THE INVERTEBRATA LONDON Cambridge University Press FETTER LANE NEW YORK • TORONTO BOMBAY • CALCUTTA • MADRAS Macmillan TOKYO Manizen Company Ltd All rights reserved ,4 THE INVERTEBRATA , Jl Manua/ for the Use of Students '^(a^ by L. A. BORRADAILE Fello • ■ r A- ■ r 1 2 3 % I . ■:^-'\r>'.'-y ' 1 I \ ' r "\.vrS ?->r;iT>-'-'..-'V^ 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. at an angle which has no necessary relation to the direction of the stimulus, and may again bring the individual into the unfavourable PROTOZOA 35 circumstances. The reaction is then repeated. Thus the organism is shepherded by its reactions in the direction of the optimum conditions. 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 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. 6 C, 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 con- tent than the medium in which they are suspended (radiolarians, Globigerina, heliozoa. ; Figs. 32, 33, 69, 71). In fresh waters their species have the same cosmopolitan distribution as other small fresh water 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 shorter periods. In this matter protozoa are particularly subject to the^H of the medium, its dissolved organic contents, and its saline contents. Thus Polytoma requires an acid medium, Spirostomum a slightly alkaline one, and Acanthocystis pronounced alkalinity. Euglena viridis and Polytoma live in highly nitrogenous infusions, Actino- sphaerium 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 con- ditions of salinity, but Polystomella ranges from the sea into brackish waters. For many holophytic protozoa the amount of sunlight is 3-2 36 THE INVERTEBRATA T/ww|«ffft!n^ B \\\ Fig 32. Radiolaria without skeleton. A, Thalassicolla pelagica, x 20. After Haeckei. B, Collozoum inerme. C, A central capsule of the same, more highly magnified. After Doflein. caL calymma; cps. central capsule; tiu. nucleus; oil. oil globule; ps. pseudopodium ; y.c. "yellow cells" (Zooxanthellae). PROTOZOA 37 important. Others, as Euglena viridis, bleach in the absence of Hght, but can still flourish if the presence of organic matter in solution makes 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 c.vac. f.vac Fig. 33. Actinosphaerium eichhornt, 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;ac. food vacuole which has just swallowed a rotifer; ps. pseudopodia. protozoan faunas. The powers, possessed by freshwater protozoa, of distribution across inhospitable regions and of surviving unfavourable conditions at home are no doubt due to the facility with which they form resistant cysts (p. 16). 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 38 THE INVERTEBRATA bodies, and those of very foul waters, are branches of the aquatic fauna: they include many flagellates, Umax amoebae (p. 63), 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 else- where. It has important effects upon the fertility of the soil, by de- vouring valuable bacteria. Perhaps the only truly subaerial members of the phylum are certain mycetozoa. Pai-asitic 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. 107), ecto- parasitic (as Oodinium, p. 49), inhabitants of internal cavities (as Opalina, p. 99), tissue parasites (as Myxobolus, p. 95), or intracellular (as Plasmodium, p. 86). They show, according to their degree of parasitism, the same peculiarities as other parasites — reduction of organs of loco- motion, 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 sub- stances, as the malaria parasites do. Many are specific to a particular host or hosts : not infrequently there are two successive hosts belonging to different phyla; thus Aggregata passes from the crab to the octopus, the malaria parasite from man to the mosquito. Symbiosis of various kinds is practised by both holophytic and holo- zoic protozoa. Instances of this are described below, on pp. 42, 63, 104. 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- Fig. 34. Oodinium poucheti, parasitic on Oikopleura. A, An Oikopleura bearing the para- sites. 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". PROTOZOA 39 trasts the Sarcodina under the name of Gymnomyxa with the other classes, or Corticata^ on the ground that the latter possess a firm ectoplasm. 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 speciaHza- tion 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 Sarco- dina ; 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 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 40 THE INVERTEBRATA 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. 24). In the Dinoflagellata 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 Eugleiia 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. 24. The mitoses (see p. 19) 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 diflference between gametes are found, 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. 51) isogamy and anisogamy are facultative ; and various species of Chlamydomonas (see p. 26) 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. 14), and the Zoomastigina, 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. MASTIGOPHORA 41 Owing to this fact it is impossible to frame a definition which will enable every member of each subclass to be recognized as such without comparison with other species. Certain characteristics, how- ever, 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 holo- phytic 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 are probably also saprophytic. Certain species (Ochromonas, 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 aspyrenoids, 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, 42 THE INVERTEBRATA 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 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 Cotivoluta roscoffensis. From Keeble. chl. chloroplast; e. eye-spot; nu. nucleus ; ^>'r. pyrenoid. 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 for which there are no chromatophores. These facts show how the colourless species may have arisen. Members of various orders of the Phytomastigina (cryptomonads, a chrysomonad, a chlamydomonad, and perhaps dinoflagellates) are known to live in the resting stage as symbionts in holozoic organisms (other protozoa, sponges, coelenterates, worms, etc.). Nearly all are PHYTOMASTIGINA 43 yellow or brown {Zooxanthellae) ; most green symbionts (Zoochlorellae) are algae belonging to the Protococcaceae. An exception to this is the 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 a symbiosis benefits by a supply of carbon dioxide and the nitrogenous excreta of its host ; the latter has waste matters removed, is suppHed with oxygen, and sometimes draws on the supply of carbohydrates manufactured by the guest, though it rarely relies upon this alone. 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. 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, 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 c\\i2ites,Noctiluca, etc., also harbour holophytic symbionts. Zooxan- thellae 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 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 Phytomastigina, that which contains the most highly organized members is the large and protean group ^ig. 37. Lithocircus annu- Dinoflagellatay characterized by the posses- laris. After Lankester. cps. sion of two flagella, one longitudinally central capsule ;«m. nucleus; directed and the other transverse, usually ^^jj-^poreplate; 3;.^. yellow in a groove around the body but in a few cases twisted about the base of the longitudinal flagellum. Three of the remaining orders differ from the rest in the possession, in the anterior part of the body, of a pit ("gullet") or groove, from which the flagella usually arise. One of these, the Cryptomonadina, has simple contractile vacuoles and its carbohydrate reserves are of starch : it is held by some authorities to be related to the ancestors of the dinoflagellates. The second, the Euglenoidma, has a more complex 44 THE INVERTEBRATA 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 Vohocina, the most plant-like of the Mastigophora, with green chromatophores (except in a few colourless genera) and starch reserves; and the Chysomonadina, by some regarded 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. 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. Known to be capable of all three modes of nutrition. 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. 38 D-D 2). One flagellum; one chromatophore. Passes most of its life in the resting stage, which by division forms a plant-like growth (see p. 42). In fresh waters. Rhizochrysis. Flagella normally lacking ; one chromatophore ; body naked and permanently amoeboid. Fig. 38. Chrysomonadina. A, Chrysamoeba 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, " Plant " of Hydrunis. D^ , Tip of a branch of the same. Dg, Flagellate stage ("swarmer") of Hydrunis. E, Syraco- sphaera pidchra, x 2000. F, Distephamis speculum, x 800. After various authors, with modifications, cph. chromatophore ; cth. coccolith; /ew.leucosin; nu. nucleus. 46 THE INVERTEBRATA 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 chromato- phores (sometimes green); a case composed of calcareous plates (coccoliths) or rods {rhabdoliths) enclosing the body. Marine, plank- tonic, 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, 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 EU GLENOID I NA 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; para- mylum reserves ; and contractile vacuole fed by accessory vacuoles. The nutrition is interesting. Some species, at least, can live and multiply, though slowly, with purely holophytic nutrition. None, however, flourish unless some organic matter be present, and the presence of peptones is particularly favourable. If the medium be ^ 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. 39. Cryptomonadina and Euglenoidina. A, Cryptomonas ovata, x 900, B, Chrysidella schaudinni, in the resting stage. C, Cyathomonas truncata, X 1000. D, Euglena viridis, x 400. D', A longitudinal section of the anterior end of the same, more highly magnified. E, Peranema trichophorufn, x 850. F, Copromonas subtilis, 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; f. vac. food vacuole;^, flagellum; gu. gullet; nu. nucleus; pmy. paramylum grains; res. reservoir; rod. stiffening rods of gullet; stch. starch grains ; tri. trichocysts. 48 THE INVERTEBRATA very rich in organic matter the chromatophores grow pale and shrink even in the Ught, and in the dark this happens even with a low per- centage of such substances. 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. 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 PHYTOMASTIGINA 49 together for a considerable time as a chain. The occurrence of syn- gamy 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 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. 6). The protoplasm contains peculiar nematocyst-like organs. Holozoic. 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. Dinamoebidium. Colourless and holozoic; completely Amoeba-\ik.Q 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 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. The animal 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-Hke growths. Marine. 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. 50 THE INVERTEBRATA Of all the Mastlgophora, the members of this order most closely resemble the typical plants. 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. 26). In fresh waters. Polytoma (Fig. 24). A colourless Chlamydomonas', retaining the eye-spot (usually) and the habit of starch formation ; but with the t-ann. 11 tc- Fig. 40. Dinoflagellata. A, Ceratium macroceras, x about 300. B, Polykrikos schwarzt, x 250. C, A discharged "nematocyst" oi Polykrikos. After various authors, with modifications, ann. annuli; cph. chromatophore ; cu. cuticle; In.fl. longitudinal flagellum ; rite. " nematocyst" ; nu. nucleus ; sul. sulcus ; sut. suture between plates of cuticle ; tr.fl. transverse flagellum. cuticle composed of some substance which does not give the cellulose reaction. Nutrition saprophytic by means of simple substances (fatty acids, amino acids, etc.). Syngamy is facultatively hologamy or merogamy, isogamous or anisogamous, according to the age of the gametes. In infusions of decaying animal substances. Carteria (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 roscoffensis . Haematococcus (= Sphaerella, Fig. 42). Differs from Chlamy- VOLVOCINA SI 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, free-swimming colonies of 16 or 32 pear-shaped zooids, each with the organization of a Chlamy do- monas, closely pressed together with the narrow end inwards and the Fig. 41- Fig. 41. Noctiluca, x 65. A, Ordinary 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. From West. A-C, Individuals in ordinary phase, showing strands of protoplasm from body to cuticle. D, F, E, Successive stages in fission. G, H, Individuals in resting phase. flagella outwards. An additional cellulose envelope containing mucilage encloses the whole colony. The colonies are reproduced in two ways : (i) asexually, by the repeated fission of each zooid to form a group of 16 like the parent colony, the dissolution of the colonial and zooid en- velopes, 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, 4-2 52 THE INVERTEBRATA the gametes differ in size. They unite indifferently, 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 envelope, 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 macrogametes, while in others each zooid divides into a bundle of 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 re- mains within the parent body. Some of the colonies have already produced flagella, and will shortly break out of the parent cell, b-g, Stages in sexual reproduction — h. 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 ygote on germination, g, New colony produced by vegetative division of the motile individual. 16-64 slender individuals (microgametes), which are set free and fertilize the individuals of a macrogamete (female) colony. Pleodorina (Fig. 3 b, 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 having a much smaller pro- portion of reproductive zooids. Those zooids which perform asexual VOLVOCINA 53 reproduction are known 2iS 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 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 repro- ductive units: daughter colonies (d.c.) produced asexually by division of a single zooid ; ripe macro gametes or young zygotes (z) ; and young " antheridia " {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 micro- gametes are seen sideways and in two, endways. remain for a time before they are set free. The clusters (antheridia) of microgametes arise in the same way. In some species the micro- 54 THE INVERTEBRATA gametes are considerably modified, 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. Vc..£ V4£. 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.2. 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 (mw) by the cell- walls; the unaltered middle layer of the walls {m.L) is still visible. Proto- plasmic strands (p.c), fine in the one species and thick in the other, connect the zooids. Otherwise each zooid, with its single curved chloroplast (ch), eye-spot and two flagella, has the structure of a Chlamydomonas . 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. 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 (w), 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 cells have been omitted. 5^ THE INVERTEBRATA 1. The Zoomastigina never have starch or other amyloid reserves. 2. They often have more than two flagella. This is very rare in the Phytomastigina. 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. 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. 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 vidth one nucleus; freshwater species multinucleate. 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. 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 pelHcle 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 1 Helkesitnastix, a coprozoic member of the Protomonadina, performs hologamy. ZOOMASTIGINA 57 ■>;; / #-^^-flf^: ^-^rh. "■-^v^ m^,^^.^^ .?^ W'r^^' - „J.^^ J'' .i^^^ W^.'. '•" •^ --- Ji'J.JJ,' J^^^^ S^i{ "vA'-.-i V- 5.V" jC^^f'^J j»Vi^&, '"'r ■-■':'. "^^"i^Stli '%^mj''^^r.- _,,,^^ ;f' ■''■"^^^^^--;;;_ •f^yio i •"i^ifot^i) • ..'''«'■ ^^1^: '''■'""I '-.-.- . i?-^2^. \fl. u.me. ,-u.me, .ha.gr. p.hj. Fig. 47. Zoomastigina. A, Mastigamoeba aspera, x about 300. B, Otkomonas termo, x 2000. C, Monas vulgaris, x 2000. D, Bodo saltans, x 2000. E, Try- panosoma brucei, 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 ; f.vac. food vacuole; ji. flagellum; fl.' trailing flagellum; M. position of mouth-spot; nu. nucleus; p.by. parabasal body; ps. pseudopodium ; rh. rhizoplast; u.me. undulating membrane. 5^ 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- mus, in whose gut they assume the form of Herpetomonas . In man they cause kala-azar and Oriental sore. -fl- -p.by. A V C V D Fig. 48. A diagrammatic comparison of various Trypanosomidae. A, Her- petomonas. B, Leishmania. C, Crithidia. D, Trypanosoma, ba.gr. basal granule ;y?. flagellum; nu. nucleus; p.by. parabasal body; u.me. undulating membrane. 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, so that the disposition of the flagellum recalls that of the trailing flagellum of Bodo. Many species, all parasitic in the blood and other fluids of vertebrates, and nearly PROTOMONADINA 59 all (not T. equiperdum^ which is transmitted by coitus) distributed by a second, invertebrate, host, which is usually an insect for terrestrial species and a leech for aquatic species. In the invertebrate the try- panosome 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.gambiense, trans- mitted by a tsetse fly) by the bite of the invertebrate, in others (e.g. T.lewisim the rat, transmitted by a flea) by the invertebrate or its faeces being swallowed by the vertebrate. The pathogenic species appear always to 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 Fig. 49. Choanoflagellata. A, Monosiga brevipes, x 1200. B, Codosiga um- bellata, x 310. Both after Saville-Kent. C, Ingestion in Codosiga. f. vac. food vacuole. capable of reinfecting the vertebrate. T. cruzt, the cause of Chagas* disease in man in South America, is non-pathogenic 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 verte- brate host, it passes most of its time, and reproduces, as 2. Leishmania form, in the tissues. T.gambiense and T. rhodesiense, causes of sleeping sickness in man when they have passed into the cerebrospinal fluid, and T. brucei, 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. Choanoflagellata (or Choanoflagellidae) . Uniflagellate , generally fixed forms; with a protoplasmic collar around the base of the flageflum. Ingestion by attraction of particles by the flagellum to the outside of the collar, adherence to this, and transference by streaming of proto- 6o THE INVERTEBRATA plasm to the base of the collar, where they are received by a vacuole which is formed between the cuticle, if present, and the protoplasm (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. Fig. 50. Trichomonas muris, semidiagrammatic. From Hegner and Talia- 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; ft. anterior flagella; fl.' posterior flagellum ; kar. karyosome; M. mouth (cytostome) ; tiu. nucleus; p.hy. parabasal body; u.me. undulating membrane; ii.me.' posterior flagellum lying along the edge of the undulating membrane. 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.) POLYMASTIGINA 6i Body roughly egg-shaped ; with four flagella, of which one is directed backward and united to the body by an undulating membrane ; a cyto- stomenear 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 organ. In the cytoplasm, a number of ''chromatic granules" are also present. Fig. 51. Giardia intestinalis, from the intestine of man. Semidiagrammatic. axs. axostyle (axoneme) ; ba.gr. basal granules; ce. centriole; cone, ventral con- cavity (" sucker ");j^. fibre around concavity; 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. Several species, parasitic in various cavities of vertebrates, including the mouth, intestine, and vagina of man. Hexamitus {=OctomituSj Fig. 4). (Diplomonadina.) Body elon- gate ; without gullet ; presenting strong bilateral symmetry ; and bearing 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 group of basal granules, with which they are connected. Intestinal parasites of vertebrates. 62 THE INVERTEBRATA Giardia {=Lamblia, Fig. 51). (Diplomonadina.) Shaped like a half- pear, broad end forwards, with, on flat side, a concavity for adhesion. Organization as Hexamitiis but all flagella in middle or hinder region. Parasitic in intestine of man and other mammals. Trichonympha (Fig. 52). (Hypermastigina.) Body narrower in front than behind ; provided with very numerous flagella arranged in three distinct sets; without gullet. At the front end is a papilla. The 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; nii. nucleus; obl.f. oblique fibres (rhizoplasts) ; ^eZ. pellicle ; ^05^.^. posterior flagella; surf.rdg. surface ridges of pellicle; tr.my. transverse myonemes. 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 myonemes. PROTOZOA 63 In the conical front region on which the first set of flagella stand, the rhizoplasts and basal granules are merged to form converging 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 centrlole. Trichonympha is symbiotic with termites, In whose gut It lives (p. 412). 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 contained In the endoplasm of the hinder part of the body of Trichonympha, Into which they are Ingested by the cupplng-ln of this region under the action of the myonemes of the fore part. 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 Foramlnlfera, 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 Foraml- nlfera by Intermediates (as Lieberkiihnia and Allogromia). There is, indeed, a continuous series from naked amoebae to such foramlnlfera as Polystomella, and the drawing of a boundary line between the groups of which they are typical Is a matter of convenience. Naegleria (Fig. 53). Small amoebae which live in various foul infusions ; 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 64 THE INVERTEBRATA 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 pseudo- podia; contractile vacuole; and no flagellate phase. Various species. The true A.proteus is the largest of the common Amoebae^ has a lens- t^ 0 Fig. 53. Naegleria bistadialis, x 800. Partly after Kiihn, in Doflein. A, Amoe- boid condition. B, Transition to flagellate condition. C, Flagellate condition. con.vac. contractile vacuole; rh. rhizoplast. shaped nucleus and longitudinal ridges on the ectoplasm, forms spores endogenously in the unencysted condition, and is not a diatom feeder. 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 individual, in which they are dissolved and set free their contents, AMOEBINA 65 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 occasionally abscesses of the liver and other organs, differs from E. coli in having a distinct ectoplasm, in the central position of the '^i-^^-A Fig. 54. Amoebae. From Hegner and Taliaferro, after Schaeffer. A, A. pro- tens, a^, Equatorial view of nucleus, a^polar view of nucleus. «^, 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. 6^, 40 X 18. c^y 40 X 32. a*, maximum 4-5. b^, maximum 2-5. c^-c^°, maxima 10 to 30. karyosome 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 organic debris. The cytoplasm contains glycogen granules (see p. 15). Bi 5 ecp. en p. nu. B J'lac. Fig. 55. Entamoeba, x about 2000. After Dobell and O'Connor. A, E. histo- lytica. B, E. colt. b.c. ingested red blood corpuscles; ecp. ectoplasm; enp. endoplasm; f.vac. food vacuole; kar. karyosome, eccentric in E. coli; nu, nucleus; ps. pseudopodium. V CD Fig. 56. Fig. 56. A diagram of the life cycle of Entamoeba coli. a-f, Encystment and formation of amoebulae. x-z. Binary fission (in gut of host). Fig. 57. Pelomyxa palustris. Partly after Doflein. f.p. undigested particles swallowed with food; ^Zy. glycogen granules; nu. nuclei (stained). 1 SARCODINA 67 Order FORAMINIFERA Sarcodina which 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. A SCO Fig. 58. Binary fission of Euglypha 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. The reproduction of the single-chambered forms (Monothalamia) is both by binary and by multiple fission. In binary fission, Lie- berkiihnia 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 this group there does not usually appear to be a regular alternation of sexual and asexual 5-2 6S THE INVERTEBRATA reproduction. In the Polythalamia binary fission does not occur, and in some cases, 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 att. ?fi- g. vac. nu. -r>«V^,^, Fig. 59. Arcella discoides, x 500. From Leidy. A, Seen from above. B, Seen from the side, optical section, sh. shell; ^5. pseudopodia; op. edge of opening into shell ; att. thread attaching animal to inner surface of shell ; nu. nucleus ; f.vac. food vacuole ; g.vac. gas vacuole. nuclei and a chromidium present. Gas vacuoles in the protoplasm are said to contain oxygen and to have a hydrostatic function. Reproduction by binary fission, or by budding to form amoebulae with fine pseudopodia (Nucleariae). In fresh waters. Difflugia (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 FORAMINIFERA 69 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. LieberkUhnia (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. 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. 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. 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 ox 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 by extension at their openings. Shells with more than one chamber 70 THE INVERTEBRATA 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. 6 B), or in a spiral, as in Polystomella, etc. (Figs. 62, 63, 65), or occasionally irregularly; and the shell may be strengthened by the deposition, upon their original 4 J3 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; b, septum; c, anterior wall of last chamber; d^ supplemental skeleton. B Fig. 63. Dimorphism of Nummulites laevigatas, Bracklesham Beds (Eocene), Selsea. From Woods. A, Section of the entire shell of the megalospheric form, X 9. B, Section of the central part of the microspheric form, x 9. 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 2Lre 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 SARCODINA 71 of generations in the life cycle, the microspheric form reproducing asexually while the megalospheric form 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 v^^aters. Rhabdammina (Fig. 6 A). Shell one-chambered, straight or forked, tubular, composed of foreign particles. Marine. Nodosaria (Fig. 6 B). 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. Globigerina (Fig. 6 C). Shell perforate, calcareous, chambers fewer and less compact than in Polystomella, arranged in a rising (helicoid) spiral, and bearing long spines. External layer of proto- plasm frothy, with large vacuoles by which the specific gravity is reduced. Marine, pelagic. 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. 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 (Spumellaria), gathered into groups in the Actipylaea {Acantharia), concentrated into one '*pore plate" in the Monopylaea (Nassellaria), and represented by three openings or "oscula" in the Tripylaea {Phaeodarta). The spicules are usually siliceous, but in one Fig. 64. Allogromia oviformis, x 230, but the pseudopodia less than one- third their relative natural length. From M. S. Schultze. sh. sheW; 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. FORAMINIFERA 73 Fig. 65. Polystomella crispa, x 45. After M. S. Schultze. sh. shell; ppm. a mass of protoplasm formed by the fusion of pseudopodia ; ps. pseudopodia. The retral processes are darkly shaded: the external protoplasm is not visible. 74 THE INVERTEBRATA o.wh. qam. ^^^nu. Fig. 66. Stages in the life cycle oi Polystojnella. 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. retral processes; i, first chamber. RADIOLARIA 75 group (Acantharia) they are said to be of strontium sulphate. They are rarely absent, occasionally loose, but usually united into a lattice-work (Figs. 67, 68), which is often very complicated, with projecting spines. The latter maybe 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 Fig. 67. Fossil Radiolaria. From Woods. A, Lithocampe tschernyschevi, Devonian. B, Trochodiscus longispinus, Carboniferous. C, Podocyrtis schom- burgki, Barbados Earth (Tertiary). A and C, Nassellaria; B, Spumellaria. belong to parasitic members of that group. It is possible, on the other 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. 19, 20). Symbiotic flagellates, known as "yellow cells" {Zooxanthellae^ see pp. 43, 46), are present in large numbers in the cytoplasm of many of the Radiolaria. Thalassicolla(Fig.22A). (Suborder Spumellaria.) Skeleton absent or represented by some loose siliceous spicules ; one nucleus ; yellow cells in extracapsular protoplasm. 76 THE INVERTEBRATA 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 skA sk.2 ^-nu. cps. Fig. 68. A, Heliosphaera inermisy x 350. After Hertwig. B, The skeleton of Actinomma. After Biitschli. sk. skeleton; cps. central capsule; nu. nucleus. The yellow cells are shown, but not labelled, in A. Structures known as " myophrisks ", surrounding the spines of this 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. RADIOLARIA 77 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. 78 THE INVERTEBRATA 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 pseudo- podia 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 press towards the body prey which has adhered to them, their axial filaments are temporarily absorbed at the bend. 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. 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 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 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. ^ Paedogamy is a kind of autogamy in which not only the nucleus but also its cytoplasm divides and reunites. i Fig. 70. Dimorpha mutans. Partly after Blochmann. A, In the flagellate phase, alive. B, In the heliozoan phase, stained, with pseudopodia as if alive. ba.gr. basal granule; esm. chromatic matter which will condense to form the endosome ; /.z^ac. food vacuole;^, flagella; nu. nucleus; ps. pseudopodia. Fig. 71. Actinophrys sol, x about 800. From Bronn. ecp. ectoplasm; enp. endoplasm; c.vac. contractile \ac\io\e;f.vac. food vacuole; nu. nucleus; ps. pseudopodium. 8o THE INVERTEBRATA Actinosphaerium (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 Fig. 72. A-F, Successive stages in the autogamy of Actinophrys sol. After Belar. ecp. ectoplasm; enp. endoplasm;y. jelly coat; ps. pseudopodium put out by cJ, the male gamiete, towards ?, the female gamete. cytoplasm divides into as many corpuscles as there are nuclei. Each of these then undergoes a process similar to that which occurs in Actinophrys y forming a zygote which hatches as an independent individual. In fresh waters. SARCODINA 8i 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. 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. 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 sclerotium. It prepares for reproduction by BI 6 82 THE INVERTEBRATA 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. Class SPOROZOA 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. 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. 32, 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 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. PROTOZOA 83 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. 26). 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. 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. 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. Eitneria (Fig. 74) is parasitic in the intestinal epithelium of various vertebrates and invertebrates. E. schubergi, from the intestine of the centipede Lithobius, may be described as a type of the suborder. The spherical trophozoite (agamont) undergoes schizogony (agamo- gony) 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 ^ The term syzygy should perhaps be restricted to such pairs. 6-2 84 THE INVERTEBRATA 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 Fig. 74. A diagram of the life 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 (syngamy). 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. female hologamete. In the male gamont the nucleus divides several times, and the daughter nuclei are set free with portions of the cyto- plasm as biflagellate male gametes, which are thus merogametes. COCCIDIOMORPHA 85 The gametes leave the host cell and unite while free in the gut cavity. The zygote nucleus undergoes what is probably a reduc- tion 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 sporocyst) in which it divides into two sporozoites. Thus sporogony takes place in two stages. In each of these there is some residual protoplasm. 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 sporoblasts, 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 difl^er 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 undergoes syzygy, the organism would appear to belong to the Adelea stock, whereas the Haemosporidia are probably related to Eimeria. Schellackia and Lankesterella, which have no syzygy, are transitional to the Haemosporidia, under which (on p. 86) their life histories are described. 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. 86 THE INVERTEBRATA (i) Schellackia (suborder Coccidia), parasitic in the gut of a tortoise, 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 red corpuscles, which then break up, setting free the schizozoites (merozoites) and also products of the metabolism 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 sporozoites free in the blood of the insect. They make their way into the salivary glands and are injected with the saliva into a HAEMOSPORIDIA 87 Fig. 75. A diagram of the life cycle of Plasmodium vivax. From 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 zygote of endoderm cell of mosquito. 15, Encystment. 16, Sporoblasts formed by division of zygote (sporont). 17, 18, Formation of sporozoites. 19, Invasion by latter of salivary gland. 20, Sporozoites injected into blood of a man. 88 THE INVERTEBRATA 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. Piroplasma (= Babesia). Organisms of doubtful affinity, possibly related to the Haemosporidia, which infest red corpuscles of mammals and are transferred by ticks. In the corpuscles, they are unpigmented bodies, round or pear-shaped according to stage, which multiply usually by binary fission. Details of the stages in the tick are un- certain. At least one species passes from generation to generation of the tick through the ovum. Piroplasma is not found in man, but is the cause of red- water fever in cattle and fevers in dogs. 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 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 TELOSPORIDIA 89 swallowed by new members of the host species, in whose intestine the spore cases are digested and the process repeated. 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 sevetal generations of schizogony, they become free gamonts, enter into syzygies, encyst, and within the 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. 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 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. 90 THE INVERTEBRATA Suborder EUGREGARINARIA Gregarinidea which have no schizogony. The adult trophozoite has a stout cuticle and the ectoplasm contains myonemes, longitudinal or transverse, or both. Partitions of the ectoplasm without myonemes may (Fig. 80 F) divide the body into 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 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 ; nii. nuclei of agamont; nu.' gamete nucleus ; nu." nuclei of enveloping (residual) protoplasm; spz. sporozoites; str. striated border of epithelium of Malpighian tubule; zyg. zygote. 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 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. EUGREGARINARIA 91 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 containing several sporozoites) are each the whole product of a zygote 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. (i.e. are oocysts), whereas in the coccidians, where the female gamete is a hologamete, the zygote forms, by means of a generation of sporoblasts, several such spores in its oocyst. 92 THE INVERTEBRATA Monocystis (Fig. 79). Without divisions of the body. Para- sitic in seminal vesicles of earthworms. Several species, some iso- gamous, others anisogamous. The spores escape either down the 1 Fig. 79. Fig. 79. Monocystis. From Borradaile. Ay M. magna, x 2$. B, M. lumbnci, 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. longa 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. vasa deferentia 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 SPOROZOA 93 reach the vesiculae semlnales, 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. 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 CNIDOSPORID I 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. 81 I), 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 /)o/^ 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. Fig. 8 1 . 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 more pansporoblasts and multiplica- tion of syncytium by plasmotomy. K, L, Development of spores in a pansporo- blast. M, Fully formed spore, before conjugation. N, Ripe spore after con- jugation, env. envelope cell; nu.' nucleus of spore case; nu." nucleus of pole capsule; sph. 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. I NEOSPORIDIA 95 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. 8i). Large syncytia in the tissues of various freshwater fishes. Some species are harmless, others dangerous pests. 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. 82). 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, pp. 13, 14 ; the contractile vacuole system, p. 16 ; the nucleus, p. 21 ; conjugation, p. 28; 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. 96 THE INVERTEBRATA Subclass CI LI AT A Ciliophora which as aduhs possess cilia; and which do not possess suctorial tentacles. } i •>_.. B m. Fig. 82. Sarcocystis li?idemanni, from the vocal cords of man. After Baraban and Saint-Remy. 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; zv. wall of parasite. The morphology of this group is much affected by the disposition of the apparatus used in obtaining nutriment. The food may be ab- sorbed through the surface: the shape of the body is then simple (Figs. 5, 86 A). Nearly always, however, there is a mouth. In some CILIATA 97 of the lower genera this is anterior and terminal, or nearly so (Fig. 88 A), but usually it is removed to one side of the body (Fig. 86 E). This side is then said to be "ventral", and that opposite to it is "dorsal". The mouth, either is merely a soft patch of exposed endoplasm or possesses 2. gullet (p. 15). In a relatively few cases (including all those in which the mouth is terminal and a few of those in which it is 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, Vorticella). 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. 16, 83 A). The higher forms have along the outer edge of the peristome a food- gathering row of cirri or membranellae, the adoral wreath (Fig. 89, ad.mae.). Typically, the spiral is open, as in Paramecium ^ but in some cases, as in Stentor (Figs. 83 B, 88 C), it has contracted, 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. 89, 90). 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 oif the peristome 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 98 THE INVERTEBRATA twisted in the opposite direction, makes this view impossible. The origin of the Peritricha may be explained as follows (Fig. 83). In 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. 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. Fig. 83. A diagram of the disposition of the peristome in various ciliata. 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 ; ^er. 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. 84) 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- I CILIATA 99 bedded in the cortex. Either the cortex or both it and the alveolar layer may be absent. In Paramecium the cortex is covered by a thick pellicle which possibly contains a minute alveolar layer. rid. tri. fH Fig. 84. Details of the ectoplasm of ciliates. After Wetzel. A, Frontonia leucas (Vestibulata). B, Paramecium. C, The same, surface view. afo. alveolar layer; ba.gr. basal granules; cor. cortex; enp. endoplasm ; y?. flagellum ; yZ.' 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.^, 21 0,85). With several, usually many, nuclei, which are all alike. In each nucleus, however, there can be distinguished Fig. 85. 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. 7-2 100 THE INVERTEBRATA two kinds of chromosomes, which are held to represent the chromatin of the mega- and micronuclei of other ciliates. 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 plasmotomy 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. 86 A). Reproduction by repeated budding at one end of the elongate body, forming a chain. Parasitic in the branchial limbs of Asellus. 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 number of meganuclei, but no micronuclei visible in the adult. Parasitic 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. 88 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. Fig. 86. Various Ciliophora. A, Anoplophryaprolifera, x 200, after Saville- Kent. B, Entodinium caudatum, after Schuberg. C, Tintinnidium inquilinum, after Faure-Fremiet. D, Trichodina pediculus, 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. 102 THE INVERTEBRATA 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. 86 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. Colpidium. As Colpoda ; but with undulating membrane. Common in infusions, freshwater and marine. Paramecium (Fig. i6). Slipper- or pear-shaped according to species; with undulating membrane^; and peristome. Common in infusions, freshwater and marine. 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. 87 A). Egg-shaped; the peristome a deep groove at the anterior end. Parasitic in the rectum of frogs, the in- testine of man (where it is occasionally harmful), etc. Nyctotherus (Fig. 87 B). Kidney-shaped; with permanent anus. Parasitic in the rectum of frogs, the intestine of man, etc. Spirostomum (Fig. 88 B). Rod-shaped; with the peristome as a long groove; meganucleus beaded; several micronuclei. In fresh waters and marine. Stentor (Fig. 88 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. 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 ^ It is said that Paramecium has four parallel undulating membranes. CILIATA 103 Fig. 87. Ciliata from the rectum of the frog. From Borradaile. A,Balantidium entozoon, x 65. B, Nyctotherus cordiformis, x 130. an. anus; c.v. contractile vacuole; meg. meganucleus; mi. micronucleus ; v. vestibule. Fig. 88. Ciliata. After various authors. A, Prorodon teres, x 500. B, Spiro- stomum ambiguum, x 150. C, Stentor coeruleus, x 50. ad.w. 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. 104 THE INVERTEBRATA Strong membranellae, and are naked on the rest of the body, save some- times for a few cilia or patches of cirri. Tintinnidiiim (Fig. 86 C). (Tintinnina.) Cup-shaped; anchored by an aboral process into a chitinoid case. In fresh waters and marine. Entodinium (Fig. S6B). (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, rendering 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. 89, 90). 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. 28, their morphology on pp.97, 98. The anus and contractile vacuole open into the deep vestibule. The meganucleus is horseshoe-shaped. Trichodina (Fig. 86 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, 91). 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. Carchesiiim (Fig. 92). As Vorticelhy but colonial. In fresh waters. I CILIATA 105 _^-ad.mae. int.u.me.'^- A-^■--rr-- 'ptx^r. Fig. 89. 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 cilia \f.vac. food vacuole ifr.cir. frontal cirri ; gu. gullet; int.u.me. internal undulating membrane; lip, projecting lower lip of peristome ; mar. dr. marginal cirri ; meg. meganucleus ; mi. micronucleus ; pre.ci. preoral cilia; pt.cir. posterior cirri; se.ci. "sensory" cilia of dorsal surface; u.me. preoral undulating membrane (another undulating membrane is present but is omitted, to simplify the figure). jX^m^L \\ Fig. 90. Stylonichia mytilus, from the left side. After Biitschli. io6 THE INVERTEBRATA Fig. 91. Fig. 92. Fig. 91. A group of individuals of Vorticella in various phases of the life history. From Borradaile. a, Ordinary individual. 6, 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). g, Conjugation. Fig. 92. Carchesium epistylidis, x 100. After Saville-Kent. CILIOPHORA 107 Epistylis. As Carchesium^ but the stalk is purely cuticular and non- contractile. In fresh waters and marine. 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. 22), (2) that they form numerous gametes, 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. 93). Shaped like a slender vase. On the gills of GammaruSj etc., in fresh and marine waters. Subclass sue TORI A 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 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. 86 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. 95 B) or formed in brood pouches (Fig. 94) from which they escape when they are ripe. External budding is the more primitive, internal the com- moner process. In either, one bud or more than one may be formed at a time. The buds (Fig. 86 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 offspring by external budding. Some species will, in unfavourable circumstances, io8 THE INVERTEBRATA 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 the other. It is not known what happens to the two zygote nuclei in these cases. The arrangement of the larval ciHa in rings, the prevalence of a 7ni. Fig. 93. Fig. 94. Fig. 93. Spirochona gettimipara, 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. 94. A diagram of the formation of an internal bud by one of the Suctoria. 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. 86 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, 95). Stalked; not seated in a cup; bearing tentacles distally. Reproduction by external, usually multiple, budding. Marine. SUCTORIA 109 Acineta (Fig. i). Stalked; the stalk expanding to form a shallow cup. Reproduction by internal budding. In fresh waters and marine. Fig. 95. 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. 86 F', 94). Dendrocometes (Fig. 86 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 osc. spi.J 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, known as the Olynthus (Fig. 96), 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^ore^, 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. |ierein it and its kind diflPer from all the Metazoa, using the principal opening not for intaking — as a mouth — but for casting out. ^he wall (Fig. 97) of the vase consists of two layers, (a) a gastral layer ^ composed of collared flagel- late cells resembling the Choanoflagellata (p. 59) 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 rim ; and {b) a dermal layer ^ which makes up the greater part of the thickness of the wall and is turned in a little way at 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 Fig. 96. The Olyfithus of a simple calcareous sponge, with part of the wall cut away to expose the paragaster. osc. os- culum; po. pore; spi. spicule. PORIFERA III 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 2iSporocytes, of a conical shape, extend through the jelly, having their base in the covering layer 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. sp por ^P<^ e^ Fig. 97. 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; j. jelly; por. pore; pc. young porocyte; pc.' fully developed porocyte; sp. spicule; sp.c. spicule cell. Besides 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. No sponge remains at this simple stage throughout its life. At the least the body branches and thus complicates its shape, and then often 112 THE INVERTEBRATA new oscula appear at the ends of the branches (Fig. 98). A higher grade is reached when, as in the calcareous sponge Sycon (Fig. 99), the greater part of the vase is covered with bhnd, thimble-shaped out- growths, regularly arranged, and touching in places, but leaving between them channels, known as inhalant (or afferent) canals, whose openings on the surface of the sponge are often narrowed and are inh.c. Fig. 99. Fig. 98. A branched calcareous sponge of the first (Ascon) type. From Sedgwick, after Haeckel. Fig. 99. A semidiagrammatic view of a simple Sycon, opened longitudinally, with a portion of the wall enlarged, inh.c. inhalent canal; fl.c. flagellated chamber. known as ostia. The thimble-shaped chambers are known sls 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 pores, now known as prosopyles, into the excurrent canals, leaves these through the openings, known as apopyles, by which they PORIFERA 113 communicate with the paragaster, and flows outwards through the osculum. A third grade is found in sponges such as the calcareous sponge Leucandra (Fig. 100), 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 . i o i ) , in which successively 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" 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, complication 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 composed solely of calcareous spicules, and their choanocytes are relatively 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 spicules, of which there exist many diff"erent types (Fig. 102), characteristic 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 sponges (Keratosa), of which the bath sponge (Euspongia, Fig. 103) 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 ramiflcations of the paragaster, the irregular appearance of numerous oscula, which 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 Hexactinelltda, in which there is always a siliceous skeleton of six- rayed spicules (Fig. 102 /), the jelly is absent, and the flagellated chambers are thimble-shaped, as in the simpler Sycons; and the Fig. loo. Diagram of a section of the wall of the sponge Leucandra aspersa, showing the direction of the currents. After Bidder. OSQ f\^ I \ ^ ^/ 0 par ei u a t] n i? % J ost S^W exhc ink c Fig. loi. 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; fix. flagellated chamber; osc. osculum; ost. ostium ;/)ar. paragaster;/)or. pore. PORIFERA "5 Fig. 1 02. Various types of sponge spicules. From Woods, a-e^ From Demo- spongiae. /, From a hexactinellid. g and h, From extinct groups of sponges, y, From Calcarea. a, With one axis (monaxon). b and c, With four axes (tetraxon: 6 is a "calthrops", c a "triaene" spicule: tetraxon spicules are found in the Tetractinellida ; the Monaxonida have monaxons only). d and e. Irregular. /, With three axes (triaxon; four six-rayed spicules united as part of a continuous skeleton by additional deposits). 7, A three- rayed compound spicule formed by the union of monaxons. 8-2 ii6 THE INVERTEBRATA Demospongiae^ 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. 105 C). Sponges have free larvae, of several different kinds, but all covered, wholly or in part, with flagellate cells, by which they swim. The remarkable feature of the metamorphoses by which these larvae be- come the fixed adults is that the flagellated cells pass into the interior, develop collars, and become the choanocytes (Fig. 105). 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 Fig. 103. A diagram of the structure of a bath sponge (Euspongia). From Borradaile. exh.c. exhalant canal; inh.c. inhalant canal; fl.c. 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. 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 PORIFERA 117 one family, the Spongillidae, occurs in fresh water, but its members are plentiful and widespread. 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 specialized and dependent upon one another than the cells of a meta- zooQ. Many of them can assume various forms, becoming amoeboid, 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 seem that they took origin from choanoflagellate mastigophora. No\v 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 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. 99). 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. 104). British. Leucandra. Canal system of the third type (Fig. 100). Nuclei of choanocytes basal. British. n8 THE INVERTEBRATA 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 pst. ih.ch ostihxh. 05^ •, / th.ch' / spi' Fig. 104. 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 cells (choanocytes); ^.c/z. flagellated chamber; spi. spicules; ap. exhalant opening (apopyle) of flagellated chamber. 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 Episoanthus. 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 119 Cliona (Monaxonida^). A cosmopolitan genus, which bores into the shells of molluscs and into calcareous rocks. Halichondria^ 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. 117. Cosmopolitan. A der.c. B Fig. 105. A, Larva {Amphiblastula) of Sycon raphanus. B, The same larva 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, showing collar cells. From Vosmaer. ap. apopyle; nu. nucleus; vac. vacuole. Euspofigia, the bath sponge (Keratosa). Mediterranean, West 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. ^ See legend to Fig. 102. 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 other features of the anatomy 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 outer layer of cells is the ectoderm, the inner the endoderm. In the phylum to which Hydra belongs, the Coelenterata, the body is always of this type, whatever form the sac or its layers may assume, though the jelly may contain cells, of various kinds and sometimes plentiful, which have migrated into it from the ectoderm or endoderm. 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 coelente rates, but mesoderm is more plentiful than 1 the cells in the jelly generally are, it contains important organs and usually definite systems of spaces (see p. 122), 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 different animals may be extraordinarily unlike, and, especially when the animal is developed from a very yolky egg, they are sometimes very difficult to recognize as such; but where the gastrula is well formed, as in the familiar development of Amphioxus or in that of a starfish (Fig. 413), 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. / r METAZOA 121 The mesoderm, whose appearance converts the gastrula into a triploblastic body, is not a single entity, but contains components which originate in two different ways, namely: (a) Cells which migrate from ectoderm or endoderm, or from mesoderm of the other kind, into the blastocoele; this kind of meso- derm is known as mesenchyme, and is comparable to the cells which invade the jelly of coelenterata. (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. 437, 413, 405, 402 A), it arises as pouches of the archenteron which separate 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 afterwards appear in it. This happens, for instance, in the tadpole. In yet other cases a single pole cell or teloblast, as in annelids and molluscs (Fig. 189), or a group of a few cells, as in arthropods, separate, on each side of the embryo, from the rudiment of the endo- derm, and multiply so as to form a band of cells in which coelomic cavities appear. In a few phyla (Platyhelminthes, Nemertea, Nematoda, Rotifera) there is no mesothelium. In most phyla, both kinds of mesoderm are formed. 1 Since mesothelium gives rise to mesenchyme, it is often difficult to distinguish between the two, and to decide what part each plays in the formation of the organs; but, broadly speaking, it can be said that the skeletal, vascular, and some muscular tissues arise from mesenchyme, while in a coelomate animal the peritoneum and the organs derived from it — gonads (ovaries and testes), mesodermal kidneys, etc. — and the principal muscles, arise from the mesothelium. After giving rise to mesoderm, the archenteron becomes the rudi- ment of the alimentary canal. Except in Platyhelminthes, the blastO;:^ pore is in various ways replaced by two openings,^ so that(lt)has both mouth and anus. Its wall, the endoderm, forms the Tming of the alimentary canal, except in those regions, known as /ore ^m^ 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. The ectoderm gives rise to the epidermis (epithelium which covers the body) to certam glands, to the excretory organs known asnephridia, ^ Chaetognatha have no mesenchyme, and it is scanty in the lower Chordata. 2 The most primitive way is probably that of Peripatus (p. 283), in which the middle of the blastopore closes and the ends become mouth and anus. 122 THE INVERTEBRATA to the principal external organs of sense, and to the nervous system (in nearly all cases: a part of the nervous system of the Echino- dermata is remarkable in being developed from the peritoneum and therefore mesodermal). The nervous system was no doubt primitively subepithelial, having arisen by specialization of cells of the epithelium for the transmission of impulses due to stimuli received upon the epithelial surface. In a number of cases (Coelenterata, some Annelida, Echinodermata, Enteropneusta, etc.) it remains in that position, but usually it is in a deeper and more protected situation. The central nervous system arose as a condensation of the primitive subepithelial nerve plexus which took place in different positions in different animals. In those which have a long axis it has the form of cords along that axis. The cords may be paired or unpaired, lateral, ventral, or dorsal. Anteriorly they pass into an enlargement which constitutes a " brain " or cerebral ganglion. In the Chordata the central nervous system is hollow; its removal from the surface of the body having taken place not, as usual, by separation from the epithelium, but by the folding-in of the strip of epithelium with which it is connected and which still remains to line its cavity. A similar condition is seen in some echinoderms. Within the massive layer of mesoderm, channels are necessary for the transport of food, excreta, hormones, germ cells and so forth, and often there must be spaces to give play to movements of the viscera. Such facilities are provided by the following two systems of cavities, of which either or both may be present: (a) The primary body cavity, sometimes known as the haemocoeky lies in the mesenchyme, and 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 it has usually the form of a branching system of vessels ("vascular system") through which the fluid is caused to circulate by the contraction of muscular fibres in the wall of some portion of it which is known as a heart. In some cases, how- ever, the haemocoele forms large "perivisceral" sinuses around the internal organs. It never contains germ cells or communicates with the exterior. (b) The secondary body cavity or coelom is from the first completely surrounded and separated from the blastocoele by the mesothelium, 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 METAZOA 123 triploblastic animals, so that these organs are unaffected by the move- ments of the body wall and are able freely to perform movements of their own, may be either coelomic or haemocoelic, but is usually coelomic. In the Arthropoda, where the perivisceral function of the coelom is entirely usurped by the haemocoele, 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 peri- visceral 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 nephridia! 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 consists of intracellular tubes of ectodermal origin, usually branched and bearing at the end of each branch a solenocyte or flame cell {see p. 177). It may be con- tinuous or divided into segmental units, the nephridia. Water 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^; (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 union in various ways of nephridia with coelomoducts or other mesodermal elements (see p. 243). In such cases the nephridia acquire a communication with the coelom, and excreta or germ cells may pass from it through them. In other groups, as in some ^ Watery excreta are sometimes concentrated by absorption during their transit through the passages of the organs of excretion. 124 THE INVERTEBRATA Crustacea, a coelomoduct is supplemented or in great part replaced by an ectodermal component, but there is no evidence that this component represents a nephridium. The body constituted by the foregoing elements has usually a bilateral symmetry^ though this is rarely exhibited completely by all the systems. In the Coelenterata and Echinodermata, however, there is a radial symmetry. It is interesting to find that a sessile life, for which such symmetry seems particularly advantageous, is characteristic of the Coelenterata, and was probably adopted by the ancestors of all the Echinodermata. The terms ventral and dorsal^ which belong by right respectively to those aspects of a bilateral animal which are normally turned to and from the ground or sub- stratum, are sometimes conveniently applied to a pair of structures by which two sides may be distinguished in the body of an animal whose symmetry is predominantly radial. They should, however, never be applied to the oral and aboral aspects of such an animal. Meristic repetition of organs of the body is common in metazoa. It may, as in parts of the body of annelids, affect practically all systems, so that there is a complete segmentaiion of the body into similar somites^ or may be confined to certain organs. In the latter case it is important to distinguish between {a) the repetition of single organs in an unsegmented animal, as the ctenidia and shell plates are in- dependently repeated in the mollusc Chiton, and {b) the condition, presented for instance by the Vertebrata and by much of the body of many arthropods, in which a formerly more complete segmentation now affects only some of the systems to which it at one time extended. The student should beware of thinking that the segmentation of all animals which present the phenomenon is derived from that of a common ancestor. The strobilization of the Cestoda in preparation for the detachment of reproductive units is a very different matter from the segmentation of the Annelida, and that again is far from being, as is sometimes assumed, certainly the same thing as the seg- mentation of the Vertebrata. The anterior end of a bilateral animal is the site of the principal sense organs, of the " brain", and usually also of the mouth, and is often obviously differentiated as a head. In a segmented animal this cephalization may extend to one or more of the anterior somites ; and these usually become part of the head, losing their individuality in the way mentioned in the preceding paragraph, and only betraying their existence by the presence of certain of their organs (ganglia, appendages, etc.). CHAPTER V THE PHYLUM COELENTERATA Metazoa, either sedentary or free-swimming, with primarily radial structure ; the body wall composed of two layers of cells, the ectoderm and endoderm, and between these a layer secreted by them which is originally a structureless lamella {mesogloea) but usually contains cells derived from the primary layers ; within the body wall a single cavity, the enteron, corresponding to the archenteron of the gastrula, having a single opening for ingestion and egestion, and often compli- cated by the presence of partitions or by the formation of diverticula or canals; digestion partly intracellular; the nervous system a net- work of cells with anastomosing processes ; commonly with the power of budding, by which either free individuals or colonial zooids may be formed; and whose sexual reproduction typically produces an ovoidal, uniformly ciliated larva, known as the planula, which has at first a solid core of endoderm. Thus defined, this phylum contains the whole of the diploblastic animals, that is, those in which the space (blastocoele) between ecto- derm and endoderm is either devoid of cells, or contains only cells derived late in development by immigration from ectoderm or en- doderm. Of such animals there are two very distinct stocks — the Cnidaria, characterized by muscular movements, which possess nematocysts (p. 127), and are reducible either to the polyp or to the medusa type (p. 128); and the Ctenophora, which retain the ciliary locomotion of the planula, are without nematocysts, and are not to be assigned either to the polyp or to the medusa type. In the Coelenterata the Metazoa are at the beginning of their evolution and we have a primitive type with great potentialities, though these animals have also already acquired strange specialized features. The tissues consist of two single layers of cells, the ectoderm and endoderm, which constitute a thin body wall surrounding the central cavity (Fig. 106): the only increase in thickness and com- plexity of the body wall that is possible is by development of a gelatinous intermediate layer. Thus, while the typical polyps like Hydra have a very thin layer of this kind, it has become thicker, very much folded and penetrated by cells in the actinozoan polyps and exceedingly thick in the large jellyfish, forming not only a kind of internal skeleton but even a reservoir of food. The principal type of cell found in the tissues, both ectoderm and endoderm, of the primitive coelenterate is the musculo-epithelial cell 126 THE INVERTEBRATA which is columnar in shape and only differs from similar epithelial cells in the higher Metazoa in the fact that it is produced into one or two contractile fibres, which are imbedded in the mesogloea. Such a tissue unit resembles a protozoon in the fact that different parts of the cytoplasm carry on different functions although they are not separated by any partition from each other nor provided with separate nuclei. An endodermal cell of Hydra has an inner border which can be produced into flagella, by means of which the fluid of the body cavity is kept in motion : or these may be retracted and the cell instead puts out pseudopodia to engulf particles of food. In the in- terior of the cell, beyond the border, the food is contained in vacuoles where it is digested, and finally the external border of the cell is pro- duced into permanent cell organs, the muscle fibres or tails already mentioned, in which the cytoplasm can contract with much greater nU. c.nem 7nsg Fig. 1 06. Diagrammatic transverse section of the body wall of Hydra. Altered from Kukenthal. c.gl. gland cell; c.s. sense cell (the other endo- dermal cells of the musculo-epithelial type may be recognized by possessing either a pair of flagella or pseudopodia); cnc. cnidocil; c.nem. cnidoblasts containing nematocysts in various stages; c.i. interstitial cells; c.n. nerve cell; ect. ectoderm; end. endoderm; msg. mesogloea; m.t. muscle tails of the musculo-epithelial cells. force and rapidity than in any other part of the cell. Among the en- dodermal cells, however, some are met with of a more specialized type : gland cells which pour into the cavity a digestive secretion (for the preparatory or extracellular digestion), and sense cells, found also in the ectoderm, which are thread-like, with a short projecting process. Both these kinds have no muscle tails. A type of cell which is even more characteristic of the Coelenterata (except the Ctenophora) than the musculo-epithelial cell is the thread cell or cnidohlast. Though this would appear to have reached the highest peak of specialization it must be pointed out that within the limits of a single cell many functions are performed and a machinery developed which would be formed from a number of different kinds of cells in the higher Metazoa. A thread cell (Fig. 107) is formed from an COELENTERATA 127 undif^erentmtedinterstittalceW^ : part of the cytoplasm becomes gland- ular and forms a large vacuole, filled with a poisonous fluid and lined with a chitinous membrane, of complicated structure. The whole of this secreted body is called the nematocyst. Another part of the cytoplasm round the nematocyst develops muscular fibrillae, and by their con- traction the "explosion" of the nematocyst is caused, an action so violent that the whole cell may be cast out of the animal as a con- sequence. The external part of the thread cell develops a short sensory ctn. Id. bar. _ _ _ .