early stages of the Diptera. A, Larva of Musca domestica. Ace- phalous amphipneustic type. B, Empty puparium of Musca domestica. C, Pupa of Musca domestica removed from puparium. D, Larva of Bibio sp. Eucephalous peripneustic type. A, B, and C after Hewitt; D, original. sp. spiracle. domen, e.g. Bihio (Fig. 349 D). In the most specialized forms, on the other hand, we find the acephalous larva whose head capsule is entirely wanting. Such acephalous larvae may be either amphi- pneustic^ with only prothoracic and posterior abdominal spiracles, or metapneustic, where only two spiracles are retained at the posterior end of the body. The first instar larva of Musca is metapneustic, subsequent instars being amphipneustic (Fig. 349 A). The eucephalous larva develops into an exarate pupa from which the adult emerges by a longitudinal slit on the thorax. The pupa INSECTA 509 resulting from the acephalous larva, on the other hand, is coarctate, the last larval skin being retained as a protective puparium, tracheal connections maintaining contact between the pupa within and the larval skin outside it. Final emergence of the fly in this case clearly involves two processes, {a) the liberation of the fly from its pupal skin, and (b) its further liberation from the puparium. The latter splits transversely (Fig. 349 B), the top being thrust away by an eversible head sac, xhtptilinum^ which such flies possess. The features of meta- morphosis just described are characteristic of many flies and by de- fining one of the suborders constitute an important basis of modern classifications (Fig. 349 C). The suborder Orthorrhapha includes all those flies which are liber- ated by means of a longitudinal split in the mid-dorsal line of the pupal case. Such flies possess no ptilinum. Many of these, the Ne- matocera, have slender antennae and usually pendulous maxillary palpi. Their larvae are eucephalous with horizontally biting mandibles and their pupae are free. To this series belong the Crane-flies (Fig. 3 50 A) , the larvae of which often damage cereal crops by devouring their roots. The Culicidae (Fig. 347) are the gnats and mosquitoes, the piercing proboscis of which has already been described. They are further distinguished by their wings which are fringed with scales. Both larvae and pupae are aquatic, the former being metapneustic, the latter propneustic (with anterior spiracles only). With the blood- sucking habit of these flies has evolved an association with certain organisms which when transmitted to man cause disease. Anopheles is concerned with the transmission of malaria. Stegomyia transmits the causative organism of Yellow Fever while Culexfatigans, a widely distributed tropical form, is a carrier of the thread-worm Filaria bancrofti^ the cause of elephantiasis. Nearly related to these are the Chironomidae (midges), the mouth parts of many of which are not adapted for piercing and sucking. A few of these, however, do suck blood, e.g. the midges of the genus Forcipomyia, whose larvae breed, some in water, others behind the bark of trees. The Cecidomyidae (Fig. 350 C) are the gall-midges distinguished by their beaded antennae adorned with whorls of setae. The larvae of a few of these are parasitic. Some are predaceous, but others, forming a large majority, are phytophagous, forming galls in plant tissues, e.g. of grasses. Contarinia pyrivora is the pear midge, the larvae pf which develop in the flowers of the pear so as to abort fruit production. Miastor lives behind tree bark in the larval state and as mentioned above is noteworthy for the phenomenon of paedogenetic partheno- genesis. Another family of blood-sucking flies, known as the Simuliidaef 510 THE INVERTEBRATA consists of small flies with a hump-backed appearance and with broad wings. The spindle-shaped larvae live in running water and are cha- racterized by the possession of prothoracic prolegs and an anal pad provided with setae by means of which they cling to rocks etc. in the Fig. 350. Types of Diptera. Tipula ochracea (Tipulidae). B, Chrysops caecutiens (Tabanidae). C, Contarinia nasturtii (Cecidomyidae). D, Hypo- derma bovis (Cyclorrhapha, Oestridae). C, from Smith after Taylor ; D, from Smith after Theobald. rapidly flowing water of their environment. Still included in the suborder Orthorrhapha are the flies with short antennae, the Brachy- cera. Though included in this scheme with the Orthorrhapha their venational characters indicate a close relation with the Cyclorrhapha. In general, the basal joints of the antennae are larger than the terminal INSECTA 511 ones, these being reduced in number as compared with the nemato- ceran condition. The maxillary palpi are porrect (not pendulous). Their larvae are hemi-cephalous with vertically biting mandibles and the pupae are free and spiny. From this vast assemblage of flies we may mention the Tabanidae or gad-flies (Fig. 350 B). These flies, to the mouth parts of which reference has already been made, are of stout build and possess large eyes occupying a large part of the head surface. Though a few transmit disease organisms (Chrysops dimidiata, the vector of the worm. Filaria loa is responsible for Calabar swelling in the natives of West Africa), the majority are harmful chiefly through the annoyance which their bites occasion. Tabanid eggs are usually laid on the leaves of plants overhanging water and their carnivorous larvae are either aquatic or ground dwellers. The robber flies (Asilidae) are large bristly flies with a backwardly directed proboscis. They feed on all kinds of insects which they paralyse with their salivary fluid, and their legs, being strong and provided with powerful claws, are well adapted for grasping the prey. The Empidae, flies of more slender build, exhibit similar habits. Their larvae are terrestrial as are also those of the preceding family. Suborder Cyclorrhapha. These flies emerge from a pupa which is enclosed in the last larval skin or puparium and the commonly trans- verse or circular split in the latter, for release of the adult, gives the name to this suborder. It is therefore really a larval feature which establishes the position of these flies in the classification. The antennae are three-jointed, the last of which is greatly en- larged, carrying a dorsal spine or arista. The maxillary palpi are one- jointed and porrect. A crescentic suture on the head lies above and encloses the bases of the antennae. This, known as the frontal suture, is a narrow slit along the margins of which the wall of the head is invaginated to form the ptilinal sac, the eversion of which enables the adult to emerge from the puparium. The extent to which the frontal suture is developed and the ptilinum persists, varies. The Syrphidae, for instance, have usually no persistent ptilinum and the frontal suture is not well-developed. All larvae have a vestigial head, and are either amphi- or metapneustic. The Syrphidae (hover-flies) form an important family of brightly coloured flies, whose most obvious mark of distinction is the posses- sion of a false longitudinal vein lying about the middle of the wing. Their larvae are amphipneustic, leathery grubs, some of which devour Aphidae (Syrphus), others hving in decaying material being saprophagous {Eristalis), others again being phytophagous (Merodon, the bulb-fly). The remainder may be considered under the heading of muscid flies. The frontal suture is prominent and the ptilinum persists. Many 512 THE INVERTEBRATA families are included here, to some of which belong such serious agricultural pests as the frit-fly of oats^ Oscinus frit^ and Chlorops taeniopus the gout-fly of barley. In such cases, the larvae bore into the growing shoot, or into the stem. Larger and better known are the saprophagous house-fly Musca and the blow-fly Calliphora. The larva of Hypoderma lineatum is parasitic in the bodies of cattle causing "warbles" on the backs of affected animals, while Gastro- philus equi, the bot-fly, is parasitic as a larva in the alimentary tract of horses. The Tachinidae are important as parasites, chiefly of larval Lepidoptera. Thus Ptychomyia remota is responsible for the very effective control of the Levuana moth, Levuana iridescens, of Fiji. Blood-sucking muscids are important, e.g. Glossma, as the vector of trypanosomiasis causing sleeping sickness of man and cattle disease in Africa. The tsetse flies are pupiparous, larvae being nourished by special glands opening into the genital tract. The larvae are deposited as soon as fully grown and pupate immediately. A number of members of this order present a greatly modified structure resulting from an ectoparasitic habit. They are known as the Pupipara, being similar in their viviparity to Glossina. The follow- ing examples may be quoted. Hypobosca is a winged leathery fly with body dorso-ventrally compressed, and is an ectoparasite of cattle. Melophagus is a wingless species, similarly associated with sheep, familiarly known as the sheep tick. Nycteribia is a wingless form parasitic on bats. Order APHANIPTERA (Fleas) Wingless insects, ectoparasitic on warm-blooded animals; laterally compressed with short antennae reposing in grooves; piercing and sucking mouth parts, maxillary and labial palps present; coxae large; tarsus five-jointed; larva legless; pupa exarate, enclosed in a cocoon. These insects are perfectly adapted to an ectoparasitic existence by their laterally compressed bodies, prominent tarsal claws, well- developed legs suitable for running between the hairs of their host and for jumping, and by their mouth parts (Fig. 351). They only exhibit slight relationship to one other order, viz. Diptera, by their metamorphic features and to a less degree by their mouth parts. The mouth parts consist of a pair of long serrated mandibles, a pair of short triangular maxillae with palps, and a reduced labium carrying palps. There is a short hypopharynx and a larger labrum- epipharynx reminiscent of the Diptera. The labial palps, held together, serve to support the other parts, a function which is performed by the labium in the Diptera. In piercing, the mandibles are most important and the blood is drawn up a channel formed by the two mandibles INSECTA 513 and the labrum-epipharynx. The thoracic segments are free and there are never any signs of wings. Though the eggs are laid on the host they soon fall off and are subsequently found in little-disturbed parts of the haunts of the host. Thus in houses they come to lie in dusty carpets and unswept corners of rooms. In a few days the larvae hatch and feed on organic debris. The legless and eyeless larva possesses a well-developed head and a body of thirteen segments. At the end Fig. 351. The life history of the flea, Ctenocephalus cams. From Imms, after Howard, a, egg; b, larva in cocoon; c, pupa; d, imago; e, larva of flea, Ceratophyllus fascia tus ; f, antenna of imago. of the third larval instar a cocoon is spun and the creature turns to an exarate pupa from which the adult emerges, the whole life cycle occupying about a month in the case of Pulex irritans. Pulex irritans is the common flea of European dwellings, but by far the most important economically is the oriental rat flea, Xeno- psylla cheopis, which transmits Bacillus pestis, the bacillus of plague from the rat to man. It appears that this bacillus lies in the gut of 514 THE INVERTEBRATA the flea and the faeces deposited on the skin of the host are rubbed into the wound by the scratching which follows the irritation from the bite. Ceratophyllus fasciatus, the European rat flea, also transmits the plague organism as also can Pulex irritanSy but since the latter does not live successfully on rats, it is never likely to prove a source of trouble* Order STREPSIPTERA Small parasitic insects, allied to the Coleoptera, with winged, free- living males and larviform females, which never leave the interior of their host. Sty lops causes great modification of its host, the bee (Andrena). CHAPTER XV THE SUBPHYLUM ARACHNIDA Arthropods with fully chitinized exoskeleton ; the anterior part of the body (prosoma), never divided into head and thorax, consisting of six adult segments, the first (preoral) with prehensile appendages (chelicerae) usually three-jointed, the second (postoral) with append- ages either sensory or prehensile (pedipalps) and the remaining four ambulatory ; the posterior part (opisthosoma) consisting of thirteen segments and a telson in the most primitive forms but tending to become shortened, the first (pregenital) segment differing from the rest, the second bearing the genital opening; respiratory mechanisms of various types usually developed in the anterior part of the opistho- soma ; coxal glands of coelomic origin in the 2nd to 5th prosomatic segments ; larval forms absent except in Limulus. As has been pointed out in the introduction to the Arthropoda, the Arachnida are distinctly marked off from the rest of the phylum by the character of their appendages and especially by their chelicerae which furnish so strong a contrast to the sensory antennae, elsewhere found in the phylum. Moreover, nowhere else (except perhaps in trilobites) are true jaws absent, the prolongation of the basal joint of the anterior limbs toward the mouth (gnathobases) serving the arach- nids for mastication. In the divisions of the group is found the greatest diversity in form, for though by no means active creatures, arachnids have become adapted to many kinds of environment. Besides the segments enumerated in the preamble, there is in the embryo of most arachnids a precheliceral segment (Fig. 352 B, C). The variation in the segments of the prosoma is confined to minor details, the chelicera preserving much the same characters throughout the group, only losing a joint in the Araneae, and being either chelate or subchelate ; the pedipalp, however, varies according to its function, being chelate in the scorpion and the Pedipalpi, which seize their prey by means of it, modified for purposes of fertilization in the spiders, and merely an ambulatory appendage in Limulus. In most forms the tergites of the segments are fused together, but in the Pedipalpi and the Solifugae the last two prosomatic segments are entirely free. It is in the opisthosoma and its segments that the greatest amount of variation can be seen. The pregenital segment (Fig. 352 B, C) is always developed in the embryo, but tends to disappear in the adult. Thus in the Palpigradi, Pedipalpi and Pseudoscorpionidea it forms a distinct segment; in Limulus it is represented by a pair of rudi- Op^'iO r ~ -iCOC. pro- 1 o-'>p--Jf- ^eox.gl- pet.--S- -(/OM. co- rnet. Fig. 352. The development of the Arachnida. A, Transverse section of a spider embryo (Theridium), after Morin, showing the coelomic sacs (coe.) and the formation of the heart (h.). n.r. nerve rudiment ; y. yolk with contained cells. B, Sagittal section of a spider embryo after Wallstabe. Coelomic sacs of pre. precheliceral segment, pro. i, pro. 6, first and last limb-bearing segments of the prosoma, prg. pregenital segment, op. 2, second, and op. 10, tenth seg- ment of the opisthosoma. C, Diagram of the scorpion embryo, altered from Dawydoff. Coelomic sacs of pre. precheliceral segment; 1-6 limb-bearing segments of the prosoma; prg. pregenital segment; gop. segment of genital operculum ; pet. segment of pectines ; Igp. segments of first three lung books ; eo.d. coelomoducts which never reach the exterior; cox.gl. coxal glands; gon. gonoducts; g. gonad. D, Embryo of the scorpion Buthus earpathieus, after Brauer. Stage showing che. the chelicerae; pp. pedipalps; the four other appendages of the prosoma; prg. the pregenital segment and appendages; 8, appendage forming genital operculum, succeeded by those which form the pectines and the lung books; 13, last of these; 7net. metasoma. ARACHNIDA 517 mentary appendages, the chilaria\ it is entirely missing in the adult scorpions. In addition to this segment there is a maximum of twelve segments and a terminal appendage, the telson, which is attained only by the embryo scorpions and the eurypterids; the Palpigradi and Pseudoscorpionidea have one less. In all these cases, there is a differentiation of the segments into two regions, the meso- and meta- soma. In Limuliis there are six segments only, but in the related extinct genus, Hemiaspis, there are three more. The Solifugae show ten. In the spiders, mites and phalangids, the body is much shortened; the phalangids have the anterior segments united to the prosoma. Lastly, the telson may be a sting in the scorpions, a jointed sensory flagellum in the Palpigradi, a fin in some eurypterids or a digging stick in others and in Limulus. A typical feature is the suctorial alimentary canal. The mouth is usually narrow and situated just behind the chelicerae; only in Limulus has it moved backwards, become enlarged and surrounded by the basal joints (gnathobases) of all the prosomatic appendages ; in the scorpions the appendages of the 2nd-4th segments form gnatho- bases; the Palpigradi and Solifugae have no gnathobases. In all arachnids, except Limulus^ the food is fluid and is drawn through a narrow oesophagus into a sucking stomach and thence into a straight mid gut, which is by far the longest part of the gut, and receives the openings of the digestive coeca ; often, as in scorpions, there are several of these, segmentally repeated, very much branched and forming a compact " liver "-hke organ. There may be important salivary glands entering the fore gut as in the scorpions. Posteriorly the mid gut, except in Limulus, gives off Malpighian tubules. The hind gut is short. The respiratory organs of the Arachnida are distributed as follows, (i) *'Gill books" in the aquatic form, Limulus, and probably in the extinct eurypterids. (2) "Lung books" in the terrestrial scorpions and Pedipalpi. (3) A combination of lung books and tracheae in the spiders. (4) Tracheae alone in the Solifugae, Pseudoscorpionidea, Phalangida and Acarina. (5) Lastly, in the Palpigradi, smaller acarines and other forms, there are no special respiratory organs and exchange of gases takes place through the skin. As the Arachnida apparently form a natural group, efforts have been made to derive these various methods of respiration one from the other. The gill books (Fig. 353) are stated to be the most primitive respiratory organs. They are piles of leaflets, in which blood circulates, attached in each segment to the posterior face of freely oscillating plates, which are possibly appendages, resembling the abdominal appendages of the Isopoda which are also respiratory in function. There is a special muscular mechanism for opening and shutting the leaflets in the water and thus facilitating gaseous exchange. In the 5l8 THE INVERTEBRATA lung books of the scorpion there are also parallel leaflets, which are sunk into pits with a confined opening (pneumostome). The air circulates between these leaflets, but there is no evidence that air is actively pumped in and out of the lung. Gaseous exchange then appears to be entirely due to diflFusion. In spiders, however, a complicated system of muscles has been described which bring about expiration by compressing the lung. Inspiration follows by the elasticity of the chitin lining. Fig- 353- Longitudinal section through the opisthosoma oi Limulus, showing four of the five gill books. From Shipley and MacBride. i, operculum; 2, second gill book; 3, muscle which moves the gills up and down; 4, blood vessels; 5, muscle which raises the operculum. Fig- 354- Diagram of respiratory organs of the Arachnida, After Kingsley. A, Two segments with appendages (gill books), bearing leaflets on their pos- terior face as in Limulus. B, Appendages partly (right) and wholly (left) withdrawn into pits of the ectoderm so that the flat appendage forms the floor of the pit and the leaflets are internal, a. anterior. It is generally supposed that the lung books of scorpions are derived from gill books by the withdrawal of the leaflets into special pouches, the lungs (Fig. 354). The appendages or plates disappear or form the floor of the lung and the leaflets appear as folds of the lining. Lung books, according to this view, are organs which, originally intended for aquatic use, have been slightly adapted for terrestrial life, but while the scorpions in their long history have shown no capacity for further ARACHNIDA 5^9 development, the rest of the Arachnida have developed the typical arthropod tracheal system. The spiders, at least, have passed through a primitive lung-book stage from which they have not all emerged. In fact they show all the stages of replacement of lung books by tracheae, which actually arise as diverticula of the lung itself. Thus we have the following stages in the spiders : (i) Two pairs of lung books and no tracheae in the families Atypidae, Liphistiidae and Aviculariidae. Fig- 355- Respiratory organs of spiders. After MacLeod. A, Horizontal section through the opisthosoma of Argyroneta. i, stigma opening into a cavity from which arise bundles of 2, terminal and 3, lateral tracheae; 4, lung book with leaflets in section. B, Longitudinal section through lung book of a spider, i, pneumostome or stigma; 2, free edge of leaflet; 3, air space between leaflets ; 4, blood space within leaflet. (2) An anterior pair of lung books and a posterior pair of stigmata, opening into tracheae, in the majority of families. (20) An anterior pair of lung books, the posterior pair of stigmata and tracheae having entirely disappeared, in the family Pholcidae. (3) Two pairs of stigmata, both opening into tracheae, in the family Caponiidae. These form a complete series. The adherents of the theory that lung books have given rise to tracheae claim that, on the whole, those spiders which have two pairs of lung books are the most primitive 520 THE INVERTEBRATA in Other respects. It may be pointed out, however, that there is also a connection between the degree of development of tracheae in a family and the activity of its members. In inert forms, there may be reduction or even total loss of the tracheal system. In all the forms in which lung books or gill books are present, there are processes in the embryo which can be identified as rudiments of appendages, on the anterior abdominal segments (Fig. 352 D). On the posterior border of these processes, leaflets develop at the same time as an invagination forms the lung cavity above them, so that the limb itself forms part of the floor of the cavity. On the whole then, embryology may be said to show the origin of lung books from gill books, and the comparative anatomy of spiders indicates that lung books have been replaced by tracheal systems. But there lie outside this series arachnid groups, like the Acarina, with tracheal systems of a different kind, which can only be derived with difficulty from the respiratory system of the other forms and may have had a separate origin. In the arachnids, the mesoblast is formed as two lateral bands which segment into somites, just as does the same tissue in the anne- lids. The somites correspond with the external segmentation and in each one of them appears a coelomic cavity. This is best seen in the scorpions (Fig. 352 C) and the spiders (Fig. 352 B). They are formed near the ventral surface and extend on the one hand into the appendage and on the other towards the dorsal middle line, where the extensions from the two sides meet and form the heart between them. They also form diverticula varying in the different groups, which are the remains of a complete series of metamerically segmented coelomoducts. In the scorpions, the embryo (Fig. 352 C) shows five pairs of these, in seg- ments 3, 4, 5, 6 and 8. In only one case, that of segment 5, do the coelomoducts reach the external surface, and persist in the adult as a pair of excretory organs, the coxal glands. In segment 8 they grow towards the middle line and form the mesodermal part of the gono- ducts. The other coelomoducts disappear and the coelomic sacs are resolved into mesenchyme which fills up the spaces of the body and forms the muscles, the blood and the fat body. In Limulus there are also a pair of coxal glands, which in development arise from the coelomic somites of no less than six segments, of which only segment 5 sends out a duct opening to the exterior. Class SCORPIONIDEA Arachnids with the prosoma covered by a dorsal carapace; the opisthosoma divided into a mesosoma and metasoma distinct from one another, containing twelve segments and a telson; chelicerae SCORPIONIDEA 521 and pedipalps both chelate; four pairs of walking legs; the first mesosomatic segment carries the genital operculum, the second the pectines, and the next four each a pair of lung books; the metasoma comprises segments reduced in size to form a flexible tail for wielding the terminal sting (the telson) and bears no appendages. Viviparous. Fig. 356. Scorpio swammerdami, x f. From Shipley and MacBride. A, Dorsal, B, Ventral view, chc. chelicera ; pp. pedipalp ; e.l. lateral and e.m.