>/*'^- Fig. 107. Nematocysts of Hydra, large penetrating type. A, Undischarged. B, Discharged. From H. attenuata. After P. Schulze. C, Discharged but re- tained within its thread cell. After Will. bar. barbs; hr.zv. (stippled) wound in chitin {ctn.) of prey possibly caused by mechanical action of smaller barbs, the continuation {con.) being due to the solvent action of a fluid from within the nematocyst; c.nc. cnidocil; fil. filament; Id. lid; la. lasso; m. muscular fibrils; nil. nucleus of thread cell. process, the cnidocily which bores through the cuticle of the musculo- epithelial cell in which it lies and comes into contact with the water. The stimulation of these cnidocils, for example if the animal is touched by the appendage of a wandering crustacean, causes a dis- turbance which is transmitted through the body of the cnidoblast to the muscle fibres of the nematocyst to cause explosion, so that within ^ This is a type of cell which preserves an embryonic character and may develop into germ cells and musculo-epithelial cells as well as cnidoblasts. 128 THE INVERTEBRATA a single cell we have the receptor and effector organs which are necessary for a very remarkable reflex action. Lastly, the nematocyst may be attached to the base of the thread cell by the lasso, an organ which helps to restrain the force of the explosion. In Hydra there are no other sense organs in the ectoderm beside the cnidocils of the thread cells. From the high degree of differentiation and the in- dependence of action these cells might almost be considered as separate organisms within the coelenterate if their development was not to be traced from the interstitial cells. The nervous system of coelenterates is one of their most character- istic organs, composed of cells of a special type which are only to be demonstrated by difficult methods of staining. Over the surface of the mesogloea on both sides among the muscle tails there is spread a network of cells (Fig. io8) with very small cell bodies and many fine branches which anastomose into each other and also connect with the sense cells in the ectoderm and endoderm. A sense cell is shown in Fig. io6. It has a rod-like process projecting from the surface and at its other end it ends in slender branches which join with those of the nerve cells. Such sense cells respond to touch and probably also to light and chemical stimuli. If a polyp is touched with a wire the disturbance is transmitted in all directions by the nerve net and results in a general contraction of the muscular system, which may last for long periods. In some cases coelenterate polyps are only capable of expansion in the absence of light. This ''nerve net " is the most primitive type of nervous system. The cells which compose it differ from the nerve cells of higher Metazoa in their simple structure, in the fact that the processes of one cell are continuous into those of the cells which surround it,^ and above all in the fact that they are arranged in a diffuse fashion, and not aggregated along particular lines. This is at any rate true for the more primitive polyps : in the medusae and the more differentiated polyps the nerve system tends to concentrate in special parts but not in such a fashion as to form any kind of a central nervous system. In the higher Metazoa such a concentration has taken place, and with the exception of the echinoderms, a nerve net only occurs in certain organs like the gut. Much of the interest of the coelenterates lies in the conflict between the two modes of life, an easy sedentary existence and a wandering or rather freely-drifting life which demands a larger measure of activity and a greater elaboration of structure and physiological development. The two types of individual which correspond to these modes of life are the Polyp and the Medusa. There are large divisions of the coelenterates in which only one type is present, while in the ^ Synaptic junctions such as occur elsewhere in the Metazoa have, how- ever, been recently demonstrated in the Scyphomedusae. COELENTERATA 129 Others they may even be united in the same species and the same colony of that species. A survey of the phylum is very largely con- cerned with the variations of these types and the combination of them in the life histories of the different coelenterates. The polyp (Fig. 109 A) is an attached cylindrical organism with a thin body wall consisting of two single layers of ectoderm and endo- derm separated by a narrow structureless lamella. At the free end an Fig. 108. Diagram of Hydra to show the nervous net. n.c. nerve cells. oral cone occurs and at its apex the mouth opening into the enteron. The oral cone (in the Hydrozoa) is surrounded by a number of tentacles, which are usually very extensible and armed with batteries of nematocysts, by which the living animals, on which the coelenterate feeds, are caught. Tentacles contain a prolongation of the endoderm which may form a tubular diverticulum of the enteron or a solid core. The medusa (Fig. 109 C) is a free-living organism differing from the 130 THE INVERTEBRATA polyp in the great widening of the body, especially along the oral surface, and the restriction of the enteron by the increase in thickness of the structureless lamella on the aboral side of the endoderm, so that while a central gastric cavity remains, the two endodermal surfaces have come together peripherally to form a solid two-layered endoderm lamella except along certain lines, where the canal system is developed radiating from the gastric cavity. The oral cone becomes the manu- can.c. Fig. 109. Diagram to illustrate the relation between polyp and medusa. A, Polyp. B, An imaginary intermediate form. C, Medusa. Ectoderm black, endoderm cross-hatched, mesogloea stippled, can.c. circular canal ; can.r. radial canal; end.l. endoderm lamella; ent. enteron; M. mouth; mh. manubrium; or.c. oral cone ; ten. tentacle ; vni. velum. The velum, present in many medusae, is absent in Ohelia. brium ; the rim which bears the original tentacles of the polyp is now separated widely from the mouth by differential growth and drawn downwards in the formation of the bell. Ver^' often a secondary set of oral tentacles are developed on the manubrium. The radial sym- metry of the polyp is more strongly emphasized in the medusa by the radial development of the canal system. The muscular system of the COELENTERATA 131 bell is greatly developed by the substitution of a type of cell in which the muscular processes form a long striated fibre while the epithelial part is greatly reduced; such a cell is capable of rapid rhythmical contraction. The nervous system may be partially concentrated to form a nerve ring and well-defined sense organs occur in connection with this. In this phylum, the lowest of the Metazoa, the gametes are of the type which is found throughout that great animal division ; the maturation divisions make their typical appearance here. Eggs and spermatozoa respectively are nearly always borne by different individuals or colonies. After fertilization the egg segments by equal divisions until firstly, a single layer of cells (ectoderm) arranged to enclose a central cavity constitutes the blastula. Then, by the migration of cells into this cavity, it becomes filled up with tissue (endoderm) while the ectoderm becomes ciliated. Such a larva with a solid core of endoderm is a planula (Fig. 1 10). It is capable of wide distribution Fig. no. Development of a hydroid polyp. After Merejkowsky. A, Forma- tion of endoderm in the blastula, by budding from the pole. B, Planula with solid core of endoderm. C, Appearance of enteron; endoderm cells beginning to arrange themselves as a single layer. by currents and may live for a considerable period before settling down. A split appears in the endoderm, the first appearance of the gastrovascular cavity, and the larva sinks to the bottom and attaches itself by one end. At the other end a mouth and tentacles appear and the creature becomes a polyp. There are a few exceptions to this in the phylum in which the egg develops directly into a medusa. SUBPHYLUM CN ID ARIA Coelenterata referable to two types, the fixed polyp and the free medusa ; locomotion usually by muscular action ; possessing nemato- cysts. 9-2 132 THE INVERTEBRATA They are divided into the following classes : Hydrozoa. Cnidaria, nearly always colonial; typically with free or sessile medusoid phase, arising as buds from the polyp-colony: no vertical partitions in the enteron; medusae with a velum and nerve ring; tentacles of polyp usually solid; ectodermal gonads; and an external skeleton. ScYPHOMEDUSAE. Cnidaria in which the polyp stage is inconspicu- ous and may be absent altogether : the polyp, where present, gives rise to medusae by transverse fission (strobilization) ; with vertical par- titions (gastric ridges) in the enteron ; velum and nerve ring absent ; endodermal gonads; and skeleton absent. AcTiNOZOA. Solitary or colonial cnidarian polyps without medusoid phase; vertical partitions (mesenteries) in the enteron; endodermal gonads; with or without a skeleton. Fig. III. The planula of a hydromedusan, Clava squamata. A and B, Swim- ming about in the sea. C, Coming to rest on a rock. D, Developing tentacles, oral cone and stolon, or.c. oral cone; stn. stolon. Magnified. From Allman. Class HYDROZOA The most typical life histories of the *'hydroids" are those in which the phenomenon of "alternation of generations" is presented. That is, there is a regular alternation of phases, hydroid colonies giving rise to free-swimming medusae and the fertilized eggs laid by the medusae each giving rise to a new colony of polyps. In the first two orders of the Hydrozoa, the Calyptoblastea and the Gymnoblastea, alternation of generations is well shown in the typical genera. As will be shown, there is a progressive suppression of the medusoid "generation" in other members of these orders. In the other orders there is, however, complete suppression of the polyp phase in the Trachomedusae and CNIDARIA 133 Narcomedusae, and in the Siphonophora remarkable colonies are found which appear to have originated by budding from a medusa. The following orders are contained in the class : Calyptoblastea (Leptomedusae) . Hydrozoa in which the coenosarc is covered by a horny perisarc, produced over the nutritive polyps as hydrothecae and over the reproductive individuals as gonothecae; the medusae flattened, with gonads on the radial canals, and usually statocysts. Gymnoblastea (Anthomedusae) . Hydrozoa in which the coenosarc is covered by a horny perisarc which stops short at the base of the polyps and reproductive individuals; the medusae bud-shaped, the depth of the bell greater than the width, with gonads on the manu- brium and eyes, but not statocysts. Hydrida. Hydrozoa existing as solitary polyps without medusoid stage; tentacles hollow; without perisarc, the polyps being capable of locomotion; gastrula forms a resting stage encased in an Qgg shell. Trachylina. Hydrozoa without "alternation of generations", the medusoid developing directly from the egg; with tentaculocysts, and with generative organs lying on the radial canals or on the floor of the gastric cavity. Hydrocorallinae. Hydrozoa existing as sessile colonies with an external calcareous skeleton into which the usually dimorphic polyps can be retracted. Siphonophora. Hydrozoa existing as free-swimming, polymorphic colonies, without perisarc, derived by budding from an original medusiform individual of anthomedusan type. The Graptolitoidea (see p. 148) are probably another order of the Hydrozoa and certainly belong to the class. Orders CALYPTOBLASTEA, GYMNOBLASTEA, HYDRIDA We will take as examples of these orders Obelia, belonging to the Calyptoblastea and Bougainvillea to the Gymnoblastea, both of which produce free-swimming medusae, and then describe Tubularia with its sessile gonophores. The series ends with Hydra (Hydrida). In a colony of Obelia (Fig. 112) root-like hollow tubes (the hydrorhiza) run over the surface of attachment, such as a seaweed, and from these spring free stems, which branch in a cymose fashion giving off the polyp heads (hydranths) on alternate sides. At the growing ends of the main branches are produced buds which develop into hydranths, and towards the base in the axils of the hydranths, polyps modified for reproduction, the blastostyles, occur. The whole system of tubes which connect up the individual polyps is the coeno- sarc, and it must be understood that the enteron or cavity of the 134 THE INVERTEBRATA colony is continuous and common to all its members. Ciliary currents distribute the food obtained by some individuals to those parts of the colony where feeding is not taking place. As in all Fig. 112. Part of a branch of Obelia sp. To the left a portion is shown in section. After Parker and Haswell. ect. ectoderm; end. endoderm; mth. mouth; coe. coelenteron; esc. coenosarc; psc. perisarc; hth. hydrotheca; his. blastostyle; med. medusa-bud; ^Z^. gonotheca. Calyptoblastea the coenosarc is completely invested by the cuticular secretion, the perisarc, composed of chitin and produced to form cups round the hydranths {hydrothecae) and the blastostyles (gonothecae). CALYPTOBLASTEA 135 The hydranth of Obelia is an expansion of the coenosarc, ending in a prominent oral cone, surrounded by a single ring of rather numerous tentacles, which have a solid core of endoderm cells. The blastostyle has a mouth but no tentacles; the body wall proliferates to form distinct individuals, the medusae. Those nearest the mouth of the gonotheca mature first, and they are liberated as they mature. The medusa of Obelia (Fig. 113), the type of the Leptomedusae, is like a shallow saucer, -^^^^^i?^ w ■■■;^^^.. the middle of the concave (subumbrel- "^ lar) surface being produced into a short manubrium. The rim of the medusa ^^| /A?^X.i i^^-^ew. bell is furnished with a large number of short tentacles. Like all medusae belonging to the Hydromedusae, it has four radial canals, running from the gastric cavity to the circular canal . ^^^- ^ ^3 • Free-swimming me- On the course of the radial canals and, ^J!;^j\/ro^T> -jI^^' ^ ^°^- i ^^ T ' and MacBnde. caw.r. radial canal; at the end of a short branch, patches ^. gonad; M. mouth at end of the subumbrellar ectoderm are of manubrium; ot. otocyst; ten. modified to form the gonads. The tentacles. germ mother-cells originate in the ectoderm of the manubrium, pass through the endoderm and along the radial canals to the gonads and then migrate into the ectoderm again. Only male or female germ cells are produced by each medusa. At regular intervals in the circumference are eight sense organs, the statocysts. They are tiny closed vesicles, lined with ectoderm and filled with fluid in which minute calcareous grains occur. The epi- thelial lining not only secretes these but is also sensory: the impact of the grains on the cells produces a stimulus which is transmitted through the nerves to the muscles, and if the position of the medusa should be abnormal the muscles contract in such a way as to right the bell of the animal. Another characteristic of the hydrozoan medusa is the velum (which is practically absent in Obelia)^ a narrow internal shelf running inside the border of the subumbrellar cavity. This is largely composed of ectodermal circular muscles, separated by a horizontal partition of structureless lamella. At its base is a double nerve ring: the inner half of this is concerned with the subumbrellar musculature and the outer with the sense organs. The ripe ova are shed into the water by the rupture of the gonad, and fertilization takes place here. Segmentation leads to the formation first, of a hollow blastula, and from this, by the immigration of cells at one pole, the elongated planula larva (Fig. no) with a solid core of endoderm is formed. It is ciliated and swims freely for a time, eventu- 136 THE INVERTEBRATA ally settling down by its broader end, while the other end develops a mouth and tentacles surrounding it. The endoderm delaminates to form the enteron. From the base of this first formed polyp there is Fig. 114. Bougainvilleafructuosa, x about 12. From Allman. A, The fixed hydroid form with numerous hydroid polyps and medusae in various stages of development. B, The free-swimming sexual medusa which has broken away from A. an outgrowth along the surface of attachment which is the beginning of the hydrorhiza. From this the rest of the colony is developed. In Bougainiillea (Fig. 114) the polyps belong to the gymno- GYMNOBLASTEA 137 blastic type to be described for Tubularia. The creeping hydrorhiza gives off branches, one of which is seen in the figure, and from these numerous individuals are budded. Most of these are polyps (hydranths), distinguished from those of the Calyptoblastea by the fact that the perisarc stops short at the base of the polyp and does not form a hydrotheca. The medusoid individuals take their origin directly from the coenosarc each as a simple bud, within which is developed a single medusa which eventually divests itself of a thin covering, breaks from its stalk and swims away. Several may spring from the same stem, but this may also bear normal polyps. There is here no 4en.or. Fig. 115. Median vertical section through a polyp of Tubularia. hist, blasto- style; can.c. circular canal; end.dig. digestive endoderm; end.vac. vacuolated endoderm, forming supporting core of tentacles ',gnph. gonophores ; M. mouth ; prs. end of perisarc; t. testis; ten.ab. aboral and ten.or. oral tentacles. Slightly altered from Kukenthal. blastostyle, or polyp modified for budding off medusae, and this con- dition, in which polyps and medusae belong to the same grade of differentiation from the coenosarc, is possibly to be regarded as primitive, that of Obelia as secondary. In Eudendrium an intermedi- ate stage occurs. Medusae are budded off from the stalk of a normal polyp, and as soon as this budding commences the polyp loses its tentacles, diminishes in length and may be said to become a blasto- style. Tubularia (Fig. 115) occurs as a colony of large polyps with long stalks springing from a hydrorhiza of insignificant extent. At the base of the 138 THE INVERTEBRATA polyp the stalk forms a swelling; there the perisarc stops. There is an oral cone surrounded by a ring of tentacles and also a ring of larger (aboral) tentacles at the broadest part of the polyp. Both kinds of tentacles are solid, with an axis of vacuolated endoderm cells placed end to end, which have a skeletal value. The reproductive individuals originate from hollow branched structures springing from the polyp itself between the oral and aboral tentacles. Each polyp has several of these branches, and from each branch about half a dozen repro- ductive individuals arise. The branch is usually termed a blastostyle^ although it is only part of an individual and not a modified polyp as in Obelia, Each of the buds it produces, however, has the structure of a medusa but remains attached to the parent polyp as long as it end. lam ^ten.on Fig. 116. Longitudinal sections through gonophores of Tw^w/ana. A, Young male. B, Female with larvae. In B the details of tissues are omitted; in A ectoderm is black, endoderm cross-hatched, end.lam. endoderm lamella; act. actinula larva; pla. planula larva with rudiments of aboral tentacles; mb. manubrium; ov. ovum; stk. stalk of polyp. Other letters as in Fig. 115. Original. lives. Like the free-swimming medusa of Bougainvillea it conforms to the anthomedusan type, the depth of the medusa bell exceeding the width and the gonads being situated on the manubrium. This sessile medusa is called a gonophore. As seen in Fig. 116 A, the radial and circular canals are formed as in Obelia^ and four very short tentacles occur opposite the radial canals on the margin of the bell ; but the entrance to the subumbrellar cavity is very much constricted compared to Obelia or a free-swimming anthomedusa. Another modification is that the eggs, when liberated from the gonad, are fertilized in the subumbrellar cavity and develop there through the planula stage into an advanced larva called the actinula (Fig. 116 B) GYMNOBLASTEA 139 which is really a polyp of Tuhularia with a short stem. At this stage it makes its way out of the shelter of the gonophore and fixes by its aboral end. As a rule, only one of these large eggs can be produced at once and a ripe gonophore generally contains two larvae of different ages, one a planula and the other an actinula, which may be seen protruding from the aperture of the bell. In such gonophores the neuromuscular structures of the bell are hardly developed at all and there are no evident sense organs. In the medusae called Lizzia and Margellium, common plankton forms whose polyp stages are not known, we see the normal anthomedusan type. In both of these there are a number of short tentacles, arranged ntc. c.end. l.n.r. II. a.h. Fig. 117. I. A, Eye of Lizzia koellikeri seen from the side, magnified. B, The same seen from in front. C, Isolated cells of the same. From O. and R. Hert- wig. Is. lens; per.c. percipient cells; pig.c. pigment cells. II. Radial section through the edge of the umbrella of Carmarina hastata showing sense organ and velum, a.n. auditory nerve; c.end. continuation of endoderm along aboral surface; cm. circular muscles of velum; /.w.r. lower nerve ring; msg. mesogloea; ntc. nematocysts; ra.v. radial canal running into circular canal, both lined by endoderm; tct. sense organ or tentaculocyst ; u.n.r. upper nerve ring; vm. velum. in groups round the margin of the bell, and four double tentacles at the end of the manubrium. Liszia possesses eight "eyes" (Fig. 117 I) which are little patches of ectoderm, in which some of the cells develop pigment while others elongate and end in rods. The latter are concluded to be the light-perceiving cells. There is also an outer enlargement of the cuticle which serves to concentrate light on the organ and may be called a lens. Though there is no direct evidence that these organs have a relation to light, they have in a simple form all the structural elements of the eye of higher animals. Margellium (Fig. 118) has no eyes but apparently suffers no disability from their absence : probably the light-perceiving cells are scattered over 140 THE INVERTEBRATA the general surface of the ectoderm. "Eyes" are however a general character of the Anthomedusa as " Ears " (as statocysts may be broadly termed) are of the Leptomedusa. Among the hydroids with sessile medusoids or gonophores there are many forms in which the medusoid structure is lost, and a bud-like structure is found in which a transverse section shows simply succes- sive layers of ectoderm with generative cells, structureless lamella and endoderm round an enteron which does not open by a mouth. In forms like this the migration of germ cells, mentioned as occurring in Obelia, are very noticeable. Thus in Eudendrium (Fig. 119 D) the germ cells are often to be distinguished making their way along the coenosarc towards developing gonophores. If this degeneration of medusae is followed to its conclusion, a stage is arrived at in which Fig. 118. Margellium, example of an anthomedusan. M. mouth; man. manu- brium; i. testis; ten. or. oral and ten.m. marginal tentacles. Original. there are no special reproductive buds at all, but the generative cells occur in the body of the hydroid. This is the condition in Hydra, where the multiplication of the interstitial cells at different positions produces testes or ovaries. In the latter case each ovary contains a single egg of a size unusual in the Hydrozoa, which grows by the ingestion of its sister oocytes and the conversion of their proto- plasm into yolk spherules. This phenomenon appears to be a con- sequence of the habitat of the genus. As in so many other freshwater animals, a free-swimming stage is omitted from the early history and the period of larval development is passed in the shelter of the egg shell; when the gastrula stage has been arrived at and the yolk is mostly absorbed, development is suspended during a resting stage of three or four weeks. After this the young Hydra pokes its oral end HYDROZOA 141 Oph/^ Fig. 119. Examples of Hydrozoa. A and B, Calyptoblastea. C and D, Gym- noblastea. A, Plumularia, small portion of a branch showing nematophores. B, Sertularia, branch with opposite hydrothecae and gonotheca. C, Pennaria, tip of branch. D, Eudendrium, showing migration of germ cells into gono- phores. gph. gonophores; gth. gonotheca; hth. hydrotheca; nah. nemato- phore ; ov-. migrating germ cells ; prs. end of perisarc ; ten.ab. aboral and ten.or. oral tentacles. A, after AlLman; B and C, original; D, after Weismann. 142 THE INVERTEBRATA out of the shell and, after creeping about for a short time, frees itself and develops a mouth and tentacles. Other characters which differ- entiate Hydra from the majority of hydroids are the solitary habit, which it shares with some Gymnoblastea, and the complete absence of a stiff perisarc, this enabling the animal to execute its characteristic looping movements. It is often pointed out that the presence of a distinct migratory phase, the medusa, would entail a serious dis- advantage on Hydra ; it is suggested that the medusae might be swept out to sea and lost. Hydra usually lives in ponds and is therefore hardly subject to this danger, but at the same time the embryo in its horny egg shell is admirably fitted for dispersal, for example in mud on the feet of migratory birds. This modification of reproductive habits in Hydra is paralleled in the freshwater sponges with their gemmules, the freshwater polyzoa with their statoblasts and the cladoceran Crustacea with their ephippial eggs. It must, however, be mentioned that a remarkable group of freshwater medusae occur which belong to the Trachylina, and a stage occurs in their life history which has sometimes been compared with Hydra and named a separate genus (Microhydra) of hydroid polyps. This is, however, an interesting case of convergence. The following genera of Calyptoblastea may be shortly mentioned : Plumularia (Fig. 119 A) with a creeping hydrorhiza, giving off plume-like branches, each of which bears a series of hydrothecae on one side only; hydrothecae small, so that the polyps cannot be com- pletely retracted within them; beside the nutritive polyps a second smaller kind (nematophore), without mouth, but with long amoeboid processes which engulf decaying polyps and larvae of other sessile forms. Sertularia (Fig. 119 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. 119 C) with two kinds of tentacles, oral capitate and aboral filiform; nematocysts of very large size. 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 Leptomedusae, are unknown. HYDROZOA 143 Order TRACHYLINA This group consists of forms in which the medusoid develops directly from the egg and the polyp has disappeared from the life cycle. The possession of sense tentacles is an important character. It consists of 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. 117 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 : Ciinina. 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. Limnocoditim was first known from the Victoria Regia tank in the Royal Botanic Gardens, but has now been discovered in various North American rivers and has even colonized ponds and canals in England. It has a polyp-like stage, Microhydra, which has a certain likeness to Hydra. Order H YDROCORALLINAE 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 hydrorhiza with frequent anastomoses. Instead of secreting a horny perisarc as the Calyptoblastea and the Gymno- blastea do, the ectoderm lays down an exoskeleton consisting of calcareous grains, which becomes bulky and solid. It may be either massive or encrusting or branching. From pits in the surface of the colony arise the polyps. These are of two types (Fig. 120). First there are the individuals of normal structure with a mouth sur- rounded by tentacles (gasterozooids) : these nourish the colony. Then there are the dactylozooids which are much longer and more slender. They have no mouth but they possess scattered capitate tentacles and may form a ring round a gasterozooid, in which case it is readily observed that their function is to catch prey and hand it to the central gasterozooid for digestion. Besides the polyps there are the medu- soids, which, as in Bougainvillea, are budded directly off from the H THE INVERTEBRATA 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. 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 individual, from the base of which springs a coenosarcal tube budding off all the other members of the colony. It usually happens ,amp. ':icanj ^can.2 tdh. Fig. 1 20. 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. that those which are developed first are needed to buoy up and propel the young colony. Consequently the first individual is either medusi- form or else forms an apical float or pneumatophore (the epithelium of which secretes gas). There may also be formed from the ectoderm of the first formed individual an oleocyst containing a drop of oil. The medusiform individuals resemble the bell of an anthomedusa, with velum and canal system but lacking the manubrium, and they are called nectocalyces : while the most primitive siphonophores have only a single one there may be a series of them. Following these the coenosarc in one type of colony (Fig. 121 A) grows to a great length and buds off at intervals along its length similar assemblages of SIPHONOPHORA 145 individuals. Such an assemblage is known as a cormidium, and may consist of (i) a shield-shaped hydrophyllium 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 mouth- less individual, the dactylozooid^ 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 Gymno- blastea 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, gastrozooids, dactylozooids and gonozooids, all being crowded together to form a compact colony. In Physalia (Fig. 122 B), the "Portuguese man- of-war", there is an enormous cap-shaped pneumatophore which floats above the surface of the water. There are no nectocalyces, but the colony is borne hither and thither by the wind and countless numbers are cast up on 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 gastro- zooids 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 stage of its digestion. Physalia can catch and devour a full- grown mackerel, and the poison of its nematocysts is so virulent as to endanger human life. In Velella (Fig. 122 A) the disc-shaped colony has a superficial resemblance to a single medusa. The pneuma- tophore 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 pneuma- tophore. 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 from which they may have sprung. Medusae like Sarsia (Fig. 121 C) 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 146 THE INVERTEBRATA Fig. 121. Development of the siphonophore colony. A, Diagram of the possible combinations of individuals in a colon3\ 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 nee.- becomes the single permanent nectocalyx of the colony; pn. 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. 7tinb. manubrium; med. daughter medusae. A, altered from Hertwig; B, after Chun; C, after Allman. SIPHONOPHORA displaced from their original position. The manubrium has come to lie outside the medusa bell as the gastrozooid, and the gonads and tentacles have also become separated. In the early stage of development Fig. 122. 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); med. medusa buds. Other letters as in Fig. 121. B, Physalia showing the "drift net" arrangement of the tentacles of the dactylozooids. of a siphonophore colony shown in Fig. 121 B, there are two necto- calyces and two groups (cormidia) of the displaced organs which 148 THE INVERTEBRATA might be supposed to belong to each other. In more advanced stages of development the displacement becomes extreme and no corre- spondence can be traced. 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 regarded 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 GRAPTOLITOIDEA Extinct, probably planktonic, Hydrozoa in which the polyp genera- tion is dominant; the medusoid generation possibly represented by gonophores; the individuals are budded off from one another and remain in contact with the parent ; there is no definite coenosarc ; and the perisarc is produced round the polyps as hydrothecae. 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 is 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, diflfer from calyptoblast hydroids because new individuals are budded off directly from older ones and not from a common coenosarc. Each colony originates from a conical body called a sicula^ the exoskeleton of the first formed individual. From the side of this a bud is formed, and from the second individual which thus develops a third, and so on. In this way a linear series of polyps is produced which are arranged in a slender lamella {stipe), the hydrothecae being in contact and the cavity of the colony being continuous. The simplest arrange- ment is in Monograptus (Fig. 123) in which the hydrothecae are all on one side of the stipe. In Diplograptus budding takes place altern- ately on the two sides of the terminal individuals so that there is a biserial stipe, and in Didymograptus (Fig. 124) the second and third hydrothecae go on budding independently, so that we have a colony with two stipes or branches. By another modification later hydro- thecae bud off two individuals instead of one, and colonies like Tetragraptus (Fig. 125) and Bryograptus are formed. 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. 1 24 B) . It is also possible that some graptolites i-si^ 6 m. hth. Fig. 123. B Fig. 124. Fig. 123. A, Part of a colony of Monograptus. B, Diagrammatic vertical section of same. C, Monograptus colonus with sicula {si.), ex. common canal; hth. hydrotheca; m. mouth of former; vr. virgula. From Woods. Fig. 124. A, Didymograptus v-fractus. Ordovician. Early part of the poly- pary. After Elles. si. sicula ; cr.c. crossing-canal ; i, first hydrotheca ; 2, second hydrotheca. x 5. B, Tetragraptus similis. Lower Ordovician. Young form with virgula and disc. After Ruedemann. x 4. Fig. 126. Fig. 125. Tetragraptus. Ordovician Rocks, a, central disc, x \. Fig. 126. Diplograptus foliaceus from the Utica Slate, New York, x After Ruedemann. For description see text. 150 THE INVERTEBRATA were independent planktonic organisms with a pneumatocyst or other kind of float. Such a pneumatocyst appears to be shown in Fig. 126 of Diplograptus 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 graptolites were true pelagic coelenterates and their floating habit gave them a uni- versal 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, Tetra- graptus 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 diflPerent genera. Certain forms, whose relationship is not clear, occur very com- monly in the Cambrian and are grouped together as "dendroid" graptolites. It is possible that they are closely related to the Calypto- blastea. Class SCYPHOMEDUSAE (SCYPHOZOA) This class contains the common jellyfishes of temperate 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. 127), are not uncommon on the British coasts, adhering to the blades of Zoster a or Laminaria. It has a narrow CNIDARIA 151 Stem arising from its exumbrellar surface, by which it attaches itself temporarily to seaweed. The edge of the bell is divided into eight lobes, on each of which are several short tentacles and the adhesive Fig. 127. 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, Trans- verse section throngh. Lucernaria along the line XX in A. D, Ephyra larva of Aurelia. can.c. circular canal; ^. gonad; ^./. gastral filament; e.s. exumbrellar stalk; mes. mesentery; ni.L' longitudinal muscle of mesentery; su.p. subum- bral pit; ten.s. tentacle becoming later a tentaculocyst ; I.R. interradius; P.R. perradius; A.R. adradius with first indication of canal. A and C, altered from Bourne; B, after Heric; D, original. organs which are called marginal anchors}- There is no velum and tentaculocysts are absent. The manubrium is well developed and the mouth opens into a spacious gastric cavity which is divided by four partitions, the interradial mesenteries ^ into four broad chambers which ^ Absent in Lucernaria. 152 THE INVERTEBRATA are said to be perradiaL The mesenteries are vertical walls projecting from the body wall and composed of endoderm with an internal layer of mesogloea. They have a free edge centrally, while on each side a vertical series oi 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 subumhral pit. The Stauromedusae only exist as individuals of this structural type, superficially more like a polyp than a medusa, but usually supposed to be a medusa, and the eg'g develops into an individual exactly re- sembling the parent. The vast majority of the Scyphomedusae belong to the subdivision Discomedusae, which includes our type Aurelia aurita (Fig. 128), 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 suhgenital 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 con- sists 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 directions (Fig. 129). The water drawn in by the mouth passes first into the gastric cavity and then the gastric pouches ; thence by the adradial SCYPHOZOA 153 canals to the circular canal. It returns thence by the branched inter- radial 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 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 Fig. 128. Aiirelia aurita. Somewhat reduced. From Shipley and MacBride. M. mouth; oa. oral arm; tn. tentacles on the edge of the umbrella; p. en. 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 alternate with these; a. en. one of the unbranched adradial canals; e.en. the circular canal; tet. marginal lappets hiding tentaculocysts ; g.fil. gastral filaments, just outside these are the genital organs. available for local needs. The gastrovascular system thus at once fulfils the functions of the digestive and circulatory systems of higher animals. The neuromuscular system is further developed than in even the medusoid individuals of the Hydrozoa. The muscles are ecto- dermal, and each cell is almost entirely converted into contractile 154 THE INVERTEBRATA protoplasm with a cross-striated pattern forming an elongated fibre ; physiologically they are capable of rapid rhythmic contraction and not of slow tonic contraction like the muscle of a sea anemone. The fibres are arranged as a circular musculature over the peripheral part of the subumbrella. The nerve net is also confined to the ecto- derm and is concentrated in the neighbourhood of the tentaculocysts. There is no true velum, but a pseudovelum consisting of an internal jy.cn- a.cn. p,cn. t.cn. Fig. 129. 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.ai. interradial canal; o.o.a. opening on oral arm. Other letters as in Fig. 128. flange which is not occupied by muscles and a nerve ring as in the Hydrozoa. The tentaculocysts are the characteristic sense organs of the Scyphomedusae (but are present also in the Trachomedusae and Narcomedusae of the Hydrozoa). They are minute tentacles which project at the end of the interradial and perradial canals, which are SCYPHOZOA 155 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 epithelium and said to be olfactory. In the neighbourhood of these tentacles, then, all the senses appear to be localized. Does this explain the fact that they are necessary for the rhythmic contraction of the medusa bell } The Scyphomedusae are excellent subjects for experiment, 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. 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. 129, o.o.fl.) 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 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. They may separate from the parent as in Hydra. At certain seasons the whole hydratuba is segmented by transverse horizontal furrows. This process is termed strobilization (Fig. 130 B). In each of the disc-like segments so produced, marginal growth at once begins, 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 ad radial canals also appear (Fig . 1 3 o 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. 127 D, 130). 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 156 THE INVERTEBRATA 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 enormously in thickness, causing the two layers of the endoderm to come to- gether 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. A Be Fig. 130. Strobilization 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 strobilization has begun. C, Strobilization 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, x 7*5. 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, e.g. Pilema here figured (Fig. 131), CNIDARIA 157 it is entirely closed. Cassiopeia is a semisedentary 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. The mode of development described above is typical in the Scyphomedusae. 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 Amelia, *'polydisc" strobilization being a secondary adaptation for the more effective spread of the species. Fig. 131. 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; stomodaeum 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. 132, and it is seen that the polyps project in life from the general surface 158 THE INVERTEBRATA of the colony. The ectoderm, mesogloea and endoderm of the polyps 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 meso- gloea 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. 132. 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 ; ect. ectoderm ; end. endoderm ; sol. solenia and end.s, solid endoderm strands ; bd. endodermal bud which will give rise to a new polyp ; 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 ALCYONARIA 159 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 de- finite positions on the mesenteries. This may be seen in the sections of a polyp in Figs. 136 and 137. It must in the first place be ex- plained 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 mesenteric filament is seen, and this consists of different elements in the different mesenteries. One pair of mesenteries, 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. 136 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 l6o THE INVERTEBRATA in Fig. 132, mes.d.y 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 fila- ments covered with an epithehum consisting largely of gland cells. Also at certain seasons the germ cells arise near the free border (Fig. 136 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 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 de- veloping 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 off, 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 y 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 ALCYONARIA l6l thickens enormously. In the red coral Corallium rubrum (Fig. 133) there is an upright branched colony with a rigid axis composed of spicules compacted together which is the precious coral of com- merce. This is clothed by the delicate tissue of the coenosarc from which the short polyps arise and which contains a network of endo- dermal 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 Mediter- ranean and the seas of Japan. Dimorphism, as described below for Pennatula, also occurs here. Fig. 133. 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 confined 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. 134). In Pennatula and its relations (suborder Pennatulacea) a single axial polyp grows to a relatively enormous length, sometimes as much as three or four metres, and contains a long horny axis which is possibly 1 62 THE INVERTEBRATA 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 siphonozooids , 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 equal and regular lateral branches on each side of the axis giving the colony its feather-like form, and the siphonozooids are mainly found Fig. 134. 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. 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, Tuhipora (the organ-pipe coral) and Heliopora (the blue coral), which are widely distributed on coral reefs, a con- tinuous calcareous skeleton is developed resembling that of reef corals. The polyps of Tuhipora are elongated and parallel and connected by stony platforms which are traversed by the endodermal tubes. But while in Tuhipora there is an internal skeleton developed as in ACTINOZOA 163 Corallium^ 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. 135) there are on the surface of the colony larger pits (thecae) occupied by the polyps and smaller pits Fig. 135. 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. which lodge tubular processes of the network of solenia: the same skeletal characters also occur in the fossil Heliolites which closely resembles it and was a dominant type in Palaeozoic coral reefs. Tubi- pora too has a Palaeozoic representative in Syringopora.^ 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. 164 THE INVERTEBRATA 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. 136 A), is found in the small burrowing sea anemone, Edwardsia (Fig. 136 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, always 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. 136 C) with eight mesen- teries. From this the hexactinian type is derived quite simply by the subsequent 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 seen in Fig. 136 E. In such a form as Peachia, often used in laboratories 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 filament, while the pairs in two exocoeles are completely absent (Fig. 136 B). The mesenteric filament of the Zoantharia (Fig. 137 B, C) is trefoil- shaped in section, and while the functions of digestion and water- ACTINOZOA i6s mes.dir. mesl mes.dir. m.l. mes.dir. mes.fi I. Fig. 136. Diagrammatic transverse section of corals and sea anemones. A, Alcyoniiim. B, Peachia. C, Edwardsia. D, Aulactinia. E, Typical actinian m the region of the oesophagus. F, Alcyonium below the oesophagus. mes.^, primary mesentery; mes.dir. directive mesenteries; mes.^-, mes.^, mes.^, secondary, tertiary and quaternary mesenteries; mes.fil. ciliated mesenteric filaments; m.l. longitudinal muscle; ov. ovaries; sip. siphonoglyph ; sip.' sulculus. i66 THE INVERTEBRATA circulation are in the Alcyonaria performed by different filaments, here they are performed by different parts of the same filament. 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 nemato- cysts, while the wings are covered with strongly ciliated epithelium which maintains a current. In the lower part of the mesentery the filament is exclusively digestive in function : the cells of 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 nem. Fig. 137. 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- gestive region, of Twe^./. mesenteric filament; ikf. 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. 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 which encloses them, and in these are formed vertical walls which ZOANTHARIA 167 rise from the plate ; the circular wall is called the theca and the radial walls septa (Fig. 138 A). The latter are formed in spaces between the mesenteries. The continued secretion by such a form as the English pot ^% pol. % Fig. 138. 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. 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. solitary coral Caryophyllia produces a cup of limestone, of which the tapering basal portion is solid but which has a shallow apical depression, which is traversed by the radiating vertical septa and contains in the l68 THE INVERTEBRATA centre a more or less regular vertical rod, the columella. The de- pression 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 hemispheri- cal mass of calcium carbonate which it has secreted. It is not sur- prising 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. 138 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. 139), 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. 138 D. 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. 138 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. 138 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. 138 C). The bud and the parent remain connected by their extrathecal portions, and this con- ZOANTHARIA 169 stitutes the coenosarc of the colony. The gaps between the thecae of the colony are filled up by calcareous material secreted by the coenosarc and called coenenchyme. The polyps of the Zoantharia attain a higher physiological grade than those found elsewhere in the coelenterates. The sea anemones, P. Mo. Fa. Fu. M. Fig. 139. 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. Meandrtna, the brain coral; Mo. Montipora, a branched coral ; P. Pontes. (Photograph by Dr S. M. Manton.) 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 170 THE INVERTEBRATA of the mesenteries cause a longitudinal retraction of the polyp, the transverse muscles of the mesenteries in the neighbourhood of the stomodaeum open the moiith when they contract, and the longi- tudinal 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. If a sea anemone is 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 action of glands and cilia is not under the control of the nervous system, but they work together with the muscles in the task of feeding. In a sea anemone like Metrtdtum, 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. So while the nature of the nerve net enables the severed tentacle of a sea anemone to execute movements just as if it was still in place, there is this remarkable co-ordination of activities in feeding. 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 Lohophyllium 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. This is especially the case in reef corals, in which the most recent investiga- tions show that the food value of the algae is practically nothing, while on the other hand the fact that they remove excreta from the coral tissues is of great 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". The Ctenophora, apart from certain aberrant forms, are globular, COELENTERATA 171 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, Pleurobrachia pileus and Hormiphora plumosa. Pleurobrachia pileus is about the size of a small 'pa.can Fig. 140. Hormiphora plumosa. After Chun. Side view. M. mouth leading via stomodaeum into infundibulum ; ab.p. aboral pole with sense organ; ah.fu. aboral funnel of infundibulum; pa.can. paragastric canal running towards 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. 140) 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 spot lying in a slight depression. The surface of the body is beset by 172 THE INVERTEBRATA 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 para- gastric 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. 13). 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 between 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 mus- culo-epithelial cells. CTENOPHORA 173 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; {n)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 Ctenophora with the Coelenterata. CHAPTER VI THE 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 para- sitic 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 constitutes the lining of the gut there exist a large number of star- shaped cells with large intercellular spaces forming a mass oi paren- chymatous tissue. The nervous system (Fig. 141) consists essentially of a network as in the Coelenterata, with the important difference that there are always 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. 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. Sense organs occur in adults only in the free-living Turbellaria, PLATYHELMINTHES 175 where they may take the form of eyes, otocysts, tentacles and ciUated pits in the ectoderm. They may also occur in the free stages in the life history of the Trematoda and Cestoda. The eyes occur on the dorsal ce.ga.-/-- 9— Fig. 141. The nervous system of Acoela, to show the network. After Steinmann. ce.ga. cerebral ganglion (brain); M. mouth; ^ and 2, male and female openings respectively. surface where they are visible as dark spots. The retina is formed of cup-shaped cells, which are heavily pigmented. The interior of the cup is filled with special nerve cells, varying in number from two 176 THE INVERTEBRATA to thirty, the fibrillae of which touch the retina, and the fibres at the other end are joined together to form an optic nerve leading to the brain. There is no lens, but the ectoderm over the eye is not pig- mented and so permits light to pass through it (Fig. 142). It should be noted that in this simple eye, as in the extremely complicated organ found in the vertebrates, the light has to pass through the sensory cells of the nervous system before it reaches the retina, for they are in front of, not behind, the retina. This type of eye is easily seen and studied in the common freshwater planarians. In Planaria torva, the eye has only two sight cells, while in Planaria lactea there are thirty. Special sensory cells, which differ from the ordinary ectoderm cells, and which are directly connected with the underlying nervous net- m • • p. fl.p. geiiat. oil. Fig. 149. 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 or penis; nius.or. muscular organ; od. oviduct; p. penis; sp. stalked gland organ (bursa copulatrix) ; ves.sem. expanded portion of vas deferens forming a vesicula seminalis. 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 seasons 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, in others, for example Planaria alpina, the eggs are only produced during the winter months. Mesostomum produces two kinds of eggs 105 THE INVERTEBRATA 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" egg 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 favourable. Asexual reproduction occurs commonly in the Turbellaria. In Microstoma lineare the hinder end buds off new individuals which remain attached for some time so that chains of three or four in- dividuals in different stages of development are often seen. Planar- ians undergo autotomy, cutting themselves in two by a ragged line which traverses the middle of the body. Lost parts are easily re- generated 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. Convoluta roscojfensis is the best known member of this division. It lives between the tidemarks on sandy shores. Imbedded in the parenchyma are algal cells which TURBELLARIA 189 live in a symbiosis (p. 43) with the Turbellarian. The photo- synthetic products of these algal cells provide a source of nourish- ment for the animal. Convoluta henseni, another member of this order, is a rare platyhelminth that has adopted a planktonic habitat. Order RHABDOCOELID A In these forms (Fig. 147) 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 forms, of which Microstomum linear e is a common ex- ample, found in fresh water, the germarium and the vitellarium are not separated. Another well-known member of this group is Dalyellia viridisy 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, a black form, and Poly cells 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 coloured with stripes down the dorsal surface. Bipalium kewense is ^ These forms, with side pouches to the gut, are sometimes placed in a separate order called Alloiocoela. 190 THE INVERTEBRATA 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 posterior end. The germarium and the vitellarium are combined into one organ but the different portions may be distinguished in sections by suitable staining. There are separate male and female openings. 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. ThysanozooUy a member of this order, has the dorsal surface covered with papillae into which run coeca from the intestine. In Yungia 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 ; ventral sucker is subdivided in some forms. 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 five tentacles at the anterior end makes the group easily recognizable (Fig. 150). The epidermis 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 opening in front of it. The nervous system is of the primitive network type. 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 PLATYHELMINTHES 191 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. 151). The 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. ^^■jten. Fig. 150. Temnocephala minor y X12. After Haswell. g.o. genital opening; M. mouth; sue. sucker; ten. tentacles. Order HETEROCOTYLEA In the Heterocotylea there is a large posterior sucker stiffened with chitinous supports. It is often subdivided, as in Octobothrium or Polystomum (Fig. 152). 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 Polystomum which occurs in the bladder of the common frog, of which from 3-10 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 192 THE INVERTEBRATA 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. en. '= — ~ha.mcmb. pane. Fig. 151. Transverse section through body wall of a trematode. After Benham. ha.ynemb. basement membrane; circ.jyi. circular muscle layer; cu. cuticle; ect.c. ectoderm cell; long.m. longitudinal muscle layer; par.c. parenchyma cell; 5^. spine; ves.c. vesicular cell (present in many trematodes). They are inconspicuous as a rule, but in Polystomum 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. 152). Corresponding ducts do not occur in the Malacocotylea. The nervous system of the Heterocotylea is more primitive than that of the Malaco- cotylea, 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 Heterocotylea TREMATODA 193 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 ,.vas def. Fig. 152. Polystomum integerrimum, ventral view, showing the reproductive system. After Zeller. g.i. genito-intestinal canal; g.o. common genital open- ing; ho. hooklet; 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. Polystomum and also in Diplosoon, where it is permanent. The eggs when laid are attached to the body of the host, Polystomum being exceptional in laying the eggs in the bladder whence they pass out to the exterior into water. The egg hatches as a ciliated larva with eye- spots and a large ventral posterior sucker. These larvae make their 13 194 THE INVERTEBRATA 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 seeks out a tadpole and it dies within twenty-four hours if one is not found. It fastens itself on to the gills, where its ciliary covering is cast and then it creeps into the bladder to wait for three years before becoming sexually mature. The larvae may, however, attach them- selves 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 meta- morphoses, and thus it never reaches the bladder. In Dlplozoon, 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. Order MALACOCOT YLEA The life history oiFasciola (Fig. 153) 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 parasitic in some verte- brate host, the sporocyst and redia stages are always parasitic in a mollusc. Three hosts 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 may form by budding a second generation of sporocysts within which the cercariae arise, (iii) the cercaria may require 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 G aster ostomutn fimhriatum^ where the sporocyst develops in the liver of Anodon, 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 macr ostomutn^ which is parasitic in the gut of thrushes, there is no free-living stage in the life history. The eggs, passed out withthe 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 develops pigment, being bright coloured in bands of green and red, while its presence stops the snail from withdrawing this tentacle. Presumably this bright object attracts the bird which devours the snail and infects itself by setting free the cercariae from the sporocyst. i Fig. 153- 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 ; ut. uterus ; ov. ovary ; sh.gl. shell gland ; a.t. anterior testis; pt.t. posterior testis ;>;. 5'/. yolk glands; vas de. vas deferens. 13-2 196 THE INVERTEBRATA Schistosoma { = Bilharzia) is a parasite of man, living as an adult in the abdominal veins (Fig. 154). It is long and thin and v^ell 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 water, -e.po. Fig. 154. Schistosoma: the male (o) is clasping the female (?) in the gynae- cophoral groove {gr.). e.po. excretory pore; M. mouth; v. sue. ventral sucker. After Fritsch. and in districts in China and Egypt where the disease is common 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 pre- vented 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 (1250-1000 B.C.). The hatching of miracidia from the egg of Schistosoma is de- pendent on the dilution of the urine by fresh water and this serves to TREMATODA 197 emphasize the fact that the stages in the Hfe history of all parasites are ultimately connected with environmental conditions. The egg of Fasciola hepatica does not hatch unless the pH of the water in which it is deposited is below 7-5, the optimum point apparently being about pH 6-5. If the eggs are kept in water more alkaline than pY{ 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 ---glory. Fig- 155* Bucephalus larva (cercaria) of Gasterostomum fimbriatum. After Benham. f.tail, forked tail; ^/.or^. glandular organ ; M. mouth ; /)Aar. pharynx. 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. 155)- 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. 156). Proglottides are usually formed. 198 THE INVERTEBRATA The Cestoda as a group have feh the influence of the parasitic habit more than the Trematoda. They have dispensed ahogether with a gut, there is no mouth, and they absorb their food through the skin. As they Hve always in the aUmentary 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 assimilating 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 sections the circular muscles appear to divide the parenchyma into ut. ect.c: cm. ;i„gc. cut. I m.f{..; /^^_ ^_ In.excca- Fig. 156. 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; ut. uterus; od. oviduct. 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. 156). 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 Elasmobranchs. 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. It is difficult from the structure to say which end is the anterior and which the posterior, for the nervous CESTODA 199 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. They are all parasitic as adults in the alimentary canals of vertebrates. The adult worm has a scolex which is provided with organs of fixation such as hooks, suckers or folds (Fig. 157). The hooks are borne on a pro- jection at the top, called the rostellum^ while the suckers are on the sides of the scolex. 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 repro- duction 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 produced in Microstoma lineare. Through each proglottis runs the excretory canals and the nervous strands which are common to all (Fig. 156). The proglottis when first cut off 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. 158). Despite its con- nection with the scolex, each proglottis must be regarded as an in- dividual for it contains a full set of generative organs both male and female. 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 is called the uterus and is used for the storage of eggs, but it is doubtful if it is homologous with the uterus of the Trematoda. The homologies of the various ducts of the genitalia of the Platy- helminthes present great difficulties (Figs. 159, 160). 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 / sue: rost. Fig. 157. Taenia solium. Slightly magnified . From Shipley and MacBride. A, Entire worm, showing head and proglottides, sue/ sucker on head; g.po. genital pores ; />ro^. ripe proglottis. B, Head. rost. rostellum; ho. hooks; sue. suckers ; strob. commencement of strobilization. C, Ripe proglottis broken ofT from worm. od. remains of vas deferens and oviduct; ut. branched uterus crowded with eggs. CESTODA 201 the Turbellaria greatly elongated and used for egg storage, while the 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 In.eivc.ca. tr.exc.ca vas de. y.'gl. sh.gl ov. Fig. 158. 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; In.n. longitudinal nerve. the exterior in the Malacocotylea but into the gut in the Hetero- cotylea. 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. 159 and 160.) 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 different shapes; in Bothriocephalus latus the covering of the embryo is ciliated, in Dipylidiiim caninum the ciliary covering is replaced by>an albuminous coat with a chitinous lining inside, in most 202 THE INVERTEBRATA tapeworms only the chitinous covering persists. The egg hatches as an onchosphere after being swallowed by the first host. The onchosphere then bores through the wall of the alimentary canal and lodges some- where in the peritoneal cavity of the host. Here it develops suckers and a scolex. In primitive forms such as Bothriocephalus^ the young cestode rests inside the first host, a Cyclops, which is then eaten by a freshwater fish. It again bores through the wall of the alimentary canal and rests in the body cavity where it grows still further, reaching the metacestode stage. 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 Schistocephalus gasterostei are commonly found. The adult in this case reaches maturity when eaten by a bird. Man acquires Bothriocephalus latus, a nearly related form, by eating pike infected with the metacestode. In the Tetrabothriata 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 pisiformis wsis 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 occur when the 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 re- markable 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 diflicult 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 cysticercus 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 Fig. 159- Fig. i6o. Figs. 159 and 160. Diagram of the arrangement of genital organs and ducts in the Platyhelminthes. i. Rhabdocoelida. 2. Tricladida. 3. Trematoda, Heterocotylea. 4. Cestoda, Dibothriata. h.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; vit. vitellarium; ^ & '^ op. common opening of genital atrium to exterior. 204 THE INVERTEBRATA the scolex. There are two divisions: (i) Dibothriata, (ii) Tetra- bothriata. The Dibothriata have two grooves, one on either side of the scolex. The uterus opens to the exterior by a birth pore. The embryo oncho- sphere has a ciliary covering. There is no cysticercus,the resting stage being a metacestode. Bothriocephalus latus belongs to this group. Triaenophorus nodulosus, another member of the group parasitic in the gut of freshwater fish, has hooks as well as grooves in the scolex and the proglottides are not marked by divisions. They are only distinguishable by the uterine birth pores. The Tetrabothriata 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. Two interesting forms may be mentioned in addition. Dipylidiuni caninum is a tapeworm inifesting the alimentary canal of dogs and cats. 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. CHAPTER VII THE NEMERTEA AND ROTIFERA 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; alimentary 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, 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. i6i, 162) 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 proboscis. 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. 162 C). The proboscis 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 is first coiled round its prey and then re- tracted to bring it within reach of the mouth. Some forms use the d.com.~-gii§ v.com pt. hrn. bm. re. pft- ps. pfi>'. rh.( St. intd. A •?• Pf- P^- ect. Fig. i6i. Fig. i6i. Cerebratulus fusciis. Dorsal view of young transparent form, x 7. After Burger, brn. cerebral ganglia; c.p. cephalic pit, a lateral slit in which is situated the cerebral organ; d.com. dorsal and v. com. ventral commissures of cerebral ganglia ; la.v. lateral vessel ; M. position of mouth seen through the body; ph. proboscis; p.al. pouches of alimentary canal; pt.hrn. posterior part of cerebral ganglion; re. rhynchocoel; rh. rhynchodaeum ; st. stomach. Fig. 162. 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. eet. ectoderm; l.v. lateral blood vessel; l.n. lateral nerve; m.e. circular muscles; m.l. longitudinal muscles; ov. ovary; p.al. pouch of intestine; par. parenchyma. Gther letters as above. C, Proboscis of a metanemertine to show the diaphragm and the stylets sy. and sy.' After Bresslau. NEMERTEA 207 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 complete nervous sheath, which in the most primitive nemerteans lies at the base of the ecto- derm cells, in others between the circular and longitudinal muscles, and in the most advanced forms within both layers of muscle. While the nervous system is thus extremely primitive there are concen- trations 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 ecto- dermal pit, the cerebral organ, which is situated in some forms in a lateral slit. There are occasionally eyes of simple structure. Inside the muscle layers the body is filled with parenchyma like that of the Platyhelminthes (Fig. 162 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 tubes 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. 163). 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 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 208 THE INVERTEBRATA 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 larv^a the young nemertean is produced (Fig. 163 A, B). Two ectodermal plates (imaginal discs) on each side of the mouth sink below the surface and are enclosed in sacs. 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 ^mesG^ pr/- ^^ect.^'---j \—al. amii." Fig. 163. 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 ; pr. prototroch ; rh. rhynchocoel. 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. 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, Geonemej'tes , Malacobdella. NEMERTEA, ROTIFERA 209 Heteronemertini. Proboscis without stylets; a second layer of longitudinal muscles outside the circular muscles ; lateral nerve cords lie between the two. Lineus^ Cerebratulus. PHYLUM ROTIFERA Minute animals, unsegmented and non-coelomate, with a ciliated trochal disc for locomotion and food collection, a complete alimentary 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 may be closely related to the Platyhelminthes. 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. 165). 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 possibly only a derivation of the segmentation cavity of the gastrula (the blastocoele), as in the trocho- sphere 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 they resemble certain specialized trematodes. Like the Nematoda they consist of a small number of cells and all the tissues, except the cells of the velum, 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. 164). BI 14 210 THE INVERTEBRATA Fig. 164. 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; brti. brain; c?2g. cingulum; d.a. cloacal aperture ; d. an. doTsal antenna ; f.c. flame cell ; g.gl. gastric gland ; ect. syncytial ectoderm of trochal disc; M. mouth; m.c. circular and m.l. longitudinal muscle cells; 7?is. muscular mastax and trophi; np. nephridium, intracellular duct represented by double dotted line; ov. ovary; oe. oesophagus; p. penis retracted; ped.gl. pedal gland; pa. papillae with large cilia; st. stomach; t. testis; tro. trochus; vit. vitellarium. ROTIFERA 211 The female is pear-shaped, the posterior end being the stalk. The anterior end is flattened and forms 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 stronger 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 l-m-c cct^"^ fi.m.C' ecdu. Fig. 165. A, Side view, diagrammatic, from Shipley and MacBride, and B, transverse section of a female rotifer. An. anus (cloaca! aperture); c.m.c. circular muscle cell; cii. 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. 164. 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 stifle cilia. 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 14-2 212 THE INVERTEBRATA free-swimming life. The dorsal surface of the rotifer is marked out by the position of the cloacal 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 hrain. There are also two lateral antennae; all three are prominences bearing stiff sense hairs. Else- where 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 trophi, which are in con- stant movement and 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 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 germar- 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. 164 B) the male has no 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. ^ Digestion is usually extracellular, but in Ascopus and other rotifers it is intracellular. ROTIFERA 213 Two kinds of reproduction occur in the rotifers as in the clado- ceran 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 Asplatichna, 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 and pass 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 reproduction can be brought on in cultures by alteration of the external conditions. Besides the environmental types which have already been men- tioned as free-swimming and inhabiting larger and smaller bodies of water, the following rotifers may also be mentioned : Stephanoceros and Floscularia are sedentary forms which secrete a protecting gelatinous tube into which they can withdraw 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 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. CHAPTER VIII THE 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 the nerves ; 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 diffusely 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, development direct, larvae only differing slightly from adult. The nematodes occupy an isolated position, and in the opinion of the writer are best considered as occupying a position like that of the rotifers, not far removed from the Platyhelminthes, and representing a precoelomate stage of organization. One of their peculiar features is certainly secondary, namely the absence of cilia. There are in some nematodes cilium-like processes to the internal border of the endo- derm cells; in one case active movement has been reported. Nearly all the other characters may be called primitive. The simplicity of organization, the absence of segmentation 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 NEMATODA 215 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 which at present there are no grounds for assuming. 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 Ascaris 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 Ascaris (Fig. 166) 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 Ascaris (Fig. 168), 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 Ascaris 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. 167) will 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 which exist in an "encysted" condition in the soil are attracted by the products of decay, and in a few days become sexually mature 2l6 THE INVERTEBRATA 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 liquid 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- mxo. laid ex.c. cut. rn,r. i\ n. Fig. i66. 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, cut. cuticle; d.n.^ v.n. dorsaland 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; 7n.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 m.r. which increase the lumen of the oeso- phagus and cause suction. section and length : in the presence of such a cuticle and the absence of circular muscles the peristaltic movements of a worm like Lumhricus are impossible. A cross-section through Rhabditis 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 h.cav. Fig. 167. 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; b.cav. buccal cavity; c.h. copulatory bursa; c.sp. copulatory spicule; cl. cloaca; ex. a. excretory aperture; gl.c. gland cells; f.g. fore gut; m.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.sew. receptaculum seminis ; t. testis; ut. uterus; z;a. vagina; vJ.vas deferens. 2l8 THE INVERTEBRATA 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 evacu- ated. The alimentary canal of the nematodes as thus seen in action represents a type simplified because the animal 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 rhabdites 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 in 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 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. NEMATODA 219 ;ex.c. -oe.t. I"h.g. Fig. 168. Fig. 169. Fig. 168. Dissection of an Ascarid to show position of the branched excretory cells. /./. lateral lines; ex.c. excretory cells. After Nassonow. Fig. 169. 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. 20 THE INVERT^BRATA The two uteri join to form the median vagina. In this the fertilized Q%g 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 contraction 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 they 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 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 wander- ing in many nematodes, and this is seen in a remarkable manner in NEMATODA 221 the classical life history of Ankylostoma (Fig. 170). These animals live 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. The life histories of the principal nematode parasites of man and domestic animals are summarized on pp. 222-3. They are arranged in a definite order passing from the simplest type in Haemonchus 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 Co W 05 CO e "^ cs 5 iH d g c« M D qH U CO ^ 5 c! :: -h; So +j CO O > lU (U t: CO a, ai! ;^-c; rt 2 oj TO >^ o a^ ■M CO 3 ^ c — t^ ^ ^ o > ^ 2 S " o o 3 j:: i O (U U u (U CO S^ a I ^ ^^ a « — DC d ^ CO e S o " S CO ^ t^ s ^ a O i? O i^ -^ »^ -^ ~^ O S O K C C ^ o cB a ::: ^ o o o ■y b CO •r; « o ^ -C S-O ^ Oj cs C *" CO O ?^ G >< tu C > « be u ^ t5 o O "G as -O -3 ♦--Co ex 3 .. O II 2 o S 4> CO C 2 03 a 13^ C >> CO — ^ -M CO ^ CO (U ^ C G g 3 c O (U g a o o (3 j:: <3 O M 1 1 r! -d £ ■B CO m « a > . o (U I CO lU c/3 rt Q +^ CJ. (U W) O o . C3 I- S ■a-s 3 C3 IS d (U I_ Ul CO CO +j ^S 2-^ CXt3 I "^ o g o .^ O lU 3 "^ cr CO ^ o ^1 "" S P.-St ^ d-S 5? Tsodox:'^ .(U'^_:35> ^ d -^ Q 5 k! >.« ^ Cd ni Ur iS-^ CO -3 "^ d 2-^ fl d^^ ►J ♦- CO CO CO 8 2 c- ^ o § c« 4; -do: 4^ ,22 2i « ^ j> iXJ 3-g 'C>, 2^ ^^ ill^ 3 « ^ ^d « CO ^^^ O O CO bjD «s :< -1-. 4-..<2 .^j Si C-^-^ d .3 > _2 r d ^ '^ S -p.B u « c K _ OT c Ci- d.g lu c3 >i ;d (1) ?». TS cvi g d o" 1^0 ^ '^•-' b C3 i3.a^|s 4> g CO. 5 CO J5 «3 -M CO ili ^ o o :r « |:?!/)q_.S § .2 d t^.d S jd S^ "^ S S =« S-^"*- S o d a jd^ a o 3 d rt o O « ^ rt 4j d >. 3 CO -^ (U > CO-- ^ ^■- d 224 THE INVERTEBRATA Fig. 170, Nematodes parasitic in man. A, B and C, Ankylostoma. 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 hancrofti, longitudinal vertical section through a mosquito (Stegomyia) 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 225 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 pene- trate the cell walls of plants. They feed on cell sap and by their inter- ference with the life of the host plant cause the formation 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 devastatriXy 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. 171 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. Inside the body thousands of larvae are produced, which escape into the soil and live there until the opportunity arises for infection of fresh roots. Insect parasites. Four of these may be mentioned, though other life histories are also of great interest. In Mermis (Fig. 169) 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 and wander into the body cavity where they nourish them- selves 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 produc- tion 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 BI 15 226 THE INVERTEBRATA 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 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) Fig. 171. Insect and plant parasites. A, Atractonema. Female showing the beginning of the prolapsus of the uterus, which has proceeded in Spherularia, 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; 1 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 causes the increase in size of the parasite, o. ovary; ut. uterus. A-C, after Leuckart; D, after Strubell. 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. 171 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 I NEMATODA 227 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. 171 A), a parasite of the Cecidomyidae (p. 439), has a similar life history. 15-2 CHAPTER IX THE PHYLUM ANNELIDA ^-ect. Segmented worms in which the perivisceral cavity is coelomic ; with a single preoral segment (prostomium) ; 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; 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 chaetae com- posed of chitin, arranged segmen- tally, imbedded in and secreted by pits of the ectoderm (Fig. 172). The cuticle is thin and not composed of chitin, thus differing from that of the Arthropoda. Four classes compose the phylum. Of these the largest and most typical is that of the Chaetopoda, which are well segmented , have a spacious peri- visceral coelom and always possess chaetae. All these characters are 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, how- ever, like Saccocirrus, retain chaetae. It is almost certain that the archian- p^g 1^2. Chaeta of Lumbricus in nelids are derived from the chaeto- body wall. Altered from Stephen- pods by a process of simplification, son. cu. cuticle; ecu ectoderm; ch. U^, T 1 TT- J- chaeta I c.7«. circular muscles ; ^r.w. The Leeches or Hirudinea are p^^^^^^',^, .^^ run. retractor mus- adapted to a specialized mode of life ^^^^ . p^y peritoneum ; jol. follicle — ectoparasitism — and their whole and fm.c. formative cell of chaeta organization is affected by it. They (with nucleus), 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 ANNELIDA 229 posterior suckers and a hermaphrodite reproductive system, closely paralleled in a subdivision of the Chaetopoda, the Oligochaeta, show the specialization of the group. The Gephyrea is a group of burrowing marine worms in which segmentation has been almost entirely sup- pressed in the adult but is sometimes shown in the larvae by meso- blastic somites and 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- stomtum 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 de- velopment of special muscles for moving the chaetae. In the other order, the Oligochaeta, there are no parapodia. The coelom is bounded by an epithelial layer, t\it peritoneum, which gives rise to the gonads, which in polychaets are usually developed in most of the segments, to the yellow cells, which play a part in the work of nitrogenous excretion, and to the coelomoducts by which the eggs and sperm pass from the coelom to the exterior. In most of the poly- chaets the eggs are fertilized 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. It is probable that in many cases the lips of the nephrostome are formed by one or more mesodermal cells so that a compound tube consisting mainly of ectoderm but partly of mesoderm exists (nephromixium). Nephro- 230 THE INVERTEBRATA mixia may take on the functions of coelomoducts where these do not exist. 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 into venous blood. The whole of the dorsal vessel (Fig. 190) is usually contractile : there may also be vertical segmental contractile vessels which are usually called "hearts". In some forms, for example, Pomatoceros (Fig. 175 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. 178)): 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. The Chaetopoda are, in this work, divided into the following orders: (i) Polychaeta, (ii) Oligochaeta. 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 fertiliza- tion; 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 CHAETOPODA 231 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. fEunicidae. Eunice, Leodice The errant Polychaeta with unmodified head and armed eversible pharynx; 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 micros- copic 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 Palolo worm). Nereidae. Nereis, Syllidae. Syllis, Myri- anida. Phyllodocidae. Eulaliay Aster ope. Polynoidae. Aphrodite^ Lepidonotus , Panthalis . 'Chaetopteridae. Chaeto- pterus. Terebellidae. Terehella, Amphitrite. Serpulidae. Pomatoceros y Filigrana. Sabellidae. Sabella^Spiro- gr aphis. IArenicolidae. Arenicola without jaws. Glyceridae. Glycera with jaws. The errant Polychaeta The external structure is known to the elementary student through the type Nereis (Fig. 173). The prostomium bears two kinds of 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 gut [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 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). 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 232 THE INVERTEBRATA forms the point of origin of the parapodiai muscles. The chaetae 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 processes as a primitive feature). But the cirri remain as the peri- tenx.^. ac^' neur. Fig. 173. Nereis. A, Dorsal view of head and first 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; p. 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 ;y. jaws; pg. paragnaths. 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. 174 A) this is constituted by a single segment, but usually two or more have been pressed forward towards the mouth POLYCHAETA 233 and modified. This is the first indication of the process of cephali- zation carried on much further in the arthropods and vertebrates. The worms in this group used to be definitely classed as the 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 recon- struct anew. The most beautiful example of tube building in the Poly- chaeta is furnished by Panthalis, a polynoid. In this the chaetal pits of the notopodium produce not stiff bristles but plastic threads which are woven by the comb-like ventral chaetae into a continuous fabric m^,". 'ph.sh. Fig. 174. 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; 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. which forms the lining of the mud-covered tube. Aphrodite, the sea mouse (Fig. 175 A), is a short, broad form which burrows in mud, and though it does not form a separate tube it covers its back with a blanket made from interwoven chaetal threads similarly formed from the notopodium. Between this blanket and the back is a space into which water is drawn by a pumping action of the dorsal bo^y wall, being filtered through the matted chaetae. In this there are special plate-like modifications of the dorsal cirri — the elytra — round which 234 THE INVERTEBRATA 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 Fig. 175. Transverse sections through different types of Polychaeta. A, Aphrodite. After Fordham. B, Aretiicola, middle region. After Ashworth. QfPomatocer OS, thoTSiii.. Original. aZ. alimentary canal ; cw. coecumof mid gut; ch.m. matted notopodial chaetae; cil. ciliated groove; ^.ti. and t>.i). dorsal and ventral blood vessels; el. elytron; m.c.,m.d.v., m.l. circular, dorsoventral and longitudinal muscles; obl.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. 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. POLYCHAETA 235 The diagnostic features of Nereis and other genera mentioned in the classification are given below. Nereis (Fig. 173). 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. 174 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. 174 C). Similar to Eulalia but a pelagic polychaet with transparent body and enormous eyes of complicated structure. Syllis (Fig. 174 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. Myrianida. Similar to Syllis but pharynx long, with a circle of teeth; no ventral cirrus. 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. 176 A), serpulids (Fig. 177) and sabellids, the appendages of the head, 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 stiflF 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 serpulids and sabellids, there are no special respiratory organs but the whole surface of the body serves for the exchange of gases. In the terebellids the tubes are composed of a soft cementing substance mixed with mud or a parchment-like material to which 236 THE INVERTEBRATA e.^" '''' ''lot. thor. Fig. 176. 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 Wilson. Chaetopterus pergamentaceiis. 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, col. peristomial collar; e. eye; tent. tentacle; up. I. upper lip, the lower lip (/./.) is a prominent structure to the right of the tentacles ; ot. otocyst ; not. notopodia ; not. 4, notopodia with en- larged chaetae in 4th chaetiferous segment ; not. 10, food- collecting notopodia ; cup, organ for forming foodballs; fan., mu.fan. muscles for moving fans; neur. neuropodia forming suckers for attachment of worm to tube; abd. abdomen ; gl.sh. mucous glands ; thor. thorax. POLYCHAETA 237 adhere sand grains, sponge spicules, foraminifera or fish-bones. It is usually porous 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 with- drawn 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. 177) 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 mew^rawe, a 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 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. 175 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. Chaetopterus (Fig. 176 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 238 THE INVERTEBRATA 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 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. Fig. 1 77 . Diagram of Pomatoceros triqueter in its tube. Original . The aperture of the tube is represented in black : the top and base of the tube are shown by vertical lines {th.), 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 fact that the tube is composed of successive rings is indicated in the neighbourhood of the aperture (ann.). ap.tb. aperture of tube ; pr. prostomium ; not. notopodia ; neur, neuropodia; op. operculum; ten. tentacle. The burrowing Polychaeta Arenicola marina (Fig. 178) 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 loco- motion 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 seg- POLYCHAETA 239 ments which have a notopodium with capillary setae and a neuro- podial ridge with chaetae resembling uncini (crotchets); the median, ac/i. per. ^^^. m.r.{ Fig. 178. Arenicola marina. Side view. After Ashworth. phar. pharynx; pr. prostomium; per. peristomium; ach. ist achaetous segment; a.r. anterior region; m.r. median region; p.r. posterior region; not. notopodium; neur. neuropodium. the segments of which have gills in addition ; and the posterior, in which parapodia and chaetae are entirely lost. 240 THE INVERTEBRATA The body wall consists of 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. Arenicola (Fig. 179) is the most convenient polychaet type for dissection and therefore the following details of internal anatomy are given. In several prominent features it dilfers from Lumbricus and also from Nereis or Eunice. The body cavity is spacious, it is not encroached 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 pharynx, however, is not well developed, the oesophagus is a thin-walled tube with no such development as the gizzard of the earthworm 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 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 in- testinal 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 I d.i: Fig. 179. General dissection of Arenicola marina. After Ashworth. aff.h.v., eff.b.v. afferent and efferent vessels of gills and body wall; au. auricle, d.v.y l.v. dorsal and lateral blood vessels; nephr. nephrostome; not.ni. 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. BI 16 242 THE INVERTEBRATA 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 it 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 blind end projecting into the coelom is fringed with solejiocytes, cellular organs which have a very close resemblance to the flame cell of Platyhelminthes and Rotifera. Such ''closed" nephridia 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. The familiar example of this is the "nephri- dium" of Lumbricus. In this type the tube mainly consists of ectoderm ; the funnel of the nephrostome is in Lumbricus (and probably other forms) derived from a mesoblast cell. Such an organ pro- duced by the fusion of two different elements is termed a nephromixium. An example which shows very clearly the dual nature of the nephro- mixium is the segmental organ of the leeches. In Fig. i8i it will be seen that the lumen of the proximal (ectodermal) and that of the distal (mesodermal) portions do not communicate and the nephro- stome cannot act as a funnel leading to the exterior. But usually the development of segmental organs has not been sufficiently studied to decide what type they belong to. In the following paragraphs such doubtful cases will be referred to as nephridia. 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 of Lumbricus are coelomoducts. In a family of the Polychaeta called the Capitellidae there are coelomoducts in most segments of the body serving as gonoducts (Fig. i8o I, D). POLYCHAETA 243 Nephridia may exist in the same segments as coelomoducts but be entirely separate in position and function. They may also occur grafted together. In the Alciopidae, for instance, nephridia and coelomoducts occur in most segments united as the nephromixium, as shown in Fig. 180 II. In Nereis 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. ,nephr. '.o.d.-- Fig. 180. Segmental organs of Polychaeta. After Goodrich. I, Transverse section (right half) of body segment showing combinations of nephridia and coelomoducts. A, Hypothetical. B, Phyllodocidae and 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. 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. The closed nephridium appears to be the most primitive type of 16-2 244 THE INVERTEBRATA 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 most of the polychaets are usually patches of the peritoneal epithelium, repeated in most of the segments, proliferating nephr. co'.d. nephr. nep'. Fig. 182. Fig. 181. Fig. 181. Segmental organ of Clepsine (Hirudinea). After Oka. Showing mesodermal part with ciliated nephrostome and a single cell of the ectodermal part, with intracellular duct. nu. nucleus. Fig. 182. 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. 180 for both figures. 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. 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. Fig. 183. 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 gemtfiipara, 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 dy discharging the eggs and sperm. In e they are emptied and sink to the bottom;/, parent worm regenerating the sexual region. 246 THE INVERTEBRATA 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 ir- regularly 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. 183 E) has acquired the strictest periodicity. As the day of the last quarter of the October-November moon dawns the Pacific Palolo breaks off 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 artificially. 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 ^m/o/j^m^ (Fig. 183 B) and Myrianida a 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 it the chain of sexual individuals which one by one detach and lead a short independent existence. In some species of Trypanosyllis (Fig. 183 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 transverse rows, amounting to more than a hundred — the fully formed sexual individual possesses a head but no vestige of an alimentary canal. The extraordinary branching form, Syllis ramosa (Fig. 183 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 notopodium during asexual life but during the maturation of the gonads the parapodium is recon- structed, a notopodium being formed from which spring bundles of long capillary swimming chaetae, while a corresponding develop- POLYCHAETA 247 I ment 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 di- gested by leucocytes before the new muscles are formed. The parapodium of the sexual form, the Heteronerets, is pro- duced into membranous frills and con- tains a new type of oar-shaped chaeta (Fig. 173 D, F). The eyes become im- mensely larger and the animal itself very sensitive to light. The Heteronereis does in fact resemble those members of the Phyllodocidae and Alciopidae which have become permanently pelagic. The in- crease 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 where the meeting of the sexes is facili- tated by the exchange of light signals, and in the nereids the discharge of sperm in borax carmine and mounted 111 1 ^ u ^ L ^u • in Canada balsam. Notice the may only be brought about by the m- ^^^^^^^^^^ developed eyes, fluence of a secretion from the swarmmg i^^g peristomial cirri, anterior female. Discharge of the gametes is unmodified trunk region, pos- nearly always followed by the death of terior modified region with the sexual individual. parapodia sloping backwards rr^i r M- 1 • • . and darker appearance owmg The fertilized egg gives rise to an un- ^^ presence of gonads, segmented larva, the trochosphere, which is described in the next section. Fig. 184. A Heteronereis . Pho- tograph of specimen stained Development of the Polychaeta The cleavage of the egg in the Polychaeta and the Archiannelida, the polyclad Turbellaria, the Nemertea and the MoUusca 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. 185,2) lying in the same plane, which are called A,B,C,D\ each cell in its further 248 THE INVERTEBRATA 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 off successively from the macromeres as A^ By 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. The cleavage is therefore said to be oi radial 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 composed of cells differing notably in size and density from those of the first three. Of the fourth quartette d^ (Fig. 185, 4) alone produces the mesoderm, while the other three, a^, b^ and t*, 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. Gastrulation (Fig. 185, 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 Arenicola, Nereis and nearly all Polychaeta and all MoUusca 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 (^^) 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 POLYCHAETA 249 Fig. 185. Partly after Dawydoff, Radial and spiral cleavage, i, Eight-cell stage in a radial type, e.g. Echinoderm. 2, Spiral cleavage, four-cell stage just before cleavage, leading to eight-cell stage. 3, Macromeres stippled. Develop- ment of Polychaeta. Nereis, 4, Diagram of segmenting egg seen from the animal pole, showing the macromeres, the first three quartettes of micromeres and the mesoblast cell ((i*) in the fourth quartette. 5, Later stage, also from 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. 250 THE INVERTEBRATA 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 ( a'^, b^, c^ ; a^, b^, c^, d^). The somatoblast (d^) breaks up into a large number of cells to form the ventral plate . The change from gastrula to trochosphere (Fig. i86) follows quickly and with little further cell division. The first quartette of micromeres have by this time been differentiated (Fig. 185, 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. 185, 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^, 6^, 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 the rudiment of a large part of the trunk of the adult 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. The trochosphere drifts hither and thither in the sea, swimming feebly by the action of the cilia of the prototroch and sometimes also by secondary rings of cilia (e.g. metatroch formed from cells of the POLYCHAETA 251 third quartette). During this pelagic existence the rudiments of the aduh 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 cylindrical 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 show at once metameric segmentation (Fig. 187), 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 inesc. oes. mes. an.v. Fig. 186. Trochosphere larva of £'z/^owafz<5. Side view. After Shearer, a/). 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 ; hl.c. blastocoele ; mesc. mesenchyme ; oes. oesophagus ; st. stomach. 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 mesen- teries 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. The advanced larva (Fig. 187 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 252 THE INVERTEBRATA the contraction of these muscles (Fig. 187 B). The larval mouth re- mains in the adult. After metamorphosis the animal sinks to the bottom and begins its adult life. A B Fig. 187. Development of Polygordius. 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. 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. CHAETOPODA 253 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, however, the great variability found in the Polychaeta. Certain main features of the reproductive system (Fig. 188) are the salient characters of the group. Its members are, without excep- tion, 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 oi 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 earthworms {Lumbricus^ Allolobophora) used in zoological laboratories in this country always possess it. Both the clitellum and the cocoon produced 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 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 254 THE INVERTEBRATA 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. i88 C) consists essentially of two pairs of testes in segments lo and ii and one pair of ovaries in segment 13, followed by ducts which open by large funnels just behind 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 oi spermathecae in the region in front of the testes. In the neighbourhood of the male external aperture there are sperniidiical {prostate) glands which do not actually open into the sperm duct. A single pair of segmental organs (nephromixia) 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 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 oi 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. Tht 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 Eutyphoeus, where the ends 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 Fig. 1 88. Reproductive organs of the Chaetopoda. A, Polychaeta (longi- tudinal section of Serpula ititestifialis 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; os. 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. 256 THE INVERTEBRATA 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 of Lumbricus is illustrated in Fig. 189. 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. The most primitive kind of nephridium (nephromixium) is that described in LumbricuSy 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 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 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 . Other modifications are those in which the nephridia open into the alimentary canal instead of to the exterior. They maybe peptonephridiay OLIGOCHAETA 257 opening into the interior part of the alimentary 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. ect.^ ^end. Fig. 189. Embryo of Lumhricus foetidiis. After E. B. Wilson. A, Lateral view of an embryo in which the mesoblast is imsegmented. 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. hrn. 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. 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. 190). 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. Bi 17 258 THE INVERTEBRATA The aquatic Oligochaets As a type of these, Stylaria, belonging to the family Naididae, will be shortly described (Fig. 191). This is a transparent worm rather 5^ Fig. 190. Fig. 191. Fig. 190. Dorsal vessel of Lwnbricus to show connections and valves. After Johnston. J.^;. dorsal vessel; vessels leading to dorsal vessel : d.i.v. irom subintestinal vessel, d.t.v. from typhlosoar 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. 191. Stylaria proboscidea. Original. Dorsal view. few. median prostomial process; e. eye; M. mouth ; p/z. pharynx; oe. oesophagus; cr.crop; /«f. 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. less than a centimetre long found crawling on water weed. The prostomium bears minute eyes and is produced into a long filiform OLIGOCHAETA 259 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 oiLumbricus, a 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. 188 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 w^orms 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 form a useful supplement to work with Lumbricus in understanding oligochaet organization. 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 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. Two common genera, Tubifex and Lumbriculus, 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. 17-2 26o THE INVERTEBRATA 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 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 Tuhifex plays the same sort of part in fresh water that the earthworms play on land. Lumbriculus resembles Tubifex superficially but has only eight d.v. pi Fig. 192. Blood circulation in Lwnbricidus 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. 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. 192). 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. CHAETOPODA 261 Class ARCHIANNELIDA Small marine annelids with simplified structure, parapodia and chaetae being usually absent. This group was founded to receive two genera, Polygordius and ProtodriluSy 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 connection of 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. 183 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 reduced vascular system and nerve cords lying in the epidermis ; with a trochosphere larva. Fig. 187. Protodrilus. As in Polygordius but with segmentation marked ex- ternally by ciliated rings and with a longitudinal ciliated groove in the middle of the ventral surface ; with a ventral muscular pharyngeal sac ; hermaphrodite. A single species, P. chaetifer, has recently been discovered with four short chaetae in each segment. Saccocirrus (Fig. 193 B). As in Protodrilus^ but with chaetae 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. 193 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), 262 THE INVERTEBRATA three pairs of sperm ducts uniting at a common median genital aperture, and two oviducts with separate genital apertures. Dinophilus (Fig. 193 D) with very short flattened body consisting of only five or six segments, a ciliated ventral surface and ciliated ring yne-p. Fig. 193. Examples of the Archiannelida. A, Nerilla, dorsal view of female. A.' Parapodiumi. B, Saccocirrus, side view of anterior end. C, Histriohdella, dorsal view of male. D, Dinophilus y 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 ;j. jaw ; nep. nephridia ; o. ovary ; od. ovi- duct; p. penis; ph. pharynx; p.f. posterior foot; rm. rectum; st. stomach; v.s. vesicula seminalis; /. eyes; p.p. palp; t. testis. A, B, after Goodrich; C, after Shearer; D, after Harmer. in every segment ; without septa, dorsal and ventral mesenteries, and a vascular system; with greatly reduced coelom and longitudinal muscles; five pairs of protonephridia ; separate sexes, male with CHAETOPODA 263 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. 193 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 inside 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 "ganglion" (Fig. 194 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. 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. 195), there is a protrusible ^ro^o^m surrounded by a.proboscis sheath. (2) A short oesophagus follows, leading into (3) the mid gut (crop) which is 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. >Mx. O ^s?>i^- .■i__i-^ft. A B Fig. 194. Anterior part of nervous system in A, Lumhricus. After Borradaile. B, Hirudo. After Leydig. The brain m both consists of a single dorsal pair of ganglia belonging to the prostomium. In Lumbricus the subpharyngeal ganglion {shp.) 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 ; j. jaws ; pr. prostomium ; ph. pharynx (with network of visceral nerves); so. sense organs. Fig. 195. Glossiphonia as example of the Rhynchobdellidae. Dorsal view. an. anus; cr. crop (black); oe. oesophagus; int. intestine (stippled); pb. pro- boscis ; ps. proboscis sheath ; rh. rhynchodaeum ; rm. rectum ; sa.gl. salivary glands. CHAETOPODA 265 (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 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. in Clepsine, Fig. 196 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 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 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 system communicates both with the coelom and the blood system. The peculiar functions of the lymphatic system are not shared by the botryoidal vessels which have no particular connection with the gut. The nervous system is of the usual annelidan type but characterized by the fusion of ganglia anteriorly (Fig. 194) 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. 181); 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 266 THE INVERTEBRATA 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 ect. n.c. Fig. 196. Transverse sections of Hirudinea to show the progressive restriction of the coelom. A, Acanthohdella, B, Clepsine, 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. hypodermal 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 (i); botryoidal tissue {b.t.) is present; ch. chaetae; cm» coecum ; cr. crop ; d.v. dorsal, l.v. lateral, v.v. ventral blood vessel ; gl. glands ; m.c. circular, m.l. longitudinal muscles; ect. ectoderm; nep. nephridium; oe. oesophagus; n.c. nerve cord; per. peritoneum; s.o. sense organs. 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. HIRUDINEA 267 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. 196 A), a parasite of salmon, is a link 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 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 OHgochaeta that the family must remain in that group. Family Rhynchobdellidae. Pontohdella^ parasitic on elasmobranch fishes. Glossiphonia (Fig, 195), a freshwater leech feeding on molluscs like Limnaea and Planorhis 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 268 THE INVERTEBRATA 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 Haemadtpsa 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. Class GEPHYREA Annelida which have lost their segmentation partly or completely : with a spacious coelomic cavity. The Gephyrea are annelids of comparatively large size and burrow- ing habits. They are divided into two orders, the Echiuroidea and the an.-^ ^an.v. Fig, 197. Echiurus. Ventral view of larva to show segmentation of posterior end. After Hatschek. an. anus; an.v. anal vesicle; M. mouth; mt. metatroch; nep. nephridium; prt. prototroch; sep. septa of transitory segments; v.n.c. ventral nerve cord. Sipunculoidea, which differ in the extent to which they depart from the chaetopod type. Order Echiuroidea, with 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 which develop fifteen pairs of mesoblastic somites (Fig. 197). Echiurus, with a spoon-shaped prostomium, two pairs of segmental organs and a trochosphere larva. Bonellia (Fig. 198 A, B), with enormously elongated prostomial proboscis bifurcated at end and extremely mobile ; a single segmental organ (brown tube); the female is the normal individual and the males are reduced to small ciliated organisms, like a turbellarian, which live in the segmental organ of the female = GEPHYREA 269 Order Sipunculoidea, without prostomium in adult; chaetae always absent, anterior part of body invaginable into posterior part ; m.retr. Fig. 198. Bonelliaviridis. A, Female. B, Male from nephridium of female. After Spengler, C, Sipiinculus. 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; bt. brown tube (nephridium); ch. position of chaetae; cil.gr. ciliated groove; cm. 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. ^ male reproductive aperture; pr. greatly enlarged prostomium; sp. spermatozoa. anus dorsal and anterior; with a single pair of segmental organs (brown tubes); no trace of segmentation even in the larva. Sipunculus (Fig. 198 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 often with one or more pairs of coelomoducts 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 entrains the necessity for joints; and the stout, jointed limbs can become jaws. 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 probably 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, v^ 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. One small group of the Arthropoda stands apart from all the rest. The Onychophora have a thin cuticle, without joints; a continuous muscular body wall; eyes (p. 274) 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, embryonic structure without external representation in the adult. 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 ARTHROPODA 271 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 undiff'erentiated. 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 pro- cesses (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, Insecta, 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 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 crayfish. 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 alteration 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 SOMITES AND LIMBS Somite Onychophora Arachnida Scorpionida Trilobita I...* Preantennae [Embr. of Spider] .? 2 Jaws Chelicerae Antennae 3 Oral Papillae Pedipalpi I St 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 Embryonic tt 9 ^ Genital operc. ? ^ lO Pectines II 0 I St Lung books 12 2nd „ 13 CO CiO 3rd „ 1 14 •i 4th „ 15 1 No limbs 1 i6 J3 0 03 ist som. Metasoma i t 17 (U 2nd cS i8 3rd .s 19 S 4th 1 0 20 CO 5th ^ 21 0 ... § 22 ... 23 24 ■•-> 1 25 ^ ... 26 ... ^ 27 ... 28 ... 29 Last pair of legs ... $(^ Postseg-j mental h Embryonic Telson Telson region) * Eyes and frontal organs belong to a presegmental region which may have median ** If the superlinguae be maxillules (see p. 407), the limbs behind them stand on t Terga fused in Scolopendra, free in Lithobius. ft Chilaria in Limulus. § This somite appears to have no limbs, because the limbs of the 8th and 9th somites ^ 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 | ist pair of legs] Collum 2nd Thoracic limb 2nd „ I St pair of legs 3rd 3rd „ 2nd ,, 2nd „ ?c?§ 4th ist Abd. som. 3rd „ 3rd pair of legs 5th 2nd „ 4th „ 4th „ j 5th „ J 6th „ ? 3rd „ 5th „ 7th 4th 6th „ 8th „ S 5th „ 7th „ be I St Abd. limb 6th 8th „ c3 9. 2nd „ 7th 9th „ 3rd 8th „ 2 loth 4th 9th ,, (styles) 3* nth a 00 5th loth „ som. 1 2th 6th nth „ (cerci) 13th „ S^ •• 14th „ 15th 1 6th 17th 1 8th „ 0 0 .. 19th „ 0 .. 20th „ c .. 2ISt „ J i ., Genital limbs ? 0 ... 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 Lithobius has 15 pairs of legs, have each moved forward one somite. RT 18 274 THE INVERTEBRATA (gnathobase) and on a more distal segment an outer branch (exopo- dite) ; but there are difficuhies in the way of assuming this in all cases, and the problem is still far from solution. The arthropod cuticle is composed of chitin, often hardened by a deposition of salts of lime. From time to time during the growth of the animal, the hard outer layers of the cuticle are separated from the inner layers, ruptured, and shed in a moult or ecdysis. The soft inner layers then expand 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 sense organs (eyes, frontal organs), and sometimes also of a median anterior element {archicerebruMy 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 somite are the deutocerebrum or mesocerebrum ; those of the third somite are the tritocerebrum or metacerebrum. The identity of some of these ganglia may be lost, even in development. The eyes of the Onychophora are a pair of simple, closed vesicles, each with its hinder wall thickened and pigmented and its cavity occupied by a lens secreted by the wall. The eyes of all other arthropods (Fig. 200) 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. 200 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. 201) com- posed of a number of bundles of cells, each bundle (ommatidium) complex in structure and containing two or more refractive bodies, but opt. I opt.n. an.com Ltr.com. Fig. 199. A plan of the nervous system of Chirocephalus . ab.z, second ab- dominal somite ; an.' antennulary nerve ; an." antennary nerve ; an." com. com- missure for fibres which unite antennary ganglia; hrn. brain; fr. nerve to frontal organ; ga. ganglion of ventral cord; m.e. nerves to median eye; 77id. mandibular nerve; mx.' maxillulary nerve; wjc." maxillary nerve; oe. oeso- phagus; oe.com. circumoesophageal commissure; opt.l. optic lobes; opt.n. optic nerve; th. i, first thoracic somite; th. 12, nerve of last thoracic somite; tr.com. transverse commissure of ventral cords. 18-2 276 THE INVERTEBRATA 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. 200 D). At its outer end is a transparent portion of the general Fig. 200. 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.nie. basement membrane of retinular layer; ex. central cell; cgn, corneagen 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. 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 ARTHROPODA 277 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 retiniila. 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 ^opt.ga. opt.n. Fig. 201. The eye of Astacus. A, The left eye. B, A portion of the cornea removed, to show the facets. C, A longitudinal section of the eye. m. muscles which move the eye; n.fi. nerve fibres; omm. ommatidia; opt.ga. optic ganglion; opt.n. optic nerve. pigmented cells, known as iris cells. The eyes of arachnids, other than the lateral eyes of Limulus, simulate the ocelli of insects, but are thought, from details of their structure, to have been formed by the degeneration of compound eyes resembling the lateral eyes of Limuliis. The median eye of the Crustacea (Fig. 215) is composed of three cups, which may (some copepods) separate widely. The paired eyes probably do not, as has been suggested, represent a pair of appendages. The foremost, or preantennal, somite, to which they would in that case belong, possesses, in Peripatus and as a rudiment 278 THE INVERTEBRATA in 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 omma- tidia exposed, and in bright light extending so as to separate the om- matidia completely. In many diurnal insects it is permanently in the latter position. Vision takes place in two ways according to the situa- tion of the pigment. When the latter is extended, in each ommatidium 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. The alimentary canal of the Arthropoda possesses at its mouth and anus involutions of ectoderm, lined by cuticle, which are known re- spectively as stomodaeum or fore guty 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 respiration of aquatic arthropods, other than those which are but little modified from terrestrial ancestors, is sometimes, if the animal be small, effected only through the general integument of the body, but usually takes place by means of 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 lung 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 ARTHROPODA 279 in the Onychophora, the Arachnida, and the Insecta and Myriapoda. Among the Crustacea, tufts of 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 ("pericardium"), 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 consequences of the structure of the vascular system are a low blood pressure and liability to severe bleeding from wounds. The latter danger is met, especially in the Crustacea, by very rapid clotting of the blood. The coelom appears in the embryo as the cavities of a series of mesoderm segments (" mesoblastic somites", Fig. 328). It never Fig. 202. Three stages in the cleavage of the egg of Astacus. After Morin and Reichenbach. nu, nuclei; yp, "yolk pyramids", due to the transitory appear- ance of divisions of the yolk corresponding to the superficial cells. assumes a perivisceral function, and in the adult is always reduced to the cavities of the gonads and of certain excretory organs. 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 (antennal) somite or a pair on the somite of the maxillae, or, rarely, both these pairs are present. In various crustaceans other glands, some ecto- dermal, some mesodermal, appear to have an excretory function, and sometimes replace both pairsof 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. Mai- pighian tubules are tubular glands which open into the alimentary 28o THE INVERTEBRATA canal near the junction of mid and hind gut in the Arachnida, Insecta, and Myriapoda, In arachnids 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 Arachnida guanin. 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 coelomoducts. 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. The ova are generally yolky, and their cleavage is typically of the kind known as "centrolecithal", in which (Fig. 202) 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 to obscure processes of immigration and delamination. The formation of the mesoblast as a pair of ventral bands, proliferated in primitive cases from behind, has already been mentioned (p. 121). As in annelids (p. 251), the mesoblast bands segment, and in most cases the segments (" mesoblastic somites") develop coelomic cavities (p. 279). 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, ob- solescence, 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 impossible 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 clearly related to the Crustacea and less closely 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 representing a branch which parted at a very early date from the main arthropod stock. The trilo- bites 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 gut and 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. 203). They are distri- buted discontinuously over the warmer parts of the world and occur in very retired positions as, for instance, beneath 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 were classed, by different investigators, with all three of these. Certain of the characters of Peripatus such as the feebly developed sense organs, the simple structure of the jaws and feet and the soft skin may be 282 THE INVERTEBRATA linked with the environment in which they lurk 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 forms, terrestrial Fig. 203. Peripatus capensis, x very slightly. From Sedgwick. 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-like limbs. The thinness of the cuticle is responsible for the absence of external segmentation (save for the repetition of the appendages). The head (Fig. 204) bears three pairs of appendages which are none of them Fig. 204. Peripatus capensis, ^ . Ventral view of anterior end. a7it. pre- antenna; o.p. oral papilla ;y. 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 maybe called preantennae (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 bite with the tip and not the side. They are moreover tucked within the oral cavity. But they are borne on muscular ONYCHOPHORA 283 papillae arising in the embryo and must without doubt be regarded as appendages. 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 haemocoeHc but the embryonic coelom is well developed. In the development of Peripatiis just after the gastrula stage the blastopore becomes elongated, the anterior part ap.ex. Fig. 205. A, Transverse section through Pm/)af?/j eJwarrfw, adult ?. al. alimentary canal (ciliated); ap.ex. aperture of excretory organs; h. heart; m.l. longitudinal and m.c. circular and diagonal muscle layers ; n.c. nerve cord ; ut. uterus. B, An excretory organ of P. edwardn. hi. bladder ; coe.s. coelomic sac; nepr. nephrostome; sec.can. secretory canal. 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 Gaffron; C, after Moseley; D, after Balfour. giving rise to the mouth, the posterior to the anus, while the median part closes (Fig. 205). 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 284 THE INVERTEBRATA this is formed, and eventually is used to form the segmental excretory organ. The other part meets its fellow in a mid-dorsal position and while in the anterior region it mostly disappears, those of the posterior segments fuse to form two longitudinal tubes which become the gonads (Fig. 206). At the same lime the gaps between the organs become filled with blood. A dorsal part of the haemocoele so formed is marked oif 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 haemocoele almost diagnoses the group as arthropods, but it was the discovery of the tracheae which led to the inclusion oi Peripatus in that phylum. The stigmata are scattered over the surface of the body most thickly on the sides and ventral surface, 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 fluid of the body cavity (Fig. 205 C). It can hardly be doubted that these organs are definitely arthropodan in type: their most significant difference from those of other forms is in their non-segmental character. Their irregular dis- tribution 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, which is ciliated. The fore gut consists of a buccal cavity into which open the large salivary glands and a muscular suctorial pharynx. The mid gut possesses no separate glands. The excretory tubules (Fig. 205 B) are composed of a distal terminal bladder, a coiled secretory canal and a funnel 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 com- plete 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 them- ONYCHOPHORA 285 selves have 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. 