^ median eyes; g.op. genital operculum; pet. pectines; 3, 4, 5, 6, walking legs of the prosoma; 9, 10, 11, 12, stigmata of right side; 13, last segment of mesosoma; pi. soft tissue of pleura ; met. i , first segment of metasoma ; tel. telson. The tergum of the prosoma bears a group of lateral eyes near the anterior border and a pair of median eyes, but some scorpions are blind. On the ventral surface there are inward projections from the 522 THE INVERTEBRATA basal joints of the pedipalps and the first two pairs of walking legs, which are masticatory in function (gnathobases). The walking legs are six-jointed and end in double claws. Between the basal joints of the last pair is a plate, the metasternite, which represents the fused sterna corresponding to these limbs; the sterna of the other pro- somatic segments are not represented. At the beginning of the meso- soma there is in the embryo a pregenital segment with two limb rudiments. This disappears without leaving a trace in the adult. The two succeeding segments bear appendages: (i) the genital operculum^ a small plate covering the openings of the genital ducts, which is formed by the union of two rudiments of appendages ; (2) the pectineSy flap-like structures attached by a narrow base with a distal border of chitinous spines like the teeth of a comb. They are tactile in function and derived from embryonic limb rudiments. There are no other ex- clusively sensory organs (except the eyes) on the body of the scorpion, but there are sense hairs scattered over the surface and more numerous on the pedipalps than elsewhere. The lung books are found on segments 3-6 of the mesosoma. The 7th segment is without any external segmental organs. As has been already mentioned, there are, in the embryo, seven pairs of meso- somatic appendages (Fig. 352 D), those on the embryonic pregenital segment and on the six succeeding segments. Of these the 4th-7th never develop to more than papillae, but folds develop on their posterior surface and the skin behind is tucked in to form the lung sacs. When the sacs are complete, the folds become the leaves of the lung book. In the internal space of these folds, the blood circulates and is presumably aerated; it contains the respiratory pigment, haemocyanin. The circulatory system of the scorpion is remarkably complete (Fig. 357). The heart consists of seven chambers (in the 7th- 13th segments), into each of which a pair of ostia opens and from each there leave a pair of lateral arteries. In addition, there is an anterior and a posterior aorta, the former dividing into many branches in the prosoma, and one of these passes backwards as a supraneural artery. The arteries end in tiny vessels and many of these communicate with the special ventral sinus, which supplies blood to the lung books. Muscles run from the roof of this to the floor of the pericardium, and when they contract the ventral sinus enlarges and draws venous blood into it. When they relax, blood is forced into the lung books, whence it is returned to the pericardium by segmental vessels. A minute mouth opens into the pharynx which is suctorial, with elastic walls which can be drawn apart by muscles. A short oeso- phagus succeeds, and into this open the salivary glands. The endo- dermal mid gut is long and narrow and receives throughout its course several pairs of ducts which lead from the digestive glands. These SCORPIONIDEA 523 524 THE INVERTEBRATA together form a bulky mass, filling up the dorsal part of the meso- somatic body cavity. The food passes into the cavity of these to be digested. It consists mainly of insects, which are chewed by the gnathobases and the juices sucked up by the action of the pharynx. The beginning of the short hind gut is marked by the Malpighian tubules. The nervous system consists of a supraoesophageal ganglion which supplies the eyes, a large suboesophageal complex which gives branches to all the adult appendages, and two ventral cords which bear ganglia in the last seven segments. The sexes are separate and the gonads constitute a network. The spermatozoa are filiform and fertilization is internal, being preceded by a courtship, described in lively fashion by Fabre as danse a deux. Scorpions are viviparous. Sometimes the eggs are rich in yolk and the young develop entirely at its expense ; in Scorpio and other genera the eggs are small and yolk is entirely absent. In this case the young develop in lateral sacs of the uterus, attached to the mother by a kind oi placenta. The young, when hatched, are sometimes carried on the mother's back. The earliest scorpions are found in the Silurian, and it is of con- siderable interest that the first genus, Palaeophonus, was a marine animal. It closely resembles the terrestrial scorpions, except in its shorter and broader limbs without claws, and in the absence of stigmata. Class EURYPTERIDA Extinct aquatic arachnids resembling the scorpions in the number and arrangement of the segments of the adult; the division of the abdomen into meso- and metasoma is not quite so marked ; chelicerae short and three-jointed, chelate; the next four segments bear append- ages which are often similar (but the pedipalps may be chelate); in the last (6th) prosomatic segment the appendages are always larger than the rest and are broad and paddle-shaped; first and second pairs of mesosomatic appendages unite to form the genital operculum ; the first five mesosomatic segments bear indications of leaf-like branchiae ; metasoma ends in a structure (telson) of variable form ; mouth has moved backwards and is surrounded by gnathobases of all the limbs. The great interest of this group lies in its similarity to the scorpions. There was, however, much more variety in external structure in these aquatic arachnids and they sometimes attained a length of six feet. Not only is there fundamental agreement in the segmentation and the division into meso- and metasoma, but also in characters like the shape and usually the size of the chelicerae, and the telson, which in primitive eurypterids has a recurved sting-like form. Slimonia (Fig. 358 B) ARACHNIDA 525 has a slightly modified telson. In one eurypterid {Glyptoscorpius) structures have been described which correspond to the pectines in position and structure. If this is substantiated, it constitutes a remarkable resemblance in detail. Fig. 358. Diagram of extinct Arachnida. A, Pterygotus osiliensis, dorsal view. After Schmidt. B, Slimonia (restoration of ventral surface by M. Laurie). C, Hemiaspis limuloides, dorsal surface. From Woods. All Silurian forms. Segments and appendages numbered to correspond ; Arabic numerals in Pterygotus and Roman in Slimonia. chc. chelicerae (segment i); 6. meta- stoma; d. compound eyes; e. simple eyes;^.op. genital operculum; tel. telson. A few special characters may be mentioned here. On the ventral surface a structure called the metastoma is seen which possibly re- presents the pregenital segment. Branchiae undoubtedly existed, but their exact nature is not known. Possibly the sterna of the segments 526 THE INVERTEBRATA which carried them were membranous and the branchiae were tucked in under them. There are five pairs and the first of these corresponds in position to the pectines of the scorpion (except possibly in Glypto- scorpius). Thus, when the ancestors of the scorpions became terrestrial, we may suppose that the first pair of respiratory appendages remained external and took on a sensory function, while the rest helped to form the lung books. Minute forms with incompletely developed abdomen and enlarged eyes have been found which are thought to be the pelagic larvae of eurypterids. The adults were in all probability carnivorous forms, which crept and swam and sometimes burrowed at the bottom of shallow seas. In Pterygotus (Fig. 358 A) and Eurypterus there are adaptive modifications of the telson for swimming and burrowing respectively. Class XIPHOSURA Aquatic arachnids with a broad prosoma divided by a hinge from the opisthosoma in which the first six segments are present and fused together dorsally; they bear six pairs of biramous appendages, of which the first form an operculum on which the genital apertures open and the remaining five carry the gill books ; chelicerae of usual arachnid type, pedipalps not distinguished from the four pairs of ambulatory appendages which follow ; mouth far back surrounded by gnathobases of all the postoral limbs ; caudal spine present possibly representing the lost abdominal segments as well as the telson; pregenital segment represented by rudimentary appendages, the chilaria. Limulus (Figs. 359, 360), which is the sole living representative of the group, is evidently more affected by specialization than either the scorpions or eurypterids, and it is on this account that the attempts which have been made to indicate the king crab as an ancestral form to higher groups have usually been regarded as ingenious but illusory. It is essentially a shore-living, burrowing animal. Like a crab, its carapace is compact, dorsoventrally flattened and expanded laterally, so that the animal can shovel its way under sand and mud. Its legs are tucked under the carapace and the hinder pair kick out the sedi- ment behind. To protect the gill books from this rough treatment, the operculum completely covers the appendages which bear them. But Limulus has not lost its tail, and an observer, watching the creature in an aquarium, will contrast it unfavourably for grace and efficiency with a crab. Its swimming movements, principally brought about by the flapping of the abdominal appendages, are slow and clumsy, and we can hardly consider it except as a sedentary animal. The chelicerae are small, chelate and three-jointed, as is usual in XIPHOSURA 529 In all the others the appendages almost meet in the middle line, but remain distinct. From the posterior surface of the exopodite arise about two hundred branchial leaflets. The appendages are provided with muscles by which the flapping movements are made which propel the animal in a leisurely way through the water and circulate water amongst the leaflets. The mouth occupies a subcentral position under the carapace, surrounded by the gnathobases. Worms and small molluscs from the shore mud are seized by the chelae and, after mastication by the gnathobases, stuffed into the mouth, which leads to the fore gut con- sisting of an oesophagus and a chitin-lined "stomach"; the mid gut is long and into it open two pairs of ducts from the digestive glands. These glands are very well developed and fill up much of the space inside the cephalothorax. There are no Malpighian tubules and no salivary glands in Limulus. The circulatory system is very complete and like that of the scorpion in its main lines. A unique feature is the complete investment of the ventral nervous system by an arterial vessel which corresponds to the supraneural vessel of the scorpion. The nervous system is of a very concentrated type. The supra- oesophageal ganglia supply the eyes and are fused with the ganglia of all the succeeding segments as far as the opercular segment to form a ring round the oesophagus. From this a double ventral cord ex- tends into the opisthosoma, swelling into ganglia in each of the "gill- book" segments. Median and lateral eyes (p. 310) are present. The coxal (brick red) glands arise from six segments in the embryo and open on the fifth pair of legs. The reproductive organs consist of a network of tubules com- municating with the exterior by paired ducts opening on the genital operculum. The eggs are laid far up on the shore at spring tides in holes dug for them by the mother, and the male, which comes ashore clinging to the carapace of the female, spreads the sperm over them, a method of fertilization very similar to that of the frog. The eggs are heavily yolked and the young hatch as a planktonic larva in a condition re- sembling the adult but with an opisthosoma showing separate segments and without the caudal spine. The larva, which swims by means of the abdominal appendages, as in the adult, has been called the *'Trilo- bite" stage, because of an extremely superficial likeness to that group. While Limulus has existed since the Trias without any modification, it is of considerable interest that in the Palaeozoic ver}'' similar animals occur, in which there are three additional segments and a rather shorter caudal spine, indicating that the latter organ has been formed at the expense of the posterior opisthosomatic segments. These ani- mals are Hemiaspis (Fig. 358 C) and Bunodes. 530 THE INVERTEBRATA Class ARANEIDA Arachnids with prosoma covered by a single tergal shield but head marked off by groove ; opisthoSoma ("abdomen ") separated by waist, soft, rarely having any trace of segmentation, two to four pairs of spinnerets and several kinds of spinning glands ; chelicerae two-jointed, subchelate; pedipalps modified in male for transmission of sperm. In the embryo spider, the segmentation of the opisthosoma is in- dicated by the presence of coelomic cavities of which there are ten (Fig. 352 B) ; there are also five pairs of rudimentary appendages, the first of these disappears, the next two assist in forming the lung books, and the fourth and fifth become the spinnerets. When more than two pairs of spinnerets are present the additional ones are split off from pre-existing spinnerets. Embryology thus shows that the existing forms with apparently unsegmented opisthosoma are descended from ancestors with nearly the full number of segments typical of arachnids. The chelicerae (Fig. 364) contain a poison gland in the basal joint. Spiders have developed to an extreme the tendency, so common in the arachnids, towards adopting a carnivorous diet. While most of the spiders on account of their size can only obtain suitable supplies of food from insect life, some are able to attack larger forms, even birds in the case of Mygale. Besides the poison glands which cause the immediate death of the prey, there are salivary glands in the under lip which produce a proteolytic ferment. A fly which is caught by a spider is pressed against the mouth by the gnathobases of the pedi- palps, a drop exudes from time to time and in a couple of hours the morsel of flesh has been externally digested and the resulting fluid sucked into the spider's alimentary canal by the pulsations of the "stomach", the chitinous exoskeleton of the prey remaining as an empty husk. This method of feeding is a leading characteristic of the group. The diagram (Fig. 361) shows the main features of the anatomy of the spider. The oesophagus, after dilating into the sucking stomach, is succeeded by the mid gut which immediately sends out two main lateral branches forward with coeca running into the limbs. It passes back through the opisthosoma and gives place to the hind gut where the Malpighian tubules are given off. The main feature is the digestive gland which is a dorsal diverticulum of the mid gut, richly branched and filling the opisthosoma on each side of the heart. In this the latter stages of digestion take place. The hind gut is short and dilated into a stercoral pocket where faeces accumulate. The heart is situated in a distinct pericardium in the opisthosoma, has three pairs of ostia, and gives off an anterior and a posterior aorta and three lateral arteries on each side. In contrast to the scorpion and Limulus there are no definite ARANEIDA 531 arterioles, but the blood is finally collected into sinuses which feed the lung books when these are present. The nervous system is more concentrated than in the scorpion, consisting of a supraoesophageal ganglion supplying the eyes (Fig. 363 ) and a suboesophageal complex supplying the rest of the body. Two non-ganglionated nerves pass backwards to the opisthosoma. The diagram (Fig. 361) shows a lung book opening in the anterior part of the opisthosoma and the details of the structure are exhibited in Fig. 355. The "leaves" of the book are seen to be thin plates with an internal space for the circulation of the blood. They are dotted with short chitinous spines (not seen) and fused with the walls of the lung. The cavity of the lung only communicates by a narrow opening d.dig. Pf"^' h. Fig. 361. Diagram of a spider, Epeira diademata, showing the arrangement of the internal organs, x about 8. From Warburton. an. anus; ar. artery; brn. brain ; chc. chelicera ; cm. caecum of mid gut in ambulatory limb ; d.dig. ducts of digestive gland ; e. eye ; ga.sb. suboesophageal ganglion ; gl.ac, gl.ag., gl.am., gl.t. aciniform, aggregate, ampulliform and tubuliform glands ; h. heart with three ostia ; Ing. lung book ; M. mouth ; m.d. dorsal muscle of sucking stomach; 7ng. mid gut; m.t. Malpighian tubule; o. ov.ary ; p.gl. poison gland; pcm. pericardium ; sp. spinneret ; s.p. stercoral pocket of hind gut ; s.st. sucking stomach; v. vessel bringing blood from lung book to pericardium. with the outside air. Respiratory movements for the renewal of the pulmonary air have not been recorded by most observers and the method of respiration cannot be very efficient. In this diagram (Fig. 361) the tracheae are not shown, but in Fig. 355 A a horizontal section through the opisthosoma is shown in which the same ingrowth has given rise to a lung book and a bundle of tracheae. The character of the tracheae is well seen. They spriAg from a long pocket in parallel series and do not branch as in the insects, but they have the typical structure, strengthened by a spiral ridge of the chitinous lining. This form [Argyroneta) shows a richly developed tracheal system, but in other forms, particularly spiders with slow movements, the number 532 THE INVERTEBRATA of tracheae is much reduced, even to a single pair from each stigma. The variations in the development of the tracheae are recorded in the opening section on the Arachnida (p. 519). Fig. 363. Fig. 362. Fig. 362. Pedipalp of Tegenaria guyonii, the large house spider. From Shipley and MacBride. i, coxa; 2, gnathobase, the so-called maxilla; 3, trochanter; 4, femur; 5, patella; 6, tibia; 7, tarsus; 8, palpal organ. Fig. 363. Front view of head of Textrix denticulata. From Warburton. I, head; 2, eyes; 3, basal joint of chelicerae; 4, claw of chelicerae. 