206. Diagram of transverse sections through embryos of Peripatus capsnsis 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. There are other derivatives of the ectoderm, the coxal glands (not homologous with those of arachnids), found on all the legs except 286 THE INVERTEBRATA the first and consisting 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 entangles enemy and prey. Fig. 207. Peripatus capensis, S , dissected to show the internal organs, x 2. After Balfour, an. anus; ant. preantenna; brn. supraoesophageal ganglia; c.oes. circumoesophageal commissure ; cx.gl. enlarged coxal 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; (^ op. male aperture. The nervous system (Fig. 207) consists of a pair of supraoesophageal ganglia from which the preantennal nerves are given off, a pair of cir- cumoesophageal commissures and two ventral cords which are widely ARTHROPODA 287 separated and connected by about ten transverse strands in each segment. There are sHght enlargements in each segment which can be regarded as incipient gangha, but the whole nervous system is primitive for an arthropod or even an annelid and can be best compared to that of Polygordius in the annelids or Chiton in the molluscs. The Onychophora are not known as fossils, but all that has been said indicates that they came off from the main arthropodan stock at a very early stage when a typical haemocoele had been developed and cephalization had commenced, but when the epithelium had not finally specialized in the production of chitin and was still ciliated in places. SuBPHYLUM TRILOBITA Palaeozoic Arthropoda with the body moulded longitudinally into three lobes; one pair of antennae; and, on all the postantennal so- mites, appendages of a common type which has two rami and a gnathobase. The Trilobita were marine organisms and were very numerous in the Cambrian and Silurian but became extinct by the Secondary period. Their body was oval and depressed, and" consisted of a head and a segmented trunk, of which the anterior somites were movable on one another, but the hindermost, in varying number, were nearly always united to form a tagma known as the pygidium. The body could usually be rolled up like that of a woodlouse. Along its whole length longitudinal grooves Fig. 208. Olenmcata- divided lateral pleural portions from a middle ractes, from the Lin- region. In the head, this middle region is known ^.^^ Flags. Natural as the glabella and transverse furrows usually ^^^^* ^°"^ mark out more or less distinctly five somites. The pleural portions of the head are known as the cheeks^ and each bears in most species a sessile compound eye. On each cheek a longitudinal facial suture divides an outer from an inner area, passing immediately internal to the eye. The posterolateral angles of the cheeks are often produced backward as spines. Under the head a large labrum or hypostoma projects backward below the mouth, behind which is a small metastoma. The antenna is uniramous and multiarticulate and is the only pre- oral appendage. Since it is the foremost of five head appendages it has the same position as the antennule of the Crustacea, with which it is probably homologous. In that case it would seem likely that a true first somite had already, as in modern Crustacea, become merged 288 THE INVERTEBRATA in the anterior region of the head. Traces of a groove which exist in some species may perhaps indicate its existence. The remaining Umbs are all of one type, though there is a gradual progressive modification from one end of the series to the other. Each has two rami. Of these, one, usually held to be the outer ("exopo- dite "), bears a long fringe of bristles, while the other (" endopodite ") is leg-like and divided into six joints. It is supposed by some authori- ties that the bristle fringe on the so-called exopodite was on the inner side of the limb, and was used for collecting food, Hke the fringes on Fig. 209. Triarthrus becki, from the Utica Slate (Ordovician) near Rome, New York. After Beecher. A, View of the ventral surface showing append- ages, etc. h, hypostome (labrum); m, metastoma. x f. B, Diagrammatic section through the second thoracic somite, a, " endopodite " ; ^, " exopodite ". C, Dorsal view of second thoracic leg. «, "endopodite"; b, "exopodite"; Cy " protopodite " with gnathobase. Enlarged. the trunk limbs of branchiopoda (p. 317). From the basal portion of the limb a process for the manipulation of food, the gnathobase, pro- jects towards the middle line. The configuration of the basal portion (protopodite), and the relation of the rami to it, are obscure. The telson is without limbs. The Trilobita hatched as a larva, the Protaspis, which was sub- circular, and consisted, like the Nauplius larva of the Crustacea, principally of head. In its further development there appear, first the pygidium, and then free somites between the pygidium and the head. TRILOBITA 289 new somites being added in front of the telson while those at the front end of the pygidium become free. Fig. 210. Development of a trilobite. After Barrande. A-D, Sao hirsuta, Cambrian, Bohemia. A, Earliest stage (Protaspis), x 12. B, Later stage, with three somites behind the head, x 12. C, With more distinct glabella furrows and four somites behind the head, x 12. D, With six somites behind the head, x 10. It is probable that the majority of the trilobites lived upon the sea- bottom in shallow to moderately deep water, but others appear to have been adapted to burrowing, pelagic, and deep-sea conditions. 19 CHAPTER XII THE SUBPHYLUM CRUSTACEA Arthropoda, for the most part of aquatic habit and mode of respira- tion ; whose second and third somites bear antennae ; and their fourth somite a pair of mandibles. The Crustacea are essentially aquatic arthropods. That fact alone makes it possible that in them the same appendages should combine the functions of locomotion (by swimming), feeding (by gathering particles from the water), respiration (by exposing a thinly covered surface to the medium), and the reception of sensory stimuli. There is perhaps no extant crustacean in which all four functions are thus combined — unless we may regard the trunk limbs of the Branchio- poda (see below) as sense organs in a minor degree — but not un- commonly three, and perhaps usually two, are performed by the same limb. In the lowest members of the subphylum — the *'phyllopod" Branchiopoda (such creatures as the fairy shrimp, Chirocephalus ^ shown in Fig. 225) — a long series of somites of the trunk bear similar appendages which all function alike in swimming, respiration, and the gathering of food. Evolution within the crustacean group appears to have proceeded by the specialization, for particular functions, of particular appendages of an ancestor which possessed along the whole length of the body a numerous series of limbs, of which all, except probably the first pair (antennules), were as much alike and capable of at least as many functions as those which the Branchiopoda now possess upon the trunk. Such a condition existed in the Trilobita, but in all modern Crustacea the appendages of the head are already specialized for various uses, and in most members of the group the specialization has gone farther. Indeed, the greater part of the vast variety displayed by the several elements of the organi- zation of the Crustacea is due to such specialization and to the fact that it has proceeded along many different lines. This evolution, which has been much affected by the presence or absence of the enveloping skin fold known as the ''shell" or carapace (see below), has involved, as the efficiency of the limbs has increased, a lessening of their number, and finally the reduction or loss of the somites whose limbs have thus disappeared. The reduction, which has oc- curred independently in every class, has taken place in the hinder part of the body, though as a rule the extreme hind end (telson) is rela- tively unaffected. The transformation of the external make-up of the body is of course reflected in the internal organization, which shows CRUSTACEA 291 corresponding concentrations of function and differentiation of the contents of somites. Five radically different lines of evolution have established the classes of the subphylum. (i) In the phyllopod orders of the Branchiopoda, it is only on the head that differentiation among the appendages has pro- ceeded to any considerable extent. Here each is specialized for some particular function, as for the service of the senses or the manducation of food. In the trunk the limbs are still, as has been said above, similar, and all subserve the functions of swimming, feeding, and respiration. In two of the three orders, however (not in the Conchostraca), there is a group of limbless segments at the hinder end of the body, and this perhaps indicates that the efficiency of the limbs has already reached a pitch which has permitted a reduction in their number. The resemblance between the limbs of the series is closest in the order AnostracUy to which Chirocephalus belongs. In the orders Conchostraca and Notostraca there is a certain progressive enfeeblement of the limbs from before backwards on the trunk, and those of the hinder pairs probably serve respiration almost exclusively, while one or two pairs at the anterior end are a good deal modified for new functions, though all are essentially alike. (The most conspicuous differences between the phyllopod orders lie, however, not in any features of their trunk limbs, but in peculiarities of the antennae, and in the fact that the Anostraca have no carapace, the Notostraca (Fig. 231) possess one as a broad dorsal shield, and in the Conchostraca (Fig. 232) it is present as a bivalve shell.) In the order Cladocera (Fig. 233) an advance in differentiation appears. In these animals a high degree of specialization of the trunk limbs for feeding enables that function to be performed by some half-dozen pairs, and accordingly the body is much shortened. Here the carapace forms (except in some aberrant genera) a part of the food-collecting apparatus, and the function of swimming, abandoned by the trunk limbs, is performed by the antennae. (2) A similar trend of evolution is even more strongly manifested by the class Ostracoda (Fig. 237), which are very short- bodied and completely enclosed in a bivalve shell formed by the carapace. Whereas, however, in the Branchiopoda it is always by trunk limbs that food is gathered, in the Ostracoda that function is performed by limbs of the head. The trunk limbs, which have lost the functions of swimming and respiration as well as that of feeding, serve relatively unimportant subsidiary purposes, and are reduced, at most, to two pairs. Some members of the class carry shortening to an extreme pitch by contriving to dispense with one or both of these pairs. (3) The members of the class Copepoda (Fig. 238) also feed by means of appendages on the head, though they use these differently from the Ostracoda. In contrast to that group they have no carapace, 19-2 SOMITES AND LIMBS Sor nite Apus Daphtiia Cypris Cyclops I No limbs No limbs No limbs ^ No limbs 2 Antennules Antennules Antennules Antennules 3 Antennae 1 Mandibles Antennae Antennae Antennae 4 Mandibles Mandibles Mandibles I Maxillules 5 Maxillules Maxillules / Maxillules ' 6 Maxillae / Maxillae (ves.) Maxillae Maxillae 7 Thor. limbs i Thor. limbs i Thor. limbs i Maxillipeds 8 •• » 2 » 2 2(??J Thor. limbs 2 / 9 3 3 3 lO 4 4 4 II 5 — 5 12 13 6 7 5 Thor. som. 7 ? lThor.som.7(cOc?$K Abd. som. i j 14 8 8 IS 9 9 CO » 2 i6 10 1 3 17 iic^? CO M 0 i8 Abd.som. I !"§ s 19 20 21 34 35 36 >> >> >> >> 2 3 4 17 J i8> 19 or (/) 0 en 0 3 2 37 38 20 21 1° 39 ,, 22. Telson Telson c? Telson Telson with rami with rami with rami with rami Abdominal" somites are those between the last genital somite and the telson. (^ indicates female, c, uniramous and vestigial, d, genital operculum of female represents seventh OF CRUSTACEA Lepas Nebalia Gammarus Astacus Somite No limbs ^ No limbs No limbs No limbs ... I Antennules Antennules Antennules Antennules .. 2 Lost in adult | Antennae 1 Antennae Antennae •• 3 Mandibles Mandibles Mandibles ^,- Maxillules Mandibles .. 4 Maxillules | Maxillules Maxillules •. 5 Maxillae Maxillae / Maxillae Maxillae .. 6 Thor. limbs i $ Thor. limbs i Maxillipeds Maxillipeds I .. 7 2 2 Legs I „ 11^ .. 8 3 3 „ n „ III .. 9 4 4 „ III Legs I ..10 5 5 „ IV „ II ..11 6d 6? „ v? „ III? ..12 7 „ VI ,, IV •.13 ^6 „ viu ,, Vc^ ..14 Abd. limbs i Abd. limbs i Abd. limbs i ...15 >> 2 3 >> 2 3 2 3 ..16 ..17 u O 4 4 4 ..18 5 6 5 6 5 6 ...19 ..20 1 g o C3 Abd. som. 7 1 ...21 1 Telson with rami Telson with rami Telson Telson the position of the m thoracic limb : the so ale opening, ? that c mite which bears it i f the female, a, joi s often reckoned as ned but distinct, the first abdomin b, fused in al. 294 THE INVERTEBRATA and they have retained a trunk of some ten somites, of which the first half-dozen bear limbs which are specialized organs of swimming. The hinder part of the trunk is without appendages, save a pair of styles on the telson, often shows coalescence of somites, and may become a mere stump. Some of those members of this class which are parasitic lose in the adult female the segmentation and most, or even all, of the appendages. In the Branchiura (Fig. 243), which are usually classed with the Copepoda but differ from the rest of that group in possessing compound eyes and in other respects, the abdomen is much reduced. (4) The class Cirripedia or barnacles, which as larvae attach themselves by their antennules to some object upon which they henceforward lead a sedentary life under the protection of a large, mantle-like carapace, bear upon the same trunk somites as the Copepoda limbs which, like those of the latter group, are biramous. These appendages, however, are used, not for swimming, but for gathering food-particles from the water ; while of the head appendages the antennae are absent and the others are much reduced and not used in gathering food. The least specialized members of this class are, in respect of segmentation and appendages, on a par with the best-segmented of the Copepoda. Most cirripedes, however (the ordinary barnacles. Fig. 244) have lost the whole of the hinder (abdominal) region of the trunk. Others are deficient in the appendages of further somites, and the series ends with the sac-like parasites of the order Rhizocephala(¥ig.2\Ci). (5) The class Malacostraca (the highest crustaceans, including various "shrimps", slaters, sandhoppers, crayfishes, etc.) obtain their food with the limbs on the anterior region (thorax) of the trunk, and, in primitive cases in which it is gathered as particles, strain it from the water with the last pair of appendages of the head (the maxillae). The thoracic limbs retain also the function of locomotion and normally are adapted for respiration by the presence upon them of gills, which are usually protected by a carapace of moderate size. Thus this region of the body of the Malacostraca is, in its own ways, as many-functioned as the corresponding part of the trunk of Chirocephalus . The Malacos- traca maintain in typical cases (Figs. 256, 268) the swimming function of the limbs on the hinder portion (abdomen) of the trunk, and some of the class have found other uses (ovigerous, copulatory, etc.) for these appendages. Accordingly there is seldom any reduction in the fixed number of fourteen (or fifteen) trunk somites which, arranged always in a thorax of eight and an abdomen of six (or seven), cha- racterizes the class. Nevertheless in a few members of the group the abdomen has become a limbless and unsegmented stump, and in the crabs (Fig. 270) and some others of the highest order {Deca- poda) it is reduced. The name Entomostraca was formerly used in the classification of CRUSTACEA 295 the subphylum, to distinguish from the Malacostraca a division con- taining all the other classes. Since, however, these differ from one another as widely as each of them does from the Malacostraca, the name is no longer used in classification but is only a convenient desig- nation for the lower crustacean classes as a whole. The restriction of feeding to a few limbs is often, though not always, accompanied by the replacement of the original habit of gathering food in small particles by other modes of feeding. Continuous and automatic straining-out of such particles, which is practised (though in different modes) by the most primitive members of various classes, is superseded in some cases by the intermittent seizure, by particular limbs, of particles of some size, and this by the graspingof larger objects, which may lead to a predatory habit. Finally, either of these modes of feeding may be replaced in parasites by suction or absorption, through organs which do not always represent appendages at all. (Parasites, however, are not known among the Branchiopoda or Ostracoda.) Needless to say, each change in the mode of obtaining nutriment has entrained numerous alterations in organs other than those by which the food is actually taken, as in the means of locomotion, sense organs, weapons of offence, etc. On the other hand, adaptations to mere differences of habitat, in the Crus- tacea, as in other arthropods, are, as a rule, strikingly small. There is, for instance, remarkably little difference between a land crustacean and its nearest marine relatives. Pelagic genera, however, are sometimes considerably modified. In the various ways which have been outlined above, the common organization of the subphylum exhibits modifications which, as will be seen from what follows, are as many and as far-reaching as those which are to be found in any division of the Animal Kingdom. The cuticle of a crustacean is, save for the joints, usually stout relative to the size of the animal, but is thinner and flexible in many parasitic genera. It is often strengthened by calcification, and in certain ostracods, barnacles, and crabs this gives it a stony hardness. In each somite there may or may not be distinguishable a dorsal plate or tergite (tergum) and a ventral sternite {sternum). The tergite may project at each side as a pleuron. There are embryological indications that the body should be re- garded as containing, besides the somites^ an anterior presegmental region, to which the eyes belong, corresponding to the prostomium of a worm, and a postsegmental region or telson, on which the anus opens. Each somite, except the first, which is purely embryonic, may bear a pair of appendages, though it is rarely that the appendages of all the somites are present at the same time. The somites never all remain distinct in the adult. Always some of them are fused together and 296 THE INVERTEBRATA with the presegmental region so as to form a head, and oftenthere is also fusion of them elsewhere. Nearly always the somites are grouped into three tagmata, dif- ferentiated by peculiarities of their shape or appendages, and known as the head, thorax, and abdomen. These, however, are not morpho- logically equivalent in different groups. The head always contains, besides the region of the eyes and the embryonic first somite, the somites of five pairs of appendages — two, the antennules and antennae, preoral; and three, the mandibles, maxillules, and maxillae, postoral. More somites are often included in the actual head, but as the additional appendages (maxillipeds) then usually show features of transition to those behind them, and as the fold of skin which forms the carapace, presently to be mentioned, first arises from the maxillary somite, the true head is held to consist only of the anterior portion of the body as far as that somite inclusive. There is evidence of an earlier head, carrying only the first three pairs of limbs which alone exist in the Nauplius larva, and still indicated in some cases (as in Chirocephalus ^ Anaspides, Fig. 256, and Mysis, Fig. 253) by a groove which crosses the cheek immediately behind the man- dible. This mandibular groove is distinct from the true cervical groove which often (as in Astacus^ Fig. 269) marks the boundary between head and thorax: the two grooves may co-exist, as in Apus and in Nephrops. The Crustacea, indeed, admirably illustrate the way in which the process of " cephalization " tends, in arthropods as in vertebrates, to extend backwards and to involve more and more segments. With it has gone a backward shifting of the mouth, which in the Crustacea now stands behind the third somite, with two pairs of appendages (antennules and antennae) in front of it. The com- missure which unites the ganglia of the antennae still passes behind the mouth, and may usually be seen, as in Astacus (Fig. 214), crossing from one of the circumoesophageal commissures to the other. The head, though it varies in extent, is of the same nature through- out the group, being, like the heads of other animals, the seat of the principal organs of special sense and of manducation. On the other hand, the two tagmata known as the thorax and abdomen, which usually can be recognized in, and together compose, the postcephalic part of the body or trunk, vary much more in extent, and each of them has in the several groups no constant feature save its position relative to the other. The precise boundary between thorax and abdomen is sometimes difficult to fix. The names, as they are com- monly used, are in this respect inconsistently applied, denoting in some groups limb-bearing and limbless regions, in others the sections of the trunk which lie before and behind the genital openings. For the sake of consistency we shall adopt the convention that the somite CRUSTACEA 297 which bears the genital openings (or the hinder such somite when, as sometimes happens, the male opening is on a somite behind that of the oviduct), is always the last somite of the true thorax. In this sense, in certain cases (copepods, cladocera), somites which are commonly called abdominal are strictly to be reckoned as thoracic. In respect of segmentation the trunk varies from the condition of a limbless stump in certain ostracods to the possession of more than sixty somites in some of the Branchiopoda. A structure very commonly found in crustaceans is the shell or carapace, a dorsal fold of skin arising from the hinder border of the head and extending for a greater or less distance over the trunk. Its size varies greatly. In the Ostracoda (Fig. 237) and most concho- stracans (Fig. 232) it encloses the whole body, extending forwards at the sides so as to shut in the head. In other cases (cirripedes. Fig. 244, most cladocera, Fig. 233), it only leaves part or the whole of the head uncovered. In typical malacostraca it covers the thorax (Fig. 269), but in some it is a short jacket, leaving several thoracic somites uncovered (Fig. 259), and in some (the Syncarida, Isopoda, and Amphipoda, Figs. 256, 260, 264) it has disappeared. In the Anostraca (Fig. 225) and Copepoda (Fig. 238) it was perhaps never present. It may be a broad, flat shield over the back, as in Apus (Fig. 231), but is usually compressed, and in the Conchostraca and Ostracoda becomes truly bivalve, with a dorsal hinge and an adductor muscle. In the Cirripedia it is an enveloping mantle, usually strengthened by shelly plates (Fig. 246). It may fuse with the dorsal side of some or all of the thoracic somites (the Cladocera, most of the Malacostraca): such somites are not on that account alone to be regarded as included in the head, though they may become so. The chamber enclosed by the carapace is known in various cases by various names as gill chamber, mantle cavity, etc., and performs important functions in sheltering gills or embryos, directing currents of water which subserve feeding or respiration, etc. In front, the carapace is continuous with the dorsal plate which represents the terga of the head, the cervical groove, if present, markingthe boundary between them. We shall apply the term dorsal shield to the structure composed of the dorsal plate of the head vnth. the carapace, if the latter be present.^ The dorsal plate of the head may be prolonged in front as a projection which is called the rostrum (Fig. 269, rs.). A glandular patch or patches on the dorsal surface of the head, near its hinder limit, in many of the Branchiopoda, in Anaspides, and in the * These terms have been used in various senses. In the usage here pro- posed, when there is no carapace fold the dorsal shield is the dorsal plate of the head together with the terga of the somites (if any) that are fused with the head. 298 THE INVERTEBRATA young stages of various other crustaceans, is known as the dorsal organ or neck gland. It is used by cladocera and conchostraca for temporary fixation. In other cases its function is not known. Possibly the organs to which this name is given are not all homologous. They must not be confused with the "neck organ" of branchiopods (see p. 307). Of the appendages or limhs of the Crustacea, the first, or antennule, is a structure sui generis, not comparable in detail with any of the others. Typically it is uniramous, and though in many of the Malacos- traca it has two rami, these are probably not homologous with the rami, described below, of other appendages. The remaining limbs may all be reduced to one or other of two types — the " biramous " limb usually so-called, to which most of them more or less clearly conform* 2ind the phyllopodiiim, to vfhxch. belong the trunk limbs of the Branchio- poda and some other appendages, chiefly maxillules and maxillae and notably the maxilla of the Decapoda. The name by which the first of these types is generally known refers to the fact that limbs which best represent it fork distally into two rami. Since, however, the phyllopodium possesses the same two rami, and bears them, though not as a distal fork, yet in the same way as a great number of limbs of the first type, it is well not to use a name which might imply that there is a constant difference in respect of the rami between the limbs of the two types. We shall therefore call the first type the stenopodium^ referring to its usually slender form (Gk. cnevo^^ narrow). In the stenopodium (Figs. 211 D-G, 212), the two rami — an inner endopodite and an outer exopodite — are set upon a common stem, the protopodite. In many cases the protopodite bears also, on its outer side, one or more processes known as epipodites (Fig. 212, ep.). In limbs in which the type is most perfectly developed the two rami are subequal and are borne distally upon the protopodite (Fig. 211 G), but in most cases the endopodite is the larger, and forms with the protopodite an axis, the corm, on which the exopodite stands laterally (Figs. 211 E, F, 212, 272). In a few instances the exopodite is the larger. The protopodite most often has two joints, a proximal coxopodite and a distal basipodite. In certain cases, however (as in the antenna of the Mysidacea and Asellus, the last three thoracic limbs of the Stomatopoda, certain swimming limbs of the Branchiura (Fig. 243 B), and less clearly in many other instances), a basal joint, the precoxa or pleiiropodite , precedes the coxopodite ; moreover the basi- podite may be divided into two joints, the probasipodite, which then usually bears the exopodite, and the metabasipodite or preischiopodite . This condition is seen most clearly on the thorax of the mala- costracan genera Anaspides and Nebalia (Fig. 211 E), where the two components of the basipodite are separate in some limbs and CRUSTACEA 299 fused in others; it is less obvious in other cases in which it occurs. Thus the full possible number of joints in the protopodite is four. Some authorities, however, prefer to regard the preischium as part of the endopodite, in which case the protopodite has only three joints Fig. 211. Limbs of Crustacea. Not drawn to scale. After various authors. A, Maxilla of Mysis larva of Penaeus (Decapoda). B, Maxilla of Acanthosoma larva of Sergestes arcticiis (Decapoda). C, Second trunk limb of female Cyclestheria hislopi (Conchostraca). D, Mandible of Calaniis. E, Thoracic limb of Nehalia. F, Mid-thoracic limb of an euphausid. G, Swimmeret of the crayfish, bp. basipodite; br. branchia (epipodite); cp. coxopodite; en. en- dopodite; ep. epipodite; ex. exopodite;^6. fiabellum (exopodite); gn. gnatho- base; pr.cp. precoxa; 1-9, endites or segments of the limb. at the most. Epipodites, when they are present, are borne upon the precoxa {proepipodites) or coxopodite {metepipodites). The endopodite is usually segmented. If the preischium be not 300 THE INVERTEBRATA reckoned to it, its maximum number of joints is five. These are found on the thoracic limbs in the subclass Eucarida of the Malacostraca as, for instance, in the crayfish, where they are named, in order from the base outwards, the ischiopodite^ meropodtte, carpopodite, propodite, and dactylopodite. In the subclass Peracarida, however, the five joints to which the above names are usually given are not homologous with those so designated in the Eucarida. Here the true carpopodite and propodite have fused, but the pre- ischiopodite, which in the Eucarida is probably fused with the ischiopodite, remains distinct, so that the distal part of the corm has still five joints. With the four possible joints of the protopodite these segments of the endopodite make up a total of nine in the corm of the limb. Sometimes a subdivision of certain of the joints into many jointlets or annuli occurs. This may be seen in some of the thoracic limbs of mysidacea and of certain prawns (Pandalus, etc.) and in many an- tennae. A slender, many-jointed, terminal portion of either ramus is known as a flagelliim. The exopodite is often unsegmented, but when segmented usually possesses a flagellum. It does not possess a standard number of joints. It is more often absent or reduced than is the endopodite. The phyllopodium (Figs. 211 A, C, 213, 227), is a broader and flatter limb than the majority of stenopodia. Its cuticle is usually thin, and then the shape of the limb is maintained largely by the pressure of blood within it. In these cases the flexibility is such that no joints are needed. There is in this limb an axial portion or corm which bears on the median side a row of lobes known as endites, and on the outer side one or more lobes known as exiles. Of the latter the more distal, standing usually opposite the third or fourth endite from the base and often known as the flabelliim, is the homologue of the exopodite of the biramous limb. Exites proximal to this are epipodites. That next to the fliabellum is the hranchia (metepipodite) ; any which maybe present proximal to the branchia are proepipodites (Fig. 227, pr.ep.). The flabellum typically overhangs its attachment proximally as well as dis- tally. In the latter direction it may extend so far as to form with the distal part of the corm a pair of equal rami (Fig.211 C). The homology of the endopodite then becomes apparent. This ramus corresponds to that part of the corm of the phyllopodium which is distal to the insertion of the flabellum or exopodite. Of the endites, that which stands at the base of the limb is usually different in form from the rest and used in one way or another for manipulating the food. It is known as the gnathohase. The number of the endites varies. In the Branchiopoda it is commonly six, but in the Anostraca (see p. 320) there are indications of seven. It is greatest in the larval maxilla of certain decapoda (Fig. 211 A), where it is nine, which, as we have CRUSTACEA 301 seen, is the maximum number of segments in the corm of the steno- podium. The suggestion, made by this fact, that the segmentation of the phyllopodium by endites corresponds with that which the corm of the stenopodium owes to the presence of joints, is strengthened by the fact that in some of the maxillae in question (Fig. 211 B), and in that of Calanus (Fig. 240), which also has nine segments, the limb is jointed and the joints fall between the endites or, where these are lacking, precisely complete their number. A less regular jointing of the same kind is present in some other phyllopodia (Apus^ Fig. 213 ; etc.). In both kinds of limb, also, the position of the exopodite Fig. 212. Fig. 213. Fig. 212. The second thoracic limb of Anaspides. After Caiman, with an addition, en. endopodite; ep. epipodite; ex. exopodite; fix. flexure of limb; I, position of precoxa or pleuropodite, represented, according to Hansen, by an isolated area of chitinization of the thoracic wall ; 2, coxa or coxopodite ; 3, basis or basipodite (probasipodite) ; 4, preischium or metabasipodite ; 5, ischium or ischiopodite ; 6, merus or meropodite ; 7, carpus or carpopodite ; 8, propus or propodite; 9, dactylus or dactylopodite. Fig. 213. Tenth thoracic limb of Apus. br. branchia; jib. flabellum; gn. gnathobase ; ep. epipodite ; ex. exopodite ; 1-5, segments of the limb ; z's', en- dites ; 6, apical endite. bears the same relation to the segmentation, being usually upon the third, occasionally upon the fourth segment, while epipodites stand on the first or second segment. Endites are rare on stenopodia, but a gnathobase is always present in the mandible (Fig. 211 D) and sometimes in other limbs, and a few other such processes occur. A limb of either type may differ from that type in the lack of any of its parts. Notably the loss of the exopodite is liable to produce from either a uniramous limb. Moreover, though the two types are very distinct in cases in which they are perfectly developed, as in the 302 THE INVERTEBRATA swimmerets of Astacus (Fig. 211 G) and the trunk limbs of Apus (Fig. 213), there are many Umbs which depart more or less from either type in the direction of the other — as, for instance, from the stenopodial type in the shape of the exopodite (Fig. 243 B), or, as stated above, in the relation of the latter to the rest of the limb, or from the phyllopodium in the proportions of the rami or the reduction of the endites. The comparison just made between the phyllopodium and the stenopodium leaves untouched the question which of them is the more primitive, that is, more resembles the limbs of the ancestral crustacean. On this point there is an old and as yet unsettled con- troversy. As proof of the primitive ness of the stenopodium it is pointed out (i) that this limb is more widespread than the phyllo- podium, (2) that it occurs in the Nauplius larva (p. 314), the early phyllopod Lepidocaris (p. 322), and the trilobites, in all of which it is likely to be primitive, (3) that it more nearly approaches the form of the majority of parapodia of the Annelida, from which the Crustacea are held to have taken origin. In demonstration of the ancestral nature of the phyllopodium it is urged (i) that typical stenopodia with subequal rami borne distally upon a proto- podite are comparatively rare and usually occur in highly specialized crustaceans (Copepoda, Cirripedia, Malacostraca), (2) that the bira- mous limbs of the Nauplius and Lepidocaris are not primitive but adaptive, the relations of the rami of the limbs of trilobites are problematical, and the admittedly primitive Branchiopoda possess phyllopodia, (3) that the unjointed, turgid, lobed phyllopodium more nearly resembles the parapodia of certain annelids in which the neuropodium is axial, than the stenopodium resembles the normal biramous parapodium. Concerning the functions of particular members of the series of limbs, and the corresponding modifications of their structure, little can be said that would hold good throughout the subphylum. There is an immense variety in these respects. The antennules and antennae are primarily sensory, and perhaps usually possess something of that function when they are also capable of swimming, prehension, attach- ment, etc. In the Nauplius larva (Figs. 224, 248) the antennules are uniramous and the antennae biramous, and they normally retain these conditions in the adult. The mandibles always play, by means of their strong gnathobase, some part in preparing the food, whether by chewing or by piercing for suction, but the distal part of the limb {palp) may aid in locomotion or set up feeding currents. They generally lose in the adult the biramous condition which they have in the Nauplius. The maxillules and maxillae tend to be phyllopodia. The maxillules have usually the function of passing food to the CRUSTACEA 303 mouth but may serve other ends. The maxillae have various functions in connection v^^ith feeding and respiration. The limbs of the thorax perform in various cases practically every function for v^hich ap- pendages are used. If a crustacean walks, it is usually by means of these limbs. Often in one or more of them the last joint can be opposed to the joint which precedes it, forming a chela (or a subchela)^ so that the appendage is adapted for grasping. Modification of the hinder thoracic or anterior abdominal limbs in connection with repro- duction is common. Abdominal limbs are lacking save in certain of the Branchiopoda and most of the Malacostraca. When they are present they are commonly used for swimming, for setting up currents of water, or for carrying eggs and young. Three elements of minor importance complete the external make-up of the Crustacea. In front of the mouth is a labrum or upper lip; behind the mandibles is a lower lip or metastoma^ usually cleft into a pair of lobes known as paragnatha ; and on the telson usually (but in no adult malacostracan except the Leptostraca Fig. 254), is a pair of caudal rami forming the caudal furca. Appendages which are lost are regenerated at subsequent moults ; and the highest members of the group possess an elaborate mechan- ism for autotomy — the breaking-off of limbs which have been injured or which have been seized by enemies. An internal skeleton is usually present in the form of ingrowths of the cuticle, known as apodemes, which serve for the insertion of muscles. Sometimes (notably in the Decapoda, Figs. 222, apo.; 276, enph.) they unite to form a framework, the endophragmal skeleton. In the Notostraca, a mesodermal tendinous plate, the endosternite , lies under the anterior part of the alimentary canal. The nervous systems of Crustacea exhibit a very complete series of stages from the ideal arthropod condition (see p. 274), to the ex- tremest concentration. That of the Branchiopoda (Fig. 199) is in a very primitive state, having the antennal ganglia behind the mouth as the first pair of the ventral ladder, distinct ganglia for the following somites, and widely separated ventral cords. In the lower members of the Malacostraca (Nebalia, some mysids, etc.), the antennal ganglia have joined the brain and the ventral cords are closer together, but otherwise the primitive condition is retained. In other crus- taceans various degrees of concentration of the ventral ladder are found, beginning with the establishment of a suboesophageal ganglion for the somites of the mouth parts (Fig. 214,5.0^5.), and ending in the formation, in the crabs (Fig, 276) and some other forms, of a single ventral ganglionic mass. In the Rhizocephala one ganglion (Fig. 250, ga.) supplies the whole body. The brain contains ganglia for the eyes (optic lobes), for the first or preantennulary somite (pro- 304 THE INVERTEBRATA n. ch St. ar Fig. 214. A semidiagrammatic view of central nervous system of Astacus. From Borradaile. ab. 1, ah. 6, the first and sixth abdominal ganglia; cer. cerebral ganglion (brain); c.oe^. circumoesophageal commissure; I.e. longi- tudinal commissures of ventral cord; n.ab.l. nerves to abdominal limbs; n.an.' nerve to antennule; n.an." nerve to antenna; n.ch. nerve to cheliped; n.m. nerves to limbs adjoining the mouth; opt.n. optic nerve; s.oes. sub- oesophageal ganglion ; 5i.«r. sternal artery; th.i, th.6, first and sixth thoracic ganglia; vis.n. nerve to proventriculus ; vis.n.' nerve to hind gut. CRUSTACEA 305 tocerebrum),^ and for the antennules {deuto- or mesocerebrum) . Except in the Branchiopoda it also contains the antennal ganglia {trito- or metacerebnim). A visceral ("sympathetic ") system is present. Sense organs are well developed in the free members of the group. Eyes are of two kinds, the compound eyes, of which a pair is usually present except in the Copepoda and adult cirripedes, and the median eye. Details of the structure of the compound eyes have been given above (p. 274). They may be sessile or stalked, and the latter con- dition has given rise to a theory that they represent a pair of append- ages. Since, however, there are no somites corresponding to their ganglia and since at their first appearance in the embryo they are sessile, this view is not generally accepted (see also p. 277). The median eye (Fig. 215) is the eye of the Nauplius larva, and it Fig. 215. A horizontal section through the median eye of Cypris. After Claus. a, position of the median (ventral-anterior) cup, which is not in the plane of section; Is. lens; n. nerve fibres; pig. pigment layer; rd. visual rod; ret. retinal cells. persists in most adults, though it is generally vestigial in the Mala- costraca. It consists of three pigmented cups, one median and two lateral, each of which is filled with retinal cells whose outer ends are continued as nerve fibres. Thus the sense cells are inverted, as in the eyes of vertebrata. Sometimes each cup has a lens. In some of the Copepoda the lateral cups are removed from the median one and de- veloped as a pair of lateral eyes. Senses other than sight are subserved by various modifications of the bristles which exist on the surface of the body and contain nerv^e fibrils in their protoplasmic contents. Most of these bristles are branched in various ways and have tactile functions, including that of appreciating the resistance of the water to movements. In the Decapoda and Syncarida on the basal ^ As in other arthropods, the name procerebrum is given to the anterior part of the brain, composed of the protocerebrum, the optic lobes, and some- times other ganglia which are not connected with paired limbs. 306 THE INVERTEBRATA joint of the antennule (Fig. 216) and in the Mysidae on the endopodite of the sixth abdominal appendage there is a pit whose wall bears such hairs while the hollow usually contains sand grains (most decapods) or a calcareous body formed by the animal (Mysidae). These organs are ,ex. aes op. D '«•/ Fig. 216. The antennule of Astacus. A, The right antennule, seen from the median side with the basal joint opened and the flagella cut short. B, Two joints of the distal part of the outer flagellum, enlarged, after Huxley. C, The basal joint of the left antennule, from above. D, Two hairs from the stato- cyst. E, An aesthetasc. aes. aesthetasc; en. inner flagellum; ex. outer flagel- lum ; grn. sand grains ; n. nerve ; n.f. nerve fibre ; op. opening of statocyst, over- hung by a fringe of hairs ; stc. statocyst. statocysts for the sense of balance. Olfactory hairs or aesthetascs (Fig. 216 B, E) with delicate cuticle stand on most antennules and on many antennae. A pair of groups of cells, sometimes surmounted by CRUSTACEA 307 setae, standing on the front of the head and known zs frontal organs, are found in many crustaceans and are supposed to be sensory. They are present as two papillae in the Nauplius larva (Fig. 247, ten.). The nuchal sense organ or ** neck organ " of many branchiopods is a group of cells on the upper side of the head containing refractive bodies and connected to the brain by a special nerve. Its function is unknown. As is well known, most crustaceans are pigmented. The pigments are of various colours — red, orange, yellow, violet, green, blue, brown, black, etc., though not all are found in any one species. The majority of them are lipochromes, though the brown and black are melanins. For the most part they are contained in branched cells (chromatophores), but some of the blue, and perhaps of certain others, is diffused in the tissues. The chromatophores may lie in the epi- dermal layer, in the dermis, or in the connective tissue of deeper organs. Their behaviour has been studied in various malacostracans. The pigment is often caused to expand or contract, which it does by flowing into and out of their processes. In this it is affected by light, responding both to intensity of illumination and to the nature of the background, but only rarely to colour (wave-length). In light of high intensity or on a light-absorbing (e.g. dull black) background it ex- pands; in light of low intensity or on a light-dispersing (e.g. dull white) background it contracts. Different pigments are affected to different degrees, and thus both the degree and the pattern of the coloration of a sensitive species (notably, for instance, of many prawns), changes with its surroundings, usually, in nature, in such a way as to render the animal inconspicuous. The response to intensity of illumination is due to direct action of the light upon the chromato- phores and will thus take place even in blinded animals ; the response to background depends upon the eyes. The eyes, however, do not act through nerves to the chromatophores, but by causing the secretion of certain endocrine glands to be poured into the blood. The alimentary canal (Figs. 222, 225, 233, 244, 264, 275) is with very rare exceptions straight, save at its anterior end, where it ascends from the ventral mouth. The fore gut and hind gut (stomodaeum and proctodaeum), lined with cuticle inturned at the mouth and anus, leave a varying length of mid gut (mesenteron) between them. The intrinsic musculature, sometimes supplemented by extrinsic muscles running to the body wall, is strongest in the fore gut, whose lining sometimes develops teeth or hairs. In the Malacostraca (Fig. 217) these elements become a more complex proventriculus ("stomach"), with a "gastric mill" and a filtering apparatus of bristles which strains coarse particles from the food which has been sufficiently finely divided, the mill and filter being often in separate " cardiac " and " pyloric " chambers. The mid gut usually bears near its anterior end 308 THE INVERTEBRATA one or more pairs of diverticula ("hepatic coeca"), which serve for secretion and absorption and may branch to form a "Hver". This gland, however, unlike the liver of vertebrates, forms all the enzymes necessary for the digestion of the food and absorbs from its lumen the products of digestion. It stores the reserves in the form of glycogen and fat. Occasionally there is an anterior median dorsal coecum. Coeca are also sometimes found at the hinder end of the mid gut : these are more often median. In a few cases the hind gut is absent and the mesenteron ends blindly. In the Rhizocephala and the monstrillid OSS.1 OSS. 5 hri. \ ^''V- u:tth. Fig. 217. The left-hand side of the fore gut and mid gut of Astacus, viewed from within, bri. bristles which form part of the filtering apparatus; cd. car- diac chamber of the "stomach"; cm. median dorsal coecum of the mid gut; la.p. lateral, finely-filtering pouch of the pyloric chamber ; la.tth. lateral teeth ; m.', 771.", left anterior and posterior gastric muscles; mdg. mid gut; me.tth. median tooth ; oe. oesophagus (not cut open) ; oss. i-oss. 6, cardiac, urocar- diac, prepyloric, pyloric, pterocardiac, and zygocardiac ossicles — calcifications of the cuticle of the stomach which constitute the mechanism of the gastric mill : when the cardiac and pyloric ossicles are pulled in opposite directions by the contraction of the muscles m.' and rn." attached to them, the teeth are brought together; py. pyloric chamber; vlv. median-dorsal and lateral valves projecting from the pyloric chamber into the mid gut. The opening of the left liver duct is seen as a dark spot behind the valves. copepods the alimentary canal is absent throughout life, for these animals take in by absorption through the skin during the parasitic period enough nutriment to last through an entire life history. Digestion is extracellular. The fore gut is frequently the seat of mechanical processes, and sometimes of chemical action by juices secreted by the mid gut diverticula, but never of absorption. The latter process as well as most of the chemical work is performed by the mid gut, including the hepatic diverticula. In the hind gut the faeces are passed to the anus, being in some entomostraca sheathed in a CRUSTACEA 309 so-called "peritrophic membrane" composed of a mucoid substance secreted by certain cells of the epithelium. The principal excretory organs of the Crustacea are two pairs of glands, known as the antennal and maxillary glands^ which open (Fig. 222, k.op^ at the bases of the appendages from which they take their names. They are very rarely (Lophogastridae) both well developed at the same stage in the same species, but one may succeed the other as a functional organ in the course of the life history : the antennal gland, for instance, is the larval excretory organ of the Branchiopoda, but the maxillary gland is that of the adult ; and the Decapoda, whose adult kidney is the antennal gland, sometimes use as larvae the maxillary gland instead. The maxillary gland is the more widespread as an adult organ, the antennary gland being functional in the adult only in certain of the Malacostraca. In the Ostracoda and Leptostraca Fig. 218. The maxillary gland of Estheria. After Cannon, sac. end sac. both are vestigial in the adult. Each of these glands (Figs. 218-220) has an end sac and a duct leading from the end sac to the exterior. The end sac is always mesodermal and doubtless represents a vestige of the coelom. The duct is sometimes (in the Malacostraca probably always), a multicellular, mesodermal structure, and sometimes intra- cellular and of ectodermal origin. At the junction of end sac and duct there is often a sphincter. The antennal gland of the Decapoda is usually very complicated. That of the crayfish lacks extensions of the bladder which lie among the viscera in many other genera, as in crabs. All the parts of the organs are excretory, and the function of the sphincter of the end sac is perhaps to prevent the passage back into that vesicle, which secretes ammoniacal compounds, of poisonous products excreted in the duct. These glands are probably the remaining members of a series of segmental excretory organs. Their mesodermal portions are B Fig. 219. Diagrams of the antennal gland of the early metanauplius of Estheria. After Cannon. A, The whole gland. B, The sphincter in section. duct, intracellular ectodermal duct; ect. ectoderm; sac. end sac (coelomic); sph. sphincter cells. Fig. 220. Diagrams of the green gland oiAstacus. From Parker and Haswell, after Marchal. I, The organ unravelled. II, A section of the organ with the parts in their natural positions, bl. bladder; c.p. cortical (green) portion of the gland; d. terminal duct; s. end sac; w.p. medullary (white) portion of the gland. CRUSTACEA 311 no doubt coelomoducts, homologous with those of the AnneHda, their ectodermal portions probably are not the homologues of nephridia but represent ectodermal glands such as are common in the Crustacea. Various other glands, mostly of doubtful morpho- logical significance, which occur in different crustaceans have been shown, or are suspected, to have an excretory function. Thus, in Nebalia, eight pairs of ectodermal glands at the bases of the thoracic limbs are excretory, while in ostracods a pair of rather complex glands, also of ectodermal origin, which lie between the folds of the shell in the antennal region, may have a similar function. Excretion appears also sometimes to be performed by coeca of the mid gut — as by some of those of the barnacles and by the posterior pair of amphipods — or by cells of the epithelium of the mid gut itself. Respiration in many of the smaller crustaceans, notably in the Cope- poda, takes place through the general surface of the body. In forms with stouter cuticle or more bulky bodies this is supplemented or replaced by the use of special organs upon which the cuticle remains thin. The most important of such organs are the lining of the cara- pace, if that structure be present, and certain epipodites which are known as gills and in many of the Malacostraca have their surface in- creased by branching or folding (Figs. 21 1 F ; 272 ; 274, 1,2). In the Decapoda incorporation of the precoxa with the flank of the body has brought it about that some of the gills (proepipodites, p. 299), stand in that position and not upon the actual limbs (Fig. 221). Such gills are known as "pleurobranchiae". In the Isopoda re- spiration is effected by the broad rami of the abdominal limbs. Renewal of the water upon the respiratory surfaces may be brought about by the movements of the limbs upon which they are located, but often certain appendages bear special lobes adapted to set up a current under the carapace and thus to flush the chamber in which the gills and the carapace lining are situated. Some land crustaceans have no special adaptations for respiration in air. In others the gill chamber is adapted, by the presence of vascular tufts of the lining of the carapace, for use as a lung. The wood- lice, which are terrestrial members of the Isopoda, are remarkable in having adopted the principle of respiration employed by normally terrestrial arthropods, for the integument of their abdominal limbs is invaginated to form branching tubes which resemble tracheae. The vascular system is seen in its most primitive condition in the Branchiopoda Anostraca {Chirocephalus ^ Fig. 225). Here the heart (h.) runs the whole length of the trunk, situated above the gut in a blood sinus known as the pericardium , with which it communicates by a pair of ostia in each somite except the last. In front it is continued into the only artery^ a short aorta, from which the blood flows direct 312 THE INVERTEBRATA into the sinuses of the head and thence through those of the trunk to the pericardium, eddies from a main ventral sinus supplying the limbs. In all other Crustacea, except the Stomatopoda, the heart, if it be present, is in some degree shortened, and in the Malacostraca Fig. 22 1 . Part of the left side of a late larva of the prawn Penaeus to show the origin of the gills. Slightly magnified. After Claus. L^-L^, the first to fifth legs ; M1-M3 , the first to third maxillipeds ; i a, 2 a, 30, ja, distal series of rudiments, standing upon the coxopodites ; from these rudiments arise the mastigobranchiae (see p. 366), and on the third maxilliped a podobranch also ; lb, 5b, I c, 2C, yc, members of two series of rudiments, standing where the membrane of the joint between the coxopodite and the body will develop ; from these respectively the anterior and posterior members of pairs of arthro- branchiae arise ; i O *0 tH +J "^ ? ^ ^ « a 2 ^ ^S ^ - 2 « . oj S $^ G -^ " ^ 2 Ji ^-'^^^^ -rt ^ Ml r -^ 'U *T: ^ S O -M C8 •3 C cs S iH '^ -^ 2 -E o • ^ ^ c ••> ^ « «5 J:; >. •"■*-> ^ C a> . i/v - n (n zi ^^^ S O >. W) ■^ O CO O , C ±2 Tj CO g c3 cT ^ c £ -' 03 >> G C ^ . D Vh 4> 314 THE INVERTEBRATA ostia; in most of the Cladocera it is a sac (Fig. 233, h!) with only one pair. In the Cirripedia and many of the Copepoda and Ostracoda the heart is absent and the blood is kept in movement only by the movements of the body and alimentary canal. In the parasitic copepod Lernanthropus and some related genera there is a remarkable system of closed blood vessels v^ithout a heart. The blood is a pale fluid, which bears leucocytes except in ostracods and most copepods. It contains in the Malocostraca the respiratory substance haemocyamn, in which copper is present in combination with a protein. In various entomostraca, notably in Lernanthropus ^ just mentioned, haemoglobin has been found. 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 Branchiopoda 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 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. 223) 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 Cirripedia, 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. 202), but sometimes are less yolky and undergo total cleavage. Occasionally they 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. Develop- ment is not infrequently direct, but in most cases involves a larval stage or stages. Typically, the crustacean hatches as a Nauplius larva (Fig. 224), a minute creature, egg-shaped with the broad end in front, unseg- mented, but provided with three pairs of appendages — the antennules, CRUSTACEA 315 Fig. 223. 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 v^hich 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. Fig. 224. A ventral view of the first Nauplius of Cyclops. After Dietrich. an.' antennule; an." antenna; gn. gnathobase; Ibr. labrum; md. mandible. 3l6 THE INVERTEBRATA 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. 305). The median eye is the only organ of vision. A pair of frontal organs (p. 307) are present as papillae or filaments. There is a large labrum. 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 Malacostraca only certain primitive genera possess it, and in the Ostracoda it is modified by having already at hatching a precociously developed bivalved cara- pace. 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 Metanauplius 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 Oi 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 may make a sudden great advance at one moult. In the Cirri- pedia 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. 334). (b) Certain structures may be precociously 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 development, before the others. Zoaeae, how- ever, most often are not preceded by a free Nauplius but appear as the first free stage (Fig. 277 A), (c) Temporary retrogression of certain organs takes place during the development of some of the Mala- costraca : this affects some of the thoracic limbs in certain Stomatopoda and the prawn Sergestes, abdominal swimmerets and the antennule in the prawn Penaeus. CRUSTACEA 317 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. The carapace is in this class very variously developed, presenting a characteristic condition in each of the principal sections of the group. In the Anostraca it is not present. The Notostraca have it as a broad shield 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 aberrant Cladocera Gymnomera 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. Usually the carapace leaves the trunk free within it, but in the Clado- cera it fuses with two (in some Gymnomera with more) of the anterior thoracic somites. Except for one small and aberrant branch (the Gymnomera), all the Branchiopoda have as trunk limbs phyllopodia (p. 300) which bear on the median side endites fringed with long 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 some of them (but not the Cladocera or Conchostraca, which have natatory antennae) swim, and all breathe and gather food. Beating rhythmi- cally forward and backward with a movement which each pair starts a little earlier than the pair in front of it, they cause, by a pumping action which shall be described presently, a flow of water into the median gully between them, outwards into the spaces between each limb and its neighbours in front and behind, and then backwards, bringing particles of food, bathing the branchiae, and causing, in the Anostraca and Notostraca, forward movement of the body. Feeding is effected by the apparatus on the inner edge, which varies in detail with the food. Some have low endites with long and close set fringes which strain off particles from the stream just mentioned. The Anostraca and Cladocera Calyptomera best illustrate this arrangement, which varies in complexity and in the higher Cladocera, such as Daphnia (see below), becomes very elaborate and confined to a correspondingly small number of limbs. Those Branchiopoda in which the endites are more prominent and the fringe less continuous have denser food material to collect — either ooze, as in many Conchostraca, or solid objects as in the Notostraca, which have 3l8 THE INVERTEBRATA powerful and mobile endites. The Gymnomera are predaceous animals, and have slender, jointed, mobile trunk limbs with which they seize their prey. The mouth parts of the Branchiopoda are small and simple in structure, a condition in which they are not primitive but exhibit reduction. That on the whole the group is a primitive one as compared with the other classes of the Crustacea, is seen in the varying and usually large number of somites, the usually small amount of differentiation in the series of limbs on the trunk, the vascular system of the lower members (p. 311), and the nervous system of all (p. 303). With the exception of a few marine cladocera and of Artemia salina (p. 321), all branchiopoda are inhabitants of fresh waters. 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. 329). The first three orders of the Branchiopoda, as they are arranged below, are sometimes collectively known as the Phyllopoda. Order ANOSTRACA Branchiopoda without carapace ; with stalked eyes ; with antennae of a fair size but not biramous ; and with the trunk limbs numerous and all alike. We may take as one example of this group, Chirocephalus dia- phanus (Fig. 225), 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 append- ages and of the abdomen, the black eyes, and often a green mass of algae in the gut. It is incessantly in motion, swimming on its back. Its delicate appearance, 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 ante- riorly to form the head, upon which the mandibular groove (p. 296) is conspicuous . The head has in front a median eye and a neck organ (p .307) , and bears at the sides : {a) the large, stalked compound eyes ; {b) the anten- «M/e5, slender, unjointed, and ending in a tuft of sense-hairs ; [c) the stout antennae, triangular in the female but in the male (Fig. 226) 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 BRANCHIOPODA 319 of them is directed towards the mouth as a process with a blunt, roughened end. Below, the head bears (a) the large lahrum which is directed backwards under the mouth ; {b) the maxillules^ a pair of ^00 p. Fig. 225. A female of Chirocephaliis diaphanus. The animal is seen from the right-hand side in the morphological position: normally it swims up- side down. ah. 1, ab. 7, first and seventh abdominal somites; al. alimentary- canal ; an/ antennule ; an." antenna ; e. compound eye ; e.' median eye ; egg p. egg pouch; h. heart; Ibr. labrum; Ir. liver; md. base of mandible; nk.on. neck organ; ov. ovary; ram, ramus of caudal fork; tel. telson; th. 11, eleventh thoracic limb; th. 12, twelfth thoracic somite. nk.on. --yan. pr.ep. Fig. 226. Fig. 227. Fig. 226. A front view of the head of a male Chirocephaliis. an.' antennule; an." antenna; e. compound eye; e.' median eye; fr.ap. frontal appendage; nk.on. neck organ. Fig. 227. A thoracic limb of Chirocephaliis, mounted flat. br. branchia; bri. bristles which strain out the food ; ep. epipodite; ex. exopodite ;yZ6. flabel- Xwcn; pr.ep. proepipodites ; 1-7, endites. small triangular plates fringed by long bristles; (c) 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. 227 shows that these possess all the 320 THE INVERTEBRATA typical features of such limbs but are remarkable for the distal position of the exopodite and for the very long basal endite, which probably represents two, of which one may be the gnathobase (p. 306), unless the latter has been lost altogether. 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 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 pro- trusible 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 stomach, than in the trunk, where it is called the intestine. From the stomach proceeds a pair of sacculated diverticula (''liver"). The food consists of small organic particles, especially unicellular algae, which are strained off from the water by the trunk limbs in the following manner (Figs. 228, 229). 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 proepipodites, exopodite, and large distal endite are 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 be- tween the limbs of the right and left sides. From this gully, there- fore, 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 backward 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 fall (the animal being on its back), dorsal wards into a median food groove of the ventral surface. There ANOSTRACA 321 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 Umbs of some of the water contained in the lateral chambers at a certain phase of the movement. The food is agglutinated by a sticky secretion produced by glands in the lab rum, and pushed by the maxillules between the mandibles, which pound it, into the mouth. The organs of excretion are a pair of maxillary glands (p. 309), situated in the hinder part of the head and the first thoracic somite. They are wholly of mesodermal origin. The nervous system (Fig. 199) and the vascular system have been described above (pp. 303 and 311). Fig. 228. A diagrammatic view of a Chirocephalns 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. Partly after Cannon. Fig. 229. 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. The gonads are a pair of tubes lying one on each side of the alimentary canal in the abdomen, ^nd are continuous in front each with a short duct. The vasa deferentia lead to the penes, the oviducts to a median 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 322 THE INVERTEBRATA concentration 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 Chirocephaliis in having only- six abdominal somites and in the form of the antennae of the male. Lepidocaiis (order Lipostraca), a minute, blind, freshwater form from the Middle Devonian, was closely related to the Anostraca, 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 exopodite lateral, and the re- maining 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 ; and the trunk limbs numerous, the first two pairs of them differing considerably from the rest. This order contains only the genera Apus and Lepidurus, which differ in but minor features. Apus cancriformis (Fig. 231) is British, but is now very rarely found in these islands. The head (Fig. 230) is Fig. 230. 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. broad and depressed, flat 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 fixation, but no nuchal sense organ. From under the carapace the hinder part of the trunk projects backwards, ending in two long, jointed caudal rami. BRANCHIOPODA 323 Fig. 231. Apiis cancriformis . 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. 1, processes of the first thoracic limb. B, Ventral view. ah. abdominal limbs; ah.' limbless somites of the abdomen; an.' antennule; car. carapace; Ihr. labrum; md. mandible; mx.' maxillule; ynx." maxilla; pgti. paragnathum; ram. ramus of caudal fork; th.i, first thoracic limb; tk.io, tenth thoracic limb; th. 11, 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. 324 THE INVERTEBRATA 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 abdomen) is reached, after which there are two to five pairs to a somite 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 long and many- jointed in Apus, though shorter in Lepidurus, and has perhaps taken over the functions of the antennae. The second thoracic limb is less modified in the same direction, the endites being shorter and unjointed : it is used in clambering and in manipulating large food particles. The remaining trunk limbs (Fig. 213) are normal phyllopodia: their structure and function have been alluded to above (p. 317). They decrease in size from before backwards. 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. Males are rare, reproduction being normally by parthenogenesis. Order CONCHOSTRACA Branchiopoda with a carapace in the form of a bivalve shell provided with a hinge and adductor muscle, and usually en- closing the head as well as the trunk ; the compound eyes sessile and apposed; the antennae large and biramous with numerous joints in the rami; and ten to twenty-seven pairs of trunk limbs, of which ^\^' ^32- Esthena *u r: * ^ j-rr r ^u 1 • i_ • rninuta, from the the nrst one or two diner from the rest in being Trias x 3 From prehensile. Woods. No member of this order is British. Estheria (Fig. 232) is a common European genus. A thoracic limb of a related but exotic form is shown in Fig. 211 C. Order CLADOCERA Branchiopoda with a carapace in the form of a compressed shell which usually covers the trunk but not the head ; the compound eyes sessile and fused; the antennae large and biramous with few joints in the rami; and six, five or four pairs of trunk limbs, some or all of which may be highly differentiated from the normal type of the Branchiopoda. The members of this order are the water fleas. They fall into two sharply contrasting divisions. One, the Calyptomera, contains the majority of the species, which have retained the ancestral habit of straining out their food from the water and have perfected for that purpose an apparatus in which the carapace as well as the trunk limbs are involved ; and the other, the Gymnomera, contains a few species of predaceous habit whose trunk limbs are modified for seizing prey BRANCHIOPODA 325 and are given free play by the reduction of the carapace so that only the dorsal brood pouch remains. Suborder CALYPTOMERA Cladocera whose carapace completely covers the trunk and its limbs, some or all of which are lamellate. The lower members of this group show affinities with the long- bodied orders (Phyllopoda) of the Branchiopoda in that their trunk limbs are all alike and all strain food from the water, gnathobases .are present on these limbs, and the heart is comparatively long. Most of the genera, however, have the trunk limbs differentiated as parts of a complex apparatus in which only some of them act as strainers, have lost the gnathobases of the straining limbs, and have a short heart, of oval outline. Sida, which may be taken among weeds in pools in various parts of Britain, is an example of the more primitive forms mentioned above (Ctenopoda). It has six pairs of trunk limbs. Daphnia and Simocephalus , common British forms, found swim- ming in ponds and ditches, are examples of the higher genera (Anomopoda). Simocephalus (Fig. 233) differs from Daphnia in possessing a cervical groove (p. 296), and in lacking a dorsal spine which in Daphnia stands on the hinder edge of the carapace. The following description applies to both genera. The head is bent down- wards, so that the median eye and the small antennules are ventral to the antennae. A large, sessile compound 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 Chirocephaliis (pp. 318, 319). The segmentation 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 dis- tinct 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 represents 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. 234. Together they form a food-gathering mechanism which is very efficient because, instead of all working in the same way as those of the Anostraca, they 326 THE INVERTEBRATA vas de. Fig. 233. A, Sidevieyv oi vcidXe Simocephalus sima. Highly magnified. From Shipley and MacBride. an.' 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 Simocephalus sima, magnified to the same extent as A. From Cunnington. an.' antennules; a?i." antennae; md. mandibles; mx.' maxillules; Th.i-Th.s, thoracic limbs ; hep. hepatic diverti- culum; h. heart; ov. ovary; bdp. brood pouch; sh.gl. shell gland; brn. brain; md.g. mid gut; nk.on. neck organ. CLADOCERA 327 Fig. 234. Thoracic limbs of Daphnia pulex. After Lilljeborg. A, First. B, Second. C, Third. D, Fifth limb. br. branchia; en. endopodite; ex. exo- podite ; fr. fringe of bristles which strains out the food : normally this fringe stands vertical to the plane of the hmb (see Fig. 235, hri.), but it has been mounted flat for drawing; "^«." "gnathobase"; ^r.e^. proepipodite. 328 THE INVERTEBRATA are differentiated in adaptation to different parts of the task. The third and fourth pairs form a pumping and straining apparatus (Fig. 235) which in principle is the same as those formed by the Umbs 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- pxh.'^ Fig. 235. A diagram of a transverse section through the thorax of Daphnia. After Storch. hri. 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. c/z. 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 ; jpr.ep, proepipodites, playing upon the carapace and closing the pumping chambers at their outer sides; th. 2-4, sections through the thoracic limbs, which being directed backwards are cut transversely : each limb underlies that behind it. base" (which is perhaps not the true gnathobase but the second endite of the ideal series) in this limb play some part — exactly what is disputed — in bringing the food to the mouth. Glands in the labrum produce a sticky secretion as in Chirocephalus . The alimentary canal resembles that of Chirocephalus (p. 320), 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. CRUSTACEA 329 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 develop 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 ephippiiim — 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 reproduction proceeds by parthenogenesis. Then, about May, a generation 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. Suborder GYMNOMERA Cladocera whose carapace does not cover the trunk and its limbs, which are slender, jointed, and prehensile. Leptodora (Fig. 236), a pelagic inhabitant of certain fresh waters in Britain and elsewhere, is the extreme member of this group. 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. The carapace has fused with the somites of this region and projects behind it as a brood pouch. The winter egg 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 biramous ; 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. There are among them freshwater and marine, pelagic 330 THE INVERTEBRATA thA -ram. Fig. 236. A female of Leptodora kindti. After Lilljeborg. an.' antennule; an." antenna; car. carapace; mdg. mid gut; ov. ovary; raw. ramus of caudal fork; tel. telson; th.i^ first thoracic limb; trk.g, ninth trunk somite. Th,i ram. Th.i Fig. 237. Lateral view of Cypris Candida. After Zenker, an.' antennules; an." antennae; md. mandibles; mx.' ist maxillae; mx." 2nd maxillae; Th.i, Th.2, thoracic limbs; ratn. ramus of caudal fork; e. eye. CRUSTACEA 331 and bottom-living forms. Parthenogenesis is common among them, and in some males have never been found. Cypris (Fig. 237) is a common British freshwater genus. It swims well, by means of its antennae, but is not pelagic. The current which sweeps the food into the shell is set up mainly by the action of the epipodites of the maxillules (whose fan of setae is conspicuous in the figure), and the food particles are gathered from the current by long bristles on the palps of the mandibles 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 crawl- ing, and the second in cleaning. Cypris lacks the compound eyes and the heart, which are found in some other members of the class. 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. 238) 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 ^^ope of this animal is that of a slender pear with a stalk. The front part of the pear is un- segmented; this is a compound head or "cephalothorax", composed 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 part of the body contains three somites, the third to fifth of the thorax. The cephalothorax and these free thoracic somites are produced at 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 convention we have adopted (p. 296), 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- 332 THE INVERTEBRATA dominal somites and a telson, which bears two styHform, 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 Fig, 238. Cyclops. A, Dorsal view of female. Partly after Hartog. an.' an- tennule; an." antenna; e. eye; ov. ovary; ut. uterus: i.e. pouch of the ovi- duct into which the eggs pass before being shed; od. oviduct; spt. sperma- theca or pouch for receiving the spermatozoa of the male; e.s. egg sacs; ram. ramus of caudal fork; An. position of anus; g.som. compound somite, consisting of the last thoracic (bearing the genital opening) and the first abdominal. B, Ventral view of male. ab. abdomen; an.' antennule; an." antenna; cop. copula; e. eye; Ibr. labrum; md. mandible; mx.' maxillule; mx." maxilla; mxp. maxilliped; pgn. paragnathum; ram. ramus of caudal fork; tel. telson; th.z, th. 6, thoracic limbs. opening are present in the female only, and in her are reduced to the condition of small valves over the openings of the oviducts. The ab- COPEPODA 333 domlnal somites are without limbs in either sex. It will be seen that the actual tagmata of Cyclops are not the head, thorax, and abdomen, however 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. 239, 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) are also uniramous ; they stand immediately internal to the maxillae. The 2nd to ^th thoracic limbs, of which the 2nd stands on the head, are biramous, with broad, flat, spiny rami (Fig. 238 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 anten- nules , and a swifter progression brought about by the use of the swimming limbs (2nd to 5th pairs) of the thorax. In the more primi- Fig. 239. Mouth parts of tive, pelagic copepods (Calanus, etc.) which Cyclops. From Sedgwick, have biramous antennae and biramous palps ^^^^^ C^^^^- ^«- endopo- . fi 1 1 11 1 dite; ex. exopodite: ma. on the mandibles, the antennules do not take mandible; mx.' maxillule; part in swimming. Such copepods /(?^^ by mx." maxilla; mxp.maxil- an automatic straining of particles from the liped. water, though their apparatus 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 mxp 334 THE INVERTEBRATA 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 sper- matophore. The eggs when laid are cemented into a packet (egg "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. 224). This is succeeded by several Metanauplius stages, and then suddenly at a moult takes on the^zr^^ 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 are an important item of food for fishes and whales, is in several respects more primitive than Cyclops, having the antennae and mandibular palps (Fig. 211 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 primitive custom of feeding by the automatic straining of food particles from the water is retained : the feeding current eddies from the swimming current which the antennae, mandibles, and maxillae set up, and is strained through a fringe of bristles on the maxillae (Fig. 240). 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 COPEPODA 335 necessarily accompanied by a high degree of degeneration. The life histories of parasites are often complicated, and may involve re- markable changes of habit. Degenerate forms usually reach one of the 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. NotodelphySy commensal in the pharynx of ascidians, is clumsy bodied, and has a large dorsal tgg 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 ex. Fig. 240. 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. 9, terminal segment 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. Chondracanthus (Fig. 241), 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 vestigial, though the mouth has not a proboscis. The males are small, retain more of the 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. 242) hatches as a Nauplius and at the first Cyclops 336 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, Fig. 242. vas d. Fig. 241. Fig. 241. ChoTidracanthus gibbosus. After Claus. A, Female. B, Male, more highly magnified, al. alimentary canal; an/ antennule; a?i/' antenna; e. eye; e.s. egg "sac"; mxp. maxilliped; t. testis; th.z and 3, thoracic limbs; vas d. vas deferens ; cJ, males attached to female. Fig. 242. 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; fix. 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.2, second thoracic limb; rafn. ramus of caudal fork. 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. COPEPODA 337 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 for the vestiges of the parasite and contains a gut-Hke prolongation of the host's coelom. Appendix to the Copepoda BRANCHIURA Crustacea, temporarily parasitic on fishes ; which possess compound eyes; carapace-like lateral expansions of a cephalothorax which is formed by the fusion of the head with two thoracic somites ; and five Fig. 243. Argulus. 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.' antennule; an." antenna; e. paired eye; ex. exopodite; mx." maxilla; mxp. maxilliped; ram. ramus of caudal furca; sip. siphon, or suctorial proboscis; sp. poison spine. pairs of thoracic limbs, of which the first is uniramous and the rest biramous, with usually a proximal extension of the exopodite. The members of this group resemble in many respects the Cope- 338 THE INVERTEBRATA / poda, with which they are generally placed, but differ from that class in certain important features, notably in the possession of compound eyes, the position of the opening of the maxillary gland on the ist thoracic limb and that of the genital ducts between the 5th pair, the inclusion of the maxillules in the proboscis, and the phyllopod-like proximal overhang of some of the thoracic exopodites (Fig. 243 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. Argulus (Fig. 243), 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 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 subphylum, 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 Asco thoracica link the class to other crustaceans. 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. 244, 246 A), found all the world over on floating 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 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. 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 CRUSTACEA 339 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 an adductor muscle By which tgm. an'. Fig. 244. A view of Lepas anatifera, cut open longitudinally to show the dis- position of the organs. From Leuckart and Nitsche, partly after Claus. stk. stalk; car. 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; cetn. cement gland and duct; add. adductor scutorum muscle, which closes the carapace ; mtl.ca. mantle cavity, i.e. the space intervening between the carapace and the body. 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 22-2 340 THE INVERTEBRATA process towards the mouth and a large, uniramous, foUaceous 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 behind, 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 /mit^/ 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. 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 commence- ment and tapers behind into an intestine. Complicated 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 opens a unicellular gland (see Fig. 247). 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 CIRRIPEDIA 341 fixed by the discs on its antennules, and its body rotates within the shell, so that the ventral surface is directed backwards (Fig. 245 A, B). Now the shell and body are rotated upwards on the antennae so that the adult position is assumed (Fig. 245 C) ; meanwhile the shell plates appear, the preoral region elongates to form the peduncle, and the abdomen disappears. Scalpellum (Fig. 246 C, D) attaches itself to fixed objects, usually in deep waters . It diff'ers from Lepas in possessing a number of additional plates on the capitulum, and scales of a similar nature on the peduncle . It is more remarkable in possessing what are known as com/)/ewe«^<2/m<2/^^. al. ^' >cu. Ih. M. e.' e. Fig. 245. 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. alimentary 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. A few species of the genus are composed entirely 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 resemble these in organization, but usually they are more or less degenerate, being sometimes even without an alimentary canal. As a rule the more de- generate live within the mantle cavity of the partner, the less degenerate on its mantle edge. In certain species, which have very 342 THE INVERTEBRATA 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 scu, ytgm. - - cnl. Fig. 246. Cirripedia Thoracica. A, Lepas anatifera. B, Balanus. 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- ingof mantle cavity ; rst. rostrum ; y^^/. rostro lateral ; scu. scutum ; ^i/e. peduncle ; t. testis; tgm. tergum; ^, dwarf males. from the settling of young hermaphrodite individuals on the stalk of old ones, which is common in stalked barnacles. Balanus (Fig. 246 B), the common acorn barnacle, differs from Lepas in the lack of a stalk, and in having an outer wall of skeletal CIRRIPEDIA 343 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 Crustacea; 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. 247-250), 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. 247 A), with the characteristic frontal horns of cirripede Nauplii but without mouth or alimentary canal. The Cypris larva (Fig. 247 B) clings to a seta of a crab by one of its 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 undiffer- entiated cells surrounded by a layer of ectoderm, passes into the host's body cavity. Carried by the blood it becomes attached to the under 344 THE INVERTEBRATA side of the intestine (Fig. 251). There rootlets begin to grow out from it and eventually permeate the body of the crab to the extremities of the 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 B A, NaiipUus. B, Cypris. undifferentiated cells ; Fig. 247. Larval stages of Sacculina. From G. Smith. A.i, antennule; A. 2, antenna; Ab. abdomen; E. F. frontal horn with gland cells; Gl. gland cells; Md. mandible; Ten. frontal tentacles (frontal organs); Tn. tendon. crabs attacked by SaccuUna. The moult at which the parasite becomes external produces a change in the secondary sexual characters in the new cuticle. The male crabs have a much broader abdomen, reduced copulatory styles (these may disappear altogether), and abdominal swimmerets (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 RHIZOCEPHALA 345 crab has regenerated a hermaphrodite gonad. Parasitic castration is the most evident expression of a remarkable and at present ill- understood interference by the parasite with the general metabolism of its host. Thompsonia (Fig. 251), 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 d.s. A B Fig. 248 . Stages in the development of Sacculina upon the mid gut of a crab. From G. Smith. A, Early stage. B, Later stage, b, swelling caused by the body of the Sacculina; c.t. central tumour upon which the body arises; d.i., d.s. inferior (posterior) and superior (anterior) diverticula of the gut of the host ; w. " 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. parasite are widely diffused through the host. Their branches in the 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. .^^\; stk. Fig. 249. \ op. Fig. 250. Fig. 249. A specimen of the shore crab (Carcinus) bearing a Sacculina. op. mantle opening; Sac. Sacculina) stk. stalk. Fig. 250. A vertical section of Sacculina at right angles to the plane of greatest breadth. From Caiman, at. atrium of oviduct; ga. ganglion; ^/. col- leteric gland opening into atrium; o. eggs in mantle cavity; op. opening of mantle cavity; oz;. ovary ; r?. roots ; stk. stalk; t. testis. Fig. 251. An abdominal limb of the prawn Synalpheus infested by Thomp- 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. CRUSTACEA 347 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 Nauplius. Laura, imbedded in the tissues of the antipatharian Gerardia, has the mantle in the form of a very spacious sac with a narrow opening. Synagoga (Fig. 252), external Fig. 252. Sj'na^o^a Tw/ra. After Nor- parasite on Antipathes, has a bi- "^^^- ^?-^' ^'^^ abdominal somite; ^ , 1 r 1-1 11 ^"- antemiule; car. carapace; M. valve mantle, from which usually ^outh; ram. ramus of caudal fork; protrudes the long abdomen of ^e/.telson; i/?. thoracic limbs, four segments and a telson. 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 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. 296), 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 348 THE INVERTEBRATA 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 the sides. The median eye is vestigial in the adult, and the compound eyes stalked. The antennules arebiramous, as they are in no crustacean of any other group. The antennae have a scale-like exopodite. The mandibles have uniramous palps and the part which projects to- wards the mouth is cleft into "incisor" and "molar" processes. The maxillules 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. 300), a natatory exopodite, and two 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 back- Fig. 253. A female of Mysts relicta. After Sars. bd.p. brood pouch; md.gr. mandibular groove; sta. statocyst. ward, and forming with the telson a tail-fan, used in rapid backward movement. There are no caudal rami. (The Leptostraca are the only members of the class which possess these rami in the adult.) Food is collected as particles in a stream set up by the action of the maxillae, which also bear the filtering fringes of bristles. This type is said to possess the caridoid fades . It is best exhibited in the small, prawn-like, pelagic forms, formerly classed together as Schisopoda but now distributed, as the orders Mysidacea (Fig. 253) and Euphausiacea, to the two main subclasses of the Malacostraca (see below). Departures from it are many and important, and most of its features have disappeared 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 MALACOSTRACA 349 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 Leptostraca 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. An exceptionally large number of members of this group have direct development. Of those which possess larvae only a few (Euphausiacea, 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. 316), in which the tagmata are present, the abdomen is better developed than the thorax, and the animal swims by biramous maxillipeds. The Malacostraca fall into two large groups and three smaller ones. Of the \a.ttery 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 pseudosomites in the anterior part of the head, certain peculiarities of the thoracic limbs, and peculiar gills on the 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 characterized by possessing a brood pouch, formed by plates {ooste- gites) upon the thoracic limbs, in which the young undergo a direct development, 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. Subclass LEPTOSTRACA Malacostraca with a large carapace provided with an adductor muscle and not fused with any of the thoracic somites; stalked eyes; the thoracic limbs all alike, without oostegites, biramous, and usually foliaceous; seven abdominal somites, of which the last bears no appendages; and caudal rami on the telson. Nebalia (Fig. 254) is the commonest and typical genus of this group. N. 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 encloses the four anterior abdominal somites. The thorax is short. Its limbs (Fig. 211 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 350 THE INVERTEBRATA as a preischium (pp. 298, 299). 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 fiagellum, and a small epipodite.) 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. 303). The excretory organs have been alluded to on pp. 309, 311. Fig. 254. A female of Nebalia hipes. From Caiman, after Claus. a.' an- temiule; a." antemia; ab.^ 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. The animal / 1 ^ ■^ 3 ti ** c e C/3 •§ . C3 a jj 0 1 a ^ % a cd a 2 d « c/3 2 to C3 4-> C« OS (U ?^ .— 1 XI . »v . ^ 0) Hi > 1 Ji: a biD 0 0 >r & 'x X ^ 0 -5 C3 a d i 0 ^ rt U( 13 •sd" :3 0 0 <3 ^ ■| CO (+-I 0 •| T? ^ d ^ "S ^ ^ ^ 0) a a « u 73 ?? ■u . as ;J d c5 5^ « W)-^ c/3 ^ -^ tr> M C« '^ be (U d JH 13 1 w 0" 0 ^ M 0 'a 0 c tup _d d (U a C y:: a 'd 0 "d 0 s hT Xi 0 si 5 03 i i 0 0 bi) Cj d a 0 t3 ! -d G 0 0 £ A .^'^ 8 f-T. "0 0 K 10 3^4 THE INVERTEBRATA ^.ti-a S V M N CO •n « £ i^ R . ^ c3 o !r! fe 'c .2 +-' o •-> . 'o ^ si::, o .t: Q, D Oi ^ o (U S > I c c ^ 73 C O o (u ji a_j o DECAPODA' 365 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 coxopodites of the legs. At the angle, the branchiostegite, viewed from above, describes the lateral part of the outline of the body. That outline begins between the eyes, where in the crayfish the rostrum stands, with the front, a low, three-toothed lobe. On each side of this is the orbit y an excavation of the surface of the head for the reception of en^ Fig. 271. Fig. 272. Fig. 271. Diagram of a transverse section through a branchial chamber of Carcinus maenas. From Borradaile. a.l.e. anterolateral edge; cp. coxo- podite; eh.sp. epibranchial space; ep.i, epipodite of the first maxilliped; ep. 3, epipodite of the third maxilliped; hy. hypobranchial space; i.r. 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. 272. The left third maxilliped of Astaciis. cp.set. coxopoditic setae; en. endopodite; ep. epipodite; ex. exopodite. the eye. From the orbit the notched anterolateral edge curves out- wards 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 /)05^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. 366 THE INVERTEBRATA 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 the 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 asphyllobranchiae. 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. 272). The first maxilliped has a mastigobranchia without a podobranchia. The gill series of Carcinus is shown in the following table : Mxpd Mxpd II Mxpd III Leg I (Cheli- ped) Leg II Leg III h^ LegV Total 2 3 2 (3) 9 + (3) Podobranchiae Anterior Arthrobranchiae Posterior Arthrobranchiae Pleurobranchiae Mastigobranchiae (i) I I (I) I I I (l) I I I I — — Total (I) 2+(l) 3 + (i) 2 I I — — The mastigobranchiae lie in the gill chamber, that of the first maxilliped in the epibranchial space above (external to) the gills and 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 DECAPODA 367 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 maxiUiped, 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 epibranchial 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 pb.mp.3 pb.mp.2 ^^^ \ / p/5,T ar.ch. Fig. 273. A dorsal view of the organs in the left branchial chamber of Car- ciniis maenas. From Borradaile. ah. abdomen; ar.ch. arthrobranchs of the cheliped; ar.7np.2, arthrobranch of the second maxilliped; ar.inp.^, arthro- branchs of the third maxilliped; ep.i, epipodite of the first maxilliped; i.r. innerlayer of the branchiostegal fold, reflected •,pb. ynp. 2, ph. nip. 2, podohr^inchs of the first and second maxillipeds; ^c./. pericardial lobe, a thin fold of the body wall, of undetermined function; ^/6. i, plh.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. maxillipedal somites are fused into a triangular mass. In front of the mouth the plate known as the epistome 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. 368 THE INVERTEBRATA 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 antennules 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 mouth parts are shown in Fig. 274. 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. 300) in the stout endopodite, but the basipodite and ischiopodite are united. The first leg has a strong chela. 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 for the attachment of the eggs. 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 second. In feeding the food is seized by the chelae, which place it between the mandibles. These do not chew it, but, unless it be soft enough for them to sever a morsel when they close upon it, they hold it while the morsel is severed by the action of the hinder mouth limbs. The basal endites of the maxillules, the mandibular palps, and the pointed lab rum push the food into the mouth. The alimentary canal resembles in general features that of the crayfish. Its mid gut is short, and bears a pair of long dorsal coeca which end, each in a coil, at the sides of the cardiac division of the proventriculus or "stomach". The hind gut, just before entering the abdomen, gives off dorsally a long tube coiled into a compact mass. The "liver" is large and enters the carapace fold. In the antennal glands the whitish medullary portion found in DECAPODA 369 cp, h 4 t.i.ad. \ 6 'Lo,ak Fig. 274. Mouth parts of the right-hand side of Carcinus maenas. From Borradaile. i, Third maxilliped. 2, Second maxilliped. 3, First maxilliped. 4, Maxilla. 5, Maxillule. 6, Mandible, ac. " accesspry " muscles which curve the surface of the scaphognathite during its scooping action; ap. apophysis; art. articular process ; b. basipodite ; c. carpopodite ; cp. coxopodite ; cp.s. coxo- poditic setae; d. dactylopodite ; e7t. endopodite; ep. epipodite; et.i, et.z, en- dites; ex. exopodite;_^. flagellum;/?^. flange to which is hinged the epipodite; h. hinge : the adjoining dotted lines show the position into which the epipodite can be flexed by pressure against the base of the cheliped ; i. ischiopodite ; i.l. inner lacinia (endite) ; 1.1,1.2, lobes (each bearing two endites) ; m. mero- podite; o.l. outer lacinia (endite); p. propodite; pb. podobranch; pip. palp; sc. scaphognathite; t.i.ad. tendon of inner adductor muscle ; t.o.ab. tendon of outer abductor; t.o.ad. tendon of outer adductor. 24 370 THE INVERTEBRATA the crayfish is lacking, and the bladder is prolonged into processes which lie among the other viscera. In the nervous system (Fig. 276) the postoral ganglia are con- centrated into a mass around the sternal artery. The vascular system (Fig. 275) is on the same plan as that of the crayfish. The, gonads are in a.div. oph.a. hep. a. vasde (I aha. Fig. 275. The shore crab, Carcinus maenas, dissected from the dorsal side to show the viscera, a.div. left anterior diverticulum of the mid gut; an.a. antennary artery; d.ah.a. dorsal abdominal artery (posterior aorta); h. heart; hep. a. hepatic artery; Ir. liver; mdg. mid gut; oph.a. ophthalmic artery (an- terior aorta); ost. ostium; p.div. posterior diverticulum of the mid gut; St. "stomach" (proventriculus) ; t. testis; vas de. vas deferens. both sexes united across the middle line and prolonged laterally into the carapace fold. Each oviduct bears a spermatheca. The first larva is a typical Zoaea (Fig. 277 A) with large compound eyes, carapace, rostrum, short unsegmented thorax, and long strong abdomen with forked telson. Of its thoracic limbs only the first two DECAPODA 371 M o -n > tooMS u u. > ►^. 3 2 g i3 3 ■:?? ^ ^ a 4; . u 4^ ^ 3 ^ -Si (U G y W « C a *t rt S «5 o o s & 8 > i/c -W ^ (Z) (D C •Oh •^-^-' C« ^ IS -^ JH I M 3 S S g ^ _^-2 3^ 2^ c g ^ S j; *i ^ w 2 G c jj ^ > a " M y => S X .b X O ^ -sf-a a ^^ ^:: g > 5 o C w (U O S c-3 S c 2 *^v •* C3 '-' Vh 3 rt ^ . G ^ § c c n c3 c £ £ £ S 24-2 372 THE INVERTEBRATA pairs are present. After its first moult, which takes place almost at once, it has a median dorsal spine. The latter two features are characteristic of the Zoaea of the crabs. A Megalopa larva (Fig. 277 B), with the cephalothorax crab -like but the abdomen macrurous and carried at length, intervenes, as in other crabs, between the Zoaea and the adult form. an', -an'.' Fig, 277. Larval stages of Carcinus maenas. A, Zoaea. After Faxon. 'R^ Me- galopa. After Bate, an . antennule; an", antenna; d.sp. dorsal spine; md. mandible; jnx'-mx" . maxillae; mxp'-mxp" . maxillipeds; rst. rostrum. Of the various examples of the order which are mentioned below, all except Leucifer, Birgus^ and Gecarcinus occur in British waters. The most aberrant member of the Decapoda is the minute, pelagic Leucifer, which has a very slender, macrurous body with an extremely elongate head, long eyestalks, no hmbs 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 Naiiplius. Leander, the common prawn, one of the Caridea ; macrurous like the crayfish, but built for swimming rather than walking, with phyllobranchiae, and with chelae only on the first two pairs of legs. Crangofi, 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. DECAPODA 373 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. Eupagurus (Fig. 278), 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, Fig. 278. Eupagurus hernhardus, ^. ah. 2, third abdominal limb; tel. telson; th.%, last thoracic limb: sc. scale of antenna. 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. Birgiis, the robber crab, is a hermit crab which has grown too large to use the shells of molluscs, and has accordingly re-developed abdominal 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. 279 j, is by origin a hermit crab. 374 THE INVERTEBRATA but has lost the habit of Hving 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. Porcellana, the china crab, related to Galathea, has a form of body resembling that of the true crabs, but possesses uropods. Fig. 279. Lithodes maia, ?, in ventral view. From the Cambridge Natural History, ab.^, lateral plates of the third abdominal somite; ab.5, left lateral plate of the fifth abdominal somite; mar. marginal plate; Te.6, telson and sixth abdominal somite, fused; th.8, brush-like last thoracic limb. Ca?icer, the edible crab, is nearly related to Carcinus but more heavily built, without the slight powers of swimming possessed by the latter, and differing in other small points. Maia, the spiny spider crab, is narrow in front, with bifid rostrum and feeble chelae, and a habit of decking itself with seaweed for con- cealment. Gecarcinus, containing land crabs of the tropics, differs from Carcinus 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. 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, end 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. 280), 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 fre- quently moulted. 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 adult, including 3.preoral and (between the antennae and the mandibles) 376 THE INVERTEBRATA acgl ■/v.n.c Fig. 280. Fig. 281. Fig. 280. Lithohius forficatus. Dorsal view of whole animal, i, antenna; 2, maxilliped; 4, pair of walking legs. Fig. 281. Lithohius forficatus, <^. Dissected to show internal organs, ant. antenna; p.cl. poison claw; amb. walking legs; ac.gl. accessory glands; al. alimentary canal ; ?nt. Malpighian tubules ; sal.gl. salivary gland ; t. testis ; ve.se. vesicula seminalis; v.n.c. ventral nerve cord. Both from Shipley and MacBride. CHILOPODA 377 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 the antennae are jointed mobile appendages varying in length ; the mandibles are toothed plates without palps, the ist maxillae consist of hd.c - t.mxp^ mxp. - a m b. 1 Fig. 282. Lithohiiis forficatus . Original. Ventral viewof 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 ; Ihr. labrum ; 7nd. mandible; mx. maxilla; mx.' maxillule; mxp. maxilliped (poison claw); t.nixp. tergum of the maxilliped. 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. 282). 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 is possibly connected with the great development of the first pair -cox. 378 THE INVERTEBRATA 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 Lithohius number eighteen. Of these, the I St 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. 283, 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 Lithohius and the group of chilopods to which it belongs, the v^——. \ terga are alternately long and short ^'^ (Fig. 280). Only the segments which Fig. 283. Lithohius forficatus, S- have long sterna have stigmata, but Hind end, ventral side. 16, last all have walking legs. In other centi- ^^^^^^^ bearing ambulatory legs ; 07 J / rnui cox. coxopoditeor lastleg; 17, pre- pedes, e.g. Scolopendra (see lable, g^^ital segment, bearing g.pod. pp. 272-3), 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 end gut into which open a pair of Malpighian tubules. The vascular system is rather 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. 281) 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 accessory glands and in the male two vesiculae seminales. Spermato- MYRIAPODA 379 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 (^^or«^) 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; Fig, 284. 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. 285 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 a ridged and toothed plate; behind them is an organ known as the 38o THE INVERTEBRATA gnathochilarium (Fig. 285 C), which, in structure and position recalls the labium of insects and, like it, is formed by the junction of paired Fig. 285. 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; md. mandible; th. thorax. B, A segment detached from the rest. stm. sternum; tgm. 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.) ch. central body; hyp. hypostoma; /./. 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. appendages, the principal part of it by the appendages of the labial segment. Also a postlabial segment contributes to it forming the DIPLOPODA 381 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 ; though the first pair of legs appears to belong to it there are no separate appendages and no stigmata. 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 may be 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 2nd segment, but really belong to the 3rd. 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. 285 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 off 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. 285 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 closely com- parable with 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 yth 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. 286). th. Fig. 286. 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 ; th. 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 Neuromere Appendage Preantennal Protocerebrum — Antennal Deutocerebrum Antenna Intercalary Mandibular Tritocerebrum Mandibular ganglion Embryonic Mandible Maxillary Labial Maxillary ganglion Labial ganglion Maxilla Labium INSECTA 383 In the embryos of most generalized insects only, are coelomic sacs present in all head segments. There are two kinds of eyes, ocelli or simple eyes and compound or faceted eyes. The ocellus consists of a single cornea, a transparent 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. These ocelli are usually the sole type of eye in the larval insect and also coexist with compound eyes in the adult. 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 omma- tidia, 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 mandibles represent the basal joint of the crustacean limb, and correspond in structure and function to the mandibles of the Crustacea but never possess a palp (Fig. 287). The i^^ and 2nd maxillae are other paired appendages which show features of resemblance to the corresponding limbs of the higher Crustacea. In the fusion of the 2nd maxillae to form a single plate, the 384 Fig. 287. Mouth parts of Machilis (Petrobiiis) maritimus. After Imms. I, Mandible. 2, Maxilla. 3, Hypopharynx (h.) and superlinguae {sL). 4, Labium, c. cardo; g. galea; gl. glossa; /. lacinia; m. mentum; mx.p. maxillary palp; pf. palpifer; pg. paraglossa; pm. prementum; pgr. palpiger; s. stipes; sm. submentum. Fig. 288 . To show the resemblance between the insectan maxilla and labium and the biramous limb of the Crustacea. From Imms, after Hansen. A, Biramous crustacean appendage. B, Insectan maxilla. C, Maxillipeds of a Gammarid crustacean. D, Insectan labium, end., end.i endites; en. endopodite; ep. epipodite; ex. exopodite: sm. submentum. INSECTA 385 labium, we have a character which is found in the maxillipeds of certain Crustacea. Fig. 288 indicates the similarity between the in- sectan and crustacean mouth parts. Such an attempt at a comparison is only possible with the more generalized mouth appendages of the Insecta. In some of the more speciahzed orders the parts are so modified as to leave no evidence of their primitive structure. Such modifications will be deah with in connection with the different orders. 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 meta- thorax, 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 cushion between them called the pulmllus). Of the many adaptations exhibited by the legs of insects the jumping type found in grasshoppers, the digging type in the mole-cricket Gryllotalpa, the swimming type in the water beetles like 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 wings 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. 301). 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 diff"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. The variations in form, consistency, and size of the wings are briefly dealt with under the different orders. BI 35 386 THE INVERTEBRATA 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 be- coming deflected the wing encounters a certain amount of pressure from behind 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 contract immensely faster than those of any other animals. It is interesting 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 em- ployed. 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. 289 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 w^ing base and raises the wing. The abdomen consists of a series of segments less differentiated 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 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 INSECTA ^ 387 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. 290) 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 ant' p.WM Fig. 289. To illustrate the mechanism of wing movement in an Aphid. Wing depression: A, side view of mesothorax; B, transverse section. Wing elevation: C, side view of mesothorax; D, transverse section, ant. anterior; dv.m. dorsoventral muscles; post, posterior; l.m. longitudinal muscles; p.w.a. pleural wing attachment; t.w.a. tergal wing attachment. Effective muscles shown by dotted lines in A and C. After Weber. 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. 309 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- 25-2 300 THE INVERTEBRATA 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. 290), 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 Fig, 290. General view of internal organs of Apis mellifica as seen from above ; musculature and tracheal system not shown. From Carpenter, an. antenna; bn. brain ; co. colon ; cr. crop ; e. eye ; ga. ganglion ; mg. mid gut ; mt. Malpighian tubule; oe. oesophagus; rm. rectum; sa.gl. salivary glands (three types are shown) ; pv. pro ventriculus ; il. ileum. 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. 291) is lined by a layer of cells all similar, which perform almost the whole task of digestion and absorption of all classes of foodstuffs. While secreting, the cells break down and their contents are discharged into the gut cavity. In the absorptive INSECTA 389 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 necessitate the constant supply of fresh cells. These are found (Fig. 291) in the trough of folds or bottom of pits into which the mid gut epithelium Fig. 291. 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; l.m. longitudinal muscles; nc. group of regenerative cells; h. striated hem; pm. peritoneal membrane. 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 different 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 peri 390 THE INVERTEBRATA trophic membrane, which is, however, secreted by special cells of the proventricular region (which may be ectodermal). Its function and place in digestion is not understood. The hind gut begins where the Malpighian tubules enter the alimentary canal and is usually divided into a small intestme 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. Though the digestive enzymes of insects belong to the same classes as those of mammalia there are many significant differences. The rapid growth of caterpillars is probably due to the possession of enzymes better adapted for the penetration of cellulose cell walls and digestion of vegetable protoplasm than mammals possess. Many insects, particularly sucking and wood-boring forms, possess ali- mentary diverticula in which bacteria or yeasts are housed, either free in the lumen or inside the cells ; it is supposed that the vegetable enzymes can be utilized in some way for the special needs of the animal. The saliva of various insects shows great variety according to their habits ; thus the larva of the tiger-beetle {Cicindela) ,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 re- liance 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. In wood-boring larvae the secretion of a mandi- bular gland softens the wood and thus assists mastication, while, in caterpillars, silk production 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- 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 insects, having been deposited there by the tubules. But in addition nitro- genous 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, 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. The circulatory system. There is, firstly , a heart, primitively consisting of thirteen chambers, each corresponding to a segment, with a pair of ostia at the base of each chamber. The blood is driven forward in NSECTA 391 these 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 hori- zontal diaphragm perforated by many holes, which separates 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. 292). By their contraction the passage of blood from the body cavity into the pericardium and heart is facilitated. The circulatory system is primitive compared with that of the decapod Crustacea and much closer to that of the phyllopods (see p. 311). It is doubtful whether there is usually any respiratory pigment in the blood, which may, however, contain pigments, such as chlorophyll, derived from the food and, in the bloodworm {Chironomus), haemoglobin. It also contains leucocytes and, associated with it, are various cellular tissues such as the fat body^ the oenocytes and nephrocytes and in various beetles the photogenic organs. Fig. 292. Transverse section through dorsal part of the abdomen of Apis ynellifica 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 ; f.b. fat body; h. heart; mg. mid gut; mt. Malpighian tubule; tra. trachea. 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 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. 293). 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 the oxygen is conveyed directly to the latter without the intervention of the blood. These end tubes, as may be seen in Fig. 294, are of the smallest calibre. 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 392 THE INVERTEBRATA 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 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 a.s. Fig. 293. 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. alimen- tary canal; a.s. air sacs; l.t. longitudinal trunk; sp. spiracles. the abdominal sclerites. Abdominal contraction with spiracles open 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- hoppers (Fig. 293), 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 INSECTA 393 circulation through the main trunks is set up aiding considerably in the diffusion of gas through the whole system. Air sacs in the form of thin-walled diverticula of the main tracheae occur in many insects (Fig. 293), particularly those with the power to fly for prolonged periods such as bees, migratory locusts and houseflies. These also assist considerably in the circulation of air through the tracheal system owing to the ease with which they can be compressed. While a neuromuscular mechanism has thus been developed to assist respiration in the typical flying insect with its active meta- bolism, there are many stages of reduction in the group, culminating in the wingless CoUembola, which have no tracheae at all, gaseous Fig. 294. Tracheal end cell and tracheoles from silk gland of caterpillar, Phalera bucephala. From Imms, after Holmgren, e. end cell; c. tracheoles; t. trachea. 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 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 suspended from the surface film (Fig. 296). The second group includes the early stages of the Odonata, Plecoptera, Ephemeroptera and Trichoptera. These have no func- 394 THE INVERTEBRATA 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 (Fig. 312). They are usually external but in certain dragonfly nymphs {Aeschna and Libelluld) the rectal wall is raised into such gills and respiration is effected by pumping water in and out through the anus. Reproduction. The sexes of insects are separate, with only one known exception, viz. Icerya purchasi, a self-fertilizing hermaphro- dite. The usual method of reproduction is by deposition of yolky eggs following copulation. The egg, except in many parasitic Hymen- optera, 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. Fig. 295. Pupa of Anopheles maculipennis . After Nuttall and Shipley. /. respiratory funnel. 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 egg 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 INSECTA 395 Fig. 296. Larva of Anopheles maculipennis . After Nuttall and Shipley. h. feeding brush; c. antenna; d. maxilla; e. thorax; /. spiracles; g. palmate hairs for suspending from surface film; /. anal gills. 39^ THE INVERTEBRATA advantage accruing from this greatly increased reproductive capacity is obvious Parthenogenesis is in certain cases, chiefly in 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 ^H^ Fig. 297. Diagram of reproductive organs of A, a male, and B, a female honey bee, C, Longitudinal section of an ovariole of Dytiscus 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. oviduct ; o. ovum ; ve.se. seminal vesicles ; so. spermatheca ; t. testis (multifollicular); vd. vas deferens; v. vagina. 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 egg. 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. Organs of reproduction (Fig. 297). In the male the testes are usually small paired organs lying more or less freely in the body cavity. The INSECTA 397 extent to which they are divided into follicles , and the form of foUicle, vary in different orders. Thus, in the Diptera, each testis is unifoUi- cular, while in the Orthoptera a multifolHcular condition prevails. Each follicle is divided into a germariiim or formative zone, a zone of growth and maturation, and a zone in which spermatids are transformed into spermatozoa. In multifollicular testes the connection between each follicle and the main duct is known as the vas efferens 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 ejaculatory duct opens between the 9th and loth abdominal sterna in association with the external genital plates of copulatory significance. Accessory glands of various kinds and little understood function are usually found associated with the genital ducts. The female organs (Fig. 297) consist of ovaries^ oviducts, sperma- thecae, colleterial glands and a bursa copulatrix. 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. 297) is tubular and contains zones corre- sponding to those met with in the follicle of the testis. In addition to the developing 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 invagination of the body wall around the genital aperture adapted for receiving the intromittent organ of the male. The nervous system of insects (Fig. 298) consists of a dorsal brain and a ventral double chain of ganglia connected by longitudinal and 398 THE INVERTEBRATA transverse commissures. The anterior three pairs of gangHa 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. 382). Fig. 298. Nervous system of a grasshopper. After Uvarov. A, Ventral chain. B, Brain and associated nerves. C, Optical section through head. ann. antennary nerve and ganglion; dc. deutocerebrum ; ocn. ocellar nerve; op.ga. optic ganglion ; pc. protocerebrum ; sug. suboesophageal ganglion ; syg. sympathetic ganglia; tc. tritocerebrum. In addition to this is the sympathetic system (Fig. 298 B, C) which supplies the muscles of the alimentary canal and of the spiracles. 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 INSECTA 399 two ecdyses is termed an instar. The animars existence is thereby made up of a succession of instars, the final one being the aduh. In the simplest and most generahzed 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 reproductive system. Where the adult is primitively wingless, as in silver fish and sp ringtails (Fig. 304), 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 Ametahola. 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. 320). The degree of metamorphosis varies con- siderably, irrespective of wings, in winged insects according as the young stages resemble their adults or not. A growth stage of a cock- roach, 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 resemblance to the final instar with its wings, elaborate body form and mouth parts (Fig. 326). Metabolous insects, those passing through a distinct metamor- phosis, are therefore further divided into two subclasses, (i) the Hetewmetabola, 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- semble their adults in body form, type of mouth parts, and the possession of compound eyes, and are known as nymphs (Fig. 299). The orders composing the Holometabola are the Neuroptera, Mecoptera, Trichoptera, Lepidoptera, Coleoptera, Strepsiptera, Hymenoptera, Diptera, and Aphaniptera. The young stages of these are known as larvae and differ from their adults in body form, mouth parts, and the absence of compound eyes. So great- is the difference between the larva and the adult that an instar known as the pupa has been specialized to bridge the gulf (Fig. 320). 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 400 THE INVERTEBRATA the alimentary canal, are broken down by phagocytic or other action and the new adult tissue is buih up from many growth centres. Fig. 299, Metamorphosis of a capsid bug (Plesiocaris vagicollis). After Petherbridge and Hussain. 1-4, nymphal instars ; 5, imago. A less obvious prepupal instar is also present, enabling the change from larva to pupa to be effected. INSECTA 401 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. 300 A). The forms of larvae vary considerably and indicate to a great extent Fig. 300. Types of coleopterous larvae. A, Campodeiform larva of Pterostichus , Carabidae (original). B, Eruciform larva of Melolontha, Scarabaeidae (original). C, Legless larva of Phyllobius urticae, Curculionidae. After Rymer Roberts. the degree of metamorphosis passed through. A campodeiform larva (Fig. 300 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. 300 B) 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, pi'olegs are often found on the abdomen, e.g. caterpillars of Lepidoptera and sawflies (Fig. 322). BI 26 402 THE INVERTEBRATA A grub (Fig. 300 C) is an apodous larva which in other respects re- sembles the eruciform type, e.g. certain Diptera 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. 320). In ohtect pupae (Fig. 317), 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 retained as a barrel-shaped ^wpanwm over the pupa within. Such protected pupae are called coarctate (Fig. 326). 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. 299). All the Heterometabola have such a wing development and therefore the alternative name Exopterygota is often given to the group. 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 adults, 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. 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 development. 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. 301). Fossil record. Though the insects form an undoubted natural group — all its members being referable to some generalized form, possessing among other things mouth parts similar to those of the cockroach, efficient for chewing solid food, an ii-segmented abdomen, a 3-seg- mented thorax and a 6-segmented head, and two pairs of membranous INSECTA 403 wings carrying parallel longitudinal veins with a reticulum of cross veins between them — the orders are clearly defined. They are not easily linked together by intermediate forms and the story of Fig. 301. 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, Instar 3. D and E, Instar $• ch. chitin; /iy. hypodermis; m.l. middle lamella; p.m. peripodal membrane ; tch. tracheo]es within the veins ; tel. tracheole cells ; tra. trachea; v. vein; wr. wing rudiment. evolution within the subphylum consists rather of disjointed sentences than a continuous theme. The two divisions already mentioned, how- ever, the Exopterygota and Endopterygota, are natural groups which we may for convenience call the " generalized " and the " specialized '* 26-2 404 THE INVERTEBRATA 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 different, 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 diflicult 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 flrst essay into the intricacies of entomology by dissecting the cockroach should keep 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. INSECTA 405 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 characteristic half-horny anterior wings appear to be the more recent development. Up to this stage none of the important endopterygote orders had made their appearance. The mandibulate Mecoptera form an order which is most 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 Permo-Carboniferous with its glacial con- ditions may have accounted for the onset of metamorphosis, the pupal stage being evolved for the purpose of surviving cold periods 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. 406 THE INVERTEBRATA Class I. Apterygota (Ametabola). Order i. Thysanura. 2. CoUembola. 3. Protura. Class II. Pterygota (Metabola). Subclass I. Exopterygota (Hetero metabola). Order 4. Orthoptera. 5. Dermaptera. 6. Isoptera. 7. Plecoptera. 8. Embioptera. 9. Psocoptera. 10. Odonata. 1 1 . Hemiptera ( Rhy nchota) . 1 2 . Ephemeroptera . 13. Mallophaga. 14. Anoplura. 15. Thysanoptera. Subclas 2. Endopterygota (Holo metabola). Order 16. Neuroptera. 17. Mecoptera. 18. Trichoptera. 19. Lepidoptera. 20. Coleoptera. 2 1 . Hy menoptera . 22. Diptera. 23. Aphaniptera. 24. Strepsiptera. 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. 287); 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. 302), the common "silver lish" which inhabits dwellings of man, and Machilis (Petrobius) maritimus, found THYSANURA 407 above high-tide mark along the sea shore and estuaries, are common examples. In M«f:Mw (Figs. 303, 287) interesting features are presented by the well-developed superlinguae and the jointed mandibles both of which are primitive characters. The superlinguae in Machilis are paired Fig. 302. Lepisma saccharina. From Imms, after Lubbock. Fig. 303. Machilis (Petrobius) maritimus. From Imms, after Lubbock. 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 408 THE INVERTEBRATA 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 hypopharynx 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. There are no tarsi on the legs, claws being borne by the tibiae. The Fig. 304. A, Axelsonia (Collembola). B, Hamula of Tomoceros showing c. basal piece, and r. its rami. From Imms, after Carpenter and Wilson. p. ocular pigmented area; v. ventral tube; h. hamula; m., d. and nic. caudal furcula. I St abdominal segment carries a ventral tube which is moistened by a glandular secretion from behind the labium poured down a ventral 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 nearly complete fusion of a pair of appendages has resulted in the formation of the hamula, which engages Xht furcula prior to leaping. The latter is a forked structure representing a pair of limbs of the 4th segment (Fig. 304). 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. INSECTA 409 Collembola have a wide distribution. They are found along the sea shore between tidemarks and submerged by each tide, e.g. Aniirida maritima. Common aquatic forms are denizens of fresh waters, e.g. Podura aquatica. They have been reported to be so 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 pre- vented 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. Class PTERYGOTA Subclass EXOPTERYGOTA Order ORTHOPTERA Insects with generalized biting mouth parts ; ligula 4-lobed, consisting 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 the Ciirsoria in which the legs are of approximately equal size and the Saltatoria in which the last pair of legs are modified for jumping (Fig. 305). The former consists of the Blattidae (cockroaches) which are swift-running, omnivorous forms, usually tropical in their distribution, the Mantidae (praying 4IO THE INVERTEBRATA 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 which give the animal a resem- blance to leaves, which is closer in the female than in the male. Fig, 305. Pachytylus migratorius . A grasshopper. Natural size. From Shipley and MacBride. The female phasmid at any rate is almost motionless, and the habit of feigning death is commonly developed in the family. All these characters 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 EXOPTERYGOTA 4II 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 numbers which invade cultivated districts causing incalculable harm. A very characteristic feature of the Saltatoria is the possession of stridulating organs. In one type, exhibited by the cricket Grylhis, a file on one of the anterior wings is rubbed over a scraper on the other. 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 longi- tudinal 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 reduction to mere scales as in certain stick insects, or to their complete absence as in Gryllohlatta. 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. 306) 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 character- istic venation and fold up along transverse as well as longitudinal furrows, thus contrasting with the Orthoptera. When unfolded, the wing presents the appearance of a half wheel, the "spokes" radiating backwards 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. 412 THE INVERTEBRATA Fig. 306. Forficula auricidaria. Male. From Imms, after Chopard. 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. 