1-.. Fig. 364. Diagrammatic view of expanded palpal organ. From Shipley and MacBride. i, tarsus; 2, bulb; 3, vesicula seminalis, and 4, the opening of its duct which is protected by 5, the conductor; 6, haematodocha which is dis- tended with blood when the palpal oigan is expanded; 7, alveolus; 8, tarsus. The spinning glands are shown in the diagram in the ventral part of the abdomen. In a web spinner like Epeira, there are five types of glands of diverse structure and function, all opening by minute pores on the spinnerets. Thus the ampulliform glands supply the radial lines ARACHNIDA 533 of the webs, and the spiral Hnes are made by the aggregate glands which furnish the viscid fluid which covers them. The egg cocoon is formed by the tubidiform glands and these glands are absent in the males. The aciniform glands manufacture the cords which are wrapped ound the prey caught in a web, and the pyriform glands make the attachment discs by which a silk thread is anchored to the ground. Such a spider as this is well adapted for its sedentary life in a web. It has immensely long legs compared to the size of the body and on the ground moves slowly and uncertainly. But its legs end in claws and spines, by which it not only can cling with absolute safety to the elastic threads of the web, but which it also uses to weave the threads of silk as they come out of the spinnerets. Thus the web spinners represent the greatest specialization of the group; there are, however, other forms like the wolf spiders (Lycosidae) and the jumping spiders (Salticidae), which are just as predaceous as the Epeiridae but by no means so sedentary. They run swiftly after their prey or jump suddenly on it. They may only possess two ampulliform glands which secrete a "drag line" which they leave behind them as they move. The web spinner relies almost entirely on its sense of touch and the vibration of the lines of the web affecting the tactile hairs of the limbs is the guide to the entangled prey. Eyes, though present, are not efficient. But the hunting spiders find their victims by sight and have a remarkable range of vision. This is not only used in the pursuit of food but also in the elaborate courtships which are characteristic of these two families, during which the male executes the most fantastic dances. The generative apertures are found between the aperture of the anterior pair of lung books and the spinnerets. Fertilization is internal and before the male is ready to fertilize the female the sperm must be transferred to his modified pedipalps (Fig. 362). The terminal joint of these is greatly enlarged and contains a complicated tubular vesicular seminalis. A drop of seminal fluid is emitted either on to a small web spun by the male or on some surface like a leaf and the palps are then applied to the fluid and the seminal vesicle charged. After this court- ship begins, and at the close the palps are inserted into the genital opening of the female; the spermatozoa are stored in spermathecae. The eggs are laid in a cocoon. Class ACARINA (Mites and ticks) Arachnids with a rounded body with no boundary between the prosoma and the opisthosoma ; basal segments of the pedipalps united behind the mouth ; no gnathobases to the four walking limbs. These forms are usually minute except in the case of the parasitic 534 THE INVERTEBRATA ticks. They are, variously, scavengers, ectoparasites on all sorts of plants and "hangers on" of all sorts of animals, but in the last case they become, by the modification of the chelicerae and pedipalps, blood-sucking parasites. In the most free-living of them, like the aquatic and predatory Hydrachnidae, the chelicerae are clawed piercing weapons and the pedipalps leg-like with sensory hairs. The chelate condition of the chelicerae may be seen in the cheese mite, Tyroglyphus (Fig. 366), which is a typical saprophyte living on cheese only when it has begun to decay. The pedipalps are here no longer leg-like. In a tick Hke Argas (Figs. 365 A, 367, 368), the pedipalps are sensory, but the chelicerae and the median hypostome are elongated and con- verted into serrated cutting tools ; a sucking channel is formed between these. The mouth is usually minute and leads into a sucking pharynx and then into an endodermal stomach which gives rise to caeca in the ticks, where there are also salivary glands of large size opening into Fig. 365. Dorsal surface of A, Argas (Argasidae) and B, Amhlyomma, $ (Ixodidae). From Nuttall and Warburton. The Argasidae are distinguished by the leathery skin, diversified by discs which mark the insertion of muscles ; the Ixodidae by the hard scutum, which covers the whole body of the male and the anterior part of the body in the female (B). the pharynx. The saliva is said to contain an anticoagulin, as in leeches, and this renders easier the gradual digestion of the blood which is taken into the stomach. A remarkable phenomenon without parallel in the Arthropoda is the occurrence of intracellular digestion in some acarines. The cells of the stomach put out pseudopodia and the blood plasma is taken into vacuoles where it is digested. The circulation is extremely degenerate. No heart has been ob- served with certainty and the blood system is lacunar in mites, but in the tick, Argas, there is a single-chambered pulsating vessel with a pair of ostia and an aorta running forward to a periganglionic sinus. The respiratory organs are tracheae, long and convoluted. These open ACAR[NA 535 Fig. 366. Tyroglyphus siro, seen from the ventral side. A, Female. B, Male. Magnified. From Leuckart and Nitsche. an. anus; chc. chelicerae; pp. pedipalps; 3, 4, 5, 6, first, second, third and fourth walking legs; rep.ap. reproductive opening, flanked by two suckers on each side ; su. suckers at side of anus. ,chc.d. hyp. chc.s. Fig- 367. Ventral view of capitulum (false head) of Argas persicus, S. From Nuttall. b.cap. basis capituli; chc.d. digit, and chc.s. shaft of chelicera; hyp. hypostome; pp. four-jointed "palp" (pedipalp); p.h., h.h. postpalpal hair, posthypostomal hair. 536 THE INVERTEBRATA by stigmata, the position of which varies in the main divisions of the group. In the Notostigmata there are four pairs of dorsal stigmata in the first four opisthosomatic segments; in the Cryptostigmata, four pairs of stigmata at the bases of the four walking legs ; in the rest there ^xhc-sh. Fig. 368. Argas persicus, S. Median longitudinal section showing the pro- boscis, alimentary canal and reproductive systems. Altered from Nuttall. The chelicerae are seen within the cheliceral sheath (chc.sh.). They are thrust forward by the contraction of dorsoventral body muscles (m.r.ch.) and cut their way into the host by the digits (chc.d.) which are moved by their flexor muscles {m.f.d.). The barbed hooks of the hypostome {hyp.) are thrust into the wound and keep the tick in place, an. anus ; b.cap. basis capituli ; brn. concentration of nervous system ; cav.sh. cavity of cheliceral sheath ; coe. caecum of the stomach; end. endosternite ; h. heart; M. mouth cavity; oe. oesophagus; ph. pharynx with radiating muscles; r.s. rectal sac; st. stomach; sal.d., sal.gl. salivary duct and gland ; ts. testis ; vd. vas deferens ; w.gl. white (accessory) gland. is a single pair of stigmata in varying positions, in front of the chelicerae (Prostigmata), between the chelicerae and pedipalps (Stomatostig- mata), between the pedipalps and ist walking legs (Heterostigmata), the 2nd and 3rd legs (Parastigmata), the 3rd and 4th (Mesostigmata), ARACHNIDA 537 and behind the 4th legs (Metastigmata). If we regard the opistho- somatic position of the breathing organs as primitive it is difficult to see how these varying arrangements have come to pass in the acarines. The life history of the parasitic forms is of great interest, especially that of the ticks or Metastigmata. These are divided into the Ixodidae (Fig. 365 B) and Argasidae. The former live permanently on one host; the life of Boophilus bovis, attached to the cow, is only interrupted by the necessity of moulting and reproduction. Though compelled to withdraw its mouth parts when the skin is being cast, the tick plunges them into the host again at the same place, as soon as possible after the completion of the process. In many other cases the ticks fall off before every moult and have to seek a new host afterwards. The argasids, however, in the full-grown state, make only short visits to the host to suck blood, lasting for a few hours. In these last cases the young can go without food for months and the full-grown tick for years. In the course of several of these meals the six-legged larva develops into an eight-legged nymph which becomes sexually mature only after further development. Copulation may take place several times, spermato- phores being inserted, but the sperm in these can only escape and reach the ovary after the female again feeds. But in all cases when fertilization of the eggs has once occurred, the female falls to the ground and after laying her eggs dies. Many kinds of ticks carry disease, e.g. in both the following cases caused by Spirochaeta, Texas fever of cattle (Boophilus annulatus) and the relapsing fever of man (Ornithodorus moubata). Also certain small parasites of the blood corpuscles {Piroplasma), in severe diseases of cattle, are carried largely by Rhipicephalus . Class PHALANGIDA Arachnids with prosoma covered by a single tergal shield and united to the opisthosoma by its whole breadth; opisthosoma always seg- mented ; chelicerae three-jointed and chelate ; pedipalps leg-like ; two simple eyes. These creatures, with their enormous elongated legs, are familiar objects in the summer; the active predaceous forms are supposed to live for a single season only, but some representatives are slow-moving and live longer. They feed on insects and other arthropods and suck their juices. The walking legs have the same number of joints as^ spiders, but the tarsus is multiarticulate. The opisthosoma contains at least ten segments. The animal breathes by tracheae and there are two stigmata on the first sternum of the opisthosoma, opening on each side of the reproductive aperture from which emerges a long pro- 538 THE INVERTEBRATA trusible process, which is an ovipositor in the female, a penis in the male. The Notostigmata mentioned above (p. 536) are forms transitional between the acarines and the phalangids. Class PANTOPODA (PYCNOGONIDA) Arachnida, in which the opisthosoma has disappeared, with the exception of the pregenital segment which bears legs on which the genital pore opens. Fig. 369. A phalangid, Oligolophus spinosus, adult : Fig. 371. A, Demodex folliculorum, ventral view. After Blanchard. A mite living in the hair follicles of man and domestic animals. B, Linguatula taenioides. After Leuckart. Ventral view, at the stage when it is eaten by the second host. al.c. alimentary canal; an. anus; cl. claws; M. mouth. The commonest example, Linguatula taenioides, lives in the nasal passages of carnivorous mammals; the larvae, in which the claws of 542 THE INVERTEBRATA the adult are borne on prominences which may be called limbs, live in other mammals, chiefly herbivorous. The eggs are* passed out of the host, the larvae climb on to plants and are eaten by hares or rabbits ; they traverse the wall of the gut and encyst in other tissues, often the liver. After a period of growth they wander once more through the body ; they may at this stage be eaten by the second host and after wandering through the body reach the nasal passages. The larvae do resemble certain parasitic mites (Fig. 371 A) and for that reason the group has been classed with the arachnids. CHAPTER XVI THE PHYLUM MOLLUSCA Unsegmented coelomate animals with a head (usually well developed), a ventral muscular/oo/ and a dorsal visceral hump ; with soft skin, that part covering the visceral hump (the mantle) often secreting a shell which is largely calcareous, and produced into a free flap or flaps to enclose partially a mantle cavity into which open the anus and the mesoblastic kidneys (usually a single pair) ; a pair of ctenidia (organs composed of an axis with a row of leaf-like branches on each side, contained in the mantle cavity, originally used for breathing) ; having an alimentary canal usually with a buccal mass, radula and salivary glands, and always a stomach into which opens a digestive gland or -^na. ped.g. Fig. 372. Comparison between annelidan and molluscan organization. Side views of A, post-trochosphere larva of Annelida with segmenting trunk; B, veliger larva of Paludina (Mollusca) before torsion. After Naef. Ali- mentary canal shown by stippling, an. anus ; brn. brain or suprapharyngeal ganglion of annelid ; ce.g. cerebral ganglion of Mollusca ; F. foot ; M. mouth ; ma. mantle; ped.g. pedal, pl.g. pleural, pa.g. parietal, sbg. subpharyngeal, vis.g. visceral ganglia; vm. velum. kepatopancreas ; with a blood system consisting of a hearty a median ventricle and two lateral auricles, arterial system and venous system often expanding into a more or less extensive haemocoele, with haemo- cyanin as respiratory pigment ; a nervous system consisting of a cir- cumoesophageal ring, often concentrated into cerebral and pleural ganglia, pedal cords or ganglia and visceral loops; coelom, varying in development, but always represented by the pericardium, the cavity of the kidneys (which communicates with the pericardium), and the cavity of the gonads; often with larvae of the trochosphere type. While we do not know exactly what the ancestral molluscs looked like, we can make a very shrewd guess at their structure. They possessed the molluscan characters given in the definition above and 544 THE INVERTEBRATA they resembled the diagrammatic creature shown in side view in Fig. 349 A. They had a head with tentacles, a flat creeping foot, a conical visceral hump covered by a mantle which possibly contained numerous calcareous spicules and not a complete shell, and a posterior mantle cavity into which opened the median terminal anus and the common apertures of the kidneys and the gonads, and which also dig.gl. gxpe. pcd. ^„ M-c. 8h.p. ma.c ped.g. p.v.C. an. k.op. A vis.g. -pa.g. ped-ij. B pig. vis.g. pcd.g.-l — ped.g. Fig- 373- Types of Mollusca. Side view. Partly after Naef. A, Ancestral mollusc. B, Amphineura. C, Gasteropoda. D, Lamellibranchiata (Nucula, a primitive type). The head-foot is stippled to contrast with the visceral hump and mantle. The course of the alimentary canal is indicated by double dotted lines. In A the mantle cavity has its original posterior position, in C it has become anterior, while in B and D it has extended forward on both sides of the body, becoming very spacious in D. a.a. anterior adductor muscle; an. anus; au. auricle; ce.g. cerebral ganglion; ct. ctenidium; dig.gl. digestive gland ; F. foot ;g.coe. genital coelom ; k. kidney ; k.op. kidney opening; M. mouth; ma. mantle; ma.c. mantle cavity; op. operculum; p.a. posterior adductor muscle; pa.g., ped.g., pl.g. parietal, pedal, pleural ganglia; plm. palp-lamella ; p.pr. palp-proboscis ; pcd. pericardium ; p.v.c. pleurovisceral (palliovisceral in B) commissure ; sh.p. shell plates ; st. stomach ; ven. ventricle ; vis.g. visceral ganglia. contained the ctenidia. In the alimentary canal the fore gut formed a muscular body, the buccal mass, and a radula (p. 557) and the mid gut an oesophagus, stomach and digestive glands and intestine. The heart had a median ventricle and a pair of auricles. The perivisceral coelom reduced by the development of an extensive haemocoele (p. 556) is represented by the pericardium with which communicates in front MOLLUSCA 545 the cavity of the gonads and at the sides the two coelomoducts ("kidneys"). In the nervous system there were as in annehds and arthropods, a circumoesophageal commissure or brain which may or may not have been ganglionated, ventral pedal cords, a visceral commissure coming from the pleural part of the brain, and a pallial commissure in the mantle edge. From this beginning diverged the different groups which we know to-day. The chitons (Amphineura), which have departed least from the ancestral structure, became elon- gated but limpet-like forms (Fig. 373 B), their visceral hump being protected by eight shell plates, their mantle cavity extended all round the foot while instead of a single pair of ctenidia many such pairs arose. The Gasteropoda remained as short creeping forms (Fig. 373 C) ; they are characterized by the growth of the visceral hump dorsally, but unequally so that it has coiled in a spiral (which is covered by a single shell). This caused a readjustment of the visceral hump which has revolved (usually to the right) on the rest of the body through 180° (torsion) and the mantle cavity is now anterior. The Lamellibranchiata (Fig. 373 D) are flattened from side to side, the whole body being covered by two mantle lobes secreting two shell valves, united by a median hinge. The ctenidia inside the greatly enlarged mantle cavity have developed into huge organs of automatic food collection and so the head, rendered unnecessary and withdrawn into the mantle cavity, has become vestigial. Similarly the foot has lost its flat sole and has to be extended out between the valves to move the animal. In the Cephalopoda, though there is an unequal growth of the visceral hump relative to the rest of the body, as in gasteropods, it is coiled in a plane spiral, but there is no torsion, the mantle cavity re- maining posterior. The primitive forms in the group (Fig. 402 A) have an external shell which is divided into chambers, and those behind the body chamber contain gas. This has had a great effect on the develop- ment of the group, for by diminishing the specific gravity of the animals it has enabled them to become more or less free-swimming. They have tended, with the loss of the shell, to become more and more efficient swimmers, and this is associated with the development of their predatory habits. The anterior region shows a kind of trans- formation new to the molluscs in its partial modification into circum- oral prehensile tentacles for seizing food. Lastly, and in connection with all these changes, the brain and sense organs have become enormously developed and the cephalopods are seen to be one of the most progressive groups of invertebrates. Characteristically the ectodermal epithelium of the mantle secretes a shell in the Mollusca and in most of them the method of secretion is the same. The original shell is laid down by the mantle of the veliger larva (Fig. 374 B), but all extension takes place by secretion at 546 THE INVERTEBRATA its edge (Fig. 377). The outer shell layer, periostracum, formed of horny conchiolin, is first produced in a groove and then the prismatic ap.o. ?^-prt. nres. -^mes. Fig. 374. Patella coerulea. A, Trochosphere larva, sagittal section. B, Early veliger larva, sagittal section C, Veliger larva, frontal section to show meso- derm bands. After Patten, ap.o. apical organ; end. endoderm; F. foot; int. intestine; M. mouth; mes. mesoblast pole cell and derivatives; m. em- bryonic muscle cells; prt. prototroch; sh. shell; st. stomach; t.t. telotroch; D and V dorsal and ventral. / , vm. an.- vm. an. sh. Fig. 375. Veliger larvae. A, Ostrea edulis, side view. After Yonge. Ciliary currents shown by arrows. Suspended material is thrown by the action of the large cilia of the velum on to the ciliated tract, ct., imbedded in mucus and carried to the mouth, M., then through the oesophagus into the stomach, St. The style, shown by stippling, projects from the style sac, s.s., in which it rotates ; many particles are imbedded in this. After leaving the stomach the material passes through the coiled intestine (dotted) and by the rectum, rm., out into the mantle cavity, ma.c. Other letters: an. anus; a.m. adductor muscle; dig.gl. digestive gland; F. foot; sh. shell; vm. velum. B, Dreissensia, ventral view. After Meisenheimer. layer, largely consisting of calcite or arragonite, is secreted underneath it by the cells of the thickened edge. The innermost nacreous layer (also mostly CaCOg) is, however, formed by the cells of the whole of MOLLUSCA 547 the mantle, and under such conditions as occur in the formation of pearls this general epithelium is capable of secreting any of the three shell layers. In the Mollusca the development of the trochosphere takes place in a fashion identical with that described for the annelid. In the diagram given here for Patella, we see the completion of gastrulation and the appearance of the ciliated rings of the trochosphere (Fig. 374 A); also the single large cell which gives rise to the mesoderm. Then in Fig. 374 B we see the early veliger with an internal organization similar to the annelid, with apical organ, larval nephridia and proto- troch. The figure shows, however, organs which are not present in the annelid. On the dorsal side between the prototroch and the anus the larval ectodermal epithelium forms the rudiment of the mantle and even at this early age secretes the first shell. On the ventral side, there is a prominence which is the foot (formed by the union of two rudiments). The single mesoderm cell gives rise first of all to two regular mesoderm bands ; and by the development of a cavity in each of these, right and left coelomic sacs are formed ; then instead of seg- menting as in the annelid, these largely break up into single cells, some elongating and becoming muscle cells (Fig. 374 C). It is because there is never any commencement of segmentation in the embryonic mesoderm in molluscs that we have the strongest grounds for be- lieving that molluscs never had segmented ancestors. The trocho- sphere is followed by a second free-swimming stage, the veliger (Fig. 375), in which the prototroch develops into an organ, the velum, of increased importance, which serves not only for locomotion but also for feeding, the cilia creating a current which brings particles into the mouth. In the veliger stage the foot increases in size and the shell often becomes coiled in the Gasteropoda. Class AMPHINEURA Mollusca with an elongated, bilaterally symmetrical body, the mouth and anus at opposite ends; with a head, without tentacles or eyes, tucked under the mantle, which occupies the whole of the dorsal surface, and contains various kinds of calcareous spicules imbedded in cuticle, sometimes united to form continuous shells; a flattened foot sometimes reduced ; a nervous system (Fig. 398 A) without definite ganglia, the ganglion cells being evenly distributed along the length of the nerve cords, and composed of a circumoesophageal commissure and two pairs of longitudinal cords [pedal and palliovisceral), each pair united by a posterior commissure dorsal to the rectum ; a radula; usually a trochosphere larva. PoLYPLACOPHORA. Shore-living amphineura with flat foot which 548 THE INVERTEBRATA occupies the whole ventral face of the body ; mantle containing eight transverse calcareous plates as well as spicules ; in the mantle groove which runs entirely round the body there is a more or less complete prst,^ prsm. I nac. I /ex.ma. I -in.ma' prsm: -^prst: Fig. 376. Fig. 377. Fig. 376. Ventral view of Chiton to show external and internal bilateral sym- metry. Mantle cavity finely stippled, the divisions of the coelom, shown above the foot, coarsely stippled, an. anus ; ct. ctenidia ; e.k.a. external kidney aperture; i.k.a. internal kidney aperture; gen. gonad; g.a. genital aperture; k. kidney ; M. mouth ; pcm. pericardium. Fig. 377. Vertical section through the edge of the mantle of Mytilus. ex.ma. external, in.ma. internal surface of mantle;^/, gland cells ; nac. nacreous layer; prsm. prismatic layer of shell; prsm.' prisms arising at external border; prst. periostracum ; prst.' periostracum arising in a fold of the mantle ; ov. ova in the mantle tissue. After Field. row of ctenidium-like gills on each side, Chiton (Figs. 373 B, 376), Craspedochilus . Aplacophora. Worm-like Amphineura in which the foot is absent or represented by a median ridge in a ventral groove and the mantle correspondingly enlarged. No shell plates but spicules only. Mantle AMPHINEURA 549 cavity perhaps represented by a small cloacal chamber at the posterior end, gills present (Chaetoderma) or absent (Neomenia). Craspedochilus is a small mollusc found underneath stones between tidemarks. It looks like an elongated limpet and has exactly the same habits, browsing on small algae and returning after excursions to a centrally situated home. In dorsal view there are seen the eight shell plates which articulate with one another and allow the animal to roll up like a woodlouse. Each plate is composed of two layers, the upper or tegmentum and lower or articulamentum. Both are calcareous, but the tegmentum is traversed by parallel canals containing ectodermal tissue which end on the surface in remarkable sense organs ; some of these have the structure of eyes (the aesthetes). Young individuals, which possess a full equipment of aesthetes are negatively phototropic. As, however, the valves become corroded and covered with encrusting organisms they become indifferent to light. The part of the mantle which surrounds the shells is called the girdle and this contains the spicules which are characteristic of the Amphineura as a whole. On the ventral surface is seen the head, which does not project from under the shelter of the mantle. It bears no eyes and no tentacles, and is separated from the foot by a narrow groove. The mantle groove is shallow, running completely round the animal and containing a vary- ing number of branchial organs, each of which resembles a ctetiidium. There may be only six on each side crowded together at the posterior