307). The termite community usually contains a dealated royal pair, the king and queen, who are the founders of the colony, and also supplementary reproductory individuals of two kinds: {a) winged, which normally serve for the formation of new colonies, and (^) wingless, which become capable of reproduction if occasion demands. There is usually a vast number of individuals of sterile wingless forms 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 militarisy which bores into ^nd does much harm to tea plants in Ceylon. The winged sexual forms in several colonies usually swarm at the ISOPTERA 413 same time, and of the countless numbers a few individuals escape the attacks of birds and other 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 Fig. 307. Hamitermes silvestri Hill. Tropical Australia. After Tillyard. A, Neoteinic queen. B, Winged male. C, Worker. D, Soldier. E, Nymph. 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 . 62) and other flagellates in the hind gut. The fragments of wood are in- gested by the Protozoa and converted into sugars, being largely stored 414 THE INVERTEBRATA Up in the form of glycogen. In some way, not properly understood, the termite takes advantage of the carbohydrate, thus made digestible, and if, as is possible by raising the temperature to 40° C, the termites are ''deprived" of their protozoa, wood is no longer digested and the animals starve. Cultures of termites have been kept for more than a year with their associated trichonymphids on a diet of pure cellulose (filter paper). It is difficult to conceive how they obtain nitrogen for building up body proteins. Termites may forage by night for plant food and the genus Termes also cultivates in its n^st fungus gardens . The fungus which grows on 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 speciaHzed for defence (Fig. 307). Both these castes consist of males and females, though secondary sexual characters 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 characteristic 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 distribution, 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. EXOPTERYGOTA 415 These insects are widely distributed in the warmer parts of the world. Many are gregarious, living in tunnels formed of silk pro- duced by tarsal glands, e.g. Emhia 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 or leaves and covered by a protecting sheath of silk by the female, e.g. Peripsocus phaep terus . 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 Carboniferous 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 important part in catching the prey and holding it while 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. 308) have a complex venation of a reticular nature, a stigma or chitinous thickening of the wing membrane found near the apex being characteristic. 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 Fig. 308. The emergence of the dragonfly Aeschna cyanea. After Latter. EXOPTERYGOTA 417 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 filaments — 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 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 difference 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 submentum 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, water boatman) 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. 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. 309). At its base the groove does not exist but the labrum 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. 309). At rest BI 27 4l8 THE INVERTEBRATA 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. 309). 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 Fig. 309. Mouth parts of the Hemiptera. A, Sagittal section through head of Graphosoma italicum. 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. Ibm. labium; Ibr. labrum; 7nd. mandible; mx. maxilla; ph. pharynx; /)/j.^. muscles of pharyngeal pump ; sty. stylets. Aphis rumicis 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. The order is divided into the Heteroptera (Fig. 310), in which save for their membranous apex the front pair of wings are harder than the hind, and the Homoptera with the front pair uniformly harder. The Heteroptera contain such families as the Capsidae living on plant HEMIPTERA 419 juices, the Cimicidae or bed bugs, flightless and blood sucking, and the families Nepidae (water scorpions), Hydrometridae or pondskaters, Notonectidae and Corixidae, whose adaptations for aquatic life are of great interest. The Homoptera contain the Cicadidae, large bugs, the larvae of which burrow and suck the root juices of trees (in Cicada septendecim this larval period lasts either thirteen or seventeen years, and the adults appear periodically in vast numbers) ; the Membracidae (tree hoppers) and the Jassidae (leaf hoppers) ; the Coccidae (scale insects),* in which the female is always wingless and usually degenerate Fig. 310, External anatomy of Leptocoris trivittatus with wings spread on one side. After Essig. aw. antenna; /?e. hemielytron. and covered by a secretion, sometimes waxy and sometimes resinous (forming shellac), and the Aphididae (green flies). 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 these eggs hatch, giving wingless parthenogenetic females which produce young viviparously. A variable number of these parthenogenetic generations is passed 27-2 420 THE INVERTEBRATA 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 Euonymus. 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. The repro- ductive capacity of these insects is most remarkable and is fortu- nately offset by the number of enemies which they possess. The following summary will assist in the understanding of the life cycle of Aphis rumicis : Fertilized eggs laid in autumn I Viviparous parthenogenetic females > Euonymus I Winged migrant parthenogenetic females I Wingless parthenogenetic viviparous females \ I \f r Viciafaha I Winged viviparous females (autumn) J Winged males x Wingless oviparous females \ \ \ Euonymus Eggs laid in autumn J 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 venation ; the hinder pair small ; caudal fila- ment and cerci very long (Fig. 311). 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. EPHEMEROPTERA 421 The nymphs at first possess no gills but subsequent instars bear on the abdomen movable tracheal gills (Fig. 312), 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. Those which live in clear still water have a stream- lined form for rapid movement, while burrowing types have fossorial legs and are often provided with protective gill opercula. The mouth Fig. 311. Ephemera vulgata . From Imms. 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: 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, 422 THE INVERTEBRATA 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. Fig. 312. Nymphal instars of Heptagonia. After Imms. A, Third instar. B, Seventh instar. C, Eighth instar. a, a\ h and c, gills belonging to these instars respectively; lo. wing rudiment. 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. EXOPTERYGOTA 423 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. 313), maybe 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 ab- md. Fig. 313. Fig. 314. Fig. 313. Hen louse, Menopon pallidum. Dorsal view, showing biting mandibles by transparency, an. antenna; md. mandible; mxp. maxillary palp; pth. prothorax; msth. mesothorax; mtth. metathorax. Fig. 314. Body louse, Pediculiis humanus. After Imms. domen. 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. 424 THE INVERTEBRATA 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 mouth parts are tubular and capable of withdrawal into the ventral side of the head. Their homologies are obscure. Pediculus humanus., the body louse (Fig. 314), 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. The louse has been found to lay about ten eggs daily, depositing in all about three hundred. Temperature plays a big part in controlling the development of these animals. Under average conditions, the life cycle is completed in about three or four weeks. 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, w^hen they pupate. Common thrips of importance are Taeniothrips inconsequens of pears and Anaphothrips striatus of grasses and cereals. INSECTA 425 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 arrange- ment of veins along the anterior border. The abdomen is without cerci. The larvae are invariably carnivorous — campodeiform , with biting or suctorial mouth parts. Aquatic larvae usually possess ab- dominal 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 fall into the water. In this larva (Fig. 315), 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 lace- wings, 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 lacewing 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 Fig. 315. Larva of Sialis lutaria. From Imms, after Lestage. 426 THE INVERTEBRATA 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, to- gether 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 wdngs, with few cross veins and held in a roof-like manner when at rest. Fig. 316. A, B, C, D, Cases of Trichoptera. A, Hydroptila maclachlani. B, Odontocerum. C, Phryganea. D, Hydropsyche, pupal case. E, Halesus guttatipennis . After Imms. 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. ENDOPTERYGOTA 427 The eggs are laid in or near water and the larvae quickly cover themselves with some foreign substances (Fig. 316), building a form of tube from the wide end of which the head projects. Respira- tion is effected by tracheal gills generally found on the abdomen, water currents being passed through the tubular case by the undula- tory movements of the body. The larvae may be herbivorous or carnivorous. 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. Order LEPIDOPTERA (Butterflies and moths) Mouth parts of the imago usually represented only by a sucking proboscis 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 elongated galeae of the maxillae, each being grooved along its inner face and locked to its neighbour (Fig. 317). The laciniae are atro- phied and the maxillary palp is usually much reduced. The mandibles are 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 cause the whole organ to roll up into its characteristic position beneath the head and thorax (Fig. 318). How the proboscis is extended 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 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 428 THE INVERTEBRATA Fig. 3 17. A, Tryphaenapronuba, with venation and frenulum (fr.) ; cJ condition on right. Original. B, Ohtect pupa oi Platyhedra gossypiella. After Metcalf and Flint. Fig. 318. Head and proboscis of a moth. A, Front view. B, Side view. After Metcalf and Flint. C, Transverse section of proboscis. After Eltring- ham. dp. clypeus; d?n. diagonal muscles; e. eye; ep. epipharynx ; j§'rt'/. galea; Ibr. labrum; l.h. locking hooks ; Ip. labial palp; wc/. mandible ; mxp. maxillary palp; n. nerve; tra. trachea. LEPIDOPTERA 429 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. 317). These latter are formed by enlarged hypodermal cells, and their main function appears to be the presentation of colour due either to the pigment they contain (like the uric acid of the Pieridae) or to striation of the surface causing interference colours. There also occur "scent scales" which may have a sexual significance. Several methods of wing coupling have been developed independently in the order. The commonest consists of a stout bristle or frenulum of the hind wing locking into a retinaculum composed of curved setae on the fore wing. 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. 322 A-C) have three thoracic and ten abdominal segments with nine pairs of spiracles situated on the pro- 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 pro legs on segments 3-6 and 10. These are different from the typical insect limbs, being conical and retractile with hooks on the apex (Fig. 322 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 (exceptions 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. 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 boll worm, Platyhedragossypiella, and the gypsy moth, Porthetria dispar, are included in this order. Order COLEOPTERA (Beetles) Biting mouth parts ; fore wings 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 430 THE INVERTEBRATA mobile; meso thorax much reduced; metamorphosis complete, larvae (see p. 401) campodeiform or eruciform or, more rarely, apodous. In the larvae the head is well developed (Fig. 300) 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), Carabidae and the Staphylinidae). They are very active in movement and often pre- daceous, with well-developed antennae and mouth parts, and chitin- ized exoskeleton. In the erucijorm type (Fig. 300 B), found among plant-eating forms like the lamellicorn beetles, the legs are shorter, Fig. 319. External anatomy of Calosoma semilaeve, with left elytron and wing extended. After Essig. an. antenna; el. elytron;/), palp; sp. spiracles. 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. 300 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 in- dicated 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 triungulin. This ENDOPTERYGOTA 431 is an active campodeiform larva which attaches itself to its host. 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 cruciform 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. In such a large order of insects it is to be expected that all manners 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 Fig. 320. The hornet, Vespa crabro. A, Larva. B, Pupa. C, Adult o . coleopteran associations. A large number, such as many coccinellids (lady birds), carabids, e.g. Carahiis violaceus, and staphylinids, e.g. Ocypiis olens, are carnivorous and to this extent useful insects. On the other hand, among the phytophagous forms are to be found the most serious agricultural pests, the boll weevil, Anthonomus grandis, causing so much damage to the cotton crop in America that it has 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 Xestobium rufomllosumy the death-watch beetle, destructive to structural timber. 432 THE INVERTEBRATA Order HYMENOPTER A (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 Fig. 321. 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 ;^^. paraglossa. 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. This order is remarkable for the great specialization of structure exhibited by its members ; for the varying degrees to which social life HYMENOPTERA 433 has developed, and for the highly evolved condition which para- sitism has reached. Specialization of structure is evidenced in the mouth parts of the 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 j Fig. 322. 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 testudinea. E, Head capsule of latter. an. antenna; dp. clypeus;/r. frons; lb. labium; Ibr. labrum; nid. mandible; mx, maxilla; oc. ocellus; v. vertex. also adapted for licking. 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. 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 ApiSy the honey bee BI 38 434 THE INVERTEBRATA (Fig. 321), 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 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 maxillary glossae undoubtedly playing an important part in maintaining a com- plete tube. The mandibles are now no longer biting organs but tools used for manipulating material such as pollen and wax. 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 promsiotttng , 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. 323) 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, egg, larval, pupal, 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 extreme 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 ^ 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) . HYMENOPTERA 435 Fig. 323. A and B, Exarate pupae of Phaenoserphus viator. C, Pupae of same projecting from empty skin of host, the ground beetle larva, Pterostichiis . After Eastham. s. spiracle; t. invagination to form tentorium. 28-2 436 THE INVERTEBRATA of the sawflies (Fig. 322 D) and the legless form of bees, wasps and ants (Fig. 320 A). The sawfly larva has a superficial resemblance to the lepidopterous caterpillar, but is distinguished by its single pair of 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. 322 clearly shows. Order DIPTERA (Flies) Insects with a single pair of functional wings, the hind pair repre- sented by stumps (halteres) (Fig. 324); 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). A further development which takes place in several families is the acquisition of blood-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. 325). The most complete system is to be found in the gadflies, e.g. Tabanus 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 epipharynx, 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, Xho^ 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. DIPTERA 437 Fig. 324. Anopheles maculipennis, $. After Nuttall and Shipley. Fig. 325. Types of mouth parts of the Diptera. A, Culex pipiens, ?. B, Glossina suhmorsitans . C, Transverse section through proboscis of Culex. D, Transverse section through proboscis of a muscid fly. E, Proboscis of a muscid fly, extended. F, Proboscis of a muscid fly, half folded, an. antenna; e. eye; f.c. food channel; hyp. hypopharynx; Ibm. labium; Ihl. labellum; Ibr.ep. labrum epipharynx; md. mandible; mx. maxilla; mxp. maxillary palp; ph. pharynx; ph. p. pharyngeal pump; pstra. pseudotracheae ; sd. salivary duct. A-D, after Patton and Cragg; E, F, original. DIPTERA 439 The mouth parts of the female mosquito (Fig. 325 A) in principle differ from those described above only in the absence of a pseudo- tracheal membrane on the labellar lobes and the more slender and elongated labium. Mandibles are absent in the males, maxillae being represented only by palps. The housefly Musca (Fig. 325 D, E, F) has lost all piercing mechanism, mandibles being absent, maxillae only being represented by the palps, and the mouth parts consist of a fold- ing labium with highly developed pseudotracheal membrane on the labellar lobes and prominent epi-hypopharyngeal tube. Musca feeds largely on fluid matter but in the presence of soluble solid food, e.g. sugar, solution is effected by regurgitating alimentary fluid on it. By means of small chitinous teeth situated round the point to which the pseudotracheae converge surfaces of solids can be scraped so enabling enzymes in the regurgitated fluids to act rapidly. The tsetse fly, Glossma {Fig. 22$ B), also possesses no mandibles and only the palps of the maxillae. It does, however, feed on mammalian 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 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. In this order are included a large number of families of the greatest economic importance. Thus, to mention a few, the Tipulidae (crane- flies) are voracious root feeders in the larval state, and the Cecido- myidae (gall midges) are, for the most part, plant feeders as larvae. The Culicidae and Simuliidae are notorious blood suckers, the former family being concerned with the transmission of malaria, yellow fever and elephantiasis. The Tabanidae (gadflies) are cattle pests. Among the large family Muscidae we find the cosmopolitan houseflies, blow- flies and the African tsetse flies. The Oestridae are endoparasitic on vertebrates in the larval state and include in their numbers the ox- warble fly and horse bot fly of this country. Lastly we may mention the Tachinidae, whose larvae are almost exclusively parasitic on other insects. 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- domen, e.g. Bibio (Fig. 326 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 amphipneustic, with only prothoracic and posterior abdominal spiracles, or metapneustic, 446 THE INVERTEBRATA 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. 326 A). Of general interest are the ectoparasitic Diptera, the Hippo- boscidae restricted to birds and cattle, the Nycteribidae and the Streblidae to bats. These forms are viviparous, nourishing their larvae within the uterus of the female. The newly deposited larva Fig. 326. Early stages of the Diptera. A, Larva oi Musca domestica. Acephalous amphipneustic type. B, Empty puparium of Musca domestica. C, Pupa of Musca domestica removed from puparium. D, Larva oiBibio sp. Eucephalous peripneustic type. A, B, and C after Hewitt; D, original, sp. spiracle. at once pupates, hence their inclusion in the group Pupipara. Among muscid Diptera the tsetse flies are similarly viviparous. The eucephalous larva develops into an exarate pupa from which the adult emerges by a longitudinal slit on the thorax. The pupa resulting from the acephalous larva, on the other hand, is coarctate,the last larval skin being retained as a protective puparium. The latter splits transversely to allow the adult to emerge (Fig. 326 C). ENDOPTERYGOTA Order APHANIPTERA (Fleas) 441 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 Fig. 327. The life history of the flea, Ctenocephalus canis. From Imms, after Howard, a, egg; b, larva in cocoon; c, pupa; d, imago; e, larva of flea, Ceratophyllus fasciatus ; f, antenna of imago. and for jumping, and by their mouth parts (Fig. 327). 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 442 THE INVERTEBRATA 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 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 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, Xenopsylla cheopis, which transmits Bacillus pestis, the bacillus of plague. It appears that this bacillus lies in the gut of 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 irritans, 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 usually three-jointed pre- hensile appendages (chelicerae), 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 oflt 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 are true jaws absent, the prolongation of the basal joint of the anterior limbs toward the mouth (gnathobases) serving the arachnids 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. 328 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. 328 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 nidi- op•^o coe. pro- 1 4n^c. prg. kiH: iocs p?y. 7-' -^OW. met. iZS Oc Fig. 328 . The development of the Arachnida. A, Transverse section of a spider embryo (Thendium), 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 precheliceral segment (pre); pro. i, pro. 6, first and last segments of the prosoma; prg. pregenital segment; op. 2, second, and op. 10, tenth segment of the opisthosoma, C, Diagram of the scorpion embryo, altered from Dawydoff. Coelomic sacs of precheliceral segment (pre.) ; 1-6 segments of the prosoma; prg. pregenital segment; g.op. segment of genital operculum; pet. segment of pectines ; /^^. segments of first three lung books ; co.d. coelomoducts which never reach the exterior ; cox.gl. coxal glands ; goti. gonoducts ; g. gonad. D, Embryo of the scorpion Buthus carpathicus, after Brauer. Stage showing chc. the chelicerae ; pp. pedipalps, the four other appendages of the prosoma ; prg. the pregenital segment and appendages; 7, appendage forming genital operculum, succeeded by the appendages forming the pectines and those which form the lung books ; 12, last of these ; met. metasoma. ARACHNIDA 445 mentary appendages, the chtlarta; it is entirely missing in the aduh 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 Limulus there are six segments only, but in the related extinct genus, Hemiaspts, 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 narro\Y 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 "-like organ. There may be important salivary glands entering the fore gut as in the scorpions. The end gut is short and, except in Limulus, gives off Malpighian tubules. 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. 329) 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 446 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 diffusion. 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. 329. Longitudinal section through the opisthosoma of Limidus, 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. 330. 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 Limuliis. 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. 330). 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 447 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. 331. 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. {2 a) 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 448 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. 328 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. 328 C) and the spiders (Fig. 328 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 coelomo- ducts. In the scorpions, the embryo (Fig. 328 C) shows five pairs of these, in segments 3, 4, 5, 6 and 9. In only one case, that of seg- ment 5, do the coelomoducts reach the external surface, and persist in the adult as a pair of excretory organs, the coxal glands. In segment 9 they grow towards the middle line and form the mesodermal part of the gonoducts. 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 Limiiliis 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 a cephalothorax (prosoma), the opisthosoma divided into a mesosoma and metasoma distinct from one another, con- taining twelve segments and a telson; the prosoma covered by a dorsal carapace ; chelicerae and pedipalps both chelate ; four pairs of ARACHNIDA 449 walking legs ; the first mesosomatic segment carries the genital oper- culum, 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. 332. Scorpio swaynmerdami, 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 basal joints of the pedipalps and the first two pairs of walking legs, Bi 29 450 THE INVERTEBRATA 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 pro- 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 pectines, 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 thicker 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. 328 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. 333). 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 cephalothorax, 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 together form a bulky mass, filling up the dorsal part of the meso- SCORPIONIDEA 451 CJ TO M 3 O «H Ui "H oj :; c =1^ ^ 3 s S ^- ^ I ^ g|. C3 fH bJD - g-^ u ^ «« a, 3 V- O •n n o " « -c .> cu • •- ^ « +j ~Ci CO Jp , cc • «^ ^ > T3 C/3 " S. "--J 1—1 ^-i ^ ■^ . .™ CO D w o bib "^ && 4-> <^ r^t ^1 g^.2 ro (S 2 55 2 rH Ph I— I O ^ cc -^ii w 29-2 452 THE INVERTEBRATA somatic body cavity. The food passes into the cavity of these to be digested. This food 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 end gut is marked by the Mal- pighian 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 Fab re 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 size of the chelicerae, and the telson, which in primitive eurypterids has a recurved sting-like form. Slimonia (Fig. 334 B) has a slightly modified telson. In one eurypterid (Glyptoscorpius) ARACHNIDA 453 structures have been described which correspond to the pectines in position and structure. If this is substantiated, it constitutes a remarkable resemblance in detail. A few special characters may be mentioned here. On the ventral Fig- 334- Diagrams 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); b. meta- stoma ; d. compound eyes ; e. simple eyes ; g.op. genital operculum ; tel. telson. 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 which carried them were membranous and the branchiae were tucked 454 THE INVERTEBRATA 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- scorpiiis). 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. 334 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 gnathohases 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. 335, 336), 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 arachnids. The succeeding four pairs of appendages are all alike in ARACHNIDA 455 structure and function, consisting of six joints, the basal one being produced into a prominent spiny gnathohase ; they are chelate (except the adult males which are clawed). The last (sixth) pair of appendages has four spines springing from the end of the last joint but one ; while Fig. 335. Limulus Polyphemus , the king crab, dorsal view, X i. From Shipley and MacBride. i, carapace covering prosoma; 2, opisthosoma; 3, caudal spine; 4, median eye; 5, lateral eye. the four anterior legs are used for walking as well as masticating, this pair is particularly concerned with digging. They also possess an external spatulate process which is inserted under the operculum and cleans the gill books. 456 THE INVERTEBRATA The chilaria, as has been stated, are the appendages of the pre- genital segment. They are flattened processes without any function that has been discovered. The appendages of the opisthosoma are shown in ventral view Fig. 336. Limulus polyphemus, x J. Ventral view. Slightly altered from Shipley and MacBride. an. anus; chc. chelicerae; chi. chilaria; g.op, genital operculum (7th appendage) ; M. mouth, surrounded by gnathobases ; opi. opisthosoma; pp. pedipalps; tel. telson or caudal spine; 3-6, prosomatic appendages; 8-12, exopodites of opisthosomatic appendages. (Fig. 336), and vertical longitudinal section (Fig. 329). They are all greatly flattened and expanded , consisting typically of a slender ' ' endo- podite" and a broad plate which is the "exopodite". The anterior pair arise in the embryo as distinct rudiments, but fuse to form the genital operculum, on the under surface of which are the genital apertures. XIPHOSURA 457 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. 274) 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 very 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 animals are Hemiaspis (Fig. 334 C) and Bunodes. 458 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. 328 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. 339) 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 My gale. 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 pedipalps, 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. 337) 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 end 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 end 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 459 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. 339) and a suboesophageal complex supplying the rest of the body. Two non-ganglionated nerves pass backwards to the opisthosoma. The diagram (Fig. 337) shows a lung book opening in the anterior part of the opisthosoma and the details of the structure are exhibited in Fig. 330. 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 d.dig. pcm. gl-ag- Pl.t. I sp. ^ gl.ac. Fig. 337. Diagram of a spider, Epeira diademata, showing the arrangement of the internal organs, X about 8. From Warburton. an. anus; ar. artery; hrn. brain; chc. chelicera; C7n. caecum of stomach; 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; mg. mid gut; m.t. Malpighian tubule; o. ovary; p. gl. poison gland; pcm. peri- cardium ; sp. spinneret ;s.p. stercoral pocket of end gut is.st. sucking stomach ; V. vessel bringing blood from lung book to pericardium. narrow opening 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. 337) the tracheae are not shown, but in Fig. 330 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 spring 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. 460 THE INVERTEBRATA the number 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. 447). '7 i Fig- 339- Fig. 338. Fig, 338. 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. 339. Front view of head of Textrix denticulata. From Warburton. I, head; 2, eyes; 3, basal joint of chelicerae; 4, claw of chelicerae. 1.. Fig. 340. 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 organ 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 of the webs, and the spiral lines are made by the aggregate glands ARACHNIDA 461 which furnish the viscid fluid which covers them. The egg cocoon is formed by the tubuliform glands and these glands are absent in the males. The aciniform glands manufacture the cords which are wrapped round 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. 338). 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 ticks. They are, variously, scavengers, ectoparasites on all sorts of 462 THE INVERTEBRATA 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. 342), 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 like ^r^^^ (Figs. 341 A, 343 , 344), 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. 341. Dorsal surface of A, Argas (Argasidae) and B, Amblyomma, $ (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 by stigmata, the position of which varies in the main divisions of the ARACHNIDA 463 Fig. 342. Tyroglyphiis siro, seen from the ventral side. A, Female. B, Male. Magnified. From Leuckart and Nitsche. pp. pedipalps ; chc. chelicerae; 3) 4> 5> 6, first, second, third and fourth walking legs; rep.ap. reproductive opening, flanked by two suckers on each side ; an. anus ; su. suckers at side of anus. .chc.d. hyp. chc.s. ;r vp- Fig. 343. Ventral view of capitulum (false head) of Argas persicus, $. From Nuttall. b.cap. basis capituli; chc.d. digit, and chc.s. shaft of chelicera; hyp. hypostome; pp. four-jointed "palp" (pedipalp); ^.^., h.h. postpalpal hair, posthypostomal hair. 464 THE INVERTEBRATA 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 is a single pair of stigmata in varying positions , in front of the chelicerae Fig, 344. Argas persicus, (^. 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 (jn.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 ; ^/z. pharynx with radiating muscles; r.s. rectal sac; st. stomach; sal.d., sal.gl. salivary duct and gland ;f^. testis ;z;^.vas deferens ; w.gl. white (accessory) gland. (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), and behind the 4th legs (Metastigmata) If we regard the opistho- somatic position of the breathing organs as primitive it is difficult to ARACHNIDA 465 see how these varying arrangements have come to pass in the acarines. The Hfe history of the parasitic forms is of great interest, especially that of the ticks or Metastigmata. These are divided into the Ixodidae (Fig. 341 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 after the completion of the process as possible. 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 the 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- trusible process, which is an ovipositor in the female, a penis in the male. Bi 30 466 THE INVERTEBRATA The Notostigmata mentioned above (p. 464) 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. 345. A phalangid, Oligolophus spinosiis, adult (J, X 2. i, chelicerae; 2, pedipalps; 3, 4, 5, 6, walking legs. From Shipley and MacBride. These extraordinary animals, e.g. Nymphon (Fig. 346 A), are all marine and semisedentary, crawling slowly over seaweed and seden- tary animals. They consist of the following regions: (i) the proboscis ^ a prolongation of the prosoma with the mouth at the tip ; (2) four segments fused together bearing four eyes, the chelicerae, the pedi- palps, the ovigerous legs which are present in both sexes and the first pair of walking legs ; (3) three free segments bearing the remaining pairs of walking legs. The body is usually very small while the legs ARACHNIDA 467 are enormously elongated. They have eight joints. The proboscis con- tains a sucking pharynx preceded by a filter of chitinous hairs which prevents any but fluid food from proceeding further. The small stomach gives off digestive coeca which extend into the legs and other appendages. The common British form, Pycnogonum littorale, is found firmly attached by the terminal claws of the legs to the sides of sea anemones in which it inserts the proboscis and sucks the juices. There is a dorsal heart with three pairs of ostia; respiration is cutaneous. The nervous system consists of supraoesophageal ganglia and a ventral chain with suboesophageal and three or four other ganglia. The sexes are separate and the males carry the eggs on the oviger- ous legs. The gonads, like the alimentary canal, are branched and open on the 4th segment of the legs (the last pair of legs in Pycnogonum or all four pairs in Phoxichilidium femoratum) . In the latter species the larvae are hatched as six-legged creatures, which form cysts in the polyps of the gymnoblast hydroid, Coryne. Four small classes, Pseudoscorpionidea, Pedipalpi, Solifugae and Palpigradi, are undoubtedly arachnids, but can merely be men- tioned here. The two small classes following have been associated with the arachnids but no sufficient reason can be advanced for this. They both exhibit simplicity of structure ; in the case of the Pentastomida this is due to parasitism, but in the Tardigrada some of the traits of primitive arthropods may be preserved. In some ways they resemble Peripatus and their development is said to be of a very primitive type. But the size and specialized habitat incline the author to regard this as a case of *' simplification" such as is met with in the Archiannelida (p. 261). Class TARDIGRADA Minute arthropods with four pairs of stumpy legs ending in claws, with oral stylets and a suctorial pharynx, without definite circulatory or respiratory systems. Representatives of this group, e.g. Macrohiotus (Fig. 346 B), are found, for instance, in moss and in the sediment of rain gutters. They are minute and often very transparent animals, with a thin and flexible cuticle. The body is usually short and flattened; the tardigrades have been compared to the tortoises among the verte- brates, from their slow and awkward gait. The mouth opens into a tube in which work the two chitinous stylets; a suctorial pharynx, the wall of which is composed of radiating muscular fibres, follows. Into the pharynx opens a pair of salivary glands. The animals pierce the wall of plant cells with the stylets and suck the sap by the action of the pharynx. Then comes a narrow oesophagus leading into a 30-2 468 THE INVERTEBRATA capacious stomach, and lastly the rectum, which is joined by two short tubes which probably represent Malpighian tubules, and by the duct of the gonad. The perivisceral cavity contains no connective tissue cells but is crowded with numerous rounded corpuscles and traversed by bands of longitudinal muscle. The nature of the cavity is not known but the existing account of the embryology describes pairs of coelomic Fig, 346. A, Ar3^m/)^ow,an exampleof the Pantopoda. After Mobius. jp^. pro- boscis with mouth; chc. chelicera; /)^. pedipalp; ol. ovigerous leg; i, 2, 3, first three ambulatory legs; ops. opisthosoma bearing, 4, last pair of ambu- latory legs. Fusion of first four segments indicated by stippling. B, Macro- biotus, ?, dorsal view. Modified from GreeflF. cl. cloaca; cor. corpuscles in body cavity; gl. dorsal accessory gland; M. mouth; m.t. Malpighian tubule; od. oviduct; oe. oesophagus; o. ovary; ph. suctorial pharynx; sa.gl. salivary glands; st. stomach; sty. stylets. pouches arising as outgrowths of the archenteron,- as in the echino- derms. The legs resemble the appendages of Peripatus and each is termin- ated by two forked claws. The last pair are terminal and the anus opens between them. The nervous system consists of suprapharyngeal, subpharyngeal and four pairs of trunk ganglia, the latter correspond- ing to the appendages. Physiologically they are interesting in their capacity for resisting ARACHNIDA 469 desiccation. Like the rotifers and nematodes with which they are associated in habitat they shrivel up with loss of water, absorbing it again and returning to life at the next rain. Class PENTASTOMIDA Elongated vermiform parasites with a secondary annulation and two pairs of claws at the sides of the mouth; without respiratory or circulatory systems. The commonest example, Linguatula taenioides, lives in the nasal passages of carnivorous mammals; the larvae, in which the claws of ale. an.--- Fig. 347. A, Demodex folliculoru?n, ventral view. After Blanchard. Amite living in the hair follicles of man and domestic animals. B, Li7iguatiila 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 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. 347 A) and it is for this reason this group has been classed with the arachnids. CHAPTER XVI THE PHYLUM MOLLUSCA Unsegmented coelomate animals with a head (usually well developed), a ventral muscular foot 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 «^:f?- vm. pa^ pcd.g. Fig. 348. 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. Note the general resemblance especially between the ventral nerve cord of A and the ventral ganglia and cords of B. 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. hepatopancreas ; with a blood system consisting of a heart, 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 circumoesophageal ring, often concentrated into cerebral and pleural ganglia, pedal cords or ganglia and visceral loops; coelom, varying in development, but always represented by t\iQ 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 MOLLUSCA 471 possessed the molluscan characters given in the definition above and 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, the common apertures of the kidneys and the gonads, and which also contained the gxpe. pcd. ,.g„ dig.gl '- ' ce.g. Fig. 349. 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; g.coe. genital coelom; k. kidney; k.op. kidney opening; M. mouth; ma. mantle; 77ia.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 (pallio visceral in B) commissure 'ysh.p. shell plates ; st. stomach ; ven. ventricle ; vis.g. visceral ganglia. ctenidia. Internally there was a gut with a radula, a heart with a median ventricle, two auricles and a pericardial cavity into which the two kidneys opened. From this beginning diverged the different groups which we know to-day. The chitons (Amphineura), which have departed least from the ancestral structure, became elongated but limpet-like forms (Fig. 349 B), their visceral hump being protected by 472 THE INVERTEBRATA 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. 349 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. 349 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 creeping character 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. 377 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. 350 B), but all extension takes place by secretion at its edge (Fig. 353). The outer shell layer, periostracum, formed of horny conchiolin, is first produced in a groove and then the prismatic 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 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 MOLLUSCA 473 a fashion identical with that described for the anneUd. In the diagram given here for Patella , we see the completion of gastrulation and the ap- pearance of the ciliated rings of the trochosphere(Fig. 350 A) ; also the ap.o. Fig. 350. 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. embryonic muscle CQ\\s;prt. prototroch; sh. shell; st. stomach; t.t. telotroch. -dig.gl. sh. Fig. 351. Veliger larvae. Ay 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.y 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. single large cell which gives rise to the mesoderm. Then in Fig. 350 B we see the early veliger with an internal organization similar to the annelid, with apical organ, larval nephridia ^nd prototroch. The 474 THE INVERTEBRATA figure shows, however, organs which are not present in the annehd. 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 promi- nence which is t\iQ 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 segmenting as in the annelid, these largely break up into single cells, some elongating and becoming muscle cells (Fig. 350 C). It is because there is never any commence- ment of segmentation in the embryonic mesoderm in molluscs that we have the strongest grounds for believing that molluscs never had segmented ancestors. The trochosphere is followed by a second free- swimming stage, the veliger (Fig. 351), in which the prototroch de- velops, with the postoral ciliated ring or metatroch, 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 MoUusca 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. 373) 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. Amphineura with flat foot which 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 row of ctenidium-like gills on each side, e.g. Chiton (Figs. 349 B, 352), Craspedochilus. Aplacophora. Worm-like Amphineura in which the foot has been greatly reduced to a median ventral ridge and the mantle corre- spondingly enlarged. Mantle cavity reduced to a small cloacal chamber at the posterior end, gills present or absent, e.g. Neomenia. Craspedochilus is a small mollusc found underneath stones be- tween tidemarks. It looks like an elongated limpet and has exactly MOLLUSCA 475 the same habits, browsing on small algae and returning after ex- cursions 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 wood louse. Each plate is composed of two layers, the upper or tegmentum and lower or articulamentum. Both are nac. prS7tiJ yCx.ma. ov. ma- Fig. 352. prsm: -^prst: Fig. 353- Fig. 352. 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; pern, pericardium. Fig. 353. Vertical section through the edge of the mantle of Mytilus. ex. ma. external, in.ma. internal surface of mantle ; gl. 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. calcareous, but the tegmentum is traversed by parallel canals which end on the surface in remarkable sense organs ; some of these have the structure of eyes. There is little evidence, however, that they are sensitive to light. Others are sense organs of quite unknown function 476 THE INVERTEBRATA (the aesthetes). 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 varying number of branchial organs, each of which resembles a ctenidium. 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 fonvard. 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. 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- ance. Their cadula may be greatly reduced or even absent. 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 pleurovisceral loop ; a radula ; often a trochosphere larva. We can safely say that the Gasteropoda are descended from sym- metrical unsegmented ancestors (p. 471), 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 GASTEROPODA 477 possessed a symmetrical body with a straight aUmentary 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 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. 354) 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. 354 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 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. 355). 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 478 THE INVERTEBRATA completing a figure of eight. The whole process takes only two or three minutes in Acmaea so that it can hardly be brought about by differential growth. Muscular contractions must play their part. The large majority of gasteropods belong to the order which exhibits 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. 356 A-C), and a more specialized one in which the right (primi- tive left) gill, its auricle and even the right kidney have disappeared Fig. 354. 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 180° of torsion and the adult condition. an. anus; m.c. mantle cavity; op. operculum; vm. velum. (Mowo^ocflr^/fl, represented by Littorina, the periwinkle, and Buccinum, the whelk) (Fig. 356 D). Some of the Diotocardia, like TrochuSy are in an intermediate state in which, though the right gill has disappeared, there is still a rudiment of the corresponding auricle. Besides this fundamental difference, there are others. For example, in the Mono- tocardia, 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 GASTEROPODA 479 of gasteropods called the Opisthobranchiata which show that the changes occurring in torsion are to a certain extent reversible. They have the ctenidium pointing backwards, the auricle behind the ventricle and the visceral loop untwisted and symmetrical. There are some forms (BuUomorpha (Fig. 363 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 Fig. 355. Diagram to illustrate torsion, when seen from above. A, Ancestral gasteropod. B, 90° of torsion. C, Torsion completed (i8o°). After Naef. an. anus ; au. auricle ; ce.g. cerebral ganglion ; M. mouth; ma.c. mantle cavity; pa.g. parietal, ped.g. pedal, pl.g. pleural, vis.g. visceral ganglia. untwisted (Aplysia (Fig. 364 A)). The Opisthobranchiata, it is plainly seen, are derived from the Monotocardia amongst the Streptoneura. They have only a single ctenidium, a single auricle and a single kidney. They have not attained to complete bilateral symmetry, because the mantle cavity is still on the right side where yet present (tecti- branchs), 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. 364 C) their de- scent is shown by the fact that they possess a veliger larva with a 480 THE INVERTEBRATA coiled visceral hump which undergoes torsion (which reverses later). The adult shows evidence of streptoneurous ancestry in the presence Fig. 356. 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, Buccinwn, 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. proboscis; rhyn. rhynchocoel; siph. siphon; ten. tentacles; vd. vas deferens; ven. ventricle; int. intestine; ^op. male aperture; rm. rectum. of the anus at the right-hand side. In Doris (Fig. 364 B) the anus is median, but the genital aperture is still situated on the right side. GASTEROPODA 481 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. 363 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, the order Pulmonata to which Helix belongs is the most specialized 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 gasteropod 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. 357). 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. 141). The gliding movement of a snail indeed resembles that of a turbellarian, and we actually find that in some marine gasteropods Bi 31 482 THE INVERTEBRATA the surface of the foot is clothed with ciUa, 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 contraction. 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 . 3 84 B ) at their tip . Both are hollow and have attached to the inside of the tip a muscle whose contraction 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 round hole on the right side which is the aperture of the mantle cavity or pneumo- stome. In the marine gasteropods the mantle cavity has a wide open- ing 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 re- striction of the respiratory aperture is one of the necessary modifica- tions. 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 towards the auricle. The floor of the cavity is arched and has a layer of muscles, which con- tract 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 decreases 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, the collar is cut 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 GASTEROPODA 483 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 difference 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 ped.art haem. Fig. 357. 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 netw^ork 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. aflferent veins; ao. aorta; art.cap. arterial capillaries; au. auricle; hue. 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. shown by the connection of the cavities by the renopericar dial canal. The coelom, though thus represented, does not constitute the perivisceral cavity. On cutting the floor of the mantle cavity and continuing 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 31-2 484 THE INVERTEBRATA described here as follows : the ventricle pumps arterial blood through a single aorta which soon divides into an anterior aorta supplying the buccal cavity and a posterior which supplies the visceral hump. The terminal branches of these arteries eventually communicate with the haemocoele (dotted in Fig. 357) and in its turn this discharges into the cir cuius venosus leading to the lung and heart. Fig. 358. Helix pomatia. A, Section of alveolus of the digestive gland. ab.c. absorptive cells; calx, 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; buc.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. The alimentary canal (Fig. 358) 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 teeth. It is formed in a ventral diverticulum of the buccal cavity called the radula sac (Fig. 359) in which proliferating tissue is constantly GASTEROPODA 485 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. v.y. p.g. m.r. Fig- 359- Vertical longitudinal section through head of Helix. After Meisen- heimer. cart.r. cartilaginous support of radula; ce.g. cerebral ganglion; y. jaw; M. month.', m.r. muscles of radula; odp. odontophore (radula sac); oe. oesophagus ; /j.gf. pedal ganglion; rad. radula; v.g. visceral ganglion. Fig. 360. 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, bm., 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 forward in the direction of the arrow and themselves reinforce the basal epithelium. The buccal cavity is succeeded by the oesophagus, which widens out into the crop, which in life contains a brown liquid secreted by the "liver". 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 by the stomach; this is imbedded in the digestive gland (liver), which 486 THE INVERTEBRATA 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 alveoli. The alveoli contain cells of three kinds, secretory, resorptive and lime-containing (Fig. 358 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 digested 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 an intra- cellular proteolytic ferment. A combination of extra- and intracellular digestion is highly characteristic of Mollusca, but in the possession of a cellulose-dissolving ferment Helix stands almost alone in the Animal Kingdom, and may be indeed said to be physiologically adapted to a plant diet. 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. 361VA), 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 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 herinaphrodite duct, the terminal portion of which is a pouch (receptaculum semifiis) 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 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 spermatozoa are compacted together 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 GASTEROPODA 487 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 o.d. sph. sjH.d. Fig. 361. 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 ; retp. retractor muscle of the penis; sph. spermatophore ; spt. spermatheca ; spt.d. spermathecal duct ; sp.d. sperm duct ; ^ gl. hermaphrodite gland (ovotestis), and ^ d. duct. 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. 361 B). The following account of further events has been given and shows, as in the earthworm, the remarkable complexity of the arrange- ments 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 488 THE INVERTEBRATA covering of the spermatophore is dissolved, and the spermatozoa set free. These now retrace their path to the junction with the female duct and then up that duct to the fertilization pouch. Fertilization 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 wgr...^ lat. cent. K!.^^ Fig. 362. 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, ??ig. marginal teeth. 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 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 develop- ment. 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 GASTEROPODA 489 the aperture upwards. The head and foot are withdrawn into the shell 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 mostly composed of Ca3(P04)2 {epiphragma). 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. 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 (pectinib ranch), 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. Buccinum^ the whelk. Purpura feeds largely on barnacles. Nassa. 49° THE INVERTEBRATA Taenioglossa : radula normally with seven teeth in each row. Natica feeds on shell fish. Littorina, the periwinkle, amphibious. S tr ombus pvogressts by leaping. Paludina 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. 362 D). ^uhordev DIOTOCARDIA Haliotis, the ormer (Figs. 356 C, 365 A), is a greatly flattened gasteropod 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), 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 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. 356 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. 356 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 related Acmaeidae there are various stages of the loss of the ctenidia and their replacement by pallial gills. The enormously elongated STREPTONEURA 491 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. Fig. 363. To illustrate origin of euthyneury in the Pulmonata, A, B, and the Opisthobranchiata, C, D. After Naef. A, Chilina. The left parietal ganglion (l.pa.g.) has moved forward owing to the shortening of its pleural 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; l.pa.g. left, r.pa.g. right parietal ganglion; ma.c. mantle cavity; v. ventricle. Suborder MONOTOCARDIA Buccinum, the whelk (Fig. 356 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 492 THE INVERTEBRATA 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 bipectinate osphradium, which is easily mistaken for a ctenidium. There is a 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. Littorina, the periwinkle, is interesting because it exhibits tendencies 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 unlike that in Helix and other pulmonates. Littorina rudis lives almost at high- water 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 not a proboscis and is not carnivorous. The radula is taenioglossate in type. 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) 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 pericardium com- pressed 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 OPI STHOBRANCH I 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. Fig. 364. 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 hep.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 seen through the transparent tissues and the direction of the ciliated currents on the epipodia indicated by arrows. After Yonge. Other letters: an. anus; ct. ctenidium; cer. cerata; ce.g. cerebral ganglion; epi. epipodia; ^ar. parapodia; ^ op. genital aperture; op.s. opening of shell sac; ^. penis; pa.g. parietal, pl.g. pleural, ped.g. pedal ganglia; sem.gr. seminal groove; ten. tentacles; vis.g. visceral ganglion; al. alimentary .canal ; au. auricle; M. mouth; ven. ventricle. 494 THE INVERTEBRATA The opisthobranchs are classified as follows : Tectibranchiata. Opisthobranchiata which often have a shell and nearly always a mantle cavity and ctenidium. Actaeon, Bulla^ Aplysia, Cavolinia. NuDiBRANCHiATA. Opisthobranchiata usually of slug-like habit which have neither a shell, nor a mantle cavity, nor a ctenidium. Eolis^ Doris. Aplysia (Fig. 364 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 possible. 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. Cavolinia (Fig. 364 D) is an example of the Pteropoda (sea butter- flies), a special group of the Opisthobranchiata which are modified for pelagic life. They 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 movement 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. Eolis (Fig. 364 C) is a nudib ranch which possesses a series of dorsal processes (the cerata), which contain diverticula of the digestive gland, each of which 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 GASTEROPODA 495 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 offensive 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. 364 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. 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 (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^ Avion, Testacella. A few pulmonates 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 496 THE INVERTEBRATA and by shortening of the visceral commissures in the Pulmonata. The important characters of the Pulmonata are those associated with the assumption of the terrestrial habit, namely the existence of the lung and the physiological characters correlated therewith. So strongly impressed are these that in 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. 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. 365 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 kind of proboscis. When the worm is swal- 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. In other respects the organization of the slugs is very similar to that of snails. The details of reproduction and development are very similar throughout 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 egg. Class SCAPHOPODA Bilaterally symmetrical MoUusca 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- MOLLUSCA 497 os, B L/ V) c Fig. 370. 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 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. 367, 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. 370 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) 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- LAMELLIBRANCHIATA 505 sions. Even in the Protobranchiata, the cihary 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 Mytilus, the common mussel (Fig. 371). Here the p. a Plicate canals Fig. 371. Diagram of the circulation in Mytilus to show the greater import- ance of the part of the system in the mantle and plicate canals. Of the blood re- turned from the viscera a much smaller proportion is sent through the ctenidia. Slightly altered from Field, Au. auricle; bl. bladder of kidney opening into the pericardium; aff.c.v. afferent, ejf.c.v. efferent ctenidial vein; l.v. longi- tudinal vein of kidney ; ^.ar. pallial artery; pc. pericardium; v. ventricle with rectum, represented by a dotted line, passing through it. 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 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 5o6 THE INVERTEBRATA 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 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 Viscera A ^^-^ ^ B Ctenidia Fig. 372. 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, ^., longitudinal afferent vessels, Iv/, and thence to the afferent system of vessels in the ctenidium, aff.c.v. On the other side the efferent system of vessels, eff.c.v., is shown returning blood to the longi- tudinal 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; ven. ventricle. the ribbon-like organs, known as plicate canals, which extend along the mantle just above the insertion of the outer ctenidium. 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 ctenidial circulation, discharging into the longitudinal afferent branchial vein, which gives off to each filament a vessel which LAMELLIBRANCHIATA 507 descends one side and ascends the other. The ascending vessels join to form a longitudinal efferent vessel, which discharges into the longi- 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. 372) it is more developed. While the Protobranchiata have a nervous system with four distinct pairs of ganglia (Fig. 349 D) in the remainder of the class the number is reduced to three by the fusion of the cerebral and pleural ganglia (Fig- 373 B). The sexes are usually separate in the Lamellibranchiata, but some species of Ostrea and Pecten are always hermaphrodite, while this con- dition is frequent in Anodonta. In the Protobranchiata the gonad dis- charges into the kidney, but in most forms there is a separate genera- tive aperture. While most marine forms and the heshwater 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. 349 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 lahial 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 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 508 THE INVERTEBRATA , condition. In this respect the Protobranchiata can hardly be held to resemble the ancestral lamellib ranch. Order F I LI BRANCH I ATA Mytilus (Fig. 368). 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 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 --^ ^ce.c. huc.c. ^b.rad.c. -p.v.c. -ped.n. \-ce.plg. ped.g. -pal.n. /-ms.flf. Fig. 373. Nervous system of A, Chiton, B, a lamellibranch. Dorsal views. The outline of the mantle edge is indicated by a dotted line, hiic.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; sh.rad.c. subradula commissure; vis.g. visceral ganglion. 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 from the byssus 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, as can be easily LAMELLIBRANCHIATA 509 seen in the laboratory. While the development of the byssus is the most outstanding characteristic of the mussel, it may also be men- tioned that a pair of simple eyes are developed, anterior 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. Pecten (Fig. 374). There are two common British species, P. maximus and P. opercularisy which are commonly known under the Fig. 374. Pecten maximus, general anatomy, right valve and ctenidium re- moved. After Dakin. add.u. unstriped and add.s. striped adductor muscle; an. anus; an. 