end, or they may occupy the whole groove from the head to the anus. It is probable that the forms with a small number of branchiae are the most primitive, and from the fact that the branchiae are graded in size it seems likely that one of them (the largest) is the original one and the others are derived from it. At any rate the repetition of the branchiae does not mean that the chitons were once metamerically segmented animals. There is no trace of any segmentation of the mesoblast in the larva and there is no correspondence between the numbers of the shell plates and of the branchiae. The mantle groove also contains the anus in the middle line posteriorly, on each side, the renal apertures just in front of it, and the genital apertures a little further forward. In this entire symmetry of the various apertures the chitons differ from any living gasteropods. The internal anatomy presents the features attributed above to the ancestral molluscs. Another feature which is probably primitive is the uniform distribution of nerve cells in the nerve cords and the consequent absence of ganglionic enlargements. The cords are con- nected by many commissures which form a nerve plexus (Fig. 398 A). A point of great interest is the palaeontological antiquity of the group, forms with eight shell valves occurring in the Ordovician. The Aplacophora are simplified forms, often worm-like in appear- 550 THE INVERTEBRATA ance. Besides the primitive character found in the chitons they have others, one of which is the free communication of the parts of the coelom. The gonads open into the pericardium and a pair of coelomo- ducts (probably corresponding to the kidneys) convey the gametes from the pericardium to the exterior. The radula varies greatly, all stages from absence to a type with several transverse rows of teeth being found. This condition may also be considered as primitive. Class GASTEROPODA Mollusca with a distinct head bearing tentacles and eyes, a flattened foot, and a visceral hump which exhibits the phenomenon of torsion in various degrees and is often coiled; always exhibiting bilateral asymmetry to a certain extent; typically with a shell secreted in a single piece; nervous system with cerebral, pleural, visceral and usually pedal ganglia and a visceral loop ; a radula ; often a trocho- sphere larva. We can safely say that the Gasteropoda are descended from sym- metrical unsegmented ancestors (p. 543), and that the most prominent differences among their present-day representatives are due to the varying degrees in which they exhibit the phenomena of torsion. The ancestors of the Gasteropoda had not been affected by torsion. They possessed a symmetrical body with a straight alimentary canal ending in a posterior anus. On each side of this was a ctenidium, that is, a breathing organ composed of an axis with a row of leaf-like branches on each side. The ctenidia may have been free on the surface when they first arose, but they were soon contained in the posterior mantle cavity which developed with the visceral hump. Many characters belonging to the primitive mollusc are still pre- served in the gasteropods, the head with tentacles, the nervous system with cerebral, pleural, and pedal ganglia, the radula, the ventricle with two auricles and the two kidneys. Lastly, there is a flat creeping foot and a visceral loop formed by a connective from each pleural ganglion uniting with its fellow in the neighbourhood of the ctenidia. In the alimentary canal of molluscs there is a tendency for digestion and resorption to be confined to a dorsal diverticulum of the alimen- tary canal which develops into the digestive gland (liver). The growth of this causes the formation of a projection, the visceral hump, and a looping of the alimentary canal. This projection grows until it falls over, and this is the first step in the coiling of the visceral hump which is such a characteristic feature of the gasteropods. Growth proceeds until, in the snail, for instance, the visceral hump would, if uncoiled, be longer than the whole of the body. Owing, however, to the fact that one side of the hump grows faster during development than the MOLLUSCA 551 Other, the whole organ is twisted into a compact spiral which can be arranged so as not to interfere with the balance of the animal while crawling. In all gasteropods with coiled shells the mantle cavity is anterior, the opening directed forward and the coiling of the visceral hump is directed posteriorly. But in the development of these forms from the larva (Fig. 378) the mantle cavity first makes its appearance behind the visceral hump, and at a particular stage the visceral hump rotates in a counter-clockwise direction through an angle of 180° on the rest of the body (Fig. 378 D). This is what is known as torsion, and as shown above it is entirely distinct from the coiling of the visceral hump which precedes it, though it may have been necessitated by the Fig. 378. To show torsion in Paludina vivipara. After Naef. Embryos seen from the side (A, B, D) and behind (C). A, Almost symmetrical stage, with mantle cavity behind but anus twisted a little to the right. B, Stage showing 90° of torsion, mantle cavity and anus to the right. C, Torsion at almost the same stage as B. D, Stage showing i8o° of torsion and the adult condition. an. anus; m.c. mantle cavity; op. operculum; vm. velum. antecedent phenomenon. Only the narrow neck of tissue (and the organs which pass through it), between the visceral hump and the rest of the body, is actually twisted ; but the orientation of the mantle cavity and its organs is changed (Fig. 379). Before torsion the ctenidia and the anus point backwards, the auricles are behind the ventricle. After torsion the ctenidia project forward, the auricles are in front of the ventricle; the mantle cavity opens just behind the head. The uncoiled visceral loop has been caught in the twisting and one connective laid over the other, one passing over the intestine and the other underneath, but both coming together near the anus and com- pleting a figure of eight. The whole process takes only two or three 552 THE INVERTEBRATA minutes in Acmaea so that it can hardly be brought about by differ- ential growth. Muscular contractions must play their part. The large majority of gasteropods belong to the order which ex- hibits torsion in full development. It is called Prosobranchiata, because of the anterior position of the gills, or Streptoneura, because of the coiled visceral loop. The periwinkles, whelks and limpets of our shores, the freshwater Paludina, and many others belong to it. The order may, however, be divided into two groups, a primitive one in which the two ctenidia and consequently the two auricles are pre- served {Diotocardia represented by Patella, Fissurella and Haliotis) (Fig. 380 A-C), and a more specialized one in which the right (primi- tive left) gill, its auricle and even the right kidney have disappeared M. max. Fig. 379. Diagram to illustrate torsion, when seen from above. A, Ancestral gasteropod. B, 90° of torsion. C, Torsion completed (180°). After Naef. an. anus; au. auricle; ce.g. cerebral ganglion; M. mouth; max. mantle cavity; pa.g. parietal, ped.g. pedal, pl.g. pleural, vis.g. visceral ganglia ; v. ventricle. {Monotocardia, represented by Littorina, the periwinkle, and Buc- cinumy the whelk) (Fig. 380 D). Some of the Diotocardia, like Trochus, are in an intermediate state in which, though the right gill has dis- appeared, there is still a rudiment of the corresponding auricle. Be- sides this fundamental difference, there are others. For example, in the Monotocardia, special generative ducts are developed (cp. also the penis of the male Buccinum), while in the Diotocardia, the generative organs open to the exterior through the right kidney. It is possible that the disappearance of the organs of one side is to be regarded as the consequence of the processes concerned in torsion and that in the Diotocardia the phenomenon cannot be regarded as having reached its climax. On the other hand, there is a large division MOLLUSCA 553 of gasteropods called the Opisthobranchiata which show that the changes occurring in torsion are to a certain extent reversible. They Fig. 380. Mantle cavities of streptoneurous gasteropods. A, Patella, ctenidia absent. B, Fissurella, ctenidia equal ; kidneys unequal, like those of Patella. C, Haliotis, right ctenidium smaller; ciliary currents shown by arrows, exhalant shown emerging from the three most recently formed holes in the shell. D, Biiccinum, male, with single set of pallial organs, an. anus ; au. auricle; ct. ctenidium; e. eye; exh.c. exhalant current; gon. gonad; inh.c. inhalant current ; k.l. left, and k.r. right, kidney ; M. mouth ; m.s. shell muscle ; mu.gl. mucous glands ; op.s. opening from mantle cavity through shell ; osp. osphradium ; op.g.k. opening from gonad into kidney ; p. penis ; pb. pro- boscis; p.c.m. pericardium; rhyn. rhynchocoele ; siph. siphon; ten. tentacles; vd. vas deferens ; ven. ventricle ; int. intestine ; Sop. male aperture ; rm. rectum. have the ctenidium pointing backwards, the auricle behind the ventricle and the visceral loop untwisted and symmetrical. There are 554 THE INVERTEBRATA some forms (Bullomorpha (Fig. 387 C, D)) included in the Opistho- branchiata which possess a complete coiled shell, but show only 90° of torsion, so that the anus and the ctenidium point laterally instead of anteriorly. The visceral loop also shows untwisting and the forms in this division are thus supposed to show partial reversion of torsion or detorsion. Forms like this pass into the typical opisthobranchs with complete detorsion, in which the shell is reduced or lost, the ctenidium directed posteriorly and the visceral loop is completely untwisted {Aplysia (Fig. 388 A)). The Opisthobranchiata, it is plainly seen, are derived from the Monotocardia amongst the Streptoneura, since they have only a single ctenidium, a single auricle and a single kidney. They have not attained to complete bilateral symmetry, be- cause the mantle cavity is still on the right side where yet present (tectibranchs), and the anus and genital aperture both open there. The disappearance of the shell and the consequent uncoiling of the visceral hump, if not the cause of detorsion, is a constant accompani- ment of the phenomenon. When it is complete, the mantle cavity and even the ctenidium may disappear and we arrive at the group known as the Nudibranchiata. In forms like Eolis (Fig. 388 C) their descent is shown by the fact that they possess a veliger larva with a coiled visceral hump which undergoes torsion (which reverses later). The adult shows evidence of streptoneurous ancestry in the presence of the anus at the right-hand side. In Doris (Fig. 388 B) the anus and renal aperture are median, but the genital aperture is still situated on the right side. The last division of the Gasteropoda is the Pulmonata, which is usually united with the Opisthobranchiata to form the group Euthy- neura. But "euthyneury " or symmetry of the nervous system (more particularly the "visceral" part of it) is arrived at in different ways in the two divisions. In the Opisthobranchiata, as shown above, it is by detorsion. In the Pulmonata, however, the shell is retained and the visceral hump coiled in typical members of the group (land snails). But the visceral loop is shortened and untwisted at the same time (Fig. 387 A, B), and finally it is incorporated with its ganglia into the circumoesophageal nerve collar, so that the nervous system becomes symmetrical. The most primitive members of the Pulmonata still show a twisted visceral loop which is beginning to shorten. All the group have lost the ctenidium but they retain the single auricle which shows them to be derived from the Monotocardia. This was brought about by a chain of circumstances involving migration from sea to shore. The type of the Gasteropoda which is usually given for dissection is Helix (either H. aspersa, the common English garden snail, or H. pomatia, the edible snail). It possesses many features which are common to the whole of the Gasteropoda, but as has been seen above, GASTEROPODA 555 the order Pulmonata to which Helix belongs is the most speciaHzed and probably the latest developed division. Helix is a terrestrial animal breathing by a kind of lung, while the majority of gasteropods are marine animals breathing by gills, and besides the complications which this involves, the reproductive system is hermaphrodite with the most elaborate provision of glands and ducts which serve to produce eggs well stored with nourishment and are arranged so as to assure cross-fertilization. In the account of Helix which follows an attempt is made to distinguish clearly between the purely gastero- pod features and the adaptive features which belong to the Pulmonata. The body of a snail is composed of three regions, the head, foot and visceral hump. The visceral hump is all that part which is covered by the shell when the animal is expanded, while the head and the foot make up the remainder outside the shell. There is no boundary be- tween the latter two regions. The German zoologists refer to the whole as the " Kopffuss " (the " head foot "), and this can be retracted as a whole within the shell by the action of the columella muscle (Fig. 381). The foot is particularly characteristic of the Gasteropoda. It possesses a flat ventral surface underlain by longitudinal muscle fibres. If a snail is observed crawling up a pane of glass, a series of rippling waves of contraction of very small amplitude are seen to pass regularly over the surface of the foot. They are co-ordinated by the action of a nervous network, such as occurs in the lower invertebrates (Fig. 1 10). The gliding movement of a snail indeed resembles that of a turbel- larian, and we actually find that in some marine gasteropods the surface of the foot is clothed with cilia, which beat in unison, though they are perhaps capable of inhibition by the central nervous system. In most water snails, however, the foot moves by muscular contrac- tion. To fit this kind of movement for passing over a hard dry surface, there is in the snail a copious secretion of slime from a pedal (mucous) gland which runs dorsal to the foot and opens just ventral to the mouth. As soon as the slime emerges it is spread out as a smooth bed of lubricating fluid along which the snail moves. There are two pairs of tentacles on the head of the snail. The first are shorter and are supposed to be the seat of the sense of smell ; the second bear a pair of simple eyes (Fig. 409 B) at their tip. Both are hollow and have attached to the inside of the tip a muscle whose con- traction turns them outside in. The mouth is a transverse slit just ventral to the first pair of tentacles. On the right side of the body not far below and behind the second pair of tentacles is the reproductive aperture. On removing the shell, the junction of the visceral hump with the rest of the body is seen anteriorly as a thickened collar which is the edge of the mantle and the seat of secretion of the principal layers of the shell. It is fused to the head of the snail except for a 556 THE INVERTEBRATA round hole on the right side which is the aperture of the mantle cavity or pneumostome. In the marine gasteropods the mantle cavity has a wide opening to the exterior, though a part of the mantle border (siphon) is modified to form a special channel by which fresh water for breathing may be drawn in by the action of the cilia clothing the gill. But in the air-breathing pulmonates where the cavity is converted into a lung, the injury of delicate respiratory tissues by evaporation must be avoided, and a pumping mechanism for renewal of air established. The restriction of the respiratory aperture is one of the necessary modifications. If a section is drawn across the lung of a snail it will be seen that the mantle forms the roof of the cavity and is covered with ridges in which run pulmonary veins converging to- wards the auricle. The floor of the cavity is arched and has a layer of muscles, which contract rhythmically. When they contract, the arch flattens and air is drawn in and at the limit of contraction a valve slides across the pneumostome. When the muscles relax, the cavity de- creases in size and exchange of gases with the blood in the roof vessels is facilitated by the increase of pressure of the contained air. Then the pneumostome opens and air is expelled ; the subsequent contraction of the floor muscles brings in a fresh supply. This "breathing " is not so regular or so frequent as in a vertebrate ; moreover, it may cease altogether in the winter when the snail hibernates. In dissection, a cut is made underneath the collar and another under the rectum and the roof of the mantle cavity turned back so as to show the pericardium enclosing the ventricle and single auricle, and the kidney, which is a yellow organ consisting of a number of folds covered by cells containing uric acid. The ureter is a thin-walled tube which runs along the right border of the mantle cavity parallel to the rectum and opens just behind the pneumo- stome and above the anus. Here again is a diff'erence from the marine gasteropods in which the anus and kidney aperture discharge inside the mantle cavity, faeces and urine being swept away by the respira- tory current. The pericardium and the kidney represent the coelom in the snail and, as is usual in Mollusca, their common derivation is shown by the connection of the cavities by the renopericardial canal. The coelom, though thus represented, does not constitute the peri- visceral cavity. On cutting the floor of the mantle cavity and con- tinuing the cut forward towards the mouth a large body cavity is revealed which contains the anterior part of the alimentary canal and the greater part of the reproductive organs. This is a haemocoele almost as well developed as that of arthropods. Its connection with the rest of the blood system and the general course of the circulation may be briefly described here as follows : the ventricle pumps arterial blood through a single aorta which soon divides into an anterior aorta GASTEROPODA 557 supplying the buccal cavity and a posterior which supplies the visceral hump. The terminal branches of these arteries eventually communi- cate with the general haemocoele (stippled in Fig. 381) and this discharges into the cir cuius venosus leading to the lung and heart. The alimentary canal (Fig. 382) commences with the buccal mass. On the roof of the mouth is a small transverse bar, the jaw, and in conjunction with this works the radula, which is a strip of horny base- ment membrane on which are fastened many rows of minute recurved ped.art haem. Fig. 381. Helix pomatia. Diagram of the circulation and haemocoelic spaces. The pulmonary veins, ventricle and arteries are shown in black; the veins and haemocoelic spaces are indicated by stippling. Only a few of the arteries are shown and a small portion of the arterial capillary network in the posterior part of the foot. The course of the columella muscle and its branches is indicated. The direction of the blood flow is shown by arrows, aff.v. afferent veins; ao. aorta; art.cap. arterial capillaries; au. auricle; buc. buccal mass; col. columella, col.m. columella muscle; cr. crop; c.v. circulus venosus; haem. haemocoelic spaces; k. kidney; n.col. nerve collar; ped.art. pedal artery; pul.v. pulmonary veins ; ten. tentacles ; ven. ventricle. teeth. It is formed in a ventral diverticulum of the buccal cavity called the radula sac (Fig. 383) in which proliferating tissue is constantly producing transverse rows of cells called odontoblasts, each of which helps to form a tooth, and other cells which secrete the basement membrane. The whole radula is pressed forward by the new growth so that fresh surfaces are constantly coming into use as the old part is worn away. The radula is supported by masses of tissue, resembling cartilage, which also serves for the attachment of muscles, and the whole forms the rounded organ which is the buccal mass. 558 THE INVERTEBRATA The buccal cavity is succeeded by the oe^phagiis, which widens out into the crop, which in Hfe contains a brown hquid secreted by the 'Miver". On the side of the crop are the branching white salivary glands, which empty their secretion by two ducts running forward into the buccal cavity. The secretion is partly mucus, partly digestive fluid containing an enzyme acting on starch. The crop is succeeded :ped.(i. muc.gh Fig. 382. Helix pomatia. A, Section of alveolus of the digestive gland. ab.c. absorptive cells; cal.c. calcareous cells; cil.c. ciliated cells of liver tube; f.c. ferment cells. After Meisenheimer. B, Diagrammatic side view of animal dissected to show the alimentary canal and nervous system. Original. An. anus; ao. aorta; au. auricle; buc. buccal mass; biic.g. buccal ganglion; ce.g. cerebral ganglion ; cr. crop ; dig.gl. digestive gland ; d.d. openings of digestive ducts (the ducts represented by black lines); F. foot; h.gl. hermaphrodite gland; int. intestine; k. kidney; muc.gl. mucous gland; n.n. nerve net in surface of foot; oc.ten. oculiferous tentacle; ot. otocyst; pa.g., ped.g., pl.g. parietal, pedal and pleural ganglia; pal.n. pallial nerve; rad.s. radula sac; sal.d. salivary duct ; sal.gl. salivary gland ; spt. spermatheca (duct broken off short) ; st. stomach ; va. valves directing food into digestive ducts ; ven. ventricle; vis.g., vis.n. visceral ganglia and nerve. by the stomach; this is imbedded in the digestive gland (liver), which occupies most of the visceral hump. The ** liver", though apparently solid, is composed of a number of tubes and the end portion {alveolus) of each tube is glandular; the rest is ciliated and serves to introduce small fragments of food into the active alveolus. The alveoli contain cells of three kinds, secretory, resorptive and lime-containing (Fig. GASTEROPODA 559 382 A). The secretory cells produce the brown fluid found in the crop ; this contains a ferment which dissolves the cellulose of plant cell walls and liberates the protoplasmic contents, no portion of which is di- gested in the crop or stomach. But these contents in the form of small granules are actually introduced into the alveoli of the liver and there taken up and digested by the resorptive cells which possess intra- odp.'^- Fig. 383. Vertical longitudinal section through head of iiTe/iA;. After Meisen- heimer. cart.r. cartilaginous support of radula; ce.g. cerebral ganglion; j. jaw ; M. mouth ; m.r. muscles of radula ; odp, odontophore (radula sac) ; oe. oesophagus ; p.g. pedal ganglion ; rad. radula ; v.g. visceral ganglion. odh. Fig. 384. Vertical longitudinal section through the radula sac of Helix pomatia. After Meisenheimer. odb. four rows of odontoblasts secreting a tooth, to. ; a. the most anterior row of odontoblasts which, together with the basal epithelium, i.ep., of the radula sac, secrete the basal membrane, b?n., to which the teeth, to., are attached. As the odontoblasts complete the secretion of a tooth they are succeeded by fresh cells from the epithelium of the radula sac, s.ep., pressing fonvard in the direction of the arrow and themselves reinforce the basal epithelium. cellular proteolytic enzymes.^ A combination of extra- and intra- cellular digestion is highly characteristic of Mollusca, but in the possession of a cellulose-dissolving ferment Helix stands almost alone ^ In carnivorous gasteropods digestion follows a different course. The glands of the alimentary canal secrete proteolytic enzymes and the digestion of protein takes place in the stomach and not in the cells of the digestive gland (see Murex, p. 566). 560 THE INVERTEBRATA in the Animal Kingdom, and may be indeed said to be physiologically adapted to a plant diet (cp. Teredo^ p. 587). The intestine runs from the stomach, within the liver, and then as the rectum in the roof of the mantle cavity. The reproductive organs are extremely complicated (Fig. 385 A), but a function has been assigned to each part of what appears to the elementary student as an unmeaning tangle of tubes. Eggs and sperm are produced in the same follicle of the ovotestis, a small white gland o.d. gph. sptd. Fig. 385. Helix pomatia. A, Reproductive organs. B, Section through the copulatory organs of two mating snails at the moment of the transference of the spermatophores. After Meisenheimer. The organs of the two individuals are indicated by shading sloping in different directions, al.gl. albumen gland ; d.s. dart sac; fl. flagellum; m.gl. mucous glands; o.d. oviduct; p. penis; rec.sem. receptaculum seminis; ret. p. retractor muscle of the penis; sph. spermatophore ; spt. spermatheca ; spt.d. spermathecal duct ; sp.d. sperm duct ; o gl. hermaphrodite gland (ovotestis), and ^ d. duct. in the apex of the visceral hump. But while ripe sperm is found throughout a large part of the year, mature eggs only occur for a very short space indeed. Both eggs and sperm pass from the ovotestis to the albumen gland through the hermaphrodite duct, the terminal portion of which is a pouch [receptaculum seminis) where sperm is stored and fertilization is said to occur. After fertilization, the eggs enveloped in albumen from the gland enter the rather voluminous female duct, which runs almost straight to the exterior. They then GASTEROPODA 561 receive a calcareous shell secreted by the epithelium of the duct. The terminal portion of the duct is the thick-walled muscular vagina, into which open the mucous glands , the dart sac and the spermathecal duct. The sperm, on the other hand, passes down a male duct which is at first only partly separate from the female duct, the cavity of both ducts being in communication until the male duct leaves the company of the female duct altogether, slips under a muscle, and joins the penis at its junction with the slender flagellum. In this latter the sperma- tozoa are compacted together and enclosed in its secretion to form spermatophores . The penis is muscular and has a special retractor penis muscle also attached to it. Both vagina and penis open into a common genital atrium, with an opening to the exterior far forward on the right side. Cross-fertilization is the rule in nearly all species of Helix but cases of self-fertilization have been known. Usually, however, there is re- ciprocal fertilization, preceded by a remarkable preparatory event in which two snails approach each other and evert the genital atrium so that the male and female apertures appear externally. The dart sac mentioned above contains a calcareous sculptured weapon, the dart, which can be secreted anew very quickly by the epithelium of the sac. This is propelled by the muscles of the sac out of the female aperture when the other snail is almost in contact — in fact the two darts are launched almost simultaneously, with such force that they pierce the body wall, traverse the cavity and are found imbedded in various internal organs. Some time after this drastic stimulation, the two snails approach each other again and reciprocal fertilization takes place, the penis of each individual being inserted in the vagina of the other (Fig. 385 B). The following account of further events has been given and shows, as in the earthworm, the remarkable complexity of the arrangements which are made to prevent self-fertilization in such common hermaphrodites. The foreign spermatophores find their way up the spermathecal duct to the terminal spermatheca, where the chitinous covering of the spermatophore is dissolved, and the sper- matozoa set free. These now retrace their path to the junction with the female duct and then move up that duct to the fertilization pouch. Ferti- lization takes place in May or June but the eggs are not laid till July. It is said that the foreign sperm remains in the pouch during this time, and that immediately before ovulation the sperm produced by the individual itself degenerates within the hermaphrodite duct so that the eggs pass down the duct without any danger of being self- fertilized and meet the foreign sperm at the end. After fertilization, the egg cell passes down the oviduct where it is enveloped with such quantities of albumen that the diameter of the albumen envelope is 20-30 times that of the egg cell itself. In the outer layer of albumen 562 THE INVERTEBRATA a skin appears, and in this crystals of calcium salts are laid down which aggregate to form a definite shell. The eggs are laid in July and August in small holes in the earth and hatch after about twenty-five days of development. In the autumn the snail loses its appetite and hides, often in company with large numbers of its fellows, under leaves, making a small hole in the ground with its foot and shell in which it lies with the aperture upwards. The head and foot are withdrawn into the shell cent. Wflr.v>, lat. aamaa cent. Fig. 386. Radula of various types. A, Docoglossate (Patella). Stout teeth used for rasping encrusting layer of algae off rocks: radula of relatively enormous length; the teeth are quickly worn away. B, Rhipidoglossate {Haliotis). Lateral and central teeth as in Patella, used in browsing on algae growing on stones. The marginals, of which only about half are shown, are probably used as a sieve to prevent fragments of food of too great size entering the oesophagus. C, Rachiglossate (Buccinum). Teeth of carnivorous type, with sharp cusps. D, Toxiglossate (Conus). Specialization of carnivorous type, in which only two teeth (laterals) remain in each row, are hollow, and are used as poisoned daggers, carrying the secretion of the salivary glands. cent, central, lat. lateral, mg. marginal teeth. and the edges of the mantle approximate to form an almost complete disc filling up the aperture, leaving only a small hole for breathing. They secrete a membrane (epiphragma) mostly composed of Ca3(P04)2 . Several such membranes may be found behind each other. In this winter sleep the snail remains for about six months ; respiratory move- ments are carried on slowly and the heart beats sink from about 10-13 to 4-6 per minute. The rate of heart beat is closely dependent on the temperature, and at a temperature of 30° C. is from 50 to 60 beats per minute. GASTEROPODA 563 Order STREPTONEURA (PROSOBRANCHIATA) Gasteropoda which exhibit torsion, nearly always with a shell and an operculum, with a visceral loop twisted in the form of a figure of eight, the mantle cavity opening anteriorly, the ctenidia in front of the heart, and separate sexes. Classification Suborder Diotocardia (Aspidobranchiata). Strep toneura always with two auricles and sometimes two ctenidia, the ctenidia with two rows of leaflets (aspidobranch), and the genital products discharging to the exterior by means of the right kidney. These are divided into two main tribes according to the characters of the radula : Rhipidoglossa possessing a radula composed of rows of numerous narrow teeth diverging like the ribs of a fan. Haliotis^ Fissurella. DocoGLOSSA possessing a radula with rows consisting each of a few strong teeth, very long and used for browsing on the algal covering of stones. Patella^ Acmaea. Suborder Monotocardia (Pectinibranchiata). Streptoneura with a single auricle and ctenidium, the ctenidium always with one row of leaflets (pectinibranch), with a single osphradium resembling an aspidobranch gill, the gonads with separate ducts opening far forward in the mantle cavity and in the male forming a penis. These are divided into four tribes, each with a distinct type of radula, of which three are mentioned below: Rachiglossa: predatory animals; radula with not more than three teeth in a row; always with a siphon. Buccinurrij the whelk. Purpura feeds largely on barnacles. Nassa. Taenioglossa : radula normally with seven teeth in each row. Natica feeds on shell fish. Littorina, the periwinkle, amphibious. S trombus progresses by leaping. Paludiha and Ampullaria, fresh water. - This tribe also includes a pelagic section, the Heteropoda (Pterotrachea). The rest are called the Platypoda. Toxiglossa: radula with two elongated teeth in each row; a poison gland. Conus (Fig. 386 D). 564 THE INVERTEBRATA Suborder DIOTOCARDIA Haliotis, the ormer (Figs. 380 C, 390 A), is a greatly flattened gastero- pod which lives between tidemarks, as far north as the Channel Islands, browsing on seaweed and eating all kinds of dead organic material. It can move with considerable speed (5-6 yards a minute), Fig- 387. To illustrate origin of euthyneury in the Pulmonata, A, B, and the Opisthobranchiata, C, D. After Naef. A, Chilina. The left parietal ganglion (Lpa.g.) has moved forward owing to the shortening of its plural connective. B, A pulmonate belonging to the Basommatophora. The corresponding con- nective on the other side has shortened also, the visceral loop has become untwisted and the nerve ganglia are concentrating. C, Actaeon, with short spire and broad shell mouth, ctenidium and anus pointing to the right. D, Bulla, showing slightly greater detorsion without spire, the shell mouth opening to the right and anus pointing posteriorly: left parietal ganglion drawn over right connective so that visceral loop is untwisted, an. anus; au. auricle; ct. ctenidium; Lpa.g. left, r.pa.g. right parietal ganglion; ma.c. mantle cavity; v. ventricle. but adheres very firmly to stones. The mantle cavity is very spacious and contains two ctenidia, the left being rather the larger, each with two rows of filaments. The mantle has a slit which runs in the roof of the mantle cavity, its position being shown by a row of holes in the shell which serve for the escape of the exhalant current. The anus opens GASTEROPODA 565 at the posterior end of the mantle cavity and the two kidneys on each side of the anus. There is a well-marked visceral loop and the pedal nerve centres have the form of long cords in which ganglion cells are evenly distributed. The gonad has no ducts but the genital cells are discharged into the right kidney. The radula has numerous marginal teeth arranged in a fan-like manner (rhipidoglossate type). Fissurella, the keyhole limpet (Fig. 380 B), is so-called because of the hole which perforates the mantle and the apex of the shell. It possesses two equal ctenidia. The visceral hump and shell are com- pletely uncoiled, but in other respects it resembles Haliotis and possesses the same type of radula. Patella, the limpet (Fig. 380 A), represents a type of complete adaptation to life on an exposed coast between tidemarks. Its conical shell only shows coiling in its early stages and offers the minimum of resistance to the waves. As in the above forms there is no operculum, but the mollusc cannot be detached from rocks without using great force, owing to the enormous power of the pallial muscles which press the shell against the rock. The mantle cavity is restricted anteriorly and the ctenidia have disappeared, though the osphradia connected with them are present as minute yellow specks. But a secondary mantle cavity extends all round between the foot and the mantle and contains a series of folds which are known as pallial gills . In the re- lated Acmaeidae there are various stages of the loss of the ctenidia and their replacement by pallial gills. The enormously elongated radula is composed of very strong teeth and there are a small number of marginals (docoglossate type). This type of radula is suited for the feeding habits of the limpet, which scrapes the crust of minute algae off the surface of rocks. Limpets have a remarkable "homing " sense, returning after excursions for food to the same spot, which may be marked by a depression in the rock. Suborder MONOTOCARDIA Biiccinum, the whelk (Fig. 380 D), lives between low-water mark and 100 fathoms. It is active and carnivorous, feeding on living and dead animals, which it grasps by means of its foot. It has a remarkable and highly developed proboscis which can be retracted within a proboscis sheath. The true mouth is situated at the end of the pro- boscis. The radula (of the rachiglossate type) is used for rasping away flesh, but it can even bore holes in the carapace of Crustacea. There is only a single ctenidium with a single row of filaments. This is the primitive right member of the pair, though situated on the left of the mantle cavity. A very prominent organ is the bipectin- ate osphradium, which is easily mistaken for a ctenidium. There is a 566 THE INVERTEBRATA single kidney which is not used for the passage of the genital products. The gonads have separate ducts and in the male there is a penis. The eggs are laid in capsules which usually contain several hundred and the capsules are attached to each other, forming the sponge-like masses so often flung up by the tide. Murex is nearly related to Buccinum and also carnivorous. It has been recently shown that the salivary glands and the "liver" all contain the same proteolytic enzymes. These have been separated by adsorption and found to comprise a proteinase, a carboxy-poly- peptidase, an aminopolypeptidase and a dipeptidase. These are just such enzymes as occur in the vertebrates and the higher Crustacea, but in contrast to vertebrates in Murex there is no division of labour amongst the digestive organs. Fig. 388. Pterotrachea. co.g. cerebral ganglion; cr. crop; ct. ctenidium; e. eye; /. foot (fin); ped.g. pedal ganglion; su. sucker; t. teeth which prevent large particles of food from passing before digestion ; vis.g. visceral ganglion ; vis.h. visceral hump. LittorinUy the periwinkle, is interesting because it exhibits tend- encies toward a terrestrial habit which is reflected in its structure. In certain species the filaments of the ctenidium are extended over the roof of the mantle cavity to form a kind of vascular network not un- like that in Helix and other pulmonates. Littorina rudis lives almost at highwater mark and spends more of its life in air than in water. The structure of this form is very similar to Buccinum but it has no proboscis and is not carnivorous. Paludina, on the other hand, is a freshwater form of common occurrence in this country which still preserves the ctenidium and so must be regarded as a direct immigrant from sea water into fresh water. It possesses a kind of uterus in which embryos of relatively enormous size are developed. Pterotrachea (Heteropoda) (Fig. 388) is an inhabitant of the open sea with many adaptations to pelagic life. It is laterally compressed ; the tissues are transparent except for the digestive gland and peri- GASTEROPODA 567 cardium compressed into a small visceral hump. The animal swims ventral surface uppermost, using its foot as a fin. The sucker is a rudiment of the crawling surface. It is predaceous, seizing worms and other animals with its radula and swallowing them whole. Order OPISTHOBRANCHI ATA Hermaphrodite gasteropods which are descended from Streptoneura which have undergone torsion but themselves show a reversal of torsion (detorsion); with the mantle cavity, where present, tending to occupy a posterior position again, the shell to become smaller, in- ternal or entirely absent and the single ctenidium to disappear and be replaced by accessory respiratory organs or by the whole external surface becoming a respiratory organ. The opisthobranchs are classified as follows: Tectibranchiata. Opisthobranchiata which often have a shell and nearly always a mantle cavity and ctenidium. Actaeon, Bullae Aplysia, Cavolinia. NuDiBRANCHiATA. Opisthobranchiata usually of slug-like habit which have neither a shell, nor a mantle cavity, nor a ctenidium. EoliSy Doris. Aplysia (Fig. 389 A), the sea hare, is found crawling on seaweeds which form its food. The younger forms occur in rather deeper water and are red in colour, matching the red algae on which they occur, while the larger individuals, between tidemarks, devour green sea- weeds such as Ulva and are olive-green. The head possesses two pairs of tentacles, the anterior being large and ear-like (hence the animal's name), while those of the second pair are olfactory in function and have each a simple eye at their base. From the sides of the foot in the posterior region rise two upwardly directed flaps, the parapodia : by using these the animal can swim. The mantle is reflected over the shell so as to cover all except a small area and the mantle cavity lies to the right of this with the ctenidium pointing backwards, while the anus is at the posterior end. In the walls of the mantle cavity are unicellular glands which secrete the purple pigment ejected by the animal when it is molested. There is a single generative aperture and a single duct for the sperm and ova but a seminal groove runs forward from the aperture to the head and reciprocal fertilization is impossible. The only internal characters which need be mentioned are the nervous system, with its well-developed but perfectly symmetrical visceral loop, and the alimentary canal which, in front of the stomach, is dilated into a crop, lined with horny plates, in which the seaweed is masticated before digestion. ^^-pten. cer.-- Fig- 389. Opisthobranchiate molluscs, in dorsal view. A, Aplysia, with parapodia {par.) turned to the side to show the mantle cavity; nervous system and buccal mass indicated by dotted lines. B, Doris, with position of heart indicated beneath the mantle. C, Eolis. After Alder and Hancock. C, One of the cerata of Eolis, shown in section, c.c. ciliated canal communicating with hcp.c. the hepatic caecum, a diverticulum of the intestine; c.s. cnidosac, opening to the exterior and containing numerous nematocysts ingested in its cells. D, Cavolinia with alimentary canal (a/.) seen through the transparent tissues and the direction of the ciliated currents on the epipodia indicated by arrows. After Yonge. Other letters: a/, alimentary canal; an. anus; an. auricle ; ce.g. cerebral ganglion ; cer. cerata ; ct. ctenidium ; epi. epipodia ; 2 op. genital aperture; op.s. opening of shell sac; p. penis; pa.g. parietal, pl.g. pleural, ped.g. pedal ganglia; par. parapodia; sem.gr. seminal groove; ten. tentacles; vis.g. visceral ganglion; M. mouth; ven. ventricle. GASTEROPODA 569 Cavolinia (Fig. 384 D) is an example of the Pteropoda (sea butter- flies), a special group of the Opisthobranchiata which are modified for pelagic life. They usually have a transparent uncoiled shell in the form of a quiver or a vase, from the aperture of which projects the foot in the form of two fins, the epipodia. By the slow flapping move- ment of these the pteropods progress through the water. There are ciliated tracts on the fins, and by the action of the cilia on these, small organisms are sifted from the water and collected in the mouth, the radula assisting in swallowing. Limacina is a pteropod with a coiled shell. Eolis (Fig. 384 C) is a nudibranch which possesses a series of dorsal processes (the cerata)^ which contain diverticula of the digestive gland, each of w'hich opens to the exterior at the tip of the process. The animal feeds on hydroids or sea anemones, and while most of the food is digested or passes out of the anus, the nematocysts are collected in terminal sacs in the cerata and when the animal is irritated they are ejected and everted. This is a unique example of the use in defence by one animal of the oflPensive weapons of another. The cerata are often brilliantly coloured and experiments with fish show that sea slugs are avoided on account of their "warning" patterns. Hermaea is another nudibranch with similar cerata, which have not, however, openings to the exterior. The animal feeds on green algae (Siphonales). The radula, in each row of which there is only a single sharp tooth, forms a saw by which the cell wall of the alga is opened. Then by dilatation of the buccal cavity the fluid protoplasm is sucked out. Doris (Fig. 384 B), the sea lemon, a short flattened nudibranch, sluggish in movement, which feeds on incrusting organisms like sponges. There is a tough mantle, which is usually pigmented and often resembles the feeding ground, and is reinforced by calcareous spicules. Anteriorly there is a single pair of short tentacles and posteriorly a median anus surrounded by a tuft of accessory gills. In front of the anus is the median kidney aperture. The nervous system is centralized round the oesophagus, and the generative aperture occurring on the right side is the only external organ which is asymmetrical. Order PULMONATA Hermaphrodite gasteropods, most of which exhibit torsion and have a shell (but no operculum), but which have a symmetrical nervous system, the symmetry being due to the shortening of the visceral connectives and the concentration of the ganglia in the circum- oesophageal mass; with a mantle cavity which has become a lung, without a ctenidium, but with a vascular roof and a small aperture 570 THE INVERTEBRATA (pneumostome) ; with a single kidney; without a larva, development being direct from an egg richly supplied with albumen. The Pulmonata are thus classified : Basommatophora. Pulmonata with eyes at the base of the posterior tentacles. Limnaea, Planorbis. Stylommatophora. Pulmonata with eyes at the tip of the posterior tentacles. Helix, Arton, Testacella. A few members of the Basommatophora are marine but these are all shore forms and breathe air. The group, like the Opisthobranchiata, must have been derived from the Streptoneura Monotocardia, as they possess a single kidney. While they are usually united with the Opisthobranchiata to form the Euthyneura, which includes all forms in which the visceral loop is untwisted, there is no real justification for the establishment of the group, for the " euthyneurous " condition is one which has been arrived at in two different ways, by detorsion in the Opisthobranchiata and by shortening of the visceral com- missures in the Pulmonata. The important characters of the Pul- monata are those associated with the assumption of the terrestrial habit, namely the existence of the lung and the physiological cha- racters correlated therewith. So strongly impressed are these that in almost all the forms which have secondarily returned to water (to fresh water as a rule), the lung continues to function as such and never contains water. Limnaea^ for example, may be observed in an aquarium to approach the surface of the water at frequent intervals, expel a bubble of air from the lung and protrude the pneumostome through the surface film for a fresh supply. There are, however, a few species {Limnaea abyssalis) which live at great depths in lakes, and here the mantle cavity is full of water. The other general characters of a pulmonate have been given at the beginning of the chapter in the description of Helix. They include the concentrated nervous system (it will be seen in Fig. 390 B that the visceral loop of Limnaea is not so much shortened as that of Helix ; in other respects also it is a more primitive form), the complicated re- productive system, with its adaptations for cross-fertilization, and the digestive tract, specialized for the consumption of vegetable food. Helix, as has been seen, is thoroughly adapted for this purpose, but in the case of some of the slugs there is an exception to the general rule in the development of the carnivorous habit. This culminates in such a form as the predaceous Testacella, which pursues earthworms underground and seizes them with the aid of the strong recurved teeth of the radula which can be thrust out of the mouth, the everted buccal cavity forming a huge proboscis. When the worm is swal- ce.g. vis.g- ped.g. ce.g os.g. Fig. 390. Comparison of gasteropod nervous systems. From Shipley and MacBride. A, Haliotis tuberculata. B, Limnaeaperegra. ce.