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. name of "scallops". The animal is found free and it moves not by 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 510 THE INVERTEBRATA border fonvard; 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. The foot is very much reduced, but it has nevertheless a ciistinct 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 oi 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 interlamellar septum. In one species, Pecten tenuicostatus, 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. 384 D), of a very complicated structure, at regular intervals all round the mantle. Order EULAMELLIBRANCHIATA Ostrea (Fig. 375). 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 alter- nately as males and females. Spawning tends to take place at full moon as in some echinoderms. Another point of physiological im- portance 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. 351 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. LAMELLIBRANCHIATA 511 Teredo (Fig. 376) is the most specialized of the boring lamelli- branchs. While most lamellibranchs burrow in mud, others tend to work in consolidated sediments, Pholas in chalk and sandstone, and Saxicava in the hardest limestone. But Teredo and Xylophaga only bore in wood. The latter makes shallow pits, but Teredo, working with extraordinary 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 constant rhythmical contraction. The saw- ea:.c. -int. d.h.c- Fig- 375- 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. dust is swallowed by the animal and is largely retained in a relatively 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, hemicellulose and probably lignin. 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 512 THE INVERTEBRATA the mouth of the tube when the siphons are retracted. The foot is very much reduced. A constant current into and out of the mantle cavity is maintained by ciUary 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. ^•^- dUjl ct'. int. cm. Fig. 376. 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; inh.c. inhalant current; pcd. pericardium; pit. palette; sh. left valve of shell; ven. ventricle. Original. 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 funnel or siphon^ 2i 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. CEPHALOPODA 513 The Cephalopoda fall into two groups, in one of which (Tetra- branchiata) there are two pairs of ctenidia and a well-developed external shell, while 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 w411 best be under- stood 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 some 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 with 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 Myopsida. Decapoda with specially modified 4th tentacles; eyes with a cornea, internal shell sometimes with rudiments of calcareous chambers. Spirula, Sepia^ Sepiola, Loligo. (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 with phosphorescent organs; some gigantic forms, like Archi- teuthis, 60 feet long. 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. 377 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 brn. oe- dig-gl- sh. rad. \ ^ Fig. 377. 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; y. jaws; i.s. ink sac; k. kidney; Ip. lips; mt. mantle; mt.' dorsal extension in Nautilus; 0. 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 515 the sides. Round the mouth are the large and mobile tentacles (arms) for seizing prey which are often considered to be part of the foot. At one side, generally called posterior, 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, discussion as to whether the tentacles are part of the head or the foot is difficult and unimportant. The shell has become internal and is a rather substantial plate which acts as an endoskeleton. The absence of a rigid 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- 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 backwards. 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 is to a certain extent a response to the character of the background. 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. It is proposed to call these surfaces dorsal and ventral respectively. 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. 378, 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 33-2 i6 THE INVERTEBRATA Fig. 378. 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 ; ^i. fin ; g.pap. genital papilla ; 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. 377. From Shipley and MacBride. CEPHALOPODA 517 anterior end of the visceral hump is seen the central anus at the end of a long papilla, the shorter renal papillae immediately on each side, and on the left side only Xh.^ 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 -abd.v. Fig. 379. Sepia offici?ialis. Dissection from the ventral side to show kidneys and blood vessels. Arrowsshowthedirectionof flow of blood, abd. z;. abdominal vein; a.ao. anterior aorta; au. auricle; a_ff.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. 377, 378. Original. nidamental glands of the female which occupy a considerable area. Between these and in front of them is the accessory nidamental gland. 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 5l8 THE INVERTEBRATA the aminoacid tyrosin by the agency of an enzyme, tyrosinase. This substance is ejected into the mantle cavity and through the funnel in moments of excitement. 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 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 Fig. 380. Vertical section of 6'epia officinalisto showthe relation of the divisions of the coelom. After Grobben. dig.gl. digestive gland ("liver"); fu. funnel ; g.coe. genital coelom ; h. heart ; i.s. ink sac ; k.pap. external opening of kidney; k.t. excretory tissue; nid. nidamental gland; o. ova; pan. "pan- creatic " tissue surrounding the duct of the digestive gland ; pcd. pericardium ; r.p.a. opening into the kidney of the renopericardial canal, r.p.c; sh. shell; St. stomach. canal running in the outer wall of the kidney and opening posteriorly into the pericardium, a wide space lying behind the kidneys which is only separated by an incomplete partition from the still more spacious genital coelom occupying the apex of the visceral hump (Fig. 380). 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 a posterior aorta ; venous blood returns to the heart from the head by a very important vessel, the vena cava, which splits in the CEPHALOPODA 519 kidney region into two branchial veins, which run to the so-called branchial hearts, special muscular dilatations which pump blood through the capillaries of the ctenidia. The blood which is oxygenated there is sucked out of the ctenidium by the expansion of the auricle. Blood is also returned directly to the branchial heart from the mantle by the abdominal veins (and a smaller pair), and by the unpaired genital and ink sac veins which run first into the right branchial vein. In describing the alimentary system it must first be mentioned that Sepia, as a type of the Decapoda, possesses ten tentacles of which the fourth pair are longer than the others. These two tentacles have a slender stem and a swollen terminal portion to which the suckers are p.saLfjL ot. ! ped.if. \ fu'.n. bru.g. Fig. 381. 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.n. nerve to funnel (fu.) coming off from pedal ganglion; y. beaks ; ma.n. mantle nerve ; oe. oesophagus ; ot. otocyst ; ped.g. pedal ganglion ; p.saLd., p.sal.gl. posterior salivary duct and gland; rad. radula; s.buc.g., i.buc.g. superior and inferior buccal ganglia ; vis.g. visceral ganglion. Original. confined. Each tentacle can be rapidly extended and attached to the living prey, and with equal rapidity retracted into a pit near the mouth, thus bringing the food into the reach of the other tentacles, which hold it while it is being devoured. Round the mouth are frilled lips and just within it are the characteristic beaks, corresponding to the jaws of the gasteropod, which bite upon each other. The buccal mass is large and contains a well-developed radula and is traversed by the narrow oesophagus. Just behind the buccal mass is the first pair of salivary glands and immediately in front of the digestive gland is the second pair, which produce not a digestive juice but a poison. In Octopus, vfhich lives largely upon crabs, the prey is seized and bitten by the beaks, a drop of the poisonous saliva entering at the same time by the punctures in the carapace and causing almost immediate death. 520 THE INVERTEBRATA The oesophagus is very narrow, but capable of distension as large masses of food are passed through it into the muscular stomach with its spiral caecum in which is a very elaborate ciliary sorting mechanism. Here the food is mixed with the secretion of the digestive gland ("liver"), a bilobed gland of solid appearance. The two ducts are covered by tissue resembling the excretory epithelium of the kidney, which was formerly termed the "pancreas ". It is now certain that this tissue is excretory and that the main part of digestion is performed by the juices of the digestive gland. The cephalopods differ from the majority of invertebrates in that the semidigested food does not penetrate into the cavity of the digestive glands but digestion is completed in the stomach. The whole of the alimentary canal is comparatively short and simple as might be expected from the carnivorous diet of the animal. Fig. 382. 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 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. 382, 383) 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. A "skull" has been developed for the protection of this large nervous mass composed of a tissue very similar to cartilage, which also forms the supports of the fins and tentacles. The brain consists of the following ganglia : dorsally the cerebral or supraoesophageal, ventrally (i) tht 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 CEPHALOPODA 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 for such complicated actions as feeding, swimming and creeping. In the -yten.n. Fig- 383. Nervous system of Sepia. After Hillig. bra.g. brachial ganglion; br.g. branchial ganglion and nerve; ce.g. cerebral ganglion; gas.g. gastric ganglion; ma.n. mantle nerve; olf.n. olfactory pit and nerve; op.g. optic ganglion; s.buc.g. superior buccal ganglion; st.g. stellate ganglion; syni.n. sympathetic nerve; ten.n. tentacular nerves; vis.g., vis.n. visceral ganglion and nerve. 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 522 THE INVERTEBRATA mantle, while in the brachial ganglia there are separate centres for gripping by the suckers and for letting go. From the cerebral ganglia there run forward a pair of nerves which end at the border of the buccal mass in a pair of superior buccal ganglia ; a circumoesophageal commissure links up these with the inferior 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 visceral loop which sends off branches to the gills and at its posterior limit bears the gastric ganglion between the stomach and the caecum. The infundibular ganglion gives off a pair of nerves to the funnel and the brachial ganglia a separate nerve to each arm which carries a ganglion on its course. 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. 381. Afterwards the dissection of the nerves coming away from the brain can be carried out. Sepia possesses very large eyes, 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 position of the lens, increasing the pressure of the vitreous humour and so pushing the lens forward (Fig. 384 C). 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. Their surface of contact with the egg is in- creased by folding. When ripe the ova escape into the genital coelom and pass into the genital duct. This has a terminal glandular enlarge- ment and there are also the nidamental glands, unconnected 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 CEPHALOPODA 523 into the funnel and then on to one of the tentacles (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 tentacle, but in other forms it is profoundly modified. In Octopus, the end of the tentacle pig.ep. op.n. Fig. 384. 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., 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. is spoon-shaped and the tentacle is extended so as to enter the mantle cavity of the female. In other octopods, a cyst is formed at the end of the tentacle in which the spermatophores are stored ; from it a long filament is protruded. Other Dibranchiata. The members of this group are classified in 524 THE INVERTEBRATA 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 extremely numerous fossil group, the Belemnitidae (Fig. 385 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 Fig. 385 . Series of Cephalopoda to illustrate the evolution of the internal shell. After Naef. A, Orthoceras, Palaeozoic. B,Belemmtes, Mesozoic. C, Spiridi- rostra, Tertiary. C, Spinila and D, Sepia, living. (D', enlargement of posterior end of D.) The reflection of the mantle over the shell is indicated by a dotted line. This is incomplete in Orthoceras, but the shell is com- pletely internal in the rest. cr^. guard ; phr. phragmocone ; prst. proostracum ; Sep. septa; sip. siphuncle. anterior plate, the proostracum. It may well have been derived from a nautiloid form like Orthoceras (Fig. 385 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. 385 C), 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. 385 C). Finally, in Sepia (Fig. 385 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, CEPHALOPODA 525 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, 385 D'). Thesiphuncle (p. 526) is a short wide funnel in between. In Loligo there is only a horny petty which represents the pro- ostracum, 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 the water that they take off from the surface and glide for considerable distances through the air, in the manner of the flying fish, aided by their spreading fins {Todarodes Sagittarius). Loligo (Fig. 386 B) 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. 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. 386 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 Opisthoteiithis , 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 TETR ABRANCHI ATA Cephalopoda with well-developed calcareous shells. Living forms with two pairs of ctenidia and kidneys ; tentacles 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. Phylloceras, 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, iht protoconch, in this position. Succeeding this are the numerous chambers, separated from each other by the curved 526 THE INVERTEBRATA 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 Fig. 386. External appearance of A, Octopus, B, a squid (Loltgo), trate the difference in manner of life. /. funnel. to illus- 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 CEPHALOPODA 527 compared with those in Sepia (Fig. 377). The shell coils forward over on the neck of the animal (exogastric) ; the mantle cavity is posterior as in all cephalopods. In other words differential growth of the visceral hump is not here associated with torsion. The mantle is thin and ad- heres to the shell ; it cannot therefore be associated with the respiratory and locomotory movements. The " head foot " is produced into two circles of tentacles which are very numerous ; they are retractile and adhesive but have no suckers. The anterior part of the region where it touches the shell is very much thickened to form the hood, 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 2iVQ 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 ''seg- mentation" may, however, be secondary. Certainly the absence of a renopericardial 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. 384 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 tentacles like Octopus (Fig. 387). The gentler swimming movements 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 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. 528 THE INVERTEBRATA The Tetrabranchiata are divided into two groups, the nautiloids and the ammonoids. The first of these contains Nautilus and other forms which agree with it in the position of the siphuncle and the shape of the septum. They reach their maximum development in the early Palaeozoic, where the dominant forms have straight shells like Orthoceras and Actinoceras, which were sometimes as much as 8 feet long. It is difficult to suppose that shelled animals of this size were anything other than sedentary organisms. There is a tendency for the shell to become coiled in later forms, exhibiting itself first in o.Un. Fig. 387. Fig. 388. Fig. 387. Nautilus macromphalus adhering to the substratum by means of its tentacles in a vertical position. It usually lies horizontally. After Willey. The shell shows alternate light and dark bands which resemble "ripple- marking", d.fn. dorsal muscular attachments of the funnel ; e. eye ; hd. hood ; mt. mantle; o.ten. ophthalmic tentacles. Fig. 388. A, Phylloceras heterophyllum, from the Lias : a part of the shell has been removed to expose the sutures, x j. B, Suture line of Phylloceras heterophyllum, from the Lias : the arrow indicates the position of the siphuncle and points towards the aperture of the shell. From Woodward. Natural size. slightly curved forms like Cyrtoceras, then in loosely coiled forms like Gyroceras and finally in the closely coiled Nautilus. There is also the reverse tendency, and in Lituites the young shell is closely coiled but in adult life it straightens out completely. The ammonoids appeared first of all in the middle of the Palaeozoic but reached their zenith in the Mesozoic. From the beginning of the Trias onward new families, genera and species are ceaselessly evolved. CEPHALOPODA 529 These are differentiated by the shape and sculpture of the shell whorls, but particularly by the patterns of the suture line, that is, the junction line of the septum and the outer shell (Fig. 388). These patterns reach the greatest complexity. A great deal of interest attaches to the fact that in these characters the earlier formed chambers of an ammonoid individual usually differ from those of the adult shell (Figs. 388, 389, 390). There may, in fact, be several changes in the life of an individual and the succession of such changes has been recorded as evidence for tracing the descent of particular ammonoids. The most striking manifestation of the phenomenon is afforded by ammonoid Fig. 390. Fig. 389. Fig. 389. Baculites chicoensis Chalk. After Perrin Smith. Fig. 390. Suture lines of Baculites to show the variation in development at different ages, a, first, b, second and c, sixth suture lines of B. chicoensis; d, adult septum of B. capensis; E. external lobe; Es. external saddle; /. in- ternal lobe; Lj, Lg, first and second lateral lobes; S^, first lateral saddle; So, internal (dorsal) saddle, a-c after Perrin Smith, d after Spath. Stocks, particularly in the Cretaceous, in which the approach of extinction is heralded by "uncoiling" in various stages. In Scaphites the shell is coiled in youth but later straightens out and finally hooks back. In Baculites (Fig. 389) only the very earliest chambers form a coiled shell ; nearly the whole of the shell is straight. But the suture lines, though tending to become simplified, show the type of the family from which the uncoiled form is derived, and it is possible to show quite definitely that such genera as " Scaphites " and " Baculites " are not natural but polyphyletic ; both scaphoid and baculoid forms occur in different lines of descent. Bi 34 CHAPTER XVII THE MINOR COELOMATE PHYLA PHYLUM POLYZOA Coelomate unsegmented animals, always sedentary and nearly always colonial ; with a circumoral ring (lophophore) of ciliated tentacles , and a U-shaped alimentary canal; usually with a ciliated free-swimming larva; asexual reproduction by budding. The ordinary individuals in a colony of polyzoa^at first sight resemble hydroid polyps — in their general shape, size and circle of tentacles. Closer inspection shows that they are triploblastic animals with a perivisceral cavity. Each individual consists of two distinct parts, the zooecium or body wall and the polypide, con- sisting of the alimentary canal, the tentacles and the tentacle sheath (which contains the tentacles when contracted). The polypide can be entirely retracted within the zooecium and, as will be seen below, has a much shorter life than the latter. In the form chosen for illustration, Plumatella (Fig. 391), the lophophore is not a simple circle, as is often the case, but is horseshoe- shaped. A small ridge, the epistome, overhangs the mouth in this genus but not in all polyzoa. The mouth opens into the oesophagus which passes into a capacious stomach with a caecum which is attached by a strand of mesoderm, xh^ funiculus , to the body wall. From the upper end of the stomach, the intestine runs to the anus which is situated just outside the lophophore. The food, consisting of small organisms like diatoms and protozoa, is collected by the cilia of the lophophore and transported through the whole of the alimentary canal by cilia. The body cavity is a true coelom containing a colourless fluid, and the cells which line it give rise to the germ cells. Polyzoa are herma- phrodite ; the testes are formed on the funiculus and the ovary on the body wall. When the germ cells are ripe the so-called intertentacular organ often appears ; this is a tube which opens within the lophophore and serves for the escape of the genital products. Part of the coelom is shut off from the rest by an incomplete septum, as the ring canal which is prolonged into the tentacles. The intertentacular organ opens internally into this. The nervous system is represented by a single ganglion, situated between the mouth and the anus, and many nerves chiefly supplying the tentacles and gut. There are no special sense organs. No trace of a vascular system exists. POLYZOA 531 A remarkable phenomenon very characteristic of the Polyzoa is the formation of the brown body. Tentacles, gut, in fact the whole of the polypide, degenerates and forms a brown, compact mass. A new poly- pide is regenerated from the zooecium and the brown body often comes to lie in the new stomach and is evacuated through the anus. m.retr. Stat Fig. 391. View of right half of P/z/wa^eZ/a/ww^o^a, slightly diagrammatic. After Allman and Nitsche. an. anus; bzo. body wall; ep. epistome;/n. funiculus; ga. ganglion; int. intestine; Iph. lophophore; M. mouth; m.retr. retractor muscles of polypide; oe. oesophagus ; ov. ovary; sp. spermatozoa; st. stomach; Stat, statoblast; t. testis; t.' the same, more mature; tb. wall of tube; ten. tentacles. This periodical renewal of the individual is a normal process in most polyzoa. It was formerly explained as due to the accumulation of excreta in the absence of specific excretory organs. It can, however, be hardly doubted that animals so small and with so great an area of 34-2 532 THE INVERTEBRATA epithelium in contact with the water are able to rid themselves of excreta in a simpler fashion. As triploblastic metazoa with a centralized nervous system the Polyzoa possess a more efficient contractile mechanism than the hydroids. The most prominent features of this are the parietal system of muscles which circle round the body wall. By their con- traction the internal pressure is raised and the lophophore extended. The retractor muscle which runs from the lophophore to the opposite end of the cell has an opposite action to the parietal system. The Polyzoa are fascinating but exasperating objects under the micro- scope : they emerge with infinite caution from the cell and withdraw with incredible rapidity. With the lophophore a flexible part of the body wall is also invaginated and this is called the tentacle sheath. The colonies of polyzoa differ greatly from those of hydrozoa in their habit and this is largely due to the absence of a connecting coenosarc. They are often incrusting like Membranipora and Flustra (hence the name of *' sea mats "), with all their cells packed closely to- gether in a single layer ; they may also be slender or massive ; in the latter case they have a superficial resemblance to the actinozoan corals. While the outer layer of the body wall is often horny or flexible it frequently becomes incrusted with calcium carbonate and thus rendered rigid. In the incrusting Polyzoa, especially the Cheilostomata, the zooecia are rigid boxes, in contact with one another along all four sides and with the substratum at the bottom. These are usually strongly calcified and only the top of the box, the frontal surface, is flexible (Fig. 392 A, B). The parietal muscles, which in primitive polyzoa formed a continuous layer of circular muscles as in Chaetopoda, here form detached groups running from the side walls through the coelom to the frontal surface. When the muscles contract the latter is de- pressed and the lophophore is protruded. The process of calcification may extend to the frontal membrane and the mechanism of protrusion has then to be altered. In one large group of the Cheilostomata, there is a membranous diverticulum of the ectoderm under the calcareous frontal surface. This is called the compensation sac (Fig. 392 C); to its lower surface the parietal muscles are attached. When they con- tract and the tentacles are extruded the sac fills with water, and when they relax the sac empties. Most of the Polyzoa are marine and' are amongst the most familiar objects of the beach. A complete division, the Phylactolaemata, are freshwater and it is one of these which is figured here (Fig. 391). The marine forms possess a variety of free-swimming larvae, which are of the trochosphere type. In the Phylactolaemata, certain internal buds called statoblasts are formed from lens-shaped masses of cells on the POLYZOA 533 funiculus and are enclosed by chitinous shells. The polypidesdie down during the winter and in the spring the statoblasts germinate and produce new colonies. Polymorphism is a feature of polyzoan as it is of hydrozoan colonies. Perhaps the most remarkable modifications are to be seen in the in- dividuals known as vibracula and avicularia of such forms as Bugula (Figs. 393 A, 394). The vibracula are nothing more than long bristles which are capable of movement and often act in concert throughout a part of the colony, sweeping backwards and forwards over the surface, preventing larvae and noxious material from settling on the colony. ten.s. ca.f.s. c.s. ten.s. ^.opc. p.m.- Fig. 392. Protrusion of the polypide in two types of cheilostomatous Polyzoa. Memhranipora. After Harmer. A, With polypide retracted. B, With polypide protruded. C, A form with a calcareous frontal wall. An. anus; ca. calcified cuticle of zooecium ; c.s. compensation sac ; f.s. frontal surface ; ga. ganglion ; int. intestine; oe. oesophagus; ope. operculum; m.retr. retractor muscle of polypide; ^.m. parietal muscles; st. stomach; ten.s. tentacular sheath. The avicularia resemble the head of a bird, possessing a movable mandible which is homologous with the operculum of an unmodified polyp, and this is provided with powerful muscles. The avicularia suddenly snap their jaws and enclose as in a vice small roving animals which touch them, particularly the larvae of incrusting animals. In the most primitive cases, an avicularium is found in the same position in the colony as an ordinary zooecium and may even possess a functional polypide. Further evolution led to displacement of the avicularia so that they became appendages of other zooecia, situated near the orifice. The two kinds of individuals thus perform 534 THE INVERTEBRATA tasks which in the Hydrozoa are allotted to the dactylozooids and in the Echinodermata to the pedicellariae. Many of the Polyzoa have free-swimming larvae which may be avc. Fig. 393. Polymorphism of Polyzoa. After Harmer, A,Bugula. Portion of a colony, avc. avicularium; ovc. ovicell; tentacles of ordinary individuals, ten. protruded, ten.' retracted. Crisia. B, Portion of colony with ovicell {ovc.), surface view. B', Section through ovicell to show emb.' primary embryo; emb." secondary embryos ;/o/. follicular tissue. Pig- 394- An avicularium of Bugula. Magnified. From Hincks. h. beak; C. chamber representing the body cavity of the modified individual; dm. muscle which opens, om. muscle which closes the mandible on the beak; md. mandible, the operculum of the modified cell; p. stalk. assigned to the " trochosphere " type. In most cases they are much modified and only the larvae of the Entoprocta and the Cvphonautes larva among the Ectoprocta possess an alimentary canal and are able POLYZOA 535 to feed. The Cyphonautes is, then, the typical form (Fig. 395). It •possesses a bivalve shell, each valve being triangular. The apical organ and ciliated ring (corresponding to the prototroch) can be seen pro- jecting from between the valves, and in addition there are various characteristic organs, such as the internal sac, by which attachment is effected, prior to metamorphosis, and the pyriform organ of unknown function. On attachment the alimentary canal degenerates and the first individual of the colony is formed by invagination froma polypide bud consisting of an internal layer of ectoderm and an external of Fig. 395. Cyphonautes larva seen A, in side view, B, in oral view. al.c. ali- mentary canal ; An. anus; ap.o. apical organ; ctl.r. ciliated ring; coe. coelom; i.s. internal sac; M. mouth; p.o. pyriform organ; ve. vestibule. mesoderm. The ectoderm gives rise to the tentacles and tentacle sheath, the ganglion and the alimentary canal of the new polypide. A polypide bud which develops in exactly the same way is formed in the course of regeneration after the formation of a brown body. In the Cyclostomata it is probable that the fertilized egg never de- velops into a single individual but always into a large number by what is known as embryonic fission, such as occurs in the parasitic Hymen- optera. A much modified zooecium, the so-called ovicell, serves as a brood pouch and in that the primary embryo is formed and attached 53^ THE INVERTEBRATA to follicular tissue which supplies nourishment. Masses of cells are nipped off to form the secondary embryos each of which becomes an aduh. Classification Class Endoprocta (Fig. 396). Simple and archaic polyzoa in which the anus is situated inside the lophophore ; without body cavity, the space between gut and body wall being filled with paren- chymatous tissue; with non-retractile tentacles which can be covered by a circular flap of the body wall, provided with a sphincter muscle; with a pair of nephridia ending in flame cells, and gonads with a duct of their own. Pedicellina^ Loxosoma. Fig, 396. Section through an endoproctous polyzoan. Altered from Ehlers. An. anus; at. atrium; ga. ganglion; g.gl. gonad; g.op. genital opening; int. intestine; M. mouth; mr. sphincter muscle of circular flap of body wall; oe. oesophagus; par. parenchyma; st. stomach. Class EcTOPROCTA. Polyzoa with anus outside the lophophore; with a coelomic body cavity and a lophophore retractile into a tentacle sheath; without definite excretory organs. Order Phylactolaemata. Freshwater Ectoprocta with a horseshoe- shaped lophophore, an epistome and statoblasts. Plumatella, Cristatella. POLYZOA, BRACHIOPODA 537 Order Gymnolaemata. Ectoprocta mostly marine, with a circular lophophore, without an epistome. Suborder Cyclostoniata with tubular zooecia, aperture without operculum, embryonic fission characteristic. Crisia. Suborder Cheilostomata, with aperture of zooecium closed by an operculum. Bugula, Flustra, Membranipora. Suborder Ctenostomata with aperture of zooecium closed by a folded membrane when the lophophore is retracted. Alcyonidium, It is, however, possible that the Endoprocta should be separated from the Ectoprocta as a distinct phylum. PHYLUM BRACHIOPODA Coelomate unsegmented animals with a bivalve shell which is always attached, the valves being respectively dorsal and ventral in position; a complex ciliated circumoral organ, the lophophore, which maintains a circulation of water in the mantle cavity and leads food currents to the mouth. The group contains only marine animals with a strong but super- ficial resemblance to the lamellibranchs among the Mollusca. In the Palaeozoic and Mesozoic it was more abundantly represented than the Mollusca, but at the present day it contains but few genera and species. Of the former Terehratula and Waldheimia are found in deep water off our own coasts and Crania occurs abundantly in shallow water in the West of Ireland. Lingula is not found in Britain, but in the tropics is sometimes exceedingly abundant in mud between tidemarks. ^ In such forms as Waldheimia and Terehratula (Figs. 397, 398), the ventral shell valve is larger than the dorsal and has a posterior beak or umbo perforated by a round aperture through which passes the stalk for attachment to a stone or rock. Each valve is secreted by a corresponding mantle flap, but in a way which differs from the corre- sponding process in the Mollusca. The mantle epithelium is produced into minute papillae which traverse the substance of the shell. The cells, of which the papillae are composed, are often of a minutely branching type which resemble the bone corpuscles of vertebrates. It must be supposed that the papillae are concerned with the secretion and growth of the shell. Each valve (Fig. 399) is composed of an outer layer of organic material (periostracum), under which is a thin layer of pure calcium carbonate and a thick inner prismatic layer composed mainly of calcareous but partly of organic material. The shell valves are opened and closed by a muscle system which is much more complicated than that of the lamellibranchs. ^'*''C\C A I ^ UjlLIBRARYJao 538 THE INVERTEBRATA m-d, stk. Fig. 397. Longitudinal section of Magellania (Waldheimia) slightly to the left of the middle line. After J. J. Lister, bzv. body wall; di.gl. digestive gland; h. heart; int. intestine; Ip. dorsal hp of Iph. lophophore; M. mouth; m.d. muscles running from dorsal valve to ventral; nphr. nephrostome; ri. vertical ridge on dorsal valve; st. stomach; stk. stalk; tn. tentacles of lophophore; tn.t. terminal tentacles. Fig. 398. Fig. 399. Fig. 398. Terehratula semiglohosa, Upper Chalk. A, Dorsal; B, Lateral view. a, posterior; b, anterior; a-h, length; c-d, breadth; e-f, thickness; ^^-A, hinge line, X f . From Woods. Fig. 399. Vertical section of shell of Magellania {Waldheimia) flavescens. a, prismatic layer; b, periostracum ; c, outer calcareous layer; d, e, canals traversing the calcareous layers, containing the mantle papillae in life. After King. BRACHIOPODA 539 The hinge hne is posterior and the mantle cavity is thus anterior. On opening the shells it is seen to be largely occupied by a compli- cated organ known as the lophophore. The mouth is placed in a trans- verse groove which is bounded, dorsally by a continuous lip and ventrally by a row of tentacles. The groove is enormously extended and its boundaries drawn out laterally into two arms which are often coiled spirally in these and other members of the phylum. The ten- tacles are long and may be protruded from the shell opening. The cilia on the tentacles and on the mantle surfaces produce two ingoing [^■-ped. Fig. 400. tentacles. Fig. 401. Crania attached to a stone in the act of feeding with protruded AA, ingoing, B, outgoing currents. After Orton. Fig. 401. Lingula in positions of life in mud (indicated by stippling). I, feeding position with peduncle (ped.) extended; 2, position when peduncle is contracted; ch. chaetae fringing entrance to shell. Arrows indicate cur- rents. currents of water at the sides opposite the two arms of the lophophore ; the outgoing current is central, between the two arms (Fig. 400). This ciliary mechanism is similar to that of the lamellibranch ctenidium. On each side the current of water is directed between the tentacles of the lophophore, and the smaller and lighter particles suspended in it are sieved away and pass into the ciliated buccal groove and so towards the mouth. Heavier particles drop on to the ventral mantle lobe and are removed by outgoing ciliary currents and sudden clapping move- ments of the valves. When the ingoing currents have passed between 540 THE INVERTEBRATA the spirals of the lophophore they unite in the median dorsal part of the mantle cavity and become the outgoing current. The lophophore is supported by calcareous processes of the dorsal valve (the brachial skeleton) which assumes diverse and diagnostic forms in the different genera. The mouth leads into a ciliated alimentary canal. There is a stomach into which opens the digestive gland composed of branching tubes in the cavity of which most of the digestion takes place. In Waldheimia the intestine ends blindly, but in Lingula and Crania there is an anus. The coelom is spacious and divided into a right and left half by a dorsoventral mesentery; transverse mesenteries also exist. It is pro- longed into the lophophore and tentacles and into the mantle as the pallial sinus. A pair of segmental organs, short tubes with large nephrostomes, which also function as generative ducts, are situated in the coelom; their external openings are at the sides of the mouth. The generative organs are developments of the coelomic epithelium and eggs and sperm alike dehisce into the body cavity. The sexes are usually separate in the brachiopods. The blood system is very little developed and consists only of a longitudinal vessel in the dorsal mesentery, in one region of which a contractile vesicle may be distinguished as the hearty and a number of vessels running forward to the mouth and backward to the mantle and generative organs ; all end blindly. The nervous system mainly consists of a supraoesophageal and a suboesophageal ganglion in front of and behind the mouth respec- tively, connected by circumoesophageal connectives. A nerve runs to each tentacle but no special sense organs are known. Lingula (Figs. 401 , 402 H) is a persistent form, which has lived since the earliest period of which we have anorganic record, the Cambrian, in precisely the same stage of development, if we can judge from the hard parts. It lives in mud or sand and has a very long contractile stalk by which it roots itself and can withdraw from the surface. The opening of the shell is usually situated near the surface and the mantle secretes chaetae, like those of annelids, which project from the anterior border, and with the help of mucus and the mantle border form in- halant siphons at the side and an exhalant siphon in the middle. The shell valves are equal in size and horny in consistency, being composed of alternating layers of chitin and calcium phosphate. Crania (Fig. 400) is a form without a stalk. The ventral valve is flat 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 BRACHIOPODA 541 Fig. 402. Development of Brachiopoda. A, Section of larva at end of gastrula- tion showing the two coelomic pouches originating from the archenteron. 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 of Lingula, corresponding to F. al.c. alimentary canal; An. anus; arch. archenteron; arm, one arm of the lophophore; coe. coelomic pouch; Ip. dorsal lip; M. mouth; tn.l. mantle lobe; stk. stalk; ten. tentacles; ch. chaetae; e. eyes. Altered from Delage and Heronard, after various authors. 542 THE INVERTEBRATA are early produced, and a posterior one, hidden by the mantle lobe, which becomes the stalk (Fig. 402 B). The mantle lobes develop four bundles of chaetae (Fig. 402 C), and then turn forward to envelop the anterior region (Fig. 402 D). This now begins to develop the lophophore (Fig. 402 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.402 A). Though the character of the body cavity, 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 Mollusca. 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. Terebratula, 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; free-swimming larva. The structure of an individual of this small and homogeneous group is shown in Fig. 403. 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 contractile 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 consists of diatoms, copepods and larvae of various kinds including fishes, in fact of most of their planktonic 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 CHAETOGNATHA 543 —al.c. "ga.v. od. ■ts. Fig. 403. Sagitta hexaptera. Ventral view, x 3!. After O. Hertwig. al.c. alimentary canal ; An. anus ;/«. fins \ga.v. ventral ganglion -J. jaw ; ikf. niouth ' od. oviduct; ov. ovary; sp. spines; ts. testis; v.d. vas deferens; v.s. vesicula seminalis. 544 THE INVERTEBRATA cavity is mainly occupied by the jaw muscles, while in the trunk and tail cavities are developed the ovaries and the testes respectively. The ovaries (Fig. 404) 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 with indefinite walls containing sperm derived from another animal. 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. The external apertures of both ducts are situated just in front of the second septum. 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 development. The sperm passes into vasa defer entia^ which are long tubes with a small internal mes. Fig. 404. 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 duct. Opening behind the testes and a terminal dilatation, the sperm vesicle^ which opens to the exterior. 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. 405 A). After gastrulation two cells become very prominent. These are the mother cells of the generative organs. The primary coelomic cavity is divided up first of all by the separation of the head cavity (Fig. 405 B) and at a later stage by a second septum between trunk and tail, which divides the genital cells, which now number four, into an anterior pair, the mother cells of the ovaries, and a posterior pair, those of the testes. Sagitta bipunctata is one of the most characteristic and cosmo- politan members of the plankton and is a typical pelagic organism CHAETOGNATHA 545 with its glassy transparent body and its powers of vertical migration ; off the coast of California it lives at a depth of 15-20 fathoms during the day and the greater part of the night, but at sunrise and sunset it rises to the surface, the light intensity and temperature there being at an optimum for the species at those times. The Chaetognatha are a very early offshoot of the coelomate stock and cannot very well be compared to any other phylum. While it is tempting to liken the tripartite division of the coelom in Chaeto- gnatha with that in echinoderms and protochordates, it must be realized that in Sagitta the two transverse septa arise at different Fig. 405. Larvae of Sagitta showing formation of coelomic pouches from the archenteron . After Burfield . In A the pouches still open into the archenteron . In B the pouches forming the head coelom have completely separated off from the archenteron and the archenteric folds have grown back so as partly to separate off the second pair of pouches, al.c. alimentary canal ; his. blastopore ; ect. ectoderm ; end. endoderm ; g.c. genital cells ; hd.coe. head coelom ; M. mouth; std. stomodaeum. times and for different reasons. (There is, however, a true tail here which is elsewhere found only in the Chordata.) The fossil, Amiskwiay occurring in the Cambrian, has been assigned to this group, but it appears to differ from the living forms in the absence of a septum between trunk and tail and in the presence of tentacles on the head. PHYLUM PHORONIDEA Coelomate unsegmented animals, sedentary, hermaphrodite and tubi- colous, with a horseshoe-shaped lophophore, an epistome, a vascular system with haemoglobin, and two segmental excretory organs. This is a very small group : the genus Phoronis (Fig. 406) includes most of the species. They are all marine animals, usually of incon- siderable size, and like all sedentary forms they have a free-swimming larva; this is called an Actinotrocha and it can be referred to the Bi . 35 546 THE INVERTEBRATA trochosphere type. It passes into the adult by a remarkable meta- morphosis which is illustrated in Fig. 407. Fig. 406. Fig. 407, Fig. 406. Phoronis. Altered from Delage and Herouard. Sagittal section to show half the lophophore, the alimentary canal, one of the nephridia and most of the vascular system. The middle part of the section is omitted. An. anus ; aff.v. afferent and eff.v. efferent vessels ; ep. epistome ; ga. ganglion ; int. intestine; Iph. lophophore; Iph.v. lophophoral vessel giving off a branch to each tentacle; ne'p.a. nephridial aperture; nephr. nephrostome; n.r. nerve ring; ov. ovary; org. lophophoral organ (paired); sep. septum between two divisions of the coelom; st. stomach; ts. testis; ten.v. tentacular vessel. Fig. 407. Actinotrocha and its metamorphosis into the adult. After various authors. A, Actinotrocha larva with ciliated lobes (cil.L) and rudimentary vis- ceral sac (vis.s.). B, Visceral sac evagina'ted. C, Growth of visceral sac and decrease in size of preoral lobe (jpr.l.). D, Metamorphosis: mouth and anus approximate, preoral lobe becomes epistome {ep.), ciliated lobes of larva are seen in the mouth and new tentacles are beginning to grow. An. anus; M. mouth. Alimentary canal shown by stippling, visceral sac by oblique shading. Phoronis is very similar to a polyzoan like Plumatella but it differs from such a form in the presence of a vascular system and in other respects. CHAPTER XVIII THE PHYLUM ECHINODERMATA Coeloraate animals; bilaterally symmetrical as larvae, radially sym- metrical as adults ; whose dermis contains calcareous ossicles ; whose coelom in the larva consists of three segments, and in the adult forms a perivisceral cavity and several intricate systems of spaces, one of the latter being a water vascular system which pushes out the surface of the body as a series of delicate tentacles, the podia or tube feet ; whose vascular system is represented by strands of lacunar tissue; whose principal nervous system remains in contact with the ectoderm from which it arose (though it may be invaginated with the latter) ; which have no nephridia ; and whose gonads discharge direct to the exterior by special ducts. The group includes the animals familiarly known as starfishes (Asteroidea), brittle stars (Ophiuroidea), sea urchins (Echinoidea), sea cucumbers or trepangs (Holothuroidea), and sea lilies (Crinoidea) (Fig. 410). The great unlikeness between these animals and all other coelomata is chiefly due to the radial symmetry which they assume at meta- morphosis and which distorts all their systems of organs to its own mould. The radii, which are nearly always five in number, diverge from the mouth. The surface of the body upon which the mouth lies is known as the oral or ambulacral, the opposite surface as the aboral or abamhulacral. The terms ' * ventral ' ' and ' ' dorsal ' ' should not be applied to these surfaces, for they correspond not to the ventral and dorsal but to the left and right sides of the larva. The anus, if present, lies usually on the aboral side, but in the Crinoidea it lies on the oral side. The alimentary canal runs a straight or devious course from mouth to anus. The other systems consist each of a ring around the axis which passes through the mouth and the middle of the aboral side, and a tube or cord along each radius. The radii are constituted by the presence of the radial members of the various systems. The areas between the radii are known as interradii. Most of the systems lie close under the ambulacral surface, and the tube feet project from it, forming radial bands known as the ambulacra. In the Asteroidea and Crinoidea the tube feet of each ambulacrum stand on either side of an ambulacral groove at the bottom of which lies the highly nervous strip of epithelium which is the radial "nerve cord". In the other classes the ambulacral groove is roofed in, forming an 35-2 548 THE INVERTEBRATA epineural canal over the nerve cord. In the Asteroidea, Ophiuroidea and Crinoidea the body is prolonged as arms in the direction of the radii, and the ambulacral and abambulacral surfaces are subequal in extent. On the other hand, in the spherical or cushion-shaped Echinoidea and the sausage-shaped Holothuroidea, the body is com- pact, and the ambulacral surface extends over most of it, leaving only Fig. 408. Asterias riihens. A, Part of an oral view: in one of the arms shown the adambulacral spines have closed over the ambulacral groove; in the others, the radial nerve can be seen. B, An aboral view of the disc, showing the madreporite. C, The tip of an adambulacral spine, showing pedicellariae. in the Echinoidea a small, and in the Holothuroidea a minute, aboral area opposite to the mouth (Fig. 409). Externally and internally the symmetry is never quite perfect. At best the presence of the madre- porite (see below), or of the anus, or of a genital opening, differenti- ates one of the interradii, and in some echinoids and holothurians a new and conspicuous bilateral symmetry has developed, and affects a number of organs. ECHINODERMATA 549 abo abo aho ^ ^^' abo abo ^or ^f% Fig. 409. Diagrams to show the relative extent of the oral and aboral sur- faces, and to compare the form of body, in the several classes of the Echino- dermata. All the diagrams are in the same morphological position. From Borradaile. i, Asteroidea. 2, Ophiuroidea. 3,*Echinoidea. 4,Holothuroidea. 5, Crinoidea. abo. aboral surface; or. oral surface. Fig. 410. Representatives of classes of the Echinodermata, in their natural positions . A, A starfish (Asteroidea) . B , The shell of a sea urchin (Echinoidea) . C, A holothurian. D, A sea lily (Crinoidea). 5S^ THE INVERTEBRATA In life, the Crinoldea are fastened to the ground by a stalk which arises from the middle of their aboral surface, and, though a few of them break free when they are adult, the mouth is directed upwards by them all. The other existing groups (Eleutherozoa) are free. In the Asteroidea, Ophiuroidea, and Echinoidea the mouth is directed downwards. The Holothuroidea apply one side of the long body to the ground, so that the mouth is directed horizontally (Fig. 4J0). The tube feet {podia), whose function was perhaps originally a sensory or food-collecting one, are (or some of them are) in the Asteroidea, Echinoidea, and Holothuroidea adapted, by the presence of suckers at their ends, to walking. Probably they always subserve respiration, and in the "irregular" echinoids some of them are modified for this function. They may also be modified for seizing food. They are protruded and retracted by alterations of the pressure of the fluid within them by the action of the water vascular system (see below). The epidermis is usually ciliated, but not in ophiuroids or, except in the ambulacral groove, in crinoids. Usually, also, it contains gland cells and sense cells, the latter with their bases prolonged into fibrils which enter a plexus, formed by them and by branched nerve cells, among the bases of the epithelial cells. The characteristic ossicles of the dermis may be scattered, so as merely to impart a leathery con- sistency to the skin, or united by muscles as a skeleton, or firmly apposed so as to constitute an armour. Some of them usually project as spines, over which the epidermis may presently wear away. Pedi- cellariae (Figs. 408 C, 424) are sets of two or three spines arranged to bite together as pincers. They are of various types, often complicated, but only occur in asteroids and echinoids. The alimentary canal differs greatly in the several groups. It is axial in the Asteroidea and Ophiuroidea, coiled in the other classes. It possesses various diverticula in different cases, but not large glands like those which are common in other phyla. The anus is lacking in the Ophiuroidea and a few asteroids, and when present is more or less excentric except in the Holothuroidea. The coelom of the adult is present as several distinct systems of spaces, of which the following are the most important: (i) the large perivisceral cavity in which lie all the principal viscera ; (2) the peri- haemal syste?n, consisting of a radial vessel (in asteroids divided longitudinally by a vertical septum in which lies the principal "blood" strand) along each radius, and a ring vessel around the mouth, all lying immediately above the main nerve cords; (3) the aboral sinus system enclosing the genital rachis and gonads (see below) ; (4) the water vascular system (Fig. 411), which lies above the peri- haemal system, and consists of a ring around the mouth, a tube, ECHINODERMATA 55- mad known as the stone canal because its wall is frequently calcified, lead- ing to an opening known as the madreporite (see below), a radial vessel along each radius, and lateral branches from the radial vessels to the tube feet, which, when the latter are used for walking, possess swell- ings known as ampullae, by whose con- tractions the feet are extended; (5) the madreporic vesicle, an inconspicuous cavity of morphological importance (see below); (6) the axial sinus. This is a space which varies greatly in its develop- ment. It is conspicuous in the Aste- roidea, small in the Echinoidea and Ophiuroidea, very small in the Holo- thuroidea, merged in the perivisceral cavity in the Crinoidea. It communi- cates with the exterior (or, as will be seen, in most holothurians with the coelom) by a pore or set of pores situated in one of the interradii. This opening constitutes the madreporite. The stone canal opens into the axial sinus just below the madreporite, and so the latter serves as the opening of the stone canal. In the Asteroidea and Echinoidea the madreporite is a conspicuous structure on the aboral side, pierced by many pores. In the Ophiuroidea it is on the oral side, and has one pore, or only a few pores. In most of the Holothuroidea it becomes detached, in the course of development, with its tiny axial sinus, from the body wall, and hangs into the perivisceral cavity, with which, instead of with the exterior, it now makes communication, by a number of pores. In this group, by meristic repetition, there may be several or many stone canals, each with an ''internal madreporite". In the Crinoidea, the stone canals, of which there are several, end each by a single opening into the perivisceral cavity, and the latter communicates by a number of pores with the exterior. In the bilateral larva (Dipleurula), the coelom (Fig. 412) is present as three pairs of sacs, of which the first is preoral. The second pair is connected by a passage with the first : the third is independent. In out- line,the relation between these sacs orsegments of the larval coelom and the coelomic spaces of the adult is as follows : the perivisceral cavity of the adult is formed by the fusion of the main portions of the hinder pair ; the aboral sinus system becomes separated from the perivisceral cavity; the perihaemal system arises as outgrowths from the left hinder cavity (in some cases it receives a component also from the left Fig. 411. A diagram of the water vascular system of a starfish. FromBorradaile. amp. ampulla ; mad. madreporite ; r.zo.v. radial water vessel; st.c. stonecanal; ^./.z;. vessel of tube foot ; iv.v.r. water vascular ring. 552 THE INVERTEBRATA anterior cavity) ; the water vascular system (" hydrocoele ") is formed by the transformation of the left second cavity (the right second cavity disappearing); the axial sinus is the persistent left anterior cavity, its madreporite being derived from a '* water pore " which puts that cavity into communication with the exterior. The opening of the stone canal into the axial sinus is the remains of the connection be- tween the left anterior cavity and the left second cavity, which latter, as we have seen, becomes the water vascular system. The madreporic vesicle is budded off from the right anterior cavity (the rest of which disappears); in the larva this vesicle pulsates; it probably repre- sents the pericardium of the Enteropneusta, which retains its con- tractile function in the adult (p. 588). All echinoderms except the Holothuroidea possess a peculiar structure known as the axial organ, composed of connective and coe. 2/ coe. zr coe. 3^ pro. , coe. 1/ M coe. 3/ Fig. 412. A diagram of the arrangement of the coelom in the ideal Dipleurula. From Sedgwick. An. anus; coe. il. left anterior coelom; coe. 2.1. left middle coelom; coe. zr. right middle coelom; coe. 3/. left hinder coelom; coe. 3^. right hinder coelom; M. mouth; n.pt. neural plate on apex of preoral lobe; pro. preoral lobe or prostomium; por. water pores: only the left of these normally appears ; it becomes the madreporite. lacunar ("vascular") tissue, with cells derived from the genital rudi- ment, known as the genital stolon. The axial organ adjoins the axial sinus where the latter is present; in the Crinoidea it lies in the axis of the body. Its function is unknown ; it has been regarded as a heart on account of contractions which it is said to perform, and as an organ of excretion because in echinoids it takes up carmine injected into the body cavity. Oi excretion in the echinoderms little is known. It appears to be performed by the wandering out, through the epithe- lia, of amoeboid cells laden with granules of excreta, and possibly by certain other organs, such as the rectal glands of asteroids, but no constant and conspicuous organs subserve it alone. There are no nephridia. Respiration is performed through a variety of structures, some of which expose the coelomic fluid to the external water, while others ECHINODERMATA 553 carry the water into the body and expose it to the fluid in the coelom. To the first class belong the podia, and the "gills" of asteroids and echinoids; to the second belong the ''genital bursae" of ophiuroids and the respiratory trees of holothurians. The vascular system of other animals is represented in the Echino- dermata by a system of strands of a peculiar lacunar tissue, containing intercommunicating spaces which have no epithelioid lining. Ulti- mately, this system is of the same nature as the blood vessels (haemo- coele) of other animals, since both are systems of spaces derived from the blastocoele and filled by a fluid matrix containing free cells ; but in appearance, and probably in the mode of its functioning, it is vtvy different. A ring of lacunar tissue surrounds the mouth, lying in or immediately above the perihaemal ring and giving off in each radius a strand or "vessel" which similarly lies above the radial peri- haemal canal. Another portion of the system lies in the axial organ and connects the oral ring with an aboral ring, which accompanies the genital rachis (see below) and sends strands to the gonads. In the Echinoidea and Holothuroidea two strong "dorsal" and "ventral" vessels from the oral ring accompany the alimentary canal, running on opposite sides of that organ and giving off a plexus of branches which ramify on it, and in holothurians also in a perforated fold of the peritoneum. A "vascular" plexus is also present on the alimen- tary canals of other groups. Contractions are said to have been ob- served in parts of the system, but it is very doubtful whether anything in the nature of a regular circulation takes place in it, though it probably maintains communication by diffusion between various parts of the body. With rare exceptions, the sexes of echinoderms are separate. The genital organs are remarkable for their simplicity. They possess neither organs of copulation, nor accessory glands, nor receptacles for the retention of ova, nor a reservoir for the storage of sperm in either sex, and they discharge direct to the exterior and not, as is usual in coelomate animals, through the coelom or through ducts proper to that cavity. Nevertheless they arise in ontogeny from the coelomic wall. The genital system consists, except in the Holothuroidea, of the genital stolon, a collection of cells in the axial organ; the genital rachis, a ring connected with the stolon (aborally in the Asteroidea, Ophiuroidea, and Echinoidea, orally in the Crinoidea) ; the gonads proper, which are sacs or tubes, often branched, borne upon long or short branches of the rachis and varying in number in the different groups; and the short ducts, lacking in the Crinoidea. In the Holo- thuroidea there is only one gonad, which lies in the "dorsal" inter- radius and has a duct in the dorsal mesentery and a vestigial stolon lying upon the duct, but no rachis. 554 THE INVERTEBRATA The nervous system consists of plexuses of fibrils and nerve cells underlying various epithelia and thickened in places into denser tracts (" nerves "). It is remarkable not only for remaining in this primitive condition, but for being partly derived from mesodermal epithelia. It is in three parts : (i) the ectoneural system underlying the w^hole ectoderm as a plexus (see p. 550) and thickened {a) along each ambulacrum as a radial nerve, (b) around the mouth as a nerve ring, which connects and has been found by experiment to co-ordinate the radial nerves (a and b are, with an epithelium, removed from the surface of the body save in asteroids and crinoids), (c) as branch nerves to such structures as tube feet, spines, etc. ; (2) the deep oral system underlying the mesodermal epithelium of the perihaemal vessels and having a distribution similar to that of the ectoneural system but less extensive than the latter and in particular defective in the Echinoidea ; (3) the aboral or apical system, also mesodermal in origin, developed from the peritoneum on the aboral side. This system is best developed in the Crinoidea, where it is removed from the general peritoneum and enclosed in the ossicles. Here it has the form of a nerve along each arm and a complex central station in the "chambered organ" (see below). In the Asteroidea it runs as a cord above the peritoneum of each arm, the cords meeting in the middle. In the Ophiuroidea and Echinoidea it is a ring in the aboral sinus. It is not found in the Holothuroidea. The mesodermal nervous systems are principally motor, innervating the muscles which move the internal skeleton. The Echinodermata are poorly provided with sense organs. There is a general sensitiveness of the epithelium of the body, at least to tactile stimuli, which is heightened in the podia and in the terminal tentacle which stands at the end of each radial water vessel in the Asteroidea, Ophiuroidea, and Echinoidea. The olfactory sense is perhaps also located in the podia or in some of them, especially in those that are situated around the mouth and in the Holothuroidea are developed into tentacles. An eye-spot is situated at the base of each terminal tentacle in the Asteroidea, and certain holothurians possess statocysts in the skin. All echinoderms are marine in habitat. Few of them are pelagic : none are parasitic. Only the Crinoidea are fixed, and some of these are only temporarily so. The majority of members of the phylum have free, pelagic larvae', though some, as Asterina, pass a considerable time in the egg mem- brane and have larvae which are not pelagic ; and a few, chiefly polar or deep-sea species, keep the young in brood pouches until they have the adult form. The eggs of the species which possess pelagic larvae are small ; the others larger and more yolky in proportion to the late- ness of the stage at which they are set free. Fertilization takes place ECHINODERMATA 555 in the sea or in brood pouches. Cleavage (radial, Fig. 185, i), is total and forms a hollow, one-layered blastula (Fig. 413 A). This, by invagi- nation or unipolar ingrowth, forms a gastrula with a wide blastocoele into which typical mesenchyme cells wander from the wall of the archenteron. The blastopore becomes the anus, and the mouth is formed by the breaking through of a stomodaeum. Meanwhile the archenteron has budded off, at the anterior end, a vesicle which, by z9i\ -ett. 1,- end. ^|3 \'^'^"i^-'~-i=p—arch. Up B ,<^#i^rffe. -hcilM. -stom. D \h Fig. 413. Stages in the development of Asterias vulgaris. After Field. A, Section of blastula. B, Section of gastrula. C, Section of older gastrula. D, Three days' larva from the right-hand side. An. anus; arch, archenteron; blc. blastocoele ; hip. blastopore ; cil.bd. ciliary band ; coe. rudiment of coelom ; ect. ectoderm; end. endoderm; e?it. enteron; mch. mesenchyme; mth. meso- thelium; stom. stomodaeum. processes that differ in detail in different cases, will give rise to the three segments of the coelom described above (p. 551). The future ventral side of the larva becomes concave. The larva is now known as the Dipleurtila. The cilia which uniformly covered the blastula be- come sparse over most of the body but, except in the Crinoidea, grow stronger and more numerous in a longitudinal band around the ventral concavity. This band is the organ of locomotion. Growing more rapidly than the rest of the ectoderm, it becomes thrown into folds, 556 THE INVERTEBRATA the larval arms (which have nothing to do with the arms of adult echi- noderms), whose length and arrangement differ so as to characterize a special type of larva in each class (Fig. 414). In the Auricularia larva of the Holothuroidea the body is elongate and the band lengthens fore and aft and outlines a strong preoral lobe . The Bipinnaria of the Asteroidea resembles the Auricularia in general features, but in it the border of the preoral lobe separates completely from the rest of the Fig. 414. Diagrams of echinoderm larvae. The postoral part of the early ciliated band is drawn heavily (except where remote), the preoral part cross- hatched. A, Early stage with simple continuous band. B, Auricularia. C, Bipinnaria. D, Pluteus. E, Crinoid larva. An. anus; M. mouth; pr.' preoral band ; pr." corresponding part of continuous band ; pt. postoral band. longitudinal band. In the Plutei of the Ophiuroidea and Echinoidea the band remains continuous, but forms only a small preoral lobe, and the postanal region of the body develops greatly, while the slender arms are supported by calcareous rods. The Pluteus of the Ophiuroidea (Ophiopluteus) has a different appearance from that of the sea urchins (Echinopluteus), owing to the fact that the former of these larvae has fewer arms than the latter and that in it the arms known as the " pos- terolateral arms" are the largest and are directed forwards, whereas these arms, if they are present in the Echinopluteus , are there small ECHINODERMATA 557 and directed outwards or backwards. The larva of the Crinoidea has no longitudinal band, but five rings of strong cilia around the body. In the development of the Holothuroidea the Auricularia is succeeded by a stage known as the pupa, in which the longitudinal band breaks up and rearranges itself into a series of five transverse rings some- what resembling those of the crinoid larva. The Bipiiinaria of the Asteroidea is succeeded by a Brachiolaria which differs from it in possessing in the preoral region three processes by which the larva can hold fast to objects. The larvae become transformed into adults by a metamorphosis which differs in the several classes. In all it involves an alteration of the position of the mouth, which in groups other than the Crinoidea is removed to the left side, and in the Crinoidea to the posterior end, taking with it the coelomic cavities of the left side. The fate of the several divisions of the larval coelom has been described above (p. 551). In the Crinoidea and Asteroidea the larva becomes fixed by the pre-' oral lobe at the time of metamorphosis, 2i fixation disc developing for the purpose. In crinoids the fixation persists, at least until the adult is completely formed. In starfishes it is only temporary. The fixation of the sea lilies, and the fact that starfishes are fixed when the bilateral symmetry of the larva changes to the radial sym- metry of the adult, are interesting facts in view of the fixation which is general in the other great group of radially symmetrical animals, the Coelenterata. Radial symmetry is essentially the symmetry of a sessile animal, which is in the same relation with its surroundings on all sides, whereas bilateral symmetry is that of a travelling animal, which needs differentiation not only of the upper side from that which faces the ground, but also of the fore from the hind end. It is likely that at one time all echinoderms were fixed, and that those which are now free retain the radial symmetry of their sessile ancestors. This supposition is supported by the fact that the earliest known fossil members of the phylum were fixed. For the rest, the Dipleurula and its metamorphosis suggest that the early sessile echinoderms were descended from a free, bilateral an- cestor; and the close resemblance between the Auricularia and the Tornaria larva of Balanoglossus , together with the history of the coelom (see p. 583), and the nature of the nervous system, indicate an affinity between that ancestor and the Enteropneusta. Class ASTEROIDEA Star-shaped or pentagonal Echinodermata ; whose arms contain caeca of the alimentary canal, and are usually not sharply marked off from the disc ; which have an aboral madreporite ; open ambulacral grooves ; and usually both suckers on the tube feet, and pedicellariae. 