^. cerebral ganglia ; ct. ctenidium; ma.n. nerve to mantle; os.g. osphradial ganglia; pa.g., ped.g., pl.g. parietal, pedal and pleural ganglia ; ped.n. non-ganglionated pedal nerves of Haliotis connected by commissures ; vis.g. visceral ganglia. 572 THE INVERTEBRATA lowed it is digested in a large crop by the action of the juices of the digestive gland. The reduction of the shell is shown in the slugs, some of which, like Testacella, have a small cap-like shell, which cannot possibly con- tain the visceral hump, while others have an internal horny disc like the shell of Aplysia and still others none at all. The mantle cavity of slugs opens by a pneumostome but there are no respiratory move- ments as in Helix. In other respects the organization of the slugs is very similar to that of snails. The details of reproduction and development are uniform through- out the group, but in some snails like Bulimus, the amount of albumen added as food for the developing embryo is so great that the egg is the size of a bantam's tgg. Class SCAPHOPODA Bilaterally symmetrical Mollusca with a tubular shell open at both ends, a reduced foot used for burrowing, a head with many pre- hensile processes, a radula, separate cerebral and pleural ganglia; ctenidia absent and circulatory system rudimentary; and a trocho- sphere larva. This is a small group of molluscs which in some ways stands be- tween the Gasteropoda and the Lamellibranchiata. They are greatly specialized for burrowing. Thus the shell is tubular and perforated at the apex. The foot emerges from the wider opening, while the apex remains above the surface of the sand when the animal is burrowing, and serves alike for the entrance of water into and its exit from the mantle cavity. The head is proboscis-like in form and has none of the usual sense organs, but in Dentalium (Fig. 391), the one common genus, there are extensible filaments, the captacula, with sucker-like ends, which arise from the dorsal side of the head and serve partly as sense organs and partly for seizing the food. The foot is conical and can be protruded for use as a digging organ. There is a well-developed radula, a mantle, which in the larva is produced into two lobes (which fuse later), a nervous system with separate cerebral and pleural ganglia and a symmetrical visceral loop. The kidneys are paired; they do not have an opening into the peri- visceral coelom. These characters, with the exception of the first and last, bring the Scaphopoda near to the primitive lamellibranch. In the two following morphological features the group is so specialized that it stands apart from any other division of the Mollusca. There are no ctenidia, respiration taking place by means of the m?ntle. The circulatory system is remarkably simplified and there is no distinct heart. MOLLUSCA 573 The gonad discharges into the right kidney as in the Diotocardia among Gasteropoda. Fig. 391. Diagram of the structure of Z)ew^a/m?«. Ahered from Naef. Head and foot stippled, an. anus; buc. buccal mass, with radula; ce.g. cerebral ganglion; eta. captacula; dig.gl. digestive gland; F. foot; gon. gonad, com- municating with the cavity of the left kidney {k.) ; M. mouth ; yyia. mantle ; max. mantle cavity; pl.g. pleural ganglion; ped.g. pedal ganglion; sh. shell; St. stomach; vis.g. visceral ganglion. The mollusc is represented buried in sand, except for the perforated narrow end, through which both the inhalant and exhalant currents (shown by arrows) flow. Class LAMELLIBRANCHIATA Mollusca in which typically the body is bilaterally symmetrical, much compressed from side to side and completely enveloped by the mantle which is divided into two equal lobes ; each lobe secretes a shell valve, the two valves being joined dorsally by a ligament and hhige and closed ventrally by the contraction of one or two transverse adductor muscles 574 THE INVERTEBRATA the head is rudimentary, eyes, tentacles and radula being absent ; there is a pair of labial palps with the mouth situated between them ; the foot is ventral, without a crawling surface but usually wedge-shaped and adapted for progression in mud or sand ; there are two ctenidia in the mantle cavity, often greatly enlarged and with a complicated structure; their cilia, together with those of the labial palps, form a mechanism for the collection of small food particles ; the sexes are nearly always separate, and there is a trochosphere and a veliger larva in the marine forms. The development of the ctenidia (Fig. 392) is the outstanding morphological and physiological character of the lamellibranchs. The arrangement of the shell valves, which allows the mantle cavity to extend the whole length of the body, also makes possible a great ex- tension of the ctenidia. The axis increases in length and the branches on each side not only increase in length, h&covmng filaments ^ but also turn up at the ends so that there is a descending and an ascending limb. The limbs of adjacent filaments are connected together by ciliary junctions (Mytilus), or by growth of tissue (Anodonta), so that thus all the filaments are joined together to form gill plates, each gill plate consisting of two lamellae formed from all the ascending and all the descending limbs respectively. The lamellae are united by cords of tissue which constitute the interlamellar concrescences. The extent to which the gills are welded together to form continuous plates is the distinction between the three main groups of the Lamellibranchiata, the Protobranchiata [Nucula), the Filibranchiata [Mytilus) and the Eulamellibranchiata (Anodonta). But even in the last-named group there are left occasional holes through which water passes into the interlamellar spaces then into the epibranchial space dorsal to the gills. Belonging to the same physiological system are the labial palps, two folds, one in front of the mouth and one behind, which are turned backwards and prolonged on each side of the visceral mass so as to form two pairs of richly ciliated triangular flaps, embracing the anterior end of the ctenidia, and enclosing a groove which leads to the mouth. In the anterior part of the mantle cavity the axis of the gill is at- tached to the side of the animal dorsal to the foot, which here forms a vertical partition dividing the cavity into a right and left half. The mantle cavity continues behind the foot, however, and here the up- turned ends of the inner rows of filaments of both ctenidia are united so that the mantle cavity is now divided by a horizontal partition into an upper or epibranchial cavity and a lower main cavity. The former opens at the dorsal siphon, the latter at the ventral siphon. A constant current of water is maintained during activity, entering by the ventral siphon, passing through the gill lamellae, and leaving by the dorsal. a.v. .1/. ^ — > - ,-»r.v. //.ci7..^ -^^^ 'a^IN. l.cil il.j.-- Fig. 392. The ctenidia of the LameHibranchiata. A and B, Mytilus. C, Anodonta. A, Vertical transverse section through ctenidium of one side. av afferent vein; a.l ascending, d.l. descending limb of filament; cil.d. ciliated discs ; e.v. efferent vein ; F. foot ; il.s. interlamellar space ; il.j. mter- lamellar concrescence ; ma. mantle ; muc. mucus travelhng ventrally (m direc- tion of arrows) and muc.' collected in the food groove; pl.c plicate canals; r V renal veins. B, Horizontal section through two adjacent filaments, bl.c. blood ceWs; ch. chitinous rods; cil.j. ciliary junction; f.cil., fi.cil., /.a/ frontal, latero-frontal, lateral cilia. The arrows denote the direction of the food current and the path of the food particles it contains. C, Horizontal section through a gill. Lettering as above. 57^ THE INVERTEBRATA From this the animal separates its food in the form of minute plants and fragments of organic debris. The current can easily be demon- strated by pipetting a suspension of carmine particles in the neigh- bourhood of the siphons, and the details of the process worked out by observing the motion of the coloured granules over the surfaces of the mantle cavity when one of the shells and its mantle lobe have been removed. In this way the direction of the ciliary currents of the ctenidia which transport the food particles can be demonstrated (Fig. 393). On entering the wide mantle cavity the velocity of the inhalant current is checked, and the heavier particles sink down and are taken up by the ciliary currents of the mantle which run towards the posterior region in the neighbourhood of the siphons. The main ingoing current with the smaller particles of carmine is drawn over the surface of the ctenidium and impinges against the individual fila- ments. Their structure and the distribution of the groups of cilia which all perform different functions is shown in the diagram of a transverse section through a ctenidium (Fig. 392 B). That the main current of water is drawn into the mantle cavity at all is the result of the activity of the lateral cilia. When the current which they have drawn to the ctenidium impinges on its surface the large latero-frontal cilia perform their task of deflecting the particles on to the face of the filaments where they come under the influence of the frontal cilia, which produce a constant stream down over the surface of the ctenidium towards its ventral edge. During the passage the particles in the stream become entangled in mucus, and on reaching the edge the string-like masses of food and mucus are directed by other cilia along the edge in the direction of the mouth, travelling partly in the ''food groove''. When the labial palps are reached the collected material may, according to its nature, either be swept straight into the mouth or come under the influence of cilia working along re- jection paths which direct it away from the mouth and toward the outgoing circulation on the mantle (Fig. 393 B). This complicated but well co-ordinated ciliary mechanism is nearly always working when the lamellibranch is covered with water, and the amount of water which passes through the mantle cavity of a single mussel is surprisingly large. But it must be remembered that this current also serves the purpose of respiration, though the ex- change of gases takes place through the medium of the mantle rather than the ctenidia. At low tide the animal must close its shell and COg accumulates within the mantle cavity. This chemical change depresses ciliary activity and finally brings the cilia to rest, so that the store of oxygen in the tissues is conserved. When the tide rises, however, the cilia immediately resume activity. Though the majority of the lamellibranchs have the power of LAM ELL IB RANG HI AT A 577 movement it is thus seen that they feed in the manner of a sedentary organism, and it is not surprising that there are many fixed and burrowing forms among them. inh.s. Fig. 393. Diagrams to show ciliary currents of Mytilus. Adapted from Orton. A, Food currents with left lobe of mantle removed to show the outer lamella only of the left gill, and the two palps of the left side separated and not embrac- ing the front end of the gill as they normally do in life. The vertical arrows re- present the currents caused by the frontal cilia, those at the bottom of the gill the main food current running to the mouth and that at the top of the gill the exhalant current, x. represents a curtain which prevents the inhalant current from directly impinging on the surface of the gill, an opportunity being thus afforded for a preliminary rejection of particles, by. byssus threads ; ct. outer lamella of left ctenidium ; exh.c. course of exhalant current shown by arrows in the epibranchial chamber, the roof of which is indicated by dots ; F. foot ; inh.s. left lip of inhalant siphon ; inh.c. inhalant current ; M. mouth ; pp. palps. B, Rejection currents. Mytilus with foot and the gills removed so as to show the interior of the right lobe of the mantle. The direction of the currents caused by the cilia is shown by arrows. The palps of the left side and the anterior end of the outer left gill remain and the rejection current marked by three parallel arrows is shown. The colleQtor current runs along the groove under the mantle edge to the pouch x. aa. anterior adductor muscle ; by.m. muscles of the byssus ; pa. posterior adductor muscle. Other letters as above. A short oesophagus leads directly into the stomach, which is a wide sac receiving on each side the ducts of the digestive gland which is 578 THE INVERTEBRATA similar to that of Helix ^ but contains only one kind of cell. This cell takes up the finely divided food which reaches the gland and digests it by intracellular ferments. The intestine runs into the foot and makes one or more loops, eventually returning to near the hind end of the stomach. It then passes through the pericardium where it is usually surrounded by the ventricle, ending as the rectum. The peculiarity of the digestive system is the presence of a diverticulum of the intestine, the cells of which secrete a crystalline style (Fig. 394) ; some cilia in the diverticulum rotate this and others move it forward so that at its free end, projecting into the stomach against a structure called the^fl5inc^/zi>/^, it is constantly worn away and the style material mixed with the contents of the stomach. It is composed of protein to which is adsorbed an amylolytic ferment and it may be broken down and re-formed periodically. There is no doubt that this represents a special Fig. 394. A, Section of part of the alimentary canal of Donax. cent, caecum of the intestine containing est. crystalline style ; g.s. gastric shield ; int. in- testine ; M. mouth ; oe. oesophagus ; st. stomach. B, Transverse section across the caecum showing cil. ciliated epithelium and est. crystalline style composed of concentric layers of material. After Barrois. provision for the digestion of carbohydrates and it is also found in somegasteropods. For the rest, digestion of proteins and absorption take place in the digestive gland, the cells of which have a surprising power of taking up solid particles. In the oyster, it may be mentioned, there is an extraordinary abundance of leucocytes which wander here, there and everywhere, through the body. It has been shown that they enter the stomach and ingest diatoms and other food particles there, speedily digesting them and wandering over the body afterwards, so that they play a unique part in the transport of food. The lamellibranchs are most conveniently classified by the structure of their ctenidia. We have firstly three groups which can be arranged in an evolutionary series, showing the ctenidia to become larger, more complex and solid organs. Lastly there is an isolated group, the Septibranchiata, in which the habit of life has completely changed and the ctenidia have practically disappeared : LAMELLIBRANCHIATA 579 Protobranchiata Nucula. FiLiBRANCHiATA Myttlus, Pecteti. EuLAMELLiBRANCHiATA Ostrea, Cyclas, Cardium, Mya, Anodonta. Septibranchiata Poromya, Cuspidaria. In Fig. 395 A, B, the difference is seen between the Protobranchiata with their short and simple filaments and the next two groups in which each filament is greatly elongated and upturned so that descending and ascending limbs can be distinguished. The contrast between the Filibranchiata and the Eulamellibranchiata is expressed by Fig. 392, in which a transverse section through a "gill" is shown, showing the component filaments separate in the first case, save for the ciliary junctions, united in the second. Lastly, in Fig. 395 C, it is seen that in the Septibranchiata, the ctenidia are replaced by a horizontal mus- cular partition (which moves up and down like the piston of a pump) Fig- 395- Vertical sections of Lamellibranchiata to show different stages of development of the ctenidia. A, Protobranchiata. B, Filibranchiata and Eulamellibranchiata. C, Septibranchiata. The arrows in C show the direction of the flow of water through the " diaphragm ", when the latter moves down- wards. After Sedgwick, from Lang. with apertures connecting the ventral and dorsal divisions of the mantle cavity. The ciliation of the filaments is the same in all the first three divi- sions. Even in the Protobranchiata, the ciliary apparatus for food- collecting has been developed as in the rest of the group, and it has been pointed out that there are ciliated discs, adjacent pairs of which act as ciliary junctions and hold the filaments together to form lamellae. There is, moreover, a subdivision of the mantle cavity into inhalant (ventral) and exhalant (dorsal) chambers in spite of the small size of the ctenidia. The blood system of the lamellibranchs is best explained by refer- ence to that of Myttlus, the common mussel (Fig. 396). Here the heart, as in Anodonta, consists of a ventricle surrounding the rectum and two auricles, each of which opens into the ventricle by a narrow canal and is attached by a broad base to the wall of the pericardium 580 THE INVERTEBRATA 'over the insertion of the ctenidia into the mantle. A single vessel, the anterior aorta (a posterior aorta is also present in Anodonta), leaves the ventricle, dilates into an aortic bulb and then divides into many arteries. Of these, the most important are the pallial arteries going to the mantle and the arteries forming part of the visceral circulation (the gastrointestinal, hepatic and terminal arteries, the last named supplying the most anterior part of the body including the foot). The arteries break up into a network of vessels in all the tissues and these Viscera p.n Plicate canals Fig. 396. Diagram of the circulation in Mytilus to show the greater import- ance of the part of the system in the mantle and pHcate canals. Of the blood re- turned from the viscera a much smaller proportion is sent through the ctenidia. Slightly altered from Field. An. auricle; bl. bladder of kidney opening into the pericardium ; aff.c.v. afferent, eff.c.v. efferent ctenidial vein ; l.v. longi- tudinal vein of kidney; p.ar. pallial artery; pc. pericardium; v. ventricle with rectum, represented by a dotted line, passing through it. join to form veins and sinuses which are largely situated on the inner side of the mantle and the superficial parts of the body. The skin, being bathed in water and devoid of any cuticular covering which might hinder diffusion, is a general organ of respiration and the mantle is the most important part of it. Most of the blood from the pallial circulation is returned to the network of vessels in the kidney through the ribbon-like organs, known as plicate canals, which extend along the mantle just above the insertion of the ctenidium. LAMELLIBRANCHIATA 581 The visceral vessels likewise return blood to the kidney network so that practically the whole of the blood passes through the excretory organ and is purified. A part of the blood from the kidney network enters the ctcjiidial circulation, discharging into the longitudinal afferent branchial vein, which gives off to each filament a vessel which descends one side and ascends the other. The ascending vessels join to form a longitudinal efferent vessel, which discharges into the longi- Viscera Ipdney eff.c.v.- .aff.c.v. Ctenidia Fig. 397. Circulation of Anodonta. A, Simplified diagram to show the course of the blood, indicating the relative importance of the various branches. Vessels returning arterial blood to the heart shown in black. B, Transverse section of Anodonta to show part of the course of the circulation. In the foot, F., the veins run into the vena cava cut in section, from which a small part of the blood is returned direct to the auricle in the dorsal wall of the bladder, bl., the rest through the kidney, k., longitudinal afferent vessels, Iv/, and thence to the afferent system of vessels in the ctenidium, ajf.c.v. On the other side the efferent system of vessels, eff.c.v., is shown returning blood to the longitudinal vessels at the base of the ctenidia, v.'\ from which it passes to the auricle, au., through an irregular system of blood spaces. The pallial circula- tion is not shown here. sbr. epibranchial space; v.c. vena cava; ven. ventricle. tudinal vein of the kidney. Into this longitudinal vein is collected the blood from the kidney network in general and by this channel blood is returned into the auricle. It will be seen that the branchial cir- culation is not important in Mytilus ; in Anodonta (Fig. 397) it is more developed. In Anodonta (Fig. 397) where the foot is larger than in Mytihis and movement more continuous the pedal artery is more impor- 582 THE INVERTEBRATA tant than the visceral arteries. The veins from the foot and the vis- cera join to form a pedal sinus and this opens into the vena cava. The junction of these is marked by a sphincter muscle (Keber's valve). This sphincter is closed when the foot is extended. The relaxation of the muscles and the pumping of the blood into the sinuses of the foot bring about the swelling of the foot. When the foot is retracted the blood is largely contained in spaces in the mantle. The pallial cir- culation is maintained during movement when the visceral circulation is interrupted as described above. While the Protobranchiata have a nervous system with four distinct pairs of ganglia (Fig. 373 D) in the remainder of the class the number is reduced to three by the iFusion of the cerebral and pleural ganglia (Fig. 398 B). The sexes are usually separate in the Lamellibranchiata, but some species of Ostrea and Pecten are always hermaphrodite, while this condition is frequent in Anodonta. In the Protobranchiata the gonad discharges into the kidney, but in most forms there is a separate generative aperture. While most marine forms and the freshwater Dreissensia have trochosphere and veliger larvae, some lamellibranchs incubate the embryos within the ctenidia, and in the family Unionidae, which includes Anodonta, the larvae are much modified {Glochidium). When they are ripe the mother liberates them if a fish swims near her, and they attach themselves to the gills or fins and become encysted there. After a parasitic life which varies greatly in length they escape from the cyst as young mussels. Order PROTOBRANCHIATA The best-known representative is Nucula (Fig. 373 D). It has a shell of very characteristic appearance with numerous teeth on the hinge line and a foot which, when fully extended, has a flat ventral surface which has been compared with that of the gasteropod. But instead of creeping by means of it the animal uses it for burrowing ; it is folded up (as is seen in the diagram), and thrust into the mud, then opened out and used as a holdfast, and the contraction of the retractor muscles draws the body below the surface. While the surface of the ctenidium is so small that the organ is of little use for feeding, the labial palp is enormous and divided into three parts. One of these is a kind of proboscis which is thrust out of the shell and collects food by ciliary currents. This is sorted and forwarded to the mouth by the other two parts without the intervention of the ctenidium. The nervous system has distinct cerebral and pleural ganglia and the gonads have retained their original connection with the kidneys. These and some less important characters show that Nucula and its LAMELLIBRANCHIATA 583 relations are probably the most primitive of living lamellibranchs. The specialization of the labial palps has had as its consequence the partial suppression of the ctenidia, which remain in an undeveloped condition. In this respect the Protobranchiata can hardly be held to resemble the ancestral lamellibranch. Order FILIBRANCHIATA Mytilus (Fig. 393). While the majority of lamellibranchs are semi- sedentary, the sea mussel has developed the sedentary tendency and marks a half-way stage to the oyster which remains fixed through Fig. 398. Nervous system of A, Chiton, B, a lamellibranch. Dorsal views. The outline of the mantle edge is indicated by a dotted line, buc.c. buccal commissure and ganglia; ce.c. cerebral commissure; ce.pl. g. cerebro-pleural ganglion; pal.n. pallial nerve; ped.g., ped.n. pedal ganglion and nerve; p.v.c. palliovisceral commissure; sb.rad.c. subradula commissure; vis.g. visceral ganglion. adult life. The mussel lives in association in beds between tidemarks where the conditions are favourable. The very extensible foot is tongue-like in shape with a groove on the ventral surface which is continuous with the byssus pit posteriorly. In this a viscous secretion is poured out which enters the groove and hardens gradually when it comes into contact with sea water. The tip of the foot is pressed against the surface to which the muSsel attaches itself, and in a cup- like hollow which ends the groove the attachment plate is formed at the end of the byssal thread. When one byssal thread has been formed the foot changes its position and secretes another thread in another place. The byssus thus consists of a mass of diverging threads arising 584 THE INVERTEBRATA from the byssiis pit and by means of it the animal is firmly attached to stones or other mussels. But mussels, particularly when young, creep about both by using the cup at the tip of the foot as a sucker and also by forming a path of threads along the surface of the substratum, as can be easily seen in the laboratory. While the development of the byssus is the most outstanding characteristic of the mussel, it may also be mentioned that a pair of simple eyes are developed, anterior ten. Fig- 399- Pecten maximus, general anatomy, right valve and ctenidium re- moved. After Dakin. add.u. unstriped and add.s. striped adductor muscle ; an. anus; au. auricle; b.gr. byssal groove; ct.' descending and ascending lamella of left ctenidium ; e. eye ; /. foot ; int. intestine ; l.p. labial palp ; M. mouth ; o. ovary ; oe. oesophagus ; st. stomach ; t. testis ; ten. tentacles of mantle; ven. ventricle; vm. velum. to the inner ctenidial lamella ; these are an inheritance from the larval mussel. The invasion of the mantle by the generative organs is another peculiar point. In the breeding season the aeration of blood in the mantle is reduced and the plicate canals (Fig. 396) become the chief organ of respiration. Pecten (Fig. 399). There are two common British species, P. maximus and P. opercularis, which are commonly known under the name of "scallops". The animal is found free and it moves not by LAMELLIBRANCHIATA 585 the ordinary lamellibranch method but by swimming. The two valves are unequal, the right being larger and more convex, and the animal rests on this valve; in P. opercularis the valves are almost equal. In swimming the valves open and close very rapidly, forcing out the water between them. Usually the water is forced out dorsally on each side of the hinge line and the animal moves with the free ventral border forward ; but on sudden stimulation the current passes out directly ventrally and the hinge line becomes anterior. There is a single large adductor muscle: this is divided into two parts and the larger of these serves for the rapid contractions which cause swimming movements ; the fibres are transversely striated ; the smaller part has fibres which are capable only of strong long-continued contraction and keep the valves closed. (Cp. Chapter iv, p. 143.) The foot is very much reduced, but it has nevertheless a distinct function, that of freeing the palps and gills from sharp and disagree- able foreign material; in the larva it is used actively in locomotion. The ctenidia, while resembling the typical filibranch gill of Mytilus in general, differ in the possession of two kinds of filaments and in the vertical folding of the gills. The larger principal filaments lie at the bottom of the troughs between successive folds and the descending and ascending limbs of each principal filament are connected by a sheet of tissue, the inter latnellar septum. In one species, Pecten tenuicostatiis, there are organic connections between filaments instead of ciliary junctions only, and the existence of this condition is a valid criticism of the classification of the lamellibranchs by ctenidial structure. Pecten is hermaphrodite. The ovary has a very vivid pink colour when the eggs are ripe. The testis lies behind it and is cream-coloured. The remaining feature to be noted is the presence of a large series of stalked eyes (Fig. 409 D), of a very complicated structure, at regular intervals all round the mantle. Order EULAMELLIBRANCHI ATA Anodonta (Figs. 392 C, 397). Many of the characters of this fresh- water genus are described above. Ostrea (Fig. 400). In this form the adult is always fixed by the left (the larger) valve. As in Pecten, there is only one adductor muscle (the posterior) in the adult (but the spat possesses two equal muscles), and this is divided into two parts, one with striated the other with non-striated fibres. The foot has disappeared entirely ; the two auricles are fused together. Of great interest are the reproductive habits : it has been established that individuals of O. edulis function alternately as males and females. Spawning tends to take place at full moon as 586 THE INVERTEBRATA in some echinoderms. Another point of physiological importance is the great part which leucocytes play in digestion ; the lumen of the alimentary canal is invaded and diatoms and similar bodies ingested, digested and transported by the leucocytes into the connective tissue. A figure of the veliger larva of Ostrea is given (Fig. 375 A) to show the ciliary currents by which food is obtained, the crystalline style, which is revolved by the action of the cilia of the style sac, and the foot, which is lost in the adult. ex.c ven.' ^int. d.bx.' Fig. 400. Ostrea edulis, general anatomy, right valve and mantle removed. After Yonge. Lettering as in Fig. 374; in addition: in.c. inhalant and ex.c. exhalant chamber; d.b.c. division between above chambers. Arrows indicate direction of currents. Teredo (Fig. 401) is the most specialized of the boring lamelli- branchs. While most lamellibranchs burrow in mud, others tend to work in consolidated sediments such as Pholas in chalk and sandstone, and Saxicava in the hardest limestone. Teredo and Xylophaga bore in wood. The latter makes shallow pits, but Teredo, working with ex- traordinary speed, excavates long cylindrical tunnels (sometimes as much as a foot in a month or two). The wood is reduced to sawdust by the rotatory action of the two shell valves, in which the adductor muscle fibres maintain a rhythmical contraction. The sawdust is swallowed by the animal and is largely retained in a relatively MOLLUSCA 587 enormous caecum of the stomach, but a great deal of the material passes into the cavity of the digestive gland and is there ingested by the epithelial cells. There is no doubt that Teredo has developed enzymes which are almost unique in the Animal Kingdom, which digest cellulose and hemicellulose. The structure of the animal is remarkable for the extraordinarily long siphons and mantle cavity; while the mantle often lays down a calcareous lining to the tube and always a pair of calcareous valves, the pallets, which close the mouth of the tube when the siphons are retracted. The foot / 1 v T V ^•^- di.(jh ct' int. cm Fig. 401. Teredo, represented boring in wood. The sawdust formed by the rotatory movement of the shell valves, sh., is shown entering the mouth, M., and the faecal pellets of undigested wood are shown as black masses in the exhalant chamber, exh.c. Other letters : an. anus ; au. auricle ; ct. ctenidium ; ct.' continuation of ctenidium as a ciliated ridge over the visceral mass; cm. caecum of stomach filled with wood; c.s. position of crystalline style sac; di.gl. digestive gland; F. foot; int. intestine; z«^.c. inhalant current; pcd. pericardium ; pit. palette ; sh. left valve of shell ; ven. ventricle. Original. is very much reduced. A constant current into and out of the mantle cavity is maintained by ciliary action, and the ctenidia, though so greatly modified and elongated, constitute a collector mechanism; but it does not seem that diatoms obtained in this way form any part of the normal food of the creature, which exists almost entirely on the carbohydrates furnished by wood which also contains small quantities of proteins. Class CEPHALOPODA (SIPHONOPODA) Bilaterally symmetrical Mollusca with a radula and a well-developed head which is surrounded by a crown of mobile and prehensile ten- tacles, sometimes held to be part of the foot, which certainly forms the 588 THE INVERTEBRATA funnel or siphon, a muscular organ, originally bilobed, used for the expulsion of water from the mantle cavity; one or two pairs of typical ctenidia; coelom sometimes exceedingly well developed, the genital part being continuous with the pericardium; typically a chambered shell in the last chamber of which the animal lives, though in most modern representatives it is reduced and internal or wholly absent; nervous system greatly centralized and eyes of great size and often complex type; eggs heavily yolked and development direct. The Cephalopoda fall into two groups, in one of which (Tetra- branchiata) there are two pairs of ctenidia and a w^ell-developed external shell, w'hile the members of the other (Dibranchiata) have one pair of ctenidia and either one internal shell or none at all. Of the Tetrabranchiata Nautilus is the only living member; of the Dibran- chiata, Sepia, a common form in the Mediterranean and elsewhere, is a convenient type. The organization of the group will best be understood from a description of these examples. As Sepia is the more easily obtained we shall describe it first and in more detail, though it is in many respects less primitive than Nautilus. Order DIBRANCHIATA Cephalopoda with a single pair of ctenidia and kidneys; shell in- ternal, enveloped by the mantle and in various degrees of re- duction; 8-10 tentacles; the two halves of the funnel only seen in the embryo; chromatophores present; eyes of complex structure. Classification Suborder Decapoda. Dibranchs with ten tentacles and wdth a well-developed coelom. Internal shell consisting of phrag- mocone, rostrum and proostracum or very much simplified. (i) Tribe Belemnoidea. Fossils from Mesozoic rocks which have given rise to the following tribes : (2) Tribe Sepioidea. Decapoda with specially modified 4th pair of tentacles which can be retracted into pits; eyes with a cornea, internal shell sometimes with phragmocone bent ventrally: fins not united posteriorly; shore and bottom living forms. Spirula, Sepia, Sepiola. (3) Tribe Oegopsida. Decapoda with anterior chamber of eye open; tentacles usually all alike; suckers often modified to form hooks ; shell only represented by a horny gladius ; strong swimmers. Includes many abyssal forms wdth phosphorescent organs; some gigantic forms, like Archi- teuthis, 60 feet long. CEPHALOPODA 589 (4) Tribe Myopsida. Decapoda with a cornea in the eye, a simple gladius, specially elongated 4th pair of tentacles, not retractile into pits; fins united posteriorly; shore forms. Loligo (Fig. 411 D). Suborder Octopoda. Dibranchs with eight tentacles and a reduced coelom. Octopus, Argonauta, Opisthoteuthis . Sepia officinalis^ is a shallow-water form, in which the shell has become internal. The general disposition of the organs remains much as it would be if the animal inhabited the last chamber of a shell like that of Nautilus (cf. Fig. 402 A and B). The whole body is cylindrical. At one end, which would have projected from the shell, is the head with the mouth in the centre and the two relatively enormous eyes at the sides. Round the mouth are the tentacles (arms) for seizing prey which are often considered to be part of the foot. Four pairs of these are short and stout and covered with suckers on their inner surface. The fourth pair (counting from the dorsal surface) are long and can be retracted into large pits at their base ; there are suckers only at their free end. The left hand member of the fifth pair in the male is slightly modified by suppression of the suckers. At one side, called pos- terior, is the mantle cavity, and protruding from its opening is the funnel, which is the remaining part of the foot. The visceral hump is the conical apex of the animal. Instead then, of being protrusible like that of a lamellibranch or used for gliding like that of a gasteropod, the main part of the cephalopod foot is greatly modified for respiratory purposes. In view of the fact that there is no boundary between the head and the foot in molluscs, dis- cussion as to whether the tentacles are part of the head or the foot is difiicult and unimportant. The shell has become internal and is a rather substantial plate which acts as an endoskeleton. The absence of a figid envelope has made it possible for the mantle to become very mobile and to develop thick muscular layers, circular muscles running round the mantle cavity and longitudinal running towards the apex of the hump. When the latter contract and the former relax the mantle cavity enlarges and draws in water which circulates round the ctenidia ; when the reverse action takes place the first effect of the contraction of the circular muscles is to draw the mantle lobe tight round the neck and then, when the contraction reaches its height, the water is expelled through the funnel. In rest these movements are gentle and rhythmic and only effect the change of water necessary for respiration. At the same time the animal is usually swimming slowly forward by the undula- ^ This description of the structure and habits of Sepia applies generally to all the well-known Decapoda. fn.^ rm. brn. oe. dig-gl- rad. V ^ \ sh. St. Fig. 402. Diagrammatic median sections through A, Nautilus and B, Sepia for comparison of the organization of the Tetrabranchiata and Dibranchiata respectively. Altered from Naef. brn. brain; cm. caecum; ct. ctenidia; dig.gl. digestive gland ; fn. funnel ; g.sep. genital septum ; h. heart ; ho. hood ; i.s. ink sac;y. jaws; k. kidney; Ip. lips; mt. mantle; mt.' dorsal extension in Nautilus; o. ovary; oe. oesophagus; rad. radula; rm. rectum; sep. septa; sh. shell; sip. siphuncle; st. stomach; t. testis. Sepia is shown in the expiratory position with the mantle pressed against the funnel, and the valve of the latter flat against its wall. In the inset C, the inspiratory phase is seen with the mantle relaxed to allow the entry of water as shown by the arrow, and the valve of the funnel opened so as to prevent the passage of water. CEPHALOPODA 59I tory movement of the lateral fins. But if Sepia is alarmed or excited the muscles contract violently and the spasmodic ejection of water through the funnel causes the animal to dart quickly backw^ards. Equally by turning the funnel backward it can move quickly forward. Not only is the mantle highly muscular but the dermis contains large cells filled with pigment, the chromatophores ^ which can be dilated by the contraction of radiating muscle fibres attached to the cell wall. By alternate contraction and expansion of the chromatophores, waves of colour are made to pass rapidly over the surface of the animal. The colour change which is brought about in this way may be to a certain extent a response to the character of the background but it is also stated to be the expression of emotions. Sepia swims with the longest axis horizontal, the upper flattened surface is that under which the shell lies and the lower the mantle- cavity surface. These surfaces are dorsal and ventral respectively and the mouth and tentacles are anterior. All round the mantle in the horizontal plane rises a horizontal fin by which the gentler swimming movements are effected. When the mantle cavity is opened as shown in Fig. 403, the funnel is seen with its narrow external and wide internal openings, and at the base of it two sockets which fit corresponding knobs on the mantle. This locking arrangement ensures that the mantle fits tightly on the neck and so that all water is expelled by the funnel. At the anterior end of the visceral hump is seen the central anus at the end of a long papilla, so placed as to discharge the faeces directly into the cavity of the funnel, the shorter renal papillae immediately on each side, and on the left side only the genital aperture, also at the end of a long papilla. More posterior still are the large and typical ctenidia. On the face of the visceral hump in mature animals the accessory genital glands are seen through the skin ; the chief of these are the shell-forming nidamental glands of the female which occupy a con- siderable area. Between these and in front of them are the accessory nidamental glands. Posterior to them is the ink sac, usually seen through the integument from which a narrow duct runs ventral to the rectum, opening into it a short distance behind the anus. The first step in dissection is to strip off the skin and then dissect out the gland and its duct as carefully as possible. It usually contains a large amount of the ink, which is composed of granules of melanin pigment formed by the oxidation of the aminoacid tyrosin by the agency of an enzyme, tyrosinase. This substance is ejected into the mantle cavity and through the funnel to form a "smoke cloud" when the animal is attacked. The next stage in dissection is the opening up of the kidneys by cutting through the thin outside wall. It will at once be seen that the 592 THE INVERTEBRATA Fig. 403. Ventral view of male of Sepia officinalis with mantle cavity opened to expose its contents, an. anus; dep.m. depressor muscle of the funnel; e. eye; /z. fin; g.pap. p^enital papilla; hec. hectocotylized arm;/, jaw; k.pap. papilla bearing external aperture of kidney ; kn. cartilaginous knob on mantle which fits into the socket, soc. ; vis.m. visceral mass. Other letters as in Fig. 402. From Shipley and MacBride. CEPHALOPODA 593 cavity of the organ contains a large amount of spongy excretory tissue, developed round the veins which run straight through the kidney. Just inside the renal papilla is a small rosette which carries the reno- pericardial aperture. This leads into the long narrow renopericardial canal running in the outer wall of the kidney and opening posteriorly into the pericardium^ a wide space lying behind the kidneys which is dep.m. i^\ ^ an. Fig. 404. Sepia officinalis. Dissection from the ventral side to show kidneys and blood vessels. Arrows show the direction of flow of blood, abd.v. ab- dominal vein; a.ao. anterior aorta; au. auricle; ajf.v. afferent branchial vein; br.ht. branchial heart ; eff.v. efferent branchial vein ; k.d. opening into dorsal sac of kidney (see arrow); k.t. excretory tissue surrounding the vena cava; pal.v. pallial vein; p.ao. posterior aorta; r.p.a. opening into kidney cavity of the renopericardial canal, r.p.c; st.g. stellate ganglion; ven. ventricle; v.cav. vena cava. Other letters as in Figs. 402, 403. Original. only separated by an incomplete partition from the still more spacious genital coelom occupying the apex of the visceral hump (Fig. 405). The median ventricle and the two lateral auricles are spindle- shaped bodies arranged in a line at right angles to the longitudinal axis of the body. Arterial blood is sent to the body from the ventricle by an anterior aorta running dorsal to the oesophagus towards the head and ^. posterior aorta\ venous blood returns to the heart fro / 'y;- ma.n VI is.g. ot. ! ped.g. \ fu'.n- bra.g. yw. Fig. 406. Vertical section through head of Sepia officinalis showing buccal mass (coarsely stippled) and brain (black) surrounded by the cartilaginous skull (finely stippled), a.sal.gl. anterior salivary gland; bra.g. brachial ganglion with brachial nerves coming off from it; ce.g. cerebral ganglion; dig.gl. "liver"; /w./z. nerve to funnel {fu.) coming off from pedal ganglion; y. beaks ; ma.n. mantle nerve ; oe. oesophagus ; ot. otocyst ; ped.g. pedal gang- lion ;/).5a/.£/., p.sal.gl. posterior salivary duct and gland; rad. radula; s.buc.g., i.biic.g. superior and inferior buccal ganglia ; vis.g. visceral ganglion. Original. glandular stomach. Here it is mixed with the secretion from the digestive gland and the digested food passes to the spiral coecum. This contains an elaborate ciliary mechanism which removes solid particles from the coecum, leaving only liquid products of digestion to be absorbed there. The digestive gland consists of a solid bilobed gland ("liver") and a more diffuse and spongy part ("pancreas"). Both are enzyme-producing, but the "pancreas" (which in Sepia is suspended in the kidney sac) is al^o partly excretory. The single duct opens into the coecum, but a groove guides its secretion into the stomach. The "liver" is the principal "storage organ" for food reserves ; it seems probable that these only reach the 'gland from the blood-stream, and that food is all absorbed in the alimentary canal, 596 THE INVERTEBRATA and does not enter the liver. In this respect the cephalopods appear to differ from the majority of invertebrates. The nervous system of Sepia is of great interest from the large size and intimate association of the ganglia round the oesophagus, which form a genuine "brain" (Figs. 407, 408) in which special centres for the co-ordination of vital activities and for the simple reflex actions have alike been detected. In contrast to vertebrates there is a con- centration of nerve cells in the brain, only a few outlying ganglia being present. For the protection of this large nervous mass a "skull " has been developed composed of a tissue very similar to cartilage, which also forms the supports of the fins and tentacles. The nerve net found in the foot of gasteropods is absent. cer. VIS. Fig. 407. Lateral view of the brain of a cephalopod (Eledone?) to show the localization of function. After Buddenbrock. al.c. alimentary canal; buc. buccal ganglion ; cer. the different divisions of the cerebral ganglion ; brae. brachial ganglion; ped. pedal ganglion; vis. visceral ganglion; the various reflex centres A for biting, B for swallowing, C for swimming forward, D for creeping and climbing, E for closing and F for relaxing the suckers, G for in-breathing and H for out-breathing. The brain consists of the following ganglia : dorsally the cerebral or supraoesophageal, ventrally (i) tho^ pedal, divided into the brachial (the motor centre for the tentacles) in front and the infundibular (supply- ing the funnel) behind, and (2) the visceral supplying the mantle and the visceral hump. The cerebral ganglia are much more differentiated than any of the others. They can be divided into separate regions which co-ordinate the movements of organs for the performance of such complicated actions as feeding, swimming and creeping. In the visceral ganglia there are also two sharply defined centres which control the movements of the whole mantle in in-breathing and out- breathing respectively as well as numerous small centres, the stimu- lation of which causes contraction of small muscle patches in the mantle, while in the brachial ganglia there are separate centres for gripping by the suckers and for letting go. CEPHALOPODA 597 From the cerebral ganglia there run forward a pair of nerves which end at the border of the buccal mass in a pair oi superior iwrca/ ganglia ; a circumoesophageal commissure links up these with the inferior ten.n. Fig. 408. Nervous system of Sepia. After Hillig. bra.g. brachial ganglion ; br.g. branchial ganglion and nerve; ce.g. cerebral ganglion; gas.g. gastric ganglion ; rna.n. mantle nerve ; olf.n. olfactory pit and nerve ; op.g. optic ganglion ; s.buc.g. superior buccal ganglion ; st.g. stellate ganglion ; sym.n. sympathetic nerve ; ten.n. brachial nerves ; vis.g., vis.n., visceral ganglion and nerve. buccal. From the visceral ganglia there is a pair of nerves running to the very prominent stellate ganglia in the mantle ; there is also a vis- ceral loop which sends off branches to the gills and a sympathetic loop ending in the gastric ganglion between the stomach and the caecum. 598 THE INVERTEBRATA The infundibular ganglion gives off a pair of nerves to the funnel and the brachial ganglia a separate nerve, which carries a ganglion on its course, to each arm. In the dissection of the nervous system a general view of the different parts of the brain is best obtained by making a longitudinal vertical section with a sharp scalpel. Such a section is shown in Fig. 406. Afterwards the dissection of the nerves coming away from the brain can be carried out. Sepia possesses very large eyes (Fig. 409 C), similar in their structure and development to those of a vertebrate. In the embryo, the eye originates as an ectodermal pit, the lining of which forms the retina and the contents of which become the vitreous humour. The pit closes up and at the point of closure the interior part of the lens is formed. Later appears a circular fold which forms the iris, limiting the pupil of the eye and forming an outer eye chamber which is finally enclosed by the growth of a cornea. The external half of the lens is formed at the same time. A special ciliary muscle regulates the posi- tion of the lens. When it is relaxed the eye is focussed on the distance : when it contracts, increasing the pressure of the vitreous humour and so pushing the lens forward, the eye is focussed on near objects. The ovaries and the testes are simply parts of the wall of the coelom. The ova are cells of large size ; they are nourished by other peritoneal cells, the follicle cells, which surround the ova and pass on food from the special blood supply. The surface of contact between these cells and the egg is increased by folding. When ripe the ova escape into the genital coelom and pass into the genital duct. This has a terminal glandular enlargement and there are also the nidamental glands, un- connected with the genital ducts, which have already been mentioned. These secrete an elastic substance which forms the egg envelope. The sperm pass similarly into the genital coelom and then by a very small aperture into the sperm duct which is modified to form in turn the seminal vesicle, the prostate gland and the terminal reservoir, called Needham's sac. All these play their part in the formation of the remarkable spermatophores, elastic tubes which by an elaborate arrangement burst and liberate the spermatozoa after copulation. The spermatophores are passed directly from the extended genital papilla into the funnel and then on to one of the arms (the hectocotylus) which is modified for the purpose of transferring the sperm to the female. In Sepia, the modification shows itself only by the suppres- sion of some rows of suckers at the base of the arm, but in other forms it is profoundly modified. In Octopus, the end of the arm is spoon-shaped and the arm is extended so as to enter the mantle cavity of the female. In other octopods, a cyst, in which the sper- matophores are stored, is formed at the end of the arm ; from it a long CEPHALOPODA 599 filament is protruded. In Philonexis and Argonauta the modified arm is charged with spermatozoa, inserted into the mantle cavity of the female and then detached. This arm was described by early observers as a parasitic worm and named Hectocotylus . 4d.. cor. cor. pig.cp. op.n. Fig. 409. Eyes of Mollusca. A, Nautilus. B, Helix. C, Sepia. D, Pecten (inverted type), cil.m. ciliary muscle; cor. cornea; d.ret. distal and pr.ret. proximal layers of the double retina of Pecten ; ir. iris ; /. lens ; Id. eyelids ; op.g.y op.n. optic ganglion and nerve; pig. ep. pigmented epithelium ; ret. retina ; tap. tapetum ; vit.h. vitreous humour. In Sepia the cartilage is shown in black. Other Dibranchiata. The members of this group are classified in two suborders, whose members respectively possess, like Sepia, ten arms (Decapoda), or, like Octopus, only eight (Octopoda). In no member of either division is there any known form in which the shell is external ; in all cases the shell is more or less rudimentary or, in the case of the Octopoda, entirely absent. There is a well-known and 6oo THE INVERTEBRATA extremely numerous fossil group, the Belemnoidea (Fig. 410 B), in which impressions of the entire creature show the internal shell, the ink sac, and the ten arms beset with hooks. The shell consists of a chambered phragmocone , protected by a thickened guard, and with an anterior plate, the proostracum. It may well have been derived from a nautiloid form like Orthoceras (Fig. 410 A), as may be seen in the accompanying series of diagrams, in which the soft parts are of course partly conjectural. In a rare living form, Spirula (Fig. 410 C), ''U:,:::;-___^ ^P' sip. A^ phr. sip. y prst. ^^w '"c Fig. 410. Series of Cephalopoda to illustrate the evolution of the internal shell. After Naef. A, Orthoceras, Palaeozoic. B, Belemnites, Mesozoic. C, Spirulirostra, Tertiary. C, Spirula and D, Sepia, living. (D', enlargement of posterior end of D.) I'he reflection of the mantle over the shell is indicated by a dotted line. This is incomplete in Orthoceras, but the shell is completely internal in the rest. gd. guard; phr. phragmocone ; prst. proostracum; sep. septa; sip. siphuncle. the chambered shell is reduced, but not quite so much as is the case in the belemnites. It is coiled and there is no guard or proostracum. Both are, however, present in the related fossil Spirulirostra (Fig. 410 C). Finally, in Sepia (Fig. 410 D) the guard is represented by the minute rostrum and, according to one interpretation, one side of the phragmocone has expanded to cover the surface of the proostracum, the septa forming the oblique calcareous partitions of the cuttle bone, while the other side forms a minute lip in which the septa are crowded together (Fig. 410 D'). The siphuncle (p. 602) is a short wide funnel in between the two sides. CEPHALOPODA 6oi pro- In LoUgo there is only a horny pen^ which represents the ostracam, while in the Octopoda there is no skeleton at all. The Dibranchiata are specialized in two ways. The first is for a pelagic life; their bodies become elongated, fins develop and they become transparent. They may, exceptionally, develop such speed in Fig. 411. External appearance of Dibranchiata. a. Octopus swimming back- ward, b, Octopus asleep, c, Sepia swimming gently, d, Loligo in the act of catching prey, e, Sepia becoming active. the water that they take off from the surface and glide for considerable distances through the air, in the mariner of the flying fish, aided by their spreading fins (Todarodes Sagittarius). Loligo (Fig. 411 ^) is a well-known example of the pelagic type and may be seen in aquaria swimming in troops, keeping their distances and turning with military precision. 6o2 THE INVERTEBRATA The second mode of specialization is for a semisedentary life on the bottom. In this the body is short and the arms, which are much larger and more mobile than in the other type, are used for crawling. Octopus (Fig. 41 1 a) hides itself among stones and seeks its prey only at night. Sepia and Sepiola^ though capable of active movement, spend long periods of rest half-covered with sand, assuming by means of chromatophore expansion brown ripple-marking on their mantles. The most sedentary form is the flattened Opisthoteuthis ^ which is almost radially symmetrical and has a remarkable resemblance to a starfish ; the arms are all joined together and form a suctorial disc by which the animal applies itself to a rock. Order TETRABRANCHIATA Cephalopoda with well-developed calcareous shells. Living forms with two pairs of ctenidia and kidneys; arms very numerous, without suckers ; eye simple ; chromatophores absent ; funnel in two halves. Suborder Nautiloidea, with membranous protoconch, central siphuncle and simple suture line, e.g. Nautilus^ Orthoceras. Suborder Ammonoidea, with calcareous protoconch, marginal siphuncle and usually complicated suture line, e.g. PhylloceraSy Baculites, A brief description of Nautilus, the only surviving cephalopod with an external chambered shell, must be given here. The shell is coiled in a plane spiral ; the earliest formed portion was membranous and is represented by a small central space. In the ammonoids there is a calcareous chamber, the protoconch, in this position. Succeeding this are the numerous chambers, separated from each other by the curved septa, each one marking a stage in the animal's growth. As the shell is added to, the animal moves forward and from time to time shuts off a space (the chamber) behind it by the secretion of a new septum. The terminal living chamber is much larger than the rest and is occupied by the body of the animal. All the others contain gas (which differs from air in its smaller proportion of oxygen) ; by means of this the heavy shell is buoyed up in the water and the animal can swim freely. The septa are perforated in the middle and traversed by the siphuncle which is a slender tubular prolongation of the visceral hump. It contains blood vessels and probably secretes gas into the chambers to maintain a constant pressure. The relations of the different parts of the body in Nautilus are easily compared with those in Sepia (Fig. 402). The shell coils forward over the neck of the animal (exogastric) ; the mantle cavity is posterior as in all cephalopods. In other words differential growth of the vis- CEPHALOPODA 603 ceral hump is not here associated with torsion. The mantle is thin and adheres to the shell; it cannot therefore be associated with the re- spiratory and locomotory movements. The "head foot" is produced into two circles of arms which are very numerous; they are re- tractile and adhesive but have no suckers. The anterior part of the region where it touches the shell is very much thickened to form the hoody and when the animal is retracted into the living chamber the hood acts as an operculum. The third region of the head foot is the funnel^ here composed of two separate lobes. The other principal points in which Nautilus differs from the rest of the living cephalopods are as follows : (i) There 2ir&four ctenidia and/owr kidneys, without renopericar- dial apertures. The pericardium opens independently to the exterior by a pair of pores. The fact that in the most primitive cephalopod now existing there is a kind of segmentation of the body cavity and mantle organs has been taken to support the origin of the cephalopods from a metamerically segmented ancestor. This "segmentation" may, however, be secondary. Certainly the absence of a renoperi- cardial connection is not a primitive feature. There is nothing to prove that the fossil chambered-shell cephalopods had four ctenidia and four kidneys. (2) There are very simple eyes (Fig. 409 A) consisting of an open pit with no lens, the surface of the retina being bathed by sea water. This appears to be a primitive feature, but Nautilus is nocturnal and the eyes may have undergone reduction. (3) There is no ink sac in Nautilus^ nor apparently in the other forms grouped in the Tetrabranchiata. Nautilus lives at moderate depths on some tropical coasts. It either swims"^near the bottom or crawls over the rocks, pulling itself along by its arms like Octopus (Fig. 411 b). The gentler swimming move- ments are caused by the contraction of the muscles of the funnel only ; the more violent movements are probably caused by the animal suddenly withdrawing into the shell, thus expelling the water from the mantle cavity. It is nocturnal and gregarious and a ground feeder. The chief interest of Nautilus lies in the fact that it is the sole living representative of a vast multitude of cephalopods with external chambered shells which flourished between the earliest Cambrian and the late Cretaceous period, a space of time embracing much the longest part of the history of life on the earth. After being the dominant type of marine invertebrate in the Mesozoic they suddenly became extinct, and the Cephalopoda are now mainly represented by the Dibranchiata with their internal shells. The Tetrabranchiata are divided into two groups, the nautiloids and the ammonoids. The first of these contains Nautilus and other 604 rUV. TNVERTERRATA forms which agree with it in the position of the sipluincle ami (he vshape of the septum. They re:icli their maxiiiunn clevelojiment in the early I'alaeo/.oic, where tlie dominant forms have straiglit shells like Orihoccras and Actinoceras, which were sometimes as much as 8 feet long. It is diflicult to suppose that shelled animals of this size were anythinjr other than sedentary organisms. There is a tendency for the shell to become coiled in odier forms, exhibiting itself first in Fijr. 412. B T^ijT- 413- Fir. 412. Nauti/us nuicrowf^liolus ;ulhon"n)^ to the suhstratviin by moans of its tontaclos in a vertical position. It usually lies horizontally. After Willey. The shell shows alternate li^ht and dark hands which resemble "ripple- marking", f//"- dorsal muscular attachments of the funnel ; c. eye ; hd. hood ; mf. mantle; o. /(•//. ophthalmic tentacles. I'^i^. 413. A, Phylhiccrds fictcrof^ltyUiim, from the Lias: a part of the shell has been removeil to expose the sutures, x ,|. H, Suture line of Phyll Fig. 427. Development of Brachiopoda. A, Section of larva at end of gastrulation showing the two coelomic pouches originating from the archen- teron. B, Larva divided into three regions. C, Differentiation of the preoral region (oblique shading) and mantle lobes (stippling). D, Turning forward of mantle lobes and shrinking of preoral region. E, Appearance of the arms of the lophophore (one shown), preoral region now represented by lip. F, In- ternal view of dorsal valve showing the first stage in development of lopho- phore as a tentacular ring. G, Further development by extension of the dorsal lip and the division of the ring into two arms. The ciliated groove is indicated by stippling and the movement of food to the mouth by arrows. H, Larva oi Lingula, corresponding to F. al.c. alimentary canal; An. anus; arch, arch- enteron ; arm, one arm of the lophophore ; coe. coelomic pouch ; Ip. dorsal lip ; M. mouth; w./. mantle lobe; stk. stalk; ten. tentacles; ch. chaetae; e. eyes. Altered from Delage and Herouard, after various authors. 6l8 THE INVERTEBRATA and fixed by its whole surface to a rock; the dorsal valve is conical. The tentacles of the lophophore are protruded from the shell margin. The Brachiopoda have free-swimming larvae which are usually divided into three regions, an anterior like the preoral region of the trochosphere, a median region in which the two lobes of the mantle are early produced, and a posterior one, hidden by the mantle lobe, which becomes the stalk (Fig. 427 B). The mantle lobes develop four bundles of chaetae (Fig. 427 C), and then turn forward to envelop the anterior region (Fig. 427 D). This now begins to develop the lophophore (Fig. 427 E, F, G) and shell valves form on the mantle lobes, while the posterior region grows into the stalk. The coelom develops as a pair of pouches or a single pouch from the archenteron (Fig. 427 A). Though the presence of mantle lobes, the presence of chaetae and the resemblance of the larva to a trocho- sphere relates the Brachiopoda to the annelid-mollusc stock, there is no evidence of segmentation and they cannot come very close to the Annelida ; but possibly are nearer to the MoUusca. On the other hand the enterocoelic development of the body cavity suggests affinities to the echinoderms and chordates. Classification EcARDiNES. Brachiopoda having shells with no hinge, no internal skeleton, and alimentary canal with an anus. Lingula^ Crania. Testicardines. Brachiopoda having shells with hinge and internal skeleton, without anus. Terehratula^ Waldheimia. PHYLUM CHAETOGNATHA Coelomate animals with an elongated body divided into three regions, head, trunk and tail, and with lateral and caudal fins ; head with a pair of eyes and two groups of chitinous teeth and jaws; cerebral ganglion and ventral ganglion (in the trunk) connected by circum- oesophageal commissures ; body wall containing a layer of longitudinal muscle cells of peculiar type arranged in four quadrants ; alimentary canal straight ; no localized excretory or respiratory organs or vascular system; hermaphrodite and cross-fertilizing; free-swimming larva. The structure of an individual of this small and homogeneous group is shown in Fig. 428. Very little need be added to the definition. The muscles are of a primitive type, each elongated cell consisting of a core of unmodified cytoplasm and an outer shell ring of con- tractile substance ; they have thus some resemblance to those of the nematodes. The chaetognaths are, however, capable of executing very rapid movement by suddenly contracting these longitudinal muscles and are able to pounce upon and capture their food, which CHAETOGNATHA 619 \—al.c. ■ga.v. consists of diatoms, copepods and larvae of various kinds including fishes, in fact of most of their plank- tonic neighbours. These are seized by the hook-like jaws and swallowed whole. The coelom is well developed with a distinct epithelial lining, and it is divided into two halves by a complete median and vertical mesentery, and also by two transverse septa into three chambers corresponding to the head, the trunk and the tail. Of these the head cavity is mainly occupied by the jaw muscles, while in the trunk and tail cavities are developed the ovaries and the testes respectively. l^\iQ ovaries (Fig. 429) are elongated solid organs attached laterally to the body wall. Traversing each ovary on its inner side is a duct with a blind anterior end [oviduct)', this encloses a second duct [sperm pouch) also with a blind anterior end and with indefinite walls, / containing sperm derived from an- ' other animal. Both ducts open into fn. a small bulblike seminal receptacle with \ an external aperture just in front of \ the second septum. The maturing egg is fertilized by a spermatozoon which passes into the ovary from the second duct and the zygote then passes through the wall of the oviduct and then to the exterior. There is a solid testis in each half of the tail cavity and from these sperm mother cells are constantly budded off into the coelom, which is thus filled with sperm in all stages of develop- ment. The sperm passes into vasa jrig. 428. Sagittahexaptera.\en- deferentia, which are long tubes with tralview, X2h- After O. Hertwig. a small internal opening behind the ale. alimentary canal; An. anus; testes and a terminal dilatation, the /": ^^^^^''•''- ^!^^^^^ ganglion; vesicula seminalis , ^\i\ch opens to the ^ol^^\sp:'^'inLluAe^^^^^ exterior. deferens ; v.s. vesicula seminalis. ■ov. od. \--ts. >—v. s. 620 THE INVERTEBRATA The eggs are laid in the sea and develop rapidly, passing through typical blastula and gastrula stages, after which the coelom is deve- loped as a pair of anterolateral pouches of the archenteron (Fig. 430 A). After gastrulation two cells become very prominent. These are the mother cells of the generative organs. The primary coelomic cavity is mes. Fig, 429. Transverse section through middle of trunk of Sagitta bipunctata. After Burfield. al.c. alimentary canal (intestine) ; gl.c. gland cells (the cells which are not stippled are absorptive cells) ; lat.fn. lateral fin ; mes. mesentery ; od. oviduct; ov. ovary (covered by endothelium); sp.d. sperm pouch. std. hd-coe. -\-