558 THE INVERTEBRATA The ossicles (Fig. 415) of the body wall of a starfish may, as in the familiar Asterias, constitute a toughening meshwork, or may have the form of more closely set plates, but are not united to form a con- tinuous shell. Along the sides of the arms run two rows of strong pieces, the supero- and infero-marginal ossicles^ which are hidden in Asterias but in many genera appear on the surface. The ossicles bear spines, which vary much in size and shape and arrangement,^ being often longer than the stumpy structures on the back of Asterias. Around and between the spines are usually pedicellariae of various sp 'ped' Fig. 415. A diagram of a transverse section of the arm of a starfish. From Borradaile. «&.m. muscle which straightens the arm; at/.05^. adambulacral ossicle; ad.sp. adambulacral spine; amb.oss. ambulacral ossicle; amp. am- pulla of tube foot; m.' muscle which opens the ambulacral groove; ped.' one of the small pedicellariae with crossed jaw ossicles; ped." one of the large pedicellariae whose jaw ossicles are not crossed; r.b.v. "radial blood vessel"; t.f. tube foot. Other letters as in Fig. 416. kinds, the most perfect of which is tliQ forcipulate , found in Asterias, which has a basal ossicle: its jaws may be straight or crossed. Over interspaces between the ossicles arise delicate, hollow outgrowths, the gills, into which the perivisceral cavity is prolonged. Above each ambulacral groove runs a double row of large, transversely placed, ambulacral ossicles, movable upon one another by muscles. Each has a smaller adambulacral ossicle at its outer end. Adambulacral spines stand on the adambulacral ossicles. In Asterias they are long, and bear groups of large pedicellariae of the kind with uncrossed jaws. They can be turned inwards to protect the ambulacral grooves. ASTEROIDEA 559 «. '^ ^ '^ C C •'•■ti "^ w r;- C u u 2 .^ ^ 2 o ^ *^ -c> ^ "5 5 M , iH . (U j# C -^ ti< -G^ • ^ .^ '^ o 395 Anoplophrya, 100 A. prolifera, lOl Anoplura, 424 Anostraca, 318; 317 Antedon, 578; 581, 582 INDEX 6ii A. bifida, 578 A. rosacea, 578; 579 Antennae, 271, 272-3 ; of Crustacea*, 292-3, 296, 298, 300, 302, 306, 318, 322, 324, 325, 331, 333, 339, 340, 348, 349, 354, 355, 356, 358, 368; of Insecta*, 383, 406, 414, 417, 430, 441 ; of Myriapoda*, 377, 379 ; of Trilobita, 287 Antennal glands, 309; 311, 359, 368 Antennules, 290; 292-3, 296, 298, 302, 306, 318, 322, 325, 333, 338, 340,343,347,348,349,355,358,368 Anterior aorta, of Araneida, 458; of Carcinus (ophthalmic artery), 370 ; of Helix, 484; of Insecta, 391 ; of Lamellibranchiata, 505 ; of Scorpi- onidea, 450; of Sepia, 518 Anterior cervical groove, 352 Anterolateral edge, 365 Antheridia, 53 Anthomedusae, 133; 138, 139, 140, 142. See also Gymnoblastea Anthonomus grandis, 43 1 Anthozoa, see Actinozoa Antipathes, 347 Anurida maritima, 409 Anus, 121 ; of Amphineura, 476; of Balanoglossiis, 586; of Brachio- poda, 540; of Echinodermata*, 547, 550, 560, 564, 570; of Gas- teropoda, 477 ; of Gephyrea*, 268, 269; of Haliotis, 490; of Helix, 483 ; of Lepas, 340 ; of Mollusca, 471; of Nemertea, 205; of Opis- thobranchiata*, 479, 494, 495 ; of Peripatus, 283 ; of Polychaeta, 250 ; of Pterobranchia, 590; of Sepia, 517. See also Alimentary canal Aorta, of Crustacea, 311; of Helix, 484. See also Anterior aorta, Posterior aorta Aperture, see Opening Aphaniptera, 441 Aphis rumicis, 419; 418, 420 Aphrodite, 233 ; 231, 234 Apical nervous system, of Crinoidea, 580; of Echinodermata, 554; of Holothuroidea, 575 Apical organ, 250, 251, 535 Apical rosette of the trochosphere, 250 Apis, 433 A. mellifica, 388, 391, 432 Aplacophora, 474 Aplysia, 494; 479, 493, 496 Apoda, 343 Apodemes, 303 Apodous larvae, see Larvae Apopyles, 112 Appendages, Paired, see Limbs Appendicularia larva, 597 Apposition image, 278 Apseudes, 354 Apterygota, 406; 385 Apus, 322; 296, 297, 301, 301, 302, 324 A. cancriformis, 322; 323 Aquatic oligochaets, 258 Arachnid section of Arthropoda, 270 Arachnida, 443 Araneida, 458 Arcella, 68; 8, 23, 24, 38, 63 A. discoides, 68 Archenteron, 120; of polychaete embryo, 250 Archiannelida, 261 ; 228 Archicerebrum of Lankester, see Procerebrum Archicerebrum sensu stricto, 274 Architeuthis, 513 Arenaceous shells, 69 Arenicola, 240 ; 230, 23 1 , 234, 242, 248 A. marina, 238; 239, 240 Argas, 462 ; 462 A. persicus, 463, 464 Argonauta, 513 Argulus,z2^; 337 A. americanus, 337 A.foliaceus, 338; 337 Argyroneta, 459 ; 447 Arion, 495 Aristocystis, 582 Aristotle's lantern, 567; of Echinus esculentus, 568 Armadillidiiim, 356 Arms, of Brachiopoda, 539; of Crinoidea, 578 ; of Dibranchiata, 515, 519,523, 524,525; of Echino- dermata, 548, 556 ; of Ptero- branchia, 590 Artemia salina, 318, 321 Arterial system, of Araneida, 459 ; of Carcinus, 370; of Helix, 484; of Lamellibranchiata, 505 ; of Litho- bius, 378; of Malacostraca, 312; of Scorpionidea, 450. See also Aorta, Vascular system 39-3 6l2 INDEX Artery, Antennary, 370; Cephalic, 378; Gastrointestinal, 505; 370; Hepatic, 505 ; Lateral, 450, 458 ; Pallial, 505; Sternal, 370; Supra- neural, 450, 457; Terminal, 505. See also Aorta, Anterior aorta, Posterior aorta Arthrobranchiae, 361 Arthropoda, 270; i, 2, 281 Articulamentum, 475 Ascaris, 215; 216, 218, 219 A. lumbricoides , 222 Ascidia, 602 Ascidiacea, 602; 601 Ascidiae compositae, 602 Ascidian tadpole, 597 ; 598 Ascon grade, 113 Ascopus, 212 n. Ascothoracica, 347 Asellus, 356; 100, 298 A. aquaticus, 357 Asexual reproduction, of Metazoa, see Budding, Strobilization ; of Porifera, 116; of Protozoa, see Agamogony, Schizogony, Sporo- gony ; of Turbellaria, 188 Aspidobranchiata, see Diotocardia Aspidochirotae, 575 Aspirigera, see Holotricha Asplanchna, 213 Astacura, 362 Astacus, 362 ; 279, 296, 302, 304, 304, 306, 308, 310, 365 A.fluviatilis, 313, 315, 363 Asterias, 561 ; 558, 560 A. rubens, 548, 561 A. vulgaris, 555 Asterina, 562; 554 Asteroidea, 557; 547 .45fero/)e, 235; 231, 233 Astomata, 100 Astroides, 167 Astropecten, 561 Atractonema, 226 Atrial opening, 592, 598, 600, 602 Atrial siphon, 592 Atrium, Genital, see Genital atrium; of Cephalochorda, 598 ; of Tuni- cata*, 594, 597, 599 Atropus pulsatoria, 415 Aulactinia, 165 Aulactinium, 76 A. actinastrum, 77 Aiirelia, 151, 157 A. aurita, 152; 153, 154; Strobiliza- tion of, 155, 156 Auricles, of Arenicola, 240 ; of Gas- teropoda, 478; of Lamellibran- chiata, 505 ; of Mollusca, 470, 471 ; of Sepia, 518 Auriculae, 567 Auricularia, 556; 556, 590 Autogamy, 28 ; 78 Autolytus, 246 ; 245 Autotomy, 303 Autozooids, 162 Avicularia, 533 Axelsonia, 408 Axial filament, 1 1 Axial organ, of Crinoidea, 580; of Echinodermata, 552; of Holothu- roidea, 575 Axial sinus, of Echinodermata, 551 ; of Echinoidea, 570 Axopodia, 11 Axostyles, 14 Babesia, see Piropiasma Bacillus pestis, 442 Baculites, 529; 529 B. capensis, 529 B. chicoenis, 529 Badhamia, 82; 81 Balanoglossida, 585 Balanoglossus, 585; 557, 583 Balantidiu7n, 102 B. entozooti, 103 Balanus, 342 ; 342 Basal disc, 166 Basal granule, 1 1 Basal ossicles, 580 Basement hiembrane, 179 Basilar plate, 381 Basipodite, 298 Basommatophora, 495 Bathynella, 351 Bdellocephala, 187 Bdelloid rotifers, 213 Beaks of Cephalopoda, 519 Beds, Mussel, 508 Behaviour of Protozoa, 33 Belemnites, 524 Belemnitidae, 524 Belemnoidea, 513 Beroe, 173 Bibio, 439; 440 Bilateral symmetry, 557; of Actino- zoa, 159; of Ciliata, 4; of Echino- INDEX 613 Bilateral symmetry {cont.) derm larvae, 547, 551, 557; of Echinoidea, 548, 570; of Holothu- roidea, 548, 572; of Metazoa, .123 , Bilharzia, see Schistosoma Binary fission of Protozoa*, 24; 32, 40,48, 64, 67,68, 69, 75, 78, 81, 107 Bipalium kewense, 189-90 Bipinnaria, 556; 556, 562 Biramous limb, see Stenopodium Birgus, 373 ; 372 Bladder worm, see Cysticercus Blastocoele, 120; i, 121, 122, 209 Blastoidea, 582 Blastopore, 120 Blastostyles, 133 Blastozoite, see Blastozooid Blastozooid, 600; 603, 604 Blastula, 120; of Coelenterata, 131; of Echinodermata, 555 ; of Obelia, 135 Blatta, 389, 404 B. orientalisy 409 Blepharoplast, 11 n. Blood, 122; of Arenicola, 240; of Chaetopoda, 230; of Ciona, 596; of Crustacea, 314; of Insecta, 391 ; of Scorpion, 450 Blood vessels, see Arterial system, Artery, Vascular system "Blood vessels" of Echinodermata, see Lacunar system Bodo, 56; 58 B. saltans, 57 B. sulcatus, Chemophobotaxis of, 34, 35 Body, of Amphipoda, 358; of Areni- cola, 238; of Balanoglossus, 585; of Cephalodiscus and Rhahdopleura, 590 ; of Crustacea, 295 ; of Cteno- phora, 171 ; of Echinodermata*, 547, 564, 572; of Isopoda, 355 ; of Medusa, 129; of Metazoa, 120; of polyp, 129; of Porifera, no, in; of Protozoa, 4; of Tubicolous Polychaeta, 237. See also Sym- metry Body cavity, of Nematoda, 215; Primary, see Haemocoele ; Second- ary, see Coelom. See also Perivis- ceral cavity Body wall, of Hirudinea, 265; of Holothuroidea, 573 ; of Metazoa, 120; of polyps, 129; of Rotifera, 209 Bonelliay 268 B. viridis, 269 Boophilus anmdatus, 465 B. hovis, 465 Bopynis, 357 B.fougerouxi, 358 Bothriocephalus, 201, 202, 204 B. latus, 201, 202 Botryllus, 603 ; 599 B. violaceus, 602 Botryoidal tissue, 265 Bougainvillea, 136; 138, 143 B.fructiiosa, 136 Brachial ossicles, 580 Brachial skeleton, 540 Brachials, 580 Brachiolaria, 557 Brachioles, 582 Brachiopoda, 537; 2; Development of, 541 Brachyura, 362 Brachyurous types, 362 Brain, 122; 124; of Arthropoda, 274; of Branchiopoda, 303, 305; ot Ciona, 596 ; of Crustacea, 303 ; of Insecta, 397, 398; of Polychaeta, 230; of Sepia, 520, 520. See also Ganglion, Cerebral (Supraoeso- phageal) Branchia, of Phyllopodium, 300, 317; of Salpa, see " Gill " Branchiae, see Gills Branchial chamber, 592 Branchial hearts, 519 Branchial opening, 592 Branchial veins, 519 Branchiobdellidae, 252, 267 Branchiogenital region, 586 Branchiopoda, 317; 290, 291, 295, 298, 303 Branchiura, 337 Breathing, see Respiratory movements Brisinga, 562 Brown body, 531 Bryograptus, 148; 150 B. callavei, 150 B. retroflexus, 150 Buccal capsule, 221 Buccal cavity, of Enteropneusta, 587 ; of Helix, 585 ; of Insecta, 387 ; of Tunicata, 592. See also Alimen- tary canal 6l4 INDEX Buccal mass, 485 ; of Helix, 484, 485 ; of Sepia, 519 Buccal tube feet, 566 Buccinum, 49 1 ; 478, 480, 488, 489, 492 Budding, of Cysticerci, 202; of Hydrozoa*, 133, 137, 138, 145; of Madreporaria, 168; of Micro- stoma, 188; of Polychaeta, 246; of Protozoa, 24, 68, 78, 107; of Pterobranchia, 590; of Stylaria, 259; of Tunicata*, 599, 600, 601, 603, 604, 606. See also Colonies Bugula, 533, 534, 537 Bulimus, 496 Bulla, 491, 494 Bunodes, 457 Bursa copulatrix, of Insecta, 397; of Turbellaria, 186, 201 Buthus, View of internal anatomy of, 451 B. carpathicus. Embryo of, 444 Byssus pit, 508 Cadophore, 607 Caecum, of Echinoidea, 569; of Polyzoa, 530; of Sepia, 520 Calabar swellings, 223 Calanus, 334; 299, 301, 333, 335 Calcarea, 117; 113 Calcareous ring, 573 Caligus, 335 Callidina, 213 Calosonia semilaeve, 430 Calotermes militaris, 412 Calymma, 75 Calyptoblastea, 133; 133-6, 142, 148, 150. See also Leptomedusae Calyptomera, 325; 324 Calyx, 578 Campodeiform larva, 401 ; of Cole- optera, 430 Canal system, 130 Cancer, 374 Capillitium, 82 Capitellidae, 242 Capitulum, 338 Caprella, 360 C. grandimana, 360 Captacula, 498 Carabus violaceus, 43 1 Carapace, 297; 290; of Branchio- poda, 291, 317; of Calyptomera, 317; of Carcimis, 362; of Cirri- pedia, 294, 297 ; of Cladocera, 291, 317, 324; of Conchostraca, 291, 297, 317; of Gymnomera, 317; of Lepas, 338; of Leptodora, 329; of Malacostraca, 297, 348 ; oiNebalia, 349; of Ostracoda, 291, 297; of Peracarida, 352 Carchesium, 104; 107 C epistylidis, 106 Carcinus, 345, 366, 374 C. maenas, 362; 364, 365, 367, 369, 370, 371, 372 Cardium, 504 Caridea, 362 Caridoid facies, 348 C?rina, 338 Carinella, 208 Carmarina, 139, 143 Carotin, 41 Carpoidea, 582 Carpopodite, 300 Carteria, 50 ; 42, 43 Caryophyllia, 167 Cassiopeia, 157 Caudal furca, 303. See also Caudal rami Caudal rami, 303, 320, 322, 325, 332, 348. . Cavolinia, 494 ; 493 Cells, 3 ; Corneagen, 276 ; Flame, 177; 123, 179, 250; Interstitial, 127 and n.; Iris, 277; Lasso, 170, 172; Musculo-epithelial, 125; of Porifera, no, in; Pole, 123; Sensory, 176; Somatic, 6; Taste, 177; Thread, 126; Yellow, 229 Cellular animals, 3 Central capsule, 71 Central nervous system, 122; of Chordata, 582; of Tunicata, 596. See also Brain, Ganglion, Nervous system Centrodorsal ossicle, 578 ; 579 Cephalization, 124; of Crustacea, 296 ; of Polychaeta, 233 Cephalochorda, 2, 582, 584. See also Amphioxus Cephalodiscus, 590; 583, 585 Cephalopoda, 512 Cephalothorax, of Arachnida, see Prosoma; of Brachyura, 362; of Branchiura, 337; of Copepoda, 331, 22?>\oiLigia, 355 Cerata, 494, 495 Ceratium, 49 ; 30 NDEX 615 C. macroceras, 50 Ceratophyllus fasciatus, 442; 441 Cerci anales, 386 Cerebral ganglia, see Brain; Gan- glion, Cerebral Cerebral organ, 207 Cerebral vesicle, 598, 599 Cerebratulus, 209 C.fuscus, 206 Cervical groove, 296 Cervical sclerites, 385 Cestoda, 197; 174 Cestoda Merozoa, 199 Cestoda Monozoa, 198 Cestus Veneris, 173 Chaetae, 228, 228; 270; of Acan- thobdella, 267 ; of Archiannelida*, 261 ; of Chaetopoda, 229 ; of Echi- uroidea, 268 ; of Oligochaeta, 253, 254, 259, 260; of Polychaeta, 231, 232, 233, 235, 237, 239 Chaetognatha, 542 ; 2 Chaetopoda, 229 ; 228 Chaetopterus, 231, 237 C. pergamentacetis, 236 Chambered organ, 580; 554, 579 Chambers, of cephalopod shells, 525 ; of Foraminifera shells, 70 Cheeks of Trilobita, 287 Cheilostomata, 537; 532 Chela, 303 Chilaria, 445, 454 Chilina, 491 Chilomonas, 46 Chilopoda, 375 ChirocephaluSy 275, 290, 291, 294, 296, 311, 319, 321, 322, 325, 328, 333 C. diaphanus, 318, 319 Chironomus, 391 ; 267 Chiton, 474; 124, 287, 475, 508 Chlamydomonas, 50; 16, 24, 25, 26, 27, 29, 33, 40, 51, 54 C. angulosa, 25 C. brauni, 26, 27 C. euchlora, 26 C. longistigma, 25 C. media, 27 C, steini, 26 Chlamydospores, 33 Chlorocruorin, 230 Chloromonadina, 48 ; 44 Chlorophyll, 41 Chloroplasts, 41 ; 14 Choanocytes, no, in, 112, 113, 116, 117 Choanoflagellata, 59 Choanoflagellidae, see Choanoflagel- lata Chondr acanthus, 335 C. gibbosiis, 336 Chondrioderma, 82 C. difforme, 81 Chonotricha, 107; 22, 28 Chordata, 583 ; 2 Chorion, 394 Chromatophores, of Cephalopoda, 515 ; of Crustacea, 307; of Phyto- mastigina*, 41 ; 14, 24, 42, 44-9 Chromidium, 23 Chromoplasts, see Chromatophores of Phytomastigina Chrysamoeba, 44 C. radians, 45 Chrysaora, 151 Chrysidella, 46 C. schaudinni, 47 Chrysomonadina, 44; 16, 56 Chrysops, 436; 223 Cicada septendecim, 419 Cicindela, 390 Cilia, 13 ; of Ciliata, 97, 98 ; of Ciona pharynx, 594, 595 ; of lamelli- branchiate gill, 501 Ciliary junctions, 499 Ciliary organ, 243 Ciliata, 96 ; 103 Ciliated band, of Dipleurula, 555 ; of Tornaria, 590 Ciliated funnel, see Dorsal tubercle Ciliated pits, 176; 180 Ciliated ring, see Ciliated band, Metatroch, Prototroch, Velum Ciliophora, 95; 19, 21, 38, 39 Ciliospores, 33 Cimicidae, 417 Cingulum, 211 Ciona, 599; 595, 596, 597, 599, 602 C. intestinalis, 592; 591, 593, 594 Circulation of blood, 122; in Ano- straca, 311; in Araneida, 458; in Arenicola, 240; in Arthropoda, 279; in Balanoglossus, 588; in Chaetopoda, 230; in Ciona, 596; in Copepoda, 314, 334; in Echino- dermata, 553; in Insecta, 390; in Lamellibranchiata, 505 ; in Mala- 6i6 INDEX Circulation of blood (cont.) costraca, 3 1 2 ; in Nemertea, 207 ; in Ostracoda, 314; in Scorpionidea, 450. See also Vascular system Circulation of food, in Alcyonium, 160; in Aurelia, 152; in Daphnia, 328; in Ohelia, 134 Circulatory system, see Vascular system Circulus venosus, 484 Circumoesophageal connectives, see Nervous system Cirrhal ossicles, 580 Cirrhals, 580 Cirri, of Crinoidea, 578 ; of Protozoa, 14; of Thoracica, 340 Cirripedia, 338; 294, 297, 314, 316 Cladocera, 324; 291, 317 Classification, vii, i ; of Brachiopoda, 542; of Cephalopoda, 512; of Gasteropoda, 489 ; of Lamelli- branchiata, 504; of Myriapoda, 375; of Opisthobranchiata, 494; of Polyzoa, 536; of Protozoa, 38; of Pulmonata, 495 ; of Radiolaria, 7 1 Clathrina, 117 Clathrulina, 81; 8, 10 Clava squamata, 132 Clavelina, 602; 599 C. lepadiformis , 602 Clavularia, 160 Cleavage or Segmentation of Ovum, in Archiannelida, 247-8 ; in Arthro- poda, 280; in Balanoglossus, 590; Centrolecithal, 280, 314; in Ciona, 597; in Crustacea, 314; in Ctenophora, 173 ; in Echinoder- mata, 555; in Medusae, 131; in Mollusca, 247-8 ; in Nemertea, 247-8 ; in Polychaeta, 247-8 ; in Polycladida, 247-8; in Pyrosoma, 684; Radial spiral, 248 Clepsine, 244, 265, 266 Cliona, 119 Clitellum, 253 ; 256, 259 Cloaca, of Holothuroidea, 575 ; of Nematoda, 220; of Rotifera, 212; of Tunicata, 595 Clypeaster, 571 Clypeastroida, 571 ; 570 Cnidaria, 131 ; 125 Cnidoblast, see Thread cell Cnidocil, 127 Cnidosporidia, 93 Coarctate pupae, 440 ; 402 Coccidia, 83 Coccidiomorpha, 83 Coccolithophoridae, 46 Coccoliths, 46 Codosiga, 60 ; 4 C. umbellata, 59 Coelenterata, 125; i, 120, 171, 173 Coeliac canal, 580 Coelom, 122; 2, 121; of Acantho- hdella, 267 ; of Annelida, 228 ; of Archiannelid genera, 261, 262, 263 ; of Arenicola, 240 ; of Arthro- poda, 279 ; of Balanoglossus, 585 ; of Brachiopoda, 540, 542 ; of Cepha- lochorda, 583, 584; of Chaeto- gnatha, 542, 544; of Chaetopoda, 228, 229; of Chordata, 583; of Crinoidea, 580 ; of Crustacea, 309 ; of Dipleurula, 551 ; of Echinoder- mata, 550, 551, 557; of Entero- pneusta, 583, 584; of Gephyrea, 229; of Hirudinea, 228, 265; of Mollusca, 470 ; of Peripatus, 283 ; of Polyzoa, 530; of Sepia, 518; of Snail, 483 ; of Tunicata, 584, 597 ; of Vertebrata, 584. See also Peri- cardium, Perivisceral cavity Coelomoducts, 123; of Arthropoda, 279; of Chaetopoda, 229; of Crustacea, 311; of Polychaeta, 242 Coeloplana, 173 Coenosarc, of Calyptoblastea, 133; of Polyzoa, 532 Colacium, 48 Coleoptera, 429 Coleopterous larvae, 401 Collar, of Balanoglossus, 585 ; of Chae- topterus, 237 ; of Choanoflagellata, 59, 60; of Pterobranchia, 590 Collar cavities, 584 ; of Balanoglossus, 585 Collar cells, see Choanocytes Collar pores, 584; of Balanoglossus, 585 Collembola, 408 Colleterial glands, 397 Collinia, 100; 28 n. Colloblasts, see Lasso cells Collozoum, 76 C. inerme, 36 Collum, 381 Colon, see Large intestine INDEX 617 Colonies, of Alcyonaria, 160; of Carchesium, 104; of Hydrozoa, 132; of Polyzoa, 530, 532; of Protozoa, 4 ; of Rhabdopleitra, 590 ; of Siphonophora, 145 ; of Syllis ramosa, 246; of Tunicata, 599; of Volvocina, 51 ; of Zoantharia, 163 Colpidium, 102 Colpoda, 102; 37 C. steini, loi Columella muscle, 481 Combinations of genes, 30 Compasses, 567 Compensation sac, 532, 533 Complemental males, 341 Compound ascidians, 600 Compound eyes, see Eyes Conchostraca, 324; 317 Conjugants, 28 Conjugation, 26. See also Syngamy Contractile vacuoles, 16; oi Euglena, 46 ; of Heliozoa, 78 ; of Protozoa, 16 Conns, 488, 490 Convoluta roscoffensis, 42 ; 43, 50, 188 C. henseni, 189 Copepoda, 331; 291, 337 Copidosoma gelechiae, 396 Copromonas, 48 ; 26 C. sub tilts, 47 Coprozoic Protozoa, 38; 26 Copulatory bursa, 220 Copulatorj^ spicules, 220 Cor allium, 16 1, 163 C. ruhrum, 161 Corals, of Alcyonaria, 162; of Hydrocorallinae, 143 ; of Madre- poraria, 163, 166-9; of Polyzoa, 532 Cordylophora, 142 Corethra, 390 Corm, 298 Cormidium, 145 Cornularia, 160 Corona, 564 Corpuscles, 122. See also Blood Cortex, of Ciliata, 98 ; of Porifera, 113 ; of Protozoa, 9 Corticata, 39 Coryne, 467 Cotton spinner, see Holothuria Course of circulation, see Circulation of blood Covering layer in Porifera, no Coxa, 385 Coxal glands, of Arachnida*, 448, 457 ; of Arthropoda, 279 ; of Peri- patus, 285 Coxopodite, 298 Crangon, 372 Crania, 540 ; 539, 542 Craspedochilus, 474 Crinoidea, 577 ; 547 Crisia, 534, 537 Cristatella, 536 Crithidia, 58 ; 57, 59 Crop, of Gasteropoda, 485 ; of In- secta, 387; of Opisthobranchiata, 494 Crotchets, 259 Crustacea, 291 ; 271 Crustacean-insect-myriapod section , 270 Cryptomitoses, 19 Cryptomonadina, 46; 43, 47 Cryptomonas, 46 C. ovata, 47 Cryptoniscus, 357 C. paguri, 358 Crystal cells, see Vitrellae Crystalline cone, 277 Crystalline style, 503 Ctenidia,47o;of Cephalopoda*, 513, 517, 527; of Chiton, 476; of Gasteropoda*, 477, 478, 479, 490, 491, 494; of Lamellibranchiata*, 499, 500, 501, 502, 504, 507, 510, 512; of Mollusca, 471 Ctenidial circulation of Lamelli- branchiata, 506 Ctenocephalus canis, 441 Ctenophora, 170; 125 Ctenoplana, 173 Ctenopoda, 325 Ctenostomata, 537 Cucumaria, 577 ; 576 Culex pipiens, 438 Cumacea, 353 Cunina, 143 Cup-shaped organs, 265 Cursoria, 409 Cuspidaria, 504 Cuticle, of Annelida, 228; of Ar- thropoda, 274 ; of Crustacea, 295 ; of Nematoda, 215; of Protozoa, 8 ; of Rotifera, 209 ; of Trematoda, 191 Cuvierian organs, 575 Cyamus, 360 Cyanea arctica, 150 6i8 INDEX Cyathomonas, 46; 11, 41 C trimcata, 47 Cyathozooid, 603 ; 605 Cyclas, 504 Cyclestheria hislopi, 299 Cyclomyaria, see Doliolida Cyclops, 331 ; 190, 202, 223, 315, 316, 332, 333, 333, 334, 335 Cyclosalpa, 607 ; 604 Cyclospora, 28 Cyclostomata, 537 Cyphonautes larva, 534; 535 Cypris, 331; 305, 330 Cypris larva, 316, 340, 341, 343, 344, 345 Cyrtoceras, 528 Cysticercus, 202; 204 C. pisiformis, 202 Cystoflagellata, see Noctiluca Cystoidea, 582 Cysts, 16 ; 8 ; Resistance, 17 ; Resting, 18. See also Gamocysts, Oocysts, Sporocysts Cytostome, 15; 97 Dactylopiiis coccus, 420 D. tomentosus, 420 Dactylopodite, 300 Dactylozooids, 143 ; 145 Dalyellia viridis, 189; 182 Daphnia, 325 ; 317, 328 D. pulex, 327 Dart, 226 Dart sac, 486 Dead men's fingers, see Alcyonium digitatum Decapoda, Cephalopoda, 513, 524; Crustacea, 361 Deep oral nervous system, 554 Deima, 577 ; 576 Demodex follictdorum, 469 Demospongiae, 118; 116 Dendritic tentacles, 573 Dendrochirotae, 577 ; 575 Dendrocoeliim lacteum, 178, 187,187, 189 Dendrocometes, 109; 28 n. D. paradoxus, loi Dendroid graptolites, 150 Dense nuclei, 19 Dentalium, 498 ; 498 Depression, 30 Dermal layer, no Dermaptera, 411 Detorsion, 479 Deutocerebrum, 274, 305 Deutomerite, 90 Development, see Embryology, Lar- vae, Life cycle Diastylis, 354 D. stygia, 354 Dibothriata, 204 Dibranchiata, 513; 523, 525 Dicyclical rotifers, 213 Didymograptus, 148; 149, 150 D. affinis, 150 D.fascicidatus, 150 D. v-f r actus y 149 Difflugia, 68 ; 8 D. urceolata, 69 Digestion, by Acarina, 462 ; by Al- cyonium, 160; by Arenicola, 240; by Arthropoda, 278 ; by Aurelia, 153; by Coelenterata, 126; by Crustacea, 308; by Daphnia, 328; by Helix, 486; by Insecta, 390; by Lamellibranchiata, 503; by Oligochaeta, 254; by Ostrea, 510; by Physalia,i45 ; by Protozoa, 15 ; by Rotifera, 212; by Teredo, 511 ; by Turbellaria, 181; by Zoan- tharia, 164. See also Alimentary canal, Circulation of food, External digestion Digestive caeca, see Digestive gland Digestive gland, of Arachnida*, 445, 450, 457, 458 ; of Brachiopoda, 540; of Mollusca*, 470, 485, 494, 503, 520. See also Liver, Mesen- teric caeca Digestive system, of Alcyonium, 159 ; of Aurelia, 152; of Medusae, 130; of Platyhelminthes*, 181, 188, 189, 190, 198; of Zoantharia, 164, 165, 166. See also Alimentary canal, Enteron Dimorpha, 78 D. mutans, 79 Dimorphic shells, 70 Dinamoebidium, 49 Dinobryon, 44 D. sertularia, 45 Dinoflagellata, 48; 43,46 n., 75 Dinophilus, 262; 261, 262, 263 Dinophysinae, 49 Dinothrix, 49 Diotocardia, 489 ; 478, 490 Diphyes, 145 INDEX 619 Dipleurula, 55S; 55i, 552 Diploblastica, 120. See also Coelen- terata Diplograptus, 148; 150 D.foliaceus, 149 Diplomonadina, 60, 61, 62 Diplopoda, 379 Diplopores, 582 Diploporida, 582 Diplozoon, 193; 194 Diptera, 436; 386, 439 Dipylidium caninum, 201, 204 Direct wing muscles, 386 Directives, 163, 164 Discomedusae, 152 Dissosteira Carolina, 392 Distephanus, 46 D. speculum, 45 Distomiim macrostomiim, 194 Division of protozoan nuclei, 19 Docoglossa, 489 Dolichoglossus kowalevskii, 586 Doliolida, 607; 600, 603, 604 Doliolum, 608 ; 599, 600, 606 Donax, 503 Doris, 494 ; 480, 493 Dorsal and ventral, 124; aspects of bilateral animals, 124; aspects of Ciliata, 97; aspects of Holothu- roidea, 572, 573 ; aspects of Sepia, 515; "blood vessels" of Echi- noidea and Holothuroidea, 553, 567, 575 ; mesenteries of Alcyo- naria, 159; structures in radial animals, 124 Dorsal antenna, 212 Dorsal blood vessel, of Arthropoda, see Aorta; of Chaetopoda, 230, 240 Dorsal "blood vessels" of Echino- dermata, 553, 567, 575 Dorsal cirrus, 23 1 Dorsal lamina, 600 Dorsal organ, 297 Dorsal plates, 562 Dorsal pores, 254 Dorsal shield, 297 and n. Dorsal siphon, 501 Dorsal tubercle, 592 Dorsolateral antennae, 212 Drag line, 461 Dreissensia, 473, 507 Drift net, of Physalia, 145 ; 147 Ductus communis, 185, 186, 199 Dytisciis, 393; 385 D. niarginalis, 396 Earthworms, 253 Ecardines, 542 Ecdysis, see Moult Echinaster sentiis, 560 Echinocardiwn, 571 E. cordatum, ^Ti Echinocyamus, 571 E. pusilhis, 571 Echinodermata, 547; 2, 122, 124 Echinoidea, 564; 547 Echinopluteus, 556 Echinosphaera, 582 Echinus, 567, 567, 570, 570, 571 E. esculentus, 564 E. miliaris, 565 Echiuroidea, 268 Echiurus, 268 ; 268 Ectoderm, 120, i ; 121, 125, 129, 248 Ectoneural system, 554; of Crinoidea, 580 Ectoplasm, 8; 15, 39, 62, 63, 65, 71, 90, 95, 97, 98 Ectoprocta, 536 Edrioaster, 582 E. bigsbyi, 581 Edrioasteroidea, see Thecoidea Edwardsia, 164; 165 Efferent canals, see Exhalant canals Egg sac, 334 , Eggs and Egg laying, of Arachnida*, 452, 457, 461 ; of Arthropoda, 280; of Balanoglossus, 590 ; of Buccinum, 492; of Chaetognatha, 544; of Ciona, 597 ; of Cnidaria*, 131, 135, 138, 139, 140, 152, 155, 160; of Crustacea*, 3 14, 3 18, 321,324, 329, 334, 338, 340, 345, 368 ; of Cteno- phora, 173 ; of Dinophilus, 263 ; of Echinodermata, 554; of Hiru- dinea, 266; of Insecta*, 411, 415, 417, 419, 420, 423, 424, 425, 426, 427, 429, 442; of lulus, 381; of Myriapoda*, 379, 381; of Nema- toda, 218, 220, 222, 223, 225, 227; of Oligochaeta, 256 ; of Pantopoda, 467 ; of Peripatus, 285 ; of Platy- helminthes*, 185, 186, 187, 193, 194, 196, 199, 201 ; of Polychaeta, 243 ; of Pulmonata, 481, 488, 496; of Rotifera, 213 ; of Sepia, 522; of Thaliacea, 604 620 INDEX Eimeria, 83 ; 85, 91 E. schuhergi, 22, 83, 84 Ejaculate ry duct, 397 Elasipoda, 577 Eledone, 520 Eleutherozoa, 582; 550 Elytra, of Coleoptera, 429 ; of Poly- chaeta, 233 Embia major, 415 Embioptera, 414 Embryology, of Arachnida, 443, 448 ; of Arthropoda, 280 ; of Asterias, 555; of Brachiopoda, 540, 541 ; of Chaetognatha, 544, 545; of Chordata, 582; of Ciona, 597; of Coelenterata, 131; of Echinodermata, 555 ; ofLumbricus, 256, 257; oi Peripattis, 283; of Polyzoa, 535; of Tardigrada, 468; of trochosphere, 248. See also Larvae Embryonic fission, see Polyembryony Enchylema, 8 End gut, see Hind gut End sac, 279, 309 Endites, 300 Endocyclica, 57 1 ; 570 Endoderm, 120; i, 121, 125, 131, 248 Endoderm lamella, 130; 156 Endomixis, 30; 21 Endophragmal skeleton, 303 Endopodite, of Crustacea, 298 ; 299 ; of Trilobita, 288 ; of Xiphosura, 456. See also Limbs Endoprocta, 536 Endopterygota, 425 Endopterygote development of wings, 402 Endosome, 19 Endosternite, 303 Endostyle, 594, 595, 597 Entamoeba, 64; 66 E. coli, 65 ; 66 E. dysenteriae, see E. histolytica E. histolytica, 65 ; 38 Enteron, 125, 129, 130, 133, 136, 140. See also Archenteron, Di- gestive system Enteropneusta, 585 ; 2, 552, 557, 584, 596 Entodiniomorpha, 102 Entodinium, 104 E. caudatum, loi Entomostraca, 294 Envelope cells, 93 £:oZw, 494; 479, 493 Epeira, 460 E. diademata, 459 Ephelota, 108 E. gemmipara, 109 Ephe?nera vidgata, 421 Ephemeroptera, 420; 393 Ephippium, 329 Ephyra larva, 155; 151 Epibolic gastrula, 248 Epibranchial space, of Decapoda, 366 ; of Lamellibranchiata, 501 Epicardial cavity, 596; diverticula, 596, 599; tube, 596, 599 Epicardium, see Epicardial cavity, etc. Epidermis, 121 Epimerite, 90 Epineural canal, 548 ; of Echinoidea, 570 ; of Ophiuroidea, 562 Epipharynx, 383. See also Mouth parts of Insecta Epiphragma, 489 Epipodites, 278, 298, 299, 300. See also Gills of Crustacea, Me- tepipodites, Oostegites, Proepi- podites Epistome, of Decapoda, 367; of Polyzoa, 530 Epistylis, 107 Epizoanthus, 118 Equitant whorls, 71 Eriatoma, 59 Eriocrania, 429 Eruciform larvae, 401 ; of Coleoptera, 430 Estheria, 324; 309, 310 E. minuta, 324 Eucarida, 361 ; 349 Eucephalous larva, 439 Eucystis, 582 Eudendrium, 137; 140, 141 Eudorina, 52; 6 Euglena, 46; 16, 33, 38, 41, 48 E. viridis, 35, 37, 4°, 47 Euglenoid movement, see Metaboly Euglenoidina, 46; 15, 43, 47 Euglypha, 68 ; 8 E. alveolata, 10 ; 67 Eugregarinaria, 90; 83 Eulalia, 235; 231, 233 Eulamellibranchiata, 501 ; 504, 510 Eumitoses, 19 INDEX 621 Eunice, 235; 231, 240 Euotiyjnus, 419, 420 Eupagurus, 373 E. bernhardus, 373 Euphausiacea, 361 ; 348 Euplectella, 118 £'M^owa^z/5,Trochosphere larva of,25l Eurypterida, 452 Eurypterus, 454 Euspotigia, 119; 113, 116 Euthyneury, 481, 495 Eutyphoeus, 254 Exarate pupae, 402 Excreta, see Excretion Excretion, of Arthropoda, 280; of Crustacea, 309 ; of Echinodermat^, 552; of Metazoa, 123; of Mol- lusca, 483; of Protozoa, 16; of Tunicata, 597. See also Excretory organs Excretory organs, 123 ; of Arachnida, 279, 280, 448 ; of Arthropoda, 279 ; of Balanoglossus, 588; of Crus- tacea, 279, 309 ; see also Antennary glands, Maxillary glands; of In- secta, 279, 390; of Myriapoda, 379, 380; of Nematoda, 215, 218; of Nemertea, 207 ; of Onychophora, 279, 284; of Platyhelminthes, 177; of Polychaeta, 242; of Rotifera, 212. ^ee«/^oCoelomoducts, Coxal glands. Glomerulus, Kidneys, Malpighian tubules, Nephridia Exhalant canals of Porifera, 113 Exhalant passage of Carctnus, 366 Exites, 300 Exocyclica, 570 Exogamous syngamy, 28 Exopodite, of Crustacea, 298, 300; of Trilobita, 288; of Xiphosura, 456, See also Limbs Exopterygota, 409 ; 402 Exopterygote development of wings, 402 External digestion, by Araneida, 458; by Insecta, 390; by Oligo- chaeta, 254; by Rhizostomeae, 156; by Turbellaria, 183 Extrathecal zone, 168 Exumbrellar surface, 151, 157 Eyes, Compound, 274, 277; Crusta- cean median, 305, 305; of Arach- nida*, 274, 449, 457, 460; of Arthropoda, 274, 276 ; of Ascidian tadpole, 598; of Crustacea*, 274, 277, 305, 318, 325, 333, 338, 340, 348, 355; of Hydrozoa, 139, 140; of Insecta, 274, 383 ; of Mollusca*, 482, 495, 509, 522, 523, 527; of Myriapoda*, 374, 379 ; of Nemer- tea, 207 ; of Onychophora, 274 ; of Polychaeta, 230, 231 ; of Trilobita, 287 ; of Turbellaria, 175, 176, 176. See also Eye-spots Eye-spots, of Asteroidea, 554; of Protozoa, 14 Eyestalk, 367 Facial suture, 287 Falciform young, 33 ; 90 Fasciola, 194; 191, 195 F. hepatica, 197 Fat body, 390, 39 1 Favia, 169 Feeding, of Actinozoa*, 160, 170; of Arachnoidea (Suctorial), 445, 450, 458, 462, 467 ; of Asteroidea, 560; of Balanoglossus, 587 ; of Brachio- poda, 539, 540; of Branchiopoda*, 317, 320, 321, 324, 325, 328; of Carcinus, 368 ; of Cephalopoda, 519; of Chaetognatha, 542; of Chaetopterus, 238; of Chordata, 580 ; of Ciliata, 97 ; of Ciona, 595 ; of Copepoda, 333 ; of Crinoidea (Antedon), 579; of Crustacea, 290- 95 ; of Cypris, 3 3 1 ; of Diotocardia*, 490, 491 ; of Echinoidea*, 567, 569, 57 1 ; of Errant Polychaeta*, 23 1 ; of Filter-feeding Malacostraca*, 348, 350, 353, 361 ; of Holothuroidea*, 575 ; of Holozoic Mastigophora*, 41, 44, 46, 48, 49, 56, 59, 61, 63 ; of Hydatina, 212 ; of Hydrocorallinae, 143; of Lamellibranchiata*, 501, 502, 507, 511, 512; of Lepas, 340; oiLimiilus, 457 ; of Monotocardia*, 491, 492; of Nematoda, 218; of Nemertea, 205 ; of Oikopleura, 600; of Ophiuroidea, 504; of Opisthobranchiata*, 494, 495 ; of Physalia, 165 ; of Polyzoa, 531 ; of Protozoa, 15 ; of Pulmonata*, 496 ; of Sarcodina*, 65, 72, 78, 81 ; of Suctoria, 107; of Temnocephala, 190 ; of Trilobita, 288 ; of Tubico- lous Polychaeta, 235; of Turbel- laria, 181 ; of Veliger, 510 622 INDEX Feet, of Histriohdella, 263 ; of Onychophora, 283 Female gametes, 26 ; of Metazoa, see Eggs ; of Forifera, 1 1 1 ; of Protozoa, 26, 27, 28, 29, 80, 84, 85, 86, 87, 91 Femur, 385 Filaria, 221 F. bancrofti, 223, 224 F. loa, 223 F. medinensis, 223 Filibranchiata, 501, 504, 508 Filograna, 231 Filopodia, 11 Finger-and-toe disease, 82 Fission, of Metazoa, see Budding, Strobilization ; of Protozoa, see Fission of Protozoa Fission of Protozoa*, 24; Binary, 24, 32, 40, 48, 64, 67, 68, 69, 75, 78, 81, 107; by budding, 24, 68, 78, 107; Longitudinal, 24, 40, 48; Multiple, 24, 32, 48, 59, 64, 67, 82, 83, 85, 88, 90; Oblique, 40; Pseudotransverse, 24, 25, 40; Radial, 24, 25, 53 ; Repeated, 24, 40, 48, 51; Transverse, 24. See also Plasmotomy Fissurella, 490 ; 478, 480, 489 Fixation disc, 557 Fixation papillae, 598 Flabellum, 300 Flagella, 11 Flagellata, see Mastigophora Flagellated chambers, 112 Flagellispores, see Flagellulae Flagellulae, 33 Flagellum, of Crustacean limbs, 300 ; of Helix, 486 Flatworms, see Platyhelminthes Floscularia, 213 Flustra, 532; 537 Follicle cells, 522 Follicles, Gonadial, 397, 486 Food, see Feeding Food groove, of Chirocephalns, 320; of Lamellibranchiata, 502 Foot, of Amphineura*, 474; of Cephalopoda*, 512, 515, 527; of Helix, 48 1 ; of Lamellibranchiata*, 499, 507, 508, 509, 512; of Mol- lusca, 470, 474; of Opisthobran- chiata*, 494; of Scaphopoda, 498 Foraminifera, 67 ; 63 Forceps, 411 Forcipulate, 558 Fore gut, see Stomodaeum Forficula auricularia, 411 ; 412 Fossil record of Insecta, 402 Frass, 226 Frenulum, 429 Frilled organ, 198, 199 Front of Carciniis, 365 Frontal appendage, 318 Frontal cilia, 501 Frontal horns, 340 Frontal organs, 307 Frontal surface, 532 Frontonia leucas, 99 Fungia, 170; 169 Fungus gardens, 414 Funiculus, 530 Funnel, of Cephalopoda*, 515, 527; of coelomoduct, 242; of nephro- mixium, 242 Furca, 303 Furcal rami, 340 Furcula, 408 Galathea, 374 Gametes, 26; of CiHophora, 28; of Cnidosporidia, 93, 94; of Fora- minifera*, 67, 69, 71, 74; of Heliozoa*, 28, 78, 80, 80; of Mastigophora, 40 ; of Metazoa, see Eggs, Spermatozoa ; of Monocystis, 27, 91; of Mycetozoa, 33, 82; of Opalina, 100; of Protozoa, 23, 24, 30, 31, 33; of Radiolaria, 75; of Telosporidia, 83-91 ; of Volvocina, 26, 27, 33, 40, 50-52 Gammariis, 358; 100, 107, 109, 181, 360 G. neglectus, 359 Gamocysts, 18; 88, 89, 90, 91 Gamogony, 32 Gamonts, 31; 32, 83-91, 87, 89, 91, 93 . Ganglia, of Ophiuroidea, 562; of ventral cord, 229, 274, 303, 334, 370,. 379, 397, 452, 457, 467, 4^8 Ganglia, System of, see Nervous system Ganglion, Antennal, 274, 303 ; Bra- chial, 520; Cerebral (Supraoeso- phageal), 122, 174, 207, 286, 379, 452, 457, 467, 468, 477, 499, 507, 520, 521; see also Brain; Gastric, Ganglion {cont ) 522 ; Inferior buccal, 522 ; Infundi- bular, 520, 522 ; of Ciona, 596, 599 ; of Rhizocephala, 303, 344; Pedal, 477, 520; Pleural, 477, 499, 507; Prostomial, see Cerebral; Suboe- sophageal, 263, 303, 340, 379, 390, 452, 457, 459, 467, 468; Superior buccal, 522 ; Supraoesophageal, see Cerebral; Trunk, of Appendicu- laria larva, 597; Visceral, 521 Gasteropoda, 476 Gasterostomimi, 197 G. fimbriatum, 194, 197 Gasterozooids, of Doliolida, 608 ; 606; of Siphonophora, 145, 147 Gastral layer, no Gastric cavity, 130 Gastric filaments, 152 Gastric glands, 212 Gastric mill, 307 Gastric shield, 503 Gas trades, 173 Gastrula, 120; of Echinodermata, 555 ; of Polychaeta, 248 Gastrulation, of Echinodermata, 555 ; of Polychaeta, 248 ; 249 Gecarcinus, 374; 372 Gemmules, 116 Generative organs, of Amphineura, 475; of Arachnida*, 452, 457, 461, 465; of Archiannelida*, 261, 262; of Arthropoda, 280; of Balanoglossus, 586, 589; of Bra- chiopoda, 540; of Chaetognatha, 544 ; of Chaetopoda, 229 ; of Chilo- poda, 378; of Cnidaria*, 135, 152, 155, 160; of Crustacea*, 314, 315, 321, 324, 329, 334, 337, 340, 346, 356, 370, 373 ; of Ctenophora, 172 ; of Diplopoda, 381 ; of Echinoder- mata, 553, 561, 564, 570, 580; of Hirudinea, 265, 267 ; of Insecta, 396, 396, 397; of Lamellibran- chiata*, 507, 509, 510; of Mol- lusca, 271 ; of Nematoda, 218; of Nemertea, 207 ; of Oligochaeta*, 253, 254, 259; of Onychophora, 285 ; of Opisthobranchiata, 494, 495; of Platyhelminthes*, 183, 190, 192,199,201,203; ofPoly- chaeta,229, 242, 244 ; of Pulmonata, 486,487, 496 ; oiiUiahditis, 216 ; of Rotifera,2i2; of Scaphopoda,499; INDEX 623 of Sepia, 522; of Streptoneura*, 478, 490, 493 ; of Tunicata, 596, 600 Generative pore, see Generative organs Genital aperture, see Generative organs Genital atrium, of Helix, 486; of Platyhelminthes, 185; oi Stylaria, 259 Genital bursae, 553, 564 Genital canal, 580 Genital coelom, 518 Genital cords, 580 Genital ducts, see Generative organs Genital opening, see Generative organs Genital operculum, 450 Genital organs, see Generative organs Genital plate, 564 Genital pleurae, 586 Genital rachis, 553; of Crinoidea, 553, 580; of Echinoidea, 533, 570 Genital stolon, see Axial organ Genital system, see Generative organs Geonemertes, 208 Gephyrea, 268; 229 Gerardia, 347 Germarium, 397; 212 Germs (gametes), see Gametes; of Cnidosporidia, 93 Geryonia, 143 Giardia, 62 G. intestinalis, 61 Gid, 202 ' ' GiW oi Salpa, 607 Gill books, 445 ; 278, 454, 455, 457 Gill chamber of Decapoda, 366 Gill clefts, of ascidian tadpole, 597; of Balanoglossus, 581, 585, 587 ; of Cephalodiscus, 581, 590; of Chor- data, 583 ; of Thaliacea, 602 ; of Tunicata, 581 Gill filaments, 499 Gill lamellae, 499 Gill plates of Lamellibranchiata, 499 Gill pouch, see Gill clefts Gill slits, see Gill clefts Gills, of Arthropoda, 278; of As- teroidea, 553, 558; of Crustacea*, 311, 340, 350, 351, 353, 354, 359, 361, 366, see also Epipodites; of 624 INDEX Gills {cont.) Echinodermata, 553 ; of Echinoi- dea*, 553, 566, 569; of Limulus, 278, 445, 454, 455, 457; of Mol- ]usca,see Ctenidia; of Polychaeta*, 230, 235, 240; of Salpida, 604; Tracheal, see Tracheal gills Girdle of Chiton, 476 Gizzard, of Earthworms, 254 ; of In- secta, see Proventriculus Glabella, 287 Glands, Aciniform, 461 ; Aggregate, 460 ; Ampulliform, 460 ; Antennal, see Antennal glands; Maxillary, see Maxillary glands; Mucous, 486; Oesophageal, 254; of ali- mentary canal, see Alimentary canal, Digestive gland. Liver; Pedal, 482; Prostate, see Prostate glands; Pyriform, 461; Shell, of Platyhelminthes,i85 ;Spermiducal, see Prostate glands ; Spinning, 460 ; TubuHform, 461 Globigerina, 35 ; 71 G. bulloides, 9 Globigerinidae, 71 Glochidium, 507 Glomeris, 381 Glomerulus, 588 ; 590 Glossina, 439; 397, 59 1 G. subynorsitans, 438 Glossiphonia, 267 ; 264 GlossobalanuSy 587, 588 Glycera, 242 ; 23 1 Glyptoscorpius , 452; 454 Gnathobase, 274 ; of Arachnida*, 445, 450, 455, 460; of Crustacea*, 300, 301,320,325,328; ofTrilobita,288 Gnathobdellidae, 267 ; 263 Gnathochilarium, 380; 380 Gonads, 121 ; of Arthropoda, 280; of Balanoglossus, 589 ;of Chaetopoda* 229, 242, 253 ; of Coelenterata*, 135, 152, 160, 172; of Crustacea* 314, 321, 329, 334, 350, 370; of Echinodermata*, 553, 561, 570, 580; of Nemertea, 205. See also Generative organs Gonapophyses, 387 Gonophore, 138 Gonopods, 378 Gonothecae, 134 Gonozooids, of Doliolida, 608; of Siphonophora, 145 Gorgonacea, 161 Grantia, 117 ' G. extusarticulata, 118 Graphosoma italicum, 418 Graptolitoidea, 148; 133 Gregarina, 93 G. cuneata, 22 G. longa, 92 Gregarinidea, 88 ; 83 Grub, 402 Grylloblatta, 411 Gryllotalpa, 411 ; 385 Gryllus, 411 Guard, 524 Gullet, of Metazoa (Oesophagus), see Alimentary canal; of Phytoma- stigina, 41, 43; of Protozoa*, 15; 41, 43, 46, 48, 97, 100, 102 Gunda segmentata, see Procerodes lobata Gymnoblastea, 133; 136-9, 140, 142. See also Anthomedusae Gymnolaemata, 537 Gymnomera, 329; 324 Gymnomyxa, 39 Gymnospores, 33 Gymnostomata, 100 Gyroceras, 528 Haetnadipsa, 268 Haematochrome, 41 ; 51 Haematococcus , 50; 35 H. lacustris, 51 Haemocoele, 122; of Arthropoda, 279; of Echinodermata, 550; of Helix, 483 ; of Insecta, 391 ; of Mollusca, 470; of Peripatus, 283, 284; of Rotifera, 209. See also Vascular system Haemocyanin, 314; 450, 470 Haemoglobin, 207, 230, 314, 391, 546 Haemogregarina, 85 Haemonchus, 221, 222 Haemopis, 267 Haemoproteiis, 86 Haemosporidia, 85 ; 83 Halcampa, 164 Halesus guttatipennis , 426 Halichondria, 119 Haliclystus, 150 Haliotis, 490; 478, 480, 488, 489 H. tiiberculata, 497 Halistemma, 145 INDEX 625 Hamitermes silvestri, 413 Hamula, 408 Haplosporidia, 95 Haplosporidium, 95 ; 20 H. limnodrili, 21 Hatschek's pit, 584 Head, 124; of Arthropoda, 271; of Chaetopoda, 230; of Crustacea*, 271, 296, 297, 318, 322, 325, 329, 331, 334> 338, 347, 349, 355, 358; of Insecta*, 271, 382, 383, 384, 407, 408, 423 ; of Mollusca*, 417, 476, 477, 481, 485, 494, 498, 513 ; of Myriapoda*, 271 ; 375, 377, 379; of Onychophora, 271, 282; of Trilobita, 287, 288 Head cavity of Chordata, 583 Head foot, 481 Head kidney, 251 Heart, 122; of Arachnida*, 448, 450, 458, 462, 467 ; of Arthropoda, 279; of Balajioglossus, 588; of Brachiopoda, 540; of Ciona, 596, 598; of Crustacea*^, 311, 325, 331, 334, 349, 350, 351, 355, 359; of Insecta, 370; of Mollusca*, 471, 477, 478, 479, 481, 482, 499, 505, 518 Hearts, Branchial, 519; oi Arenicola, 240 ; of Chaetopoda, 230 ; of Oligo- chaeta, 257 Heliolites, 163 Heliopora, 162, 163 Heliosphaera, 76 H. inermis, 76 Heliozoa, 78 Helix, 481 ; 485, 486, 492, 495, 496, 523 H. aspersa, 481 H. pomatia, 481, 483, 484, 485, 487 Helkesimastix, 56 n. Hemiaspis, 445 ; 457 H. limuloides, 453 Hemichorda, see Enteropneusta Hemimetabola, see Exopterygota Hemimyaria, see Salpida Hefnhnysis, 353 Hemiptera, 417 Hepatic caeca, see Mesenteric caeca Hepatic diverticula, see Mesenteric caeca Hepatopancreas, see Digestive gland Heptagonia, Nymphal instars of, 422 Hermaea, 495 Hermaphrodite duct, 486 Hermaphroditism, of Chaetognatha, 544; of Crustacea*, 314, 337, 340, 341,343, 357; of Ctenophora, 172; of Hirudinea, 263 ; of Icerya, 394 ; of Mollusca*, 481, 493, 494, 495, 507, 510; of Nematoda, 220; of Oligochaeta, 253 ; of Platyhel- minthes, 183; of Polyzoa, 530; of Protodrilus, 261; of Protozoa, 28; of Tunicata*, 596, 600 Herpetomonas, 58 Herpyllohius, 337 Heterocotylea, 191 Heterodera, 225 ; 226 Heteronemertini, 209 Heteronereis, 235; 232, 247 Heteropoda, 492 Heteroptera, 418 ; 419 Heterotricha, 102 Hexactinellida, 118; 113 Hexactinian, 164 Hexamitus, 61 ; 6, 62 H. intestinalis, 7 Hexapoda, see Insecta Hind gut, see Proctodaeum Hinge of Lamellibranchiata, 499 Hippospongia, 119 Hirudinea, 263 ; 228 Hirudo, 267; 263, 264, 265, 266 Histriobdella, 263 ; 262 Holectypoida, 571 Hologametes, 26 Hologamy, 26 Holomastigina, 56 Holometabola, see Endopterygota Holophytic nutrition (Photosynthe- sis), 14 n.; I, 39, 40; of Phyto- mastigina*, 14, 36, 40, 41, 43, 44, 46, 49; of Protozoa, i, 14 Holophytic Protozoa, see Holophytic nutrition Holothiiria, 572, 573, 575, 576 H. tiibidosa, 574 Holothuroidea, 572; 547 Holotricha, 99 Holozoic nutrition, 14 n. ; of Phyto- mastigina*, 41, 44, 46, 48, 49; of Protozoa, 14, 37. See also Diges- tion, Feeding Holozoic Protozoa, see Holozoic nutrition Homarus, 373 Homoptera, 418; 387, 419 40 626 NDEX Hood of Tetrabranchiata, 527 Hoplocampa testiidinea, 433 Hoplocarida, see Stomatopoda Hormiphora plumosa, 171 House of Larvacea, 600 Houses of Protozoa, 8 Hyalonema, 118 Hyaloplasm, 8 n. Hydatid cyst, 202 Hydatina, 211, 212, 213 H. senta, 209, 210 Hydra, 120; 104, 125-9, i40> 142, 155, 169, 213 H. attenuata, 127 H. viridis, 170 Hydractinia, 142 Hydranths, 133, 135, 137. See also Polyps Hydratuba, 155 Hydrida, 133, See also Hydra Hydrocoele, see Water vascular system Hydrocorallinae, 133; 143 Hydroids, see Hydrozoa Hydrophyllium, 145 Hydropsyche, 426 Hydroptila maclachlani, 426 Hydro rhiza, 133 Hydrospires, 582 Hydrothecae, 150; 134 Hydrozoa, 132; 141 Hydrurus, 44 ; 45 Hylobiiis, 226 Hymetiolepis nana, 204 Hymenoptera, 432; 386 Hymenostomata, see Vestibulata Hypermastigina, 60, 62 Hyperparasite, see Cryptoniscus Hyperpharyngeal band, 594 Hypobranchial space, 366 Hypopharynx, 436; of Machilis, 407 Hypostoma, see Labrum Hypostome, 462 Hypotricha, 104; 97 Icerya purchasi, 394 Ichthyophthirius, 100 Ideal malacostracan, 347 Ideal mollusc, 471 Idiochromatin, 22 Idotea, 356 Ileum, see Small intestine Imaginal discs, 402 Imperforate shells, 69 Incisor process, 348 Indirect wing muscles, 386 Infero -marginal ossicles, 558 Inhalant canals, 112 Ink sac, 517 Insecta, 382; 271 Instar, 399 Intercalary segment of Chilopoda, 375 Interlamellar concrescences, 499 Interlamellar septum, 510 Interlamellar spaces, 501 Internal gills, see Stewart's organs Internal longitudinal bars, 594 Internal madreporite, 551 Internal sac of Polyzoa, 535 Internal skeleton, of Alcyonaria, 160-3; of Crustacea, 303; of Echinodermata*, 550, 558, 562, 564-7, 573, 579; of Metazoa, 121 ; of Porifera, 1 13 ; of Protozoa, 3, 8, 14 Interradial mesenteries, 151 Interradii of Echinodermata, 547 Intertentacular organ, 530 Intestine, see Alimentary canal Intracellular body ca\dt}^, 215 Invertebrata, i ; 2 Ips, 226 Irregular echinoids, 550, 570 Ischiopodite, 300 Isogamy, 26; 27, 40, 50, 69, 83. See also Gametes Isopoda, 354 Isoptera, 412 Isospores, 75 Iidus, 379; 375, 381 /, terrestris, 379, 380 Japyx, 406 Jaws, of Arthropoda, 271 ; see also Mouth parts; of Chaetognatha, 542 ; of Echinoidea, 567 ; of Helix, 484; of Onychophora, 270, 282; of Sepia (beak), 519 Jelly, of Coelenterata, see Structure- less lamella (Mesogloea) ; of Pori- fera, III, 113 Kakothrips rohiistus, 424 Karyogamy, 26 n. See also Syngamy Kar^'^olymph, 8 Karyosome, 19 Keratosa, 113 Kermes ilicis, 420 Kerona, 104 Kidneys of molluscs, 483 ; 471, 477, 478, 479, 482, 490, 492, 499, 506, 507, 518, 527. See also Renal openings, Renopericardial open- ings Kineto nucleus, 11. See also Para- basal body Labial hooks, 417 Labial palps, of Insecta, see Mouth parts of Insecta; of Lamellibran- chiata, 499, 501 ; of Protobran- chiata, 507 Labidiira, 411 Labium, 383, See also Mouth parts of Insecta Labrum, of Crustacea, 303 ; of In- secta, 383; of Trilobita, 287. See also Mouth parts Lacinia mobilis, 352; 355 Lacunar system, 553 ; of Antedoii, 580; of Echinoidea, 553, 570; of Holothuroidea, 553, 575 Lacunar tissue, 553 Lamhlia, see Giardia Lamellibranchiata, 499 Languets, 594 Lankesterella, 86 ; 85 Lantern coelom, 567 Large intestine of Insecta, 390 Larvacea, 601 ; 600 Larvae, Actimda, 138; Amphi- blastula, 1 19; Appendictdaria, 597 ; Auricularia, 556, 590; Bipinnaria, 556; Brachiolaria, 557; Crinoid, 557; "Cyclops", 334; 316, 335; Cyphonautes, 534, 535; "Cypns'\ 340; 316, 343, 345; Dipleurida, 555; 551; Echinoderm, 554-7, 556; Echinopluteus y 556, 557; Ephyra, 155; Euphausid, 361; Glochidium, 507; Insect, 399; see also names of orders; Megalopa, 372; MetanaupliuSy 316, 334; Nauplius, 314; 288, 296, 302, 305, 307, 315, 316, 321, 329, 334, 335, 340, 343, 344, 347, 349, 361, 372; Nematode, 221-7; Ophiopluteits , 556 ; of Pantopoda, 467 ; of Penta- stomida, 469; of Porifera, 116; Pilidium, 207; Planula, 131, 135, 155, 160; Pliitetis, 556; Protaspis, INDEX 627 288; Rhabditoid, 221; Stomato- pod, 351; Tornaria, 590; 557; "Trilobita", of Litntdus, 457; Trochosphere, i, 2, 229, 230, 247, 250, 472, 507, 532, 534; Veliger, 474, 507; Zoaea, 316, 349, 351, 361, 370, 372, 373 Larval arms of Echinodermata, 556 Larval nephridia, 250 Lasso cells, 170 Lasso of nematocyst, 128 Lateral cilia, 501 Lateral lines of Nematoda, 214 Latero-frontal cilia, 501 Laura, 347 Laurer's canal, 201 Leander, 372 Legs, of Arachnida*, 449, 455, 461, 465; of Insecta*, 385, 408, 409, 429, 430, 441; of Malacostraca*, 355, 356, 359, 362, 368, 372, 373; of Myriapoda*, 378, 381 Leishmania, 58 ; 59 Leodice, 231; 245 L.fucata, 246 L. viridis, 246 Lepadocrinus, 582 Lepas, 338; 340, 341, 341, 342 L. anatifera, 339, 342 Lepidocaris, 322 Lepidonotus, 231, 234 Lepidoptera, 427; 386 Lepiduriis, 322; 324 L. glacialis, 322 Lepisma, 404 L. saccharina, 406 ; 407 Leptocoris trivittatus, 419 Leptodora, 329 L. kindti, 330 Leptomedusae, 133; 135, 140, 142. See also Calyptoblastea Leptomonas, see Herpetonionas Leptostraca, 349 Lernaea, 335 ; 336 Lernanthropus, 314 Leucandra, 117; 113 L. aspersa, 114 Leucifer, 372 Leucochrysis, 46 Leucon grade, 113 Leucosolenia, 117 Libellula, 394 Lieherkiihnia, 69; 63, 67 L. wagneri, 69 40-2 628 NDEX Life cycle, 4 ; of Actinomyxidea, 93 , 95 ; of Aphididae, 419 ; of Cecido- myidae, 396; of Cestoda, 201; of Cladocera, 329; of Cnidosporidia, 93> 94> 95 5 of Coccidia*, 83, 84, 85, 86; of Coccidiomorpha, 83; of Coelenterata, 129; of Doliolida, 606 ; of Foraminifera, 67 ; of Gre- garinidea*, 88, 89, 90, 91, 92; of Haemosporidia*, 85, 86 ; of Hydro- zoa*, 132, 135-44; of Malacocoty- lea*, 194, 196; of Mycetozoa, 81; of Neosporidia, 82 ;of Polythalamia, 68, 70, 71, 74; of Radiolaria, 75; of Rotifera, 213 ; of Scyphomedu- sae, 155, 156, 157 ; of Sporozoa, 82 ; of Telosporidia, 83 ; of Trypano- somidae, 59; of Tunicata, 599 Life history, of Alcyonaria, 160; of Arthropoda, 250; of ascidians, 597; of Brachiopoda, 540; of Chaetognatha, 544; of Copepoda, 334 ; of Crustacea, 3 14 ; of Echino- dermata, 554; of Heterocotylea, 193; of Insecta, 398: see also names of orders; of Leptostraca, 350; of Malacostraca, 349; of Mollusca, 472 ; of Nematoda, 221- 7 ; of Nemertea, 207 ; of Peracari- da*, 352, 356, 359; of Polychaeta, 250; of Polyzoa, 534; of Porifera, no; of Protozoa, 33; of Scypho- medusae, 157; of Siphonophora, 144; of Trilobita, 288. See also Embryology, Larvae, Life cycle Ligia, 354; 356, 358 L. oceanica, 355 Limax amoebae, 38; 63, 64 Limbs, of Arthropoda, 270, 271, 272- 3, 274.; of Crustacea, 290, 292-3, 298 ; of Onychophora, 281, 282 ; of Trilobita, 288 . See also Abdominal limbs, Antennae, Antennules, Legs, Mandibles, Maxillae, Maxil- lules, Mouth parts, Pleopods, Thoracic limbs, Trunk limbs, Uropods Limnaea, 267, 495, 496 L. peregra, 497 Limnocnida, 143 Limnocodium, 143 Limnophiliis, 427 Limulus, 454; 274, 276, 277, 443, 445, 446, 448, 457, 458 L. Polyphemus, 455, 456 Lineus, 209 Linguatula taenioideSy 469, 469 Lingula, 540; 539, 541, 542 Linin, 8 Lithobius, 375 ; 83, 85, 272n.,378, 379 L.forficatus, 376, 377, 378 Lithocampe tschernyschevi, 75- Lithocircus, 76 L. annularis, 43 Lithodes, 373 L. maia, 374 Littorina, 492 ; 478, 490 L. rudis, 492 Lituites, 528 Liver, of Arachnida, 445 ; of Crusta- cea, 308, 320, 351, 361, 368; of Helix, 485, 486 ; of Sepia, 520. See also Digestive gland. Mesenteric caeca Living chamber of Tetrabranchiata, 526 Lizzia, 139 L. koellikeri, 139 Lohophy Ilium, 170 Lobopodia, 11 Locomotion of Protozoa, 11, 78 Locusta, 411 Loimia, 236 Lo/zlg^o, 513, 525, 526 Longitudinal band, see Ciliated band Longitudinal fission of Protozoa*, 24; 40, 48 Lophohelia, 168; 167 Lophophore, of Brachiopoda, 539; 537; of Polyzoa, 530 Loxodes, 100 Loxosoma, 536 Lucernaria, 150; 151, 152, 155 Luciae, see Pyrosomatida Lumbricidae, 253, 254, 256 Lumbriculus, 260; 259 L. variegatus, 260 Lumbricus, 253; 216, 228, 240, 242, 254, 256, 259, 260, 264, 266 L.foetidus, 257 L. terrestris, 255 Lung, 481, 482, 484, 496 Lung books, 278, 445-8, 449, 450, 458, 459 Machilis (Petrobius) maritimus, 384, 406, 407 Macrobiotus, 467, 468 1 Macrogametes, see Female gametes Macromeres, 248 Macrurous type, 362 Madreporic vesicle, 559; 551, 564 Madreporite, 551; 547, 552, 560, 564, 566, 570, 572, 577, 580 Magellania {Waldheimid) flavescens, 538 Maia, 374 Malacobdella, 208 Malacocotylea, 194; 191 Malacostraca, 347; 294, 300, 307, 309, 311, 312, 314 Malaria, 86 Malaria parasite, see Plas?nodiutn Male eggs of Rotifera, 213 Male gametes, 26 ; of Metazoa, see Spermatozoa ; of Porifera, 1 1 1 ; of Protozoa, 26, 27, 28, 29, 29, 78, 80, 84, 85, 86, 87, 91 Mallophaga, 423 Malpighian tubules, 279 ; of Arach- nida*, 279, 280,445, 452, 458, 468 ; of Insecta,279,28o,39o; ofMyria- poda*, 279, 280, 378, 381 Mandibles, 271; of Crustacea, 271, 296, 302; of Insecta, 271, 383; of Myriapoda*, 271, 377, 379. See also Mouth parts Mandibular groove, 296 Mandibular palps, 302, 331, 333, 334, 340, 348, 355, 358, 368 Mantis, 385 Mantle, of Brachiopoda, 537, 539, 540, 542 ; of Cirripedia*, 297, 338, 340, 341, 344, 347; of Mollusca*, 470, 471, 472, 474, 479, 482, 490, 492, 494, 498, 499, 501, 502, 503, 505, 506,511, 515, 519, 525, 527; of Tunicata*, 592, 602 Mantle cavity, see Mantle Mantle flap, see Mantle Mantle groove, 474 ; 476 Manubrium, 130, 135, 138, 139, 144, 145, 146, 151, 152 Margellium, 139; 140 Marginal anchors, 151 Maricola, 189 Mass provisioning, 434 n. Mastax, 212 Mastigamoeba, 56 M. aspera, 57 Mastigobranchia, 366 Mastigophora, 40; 4, 38, 39 INDEX 629 Maxillae, 271; of Crustacea, 271, 296, 299, 302, 335 ; of Insecta, 383 , 384, 385 ; of Myriapoda*, 271, 377, 379. See also Mouth parts Maxillary glands, 309, 309, 321, 328, 334,. 338, 340, 351 Maxillipeds, 296; of Crustacea*, 292-3, 296, 333, 353, 355, 358, 361, 368 ; of Lithohius, 378 Maxillules, 296, 302. See also Mouth parts Meandrina, 169 Mecoptera, 425 Medusa, 129; 128, 130, 132 Medusae, 133, 135, 137, 138-40, 143, 144-8, 150-7 Megachromosomes, 22 Megalopa larva, 372 ; 372 Megalospheric form, 70 Meganephridium, 256 Meganucleus, 21; 19, 22, 39 Megascolecidae, 253, 256 Megascolides, 256 M. australis, 244 Melanin(s), 517; 85, 307 Melicerta, 213 Meloe, 430 Melolontha, 401 Membranellae, 14, 97 Membranipora, 532, 533, 537 Menopon pallidum, 423 ; 423 Mertnis, 225 ; 219 M. nigrescens , 225 Merogametes, 26, 27, 85 Meropodite, 300 Merozoites, see Schizozoites Mesenchyme, 121; 122, 123; of Ctenophora, 172; of Echinoder- mata, 555; of Hirudinea, 265; of Nemertea, 207 ; of Platyhelmin- thes, 181 ; of the trochosphere, 248 Mesenteric caeca, of Aphrodite, 234 ; of Arachnida, 280, 445, 462, 467; of Crustacea*, 308, 320, 328, 340, 350, 351, 352, 355, 357, 368; of Echinodermata*, 550, 560, 569, 580; of Insecta, 389. See also Digestive gland. Liver Mesenteric filaments, 159, 164 Mesenteries, of Actinozoa, 159, 160, 164, 165, 166, 167; of Holothu- roidea, 573 ; of Polychaeta, 251 Mesenteron (Mid gut), 121; of Crustacea*, 307, 320, 328, 368; of 630 INDEX Mesenteron {cont.) Hirudinea, 263; of Insecta, 388; of Nematoda, 218 Mesoblast, see Mesoderm Mesoblastic somites, see Mesoderm segments Mesocerebrum, see Deutocerebrum Mesoderm, 120; 121, 122, 123; in the trochosphere, 251; of Arach- nida, 448 ; of Arthropoda, 280 ; of Chordata, 584, 597. See also Mesenchyme, Mesoderm seg- ments (Mesoblastic somites), Me- sothelium Mesoderm segments (Mesoblastic somites), 280; of Arachnida, 444, 448; of Arthropoda, 279, 280; of Chaetopoda, 229, 251; of Chor- data, 584; of Onychophora, 283 Mesogloea, see Structureless lamella Mesosoma, 271, 449, 452 Mesostoma, 186; 177, 178 M. ehrenbergi, 189 M. quadrangular e, 189 Mesothelium, 121 ; 122. See also Mesoderm segments (Mesoblastic somites) Metabasipodite, see Preischiopodite Metabola, see Pterygota Metaboly, 14, 46, 47 Metacerebrum, see Tritocerebrum Metacestode stage, 202, 204 Metachronai rhythm, 13 ; 172 Metameric segmentation, 251 Metamorphosis, see Life history Metanemertini, 208 ; 205 Metapneustic, 439 Metasoma, 271, 449, 452 Metasome, 333 Metasternite, 450 Metastoma, of Crustacea, 303 ; of Eurypterida, 453 ; of Trilobita, 287 Metazoa, 3; i, 4, 19, 23, 30, 39, 117 Metepipodites, 300; 299. See also Branchia Metridiuni, 170 Miastor, 396 Microchromosomes, 22 Microfilaria diurna, 223 M. nocturna, 223 Microgametes, see Male gametes Microhydra, 143; 142 Micromeres, 248 Micronephridia, 256 Micronuclei, 21 ; 22, 28, 29, 30 Micropteryx, 429 Microspheric form, 70 Microsporidia, 95 Microstoma linear e, 188; 189, 199 Mid gut, see Mesenteron Mid-gut caeca, see Mesenteric caeca Milk glands of tsetse fly, 394 " Millepora, 144 Mitoses of Protozoa, 19 Molar process, 348 Mollusca, 470; 1,2; Types of, 471 Molpadida, 577 Monas, 56; 12, 37, 41 M. vulgaris, 57 Mono cyclical rotifers, 213 Monocystis, 92; 8, 27, 32, 33, 91, 92 M. lumbrici, 92 M. magna, 92 Monograptus, 148; 149, 150 M. colonus, 149 Monopylaea, 71 Monosiga, 60 M. brevipes, 59 Monothalamia, 68 ; 67 Monotocardia, 489; 478, 491 Monstrilla, 335 Montipora, 169 Mosaic disease, 417 Mosaic vision, 278 Motile organs of Protozoa, 1 1 Moulting, 274, 334, 398 Mouth, of Arthropoda, 271; of ascidian tadpole, 597, 598; of Balanoglossus, 585 ; of Brachio- poda, 539; of Chilopoda, 377; of Coelenterata*, 129, 130, 131, 140, 143, 145, 151, 152, 156, 159, 168, 170, 171, 172; of Echinodermata*, 546, 555, 564, 571, 578; oi Helix, 482 ; of Hirudinea, 263 ; of Hyda- tina, 211; of Insecta, 383; of Lepas, 339; oi Peripatus, 283; of Platyhelminthes*=, 174, 181, 189, 190; of Protozoa*, 15, 97, 100; of Trilobita, 287 ; of Triploblastica, 121. See also Alimentary canal. Branchial opening, Mouth parts Mouth parts (limb-jaws and lips), of Arthropoda, 270, 271, 272-3; of Crustacea*, 292-3, 302, 303, 318, 325, 331, 333, 334, 335, 339, 348, 355, 356, 358, 359, 368, 369; of INDEX 631 Mouth parts (cont.) Insecta*, 383, 384, 385, 406, 407, 408, 409, 411, 412, 415, 417, 420, 423, 424, 425, 426, 427, 428, 429, 430, 432, 433, 436, 438, 439, 441 ; of Myriapoda*, 377, 379; of Ony- chophora, 282 Mucous glands of Helix, 486 Muggiaea, 145, 146 Multicilia, 56 Multiple fission of Protozoa*, 24; 32, 48, 59, 64, 67, 82, 83, 85, 88, 90 Musca, 439; 440 M. domestica, 440 Muscle(s), Alary, 391, 391; Ad- ductor, see Adductor muscles ; Columella, 481; Retractor, see Retractor muscles. See also Mus- culature Muscle fibres, of Arthropoda, 280; of Chaetognatha, 542 ; of Coelen- terata*, 125, 131, 153; of Nema- toda, 214; of Pectin, 510; of Peripatus, 280; of Platyhelmin- thes, 177 Muscular gland organ, 186 Musculature, of Actinozoa*, 159, 164, 165; of Arthropoda, 270; of as- cidian tadpole, 597 ; of Asteroidea, 558, 559; of Brachiopoda, 537; of Cephalopoda, 515; of Chaeto- gnatha, 542; of Chaetopoda, 229, 232, 240; of Ciona, 592; of Cri- noidea, 580; of Ctenophora, 172; of Echinoidea, 566, 567, 568; of Gasteropoda, 481; of gill books and lungs, 445, 446 ; of Hirudinea, 265; of Holothuroidea, 573; of Medusae, 130, 135, 154, 155; of Nematoda,2i4,2i6; of Nemertea, 207; of Onychophora, 270, 282; of Ophiuroidea, 562, 563; of Polyzoa, 532; of Rotifera, 289; of Thaliacea, 603 ; of the trocho- sphere, 251 ; of wings, 386 Mutations, 30 Mutual fertilization, by Ciliophora, 28; by Gasteropoda, 487, 494; by Oligochaeta, 254; by Platyhelmin- thes, 186 Mya, 504 Mycetozoa, 81 ; 4, 7 My gale, 458 Myoblast, 177 Myonemes, 14 Myophrisks, 76 Myopsida, 513 Myrianida, 235; 231, 246 Myriapoda, 375 ; 271 Mysidacea, 352; 348 ikTy^V, 353; 296, 299, 353 M. relicta, 348 Mytilus, 508 ; 475, 499, 500, 501, 502, 504, 505, 505, 507, 510 Myxobolus, 95 ; 38 Myxosporidia, 95 Nacreous layer, 472 Naegleria, 63 ; 20, 78 N. bistadialis, 64 ; 18 Narcomedusae, 143; 133, 154 Nassa, 489 Nassellaria, see Monopylaea Natica, 490 Nautiloidea, 525, 528 Nautilus, 525; 513, 514, 523 N. macromphaliis , 528 Nebalia, 349; 298, 299, 303, 3" N. bipes, 349, 350 Neck of Cestoda Merozoa, 199 Neck gland, see Dorsal organ Neck organ (Nuchal sense organ), 307; 298, 318, 325 Nectocalyces, 144 Needham's sac, 522 Nematocyst, 127 Nematoda, 214; 2, 121 Nemertea, 205; 121 Neomenia, 474 Neosporidia, 93 ; 82 Neotermes, 412 Nepa, 417 Nephridia, 123 ; 229, 242-4, 244, 265 Nephridial system, 123. See also Nephridia Nephrocytes, 390; 391 Nephromixia, 229 ; 242 Nephrops, 372; 296, 373 Nephrostome, 229; 242, 256 Nereis, 235; 231, 232, 240, 243, 248, 249 Nerilla, 261 ; 262 Nerve cord, see Nervous system Nerve net, see Nerve plexus Nerve plexus, 122; of Balanoglossus, 587; of Coelenterata, 128, 154; of Echinodermata, 550, 554 ; of Platy- helminthes, 174, 190 632 INDEX Nerve rings, of Echinodermata, 554; of Medusae, 131, 132, 135, 154 Nervous system, 122; 121; of An- nelida*, 228, 229, 230, 250, 254, 256, 261, 265; of Arachnida*, 452, 457, 459, 467 ; of Arthropoda, 274; of Brachiopoda, 540 ; of Chaeto- gnatha, 542; of Chordata, 582; of Ciona, 596, 597, 599; of Coelente- rata*, 128, 131, 132, 135, 154; of Crustacea*, 303, 334, 340, 344, 350, 351, 355, 370; of Echinoder- mata*, 554, 563, 570, 575, 580; of Enteropneusta*, 587, 590; of Insecta, 397; of Lithobiiis, 379; of Mollusca*, 470, 474, 477, 479, 481 , 499, 507, 52o;of Nematoda, 215; of Nemertea, 207 ; of Onycho- phora, 286 ; of Platyhelminthes*, 174, 175, 190, 199; of Polyzoa, 530 ; of Rotifera, 209 Neuropodium, 231 Neuroptera, 425 Nidamental glands, 517 Noctiliica, 49; 43, 51 Nodosaria, 71 N. hispida, 9 Non-cellular animals, 3 Notochord, 584; of ascidian tadpole, 597; of Enteropneusta, 585, 587 Notodelphys, 335 Notonecta, 417 Notopodium, 231 Notostraca, 322; 317 Nuchal sense organ, see Neck organ Nucleariae, 68 Nuclei, in Metazoa and Protozoa, 3 ; of pansporoblasts, 93 ; of Pro- tozoa, 18-23, 18; Plurality of, in Protozoa, 6 ; Position of, in cho- anocytes, 117 Nucleoli, 19 " Nucleus ", of Rhizocephala, 345 ; of Thaliacea, 607 Niicula, 507; 471, 501, 504 Nuda, 173 Nudibranchiata, 494 ; 479 Nummulites laevigatus, 70, 71 Nuptial chamber, 413 Nutrition, 14 n.; of Mastigophora, 40; of Phytomastigina, 41, 44; of Protozoa, 14; of symbionts, 43. See also Holophytic nutrition, Holo- zoic nutrition. Saprophytic nutri- tion Nyctiphanes, 361 AT", norwegica, 361 Nyctotherus, 102 N. cordiformis, 103 Nymphon, 466, 468 Nymphs, 399 Obelia, 133; 130, 134, 135, 137, 138, 140, 152 Oblique fission, 40 Obtect pupae, 402 Ochromonas, 44; 41, 45, 56 Octohothriiim, 191 Octoniitiis, see Hexamitus Octopoda, 513; 524 Octopus, 519; 513, 523, 524, 525, 526, 527 Ocular plate, 564 Ocypus olens, 43 1 Odonata, 415; 386, 393 Odontoblasts, 485 Odontocerum, 426 Odynerus, 434 n. Oegopsida, 513 Oenocytes, 391 Oesophageal bulbs, 218 Oesophageal pouches, 254 Oesophagus, see Alimentary canal Oikomonas, 56 O. termo, 57 Oikopleura, 38, 601 O. albicans, 601 Olenus cataractes, 287 Olfactory hairs, 306 Oligochaeta, 252 ; 229, 267 Oligolophus spinosus, 466 Oligotricha, 102 Olynthus, no; no, in Ommatidium, 274; 383 Onchosphere, 201, 202, 204 Onychophora, 281 ; 270 Oocysts, 18 ; of Gregarinidea, 88, 90, 91 Oodmium, 49; 38 O. pouched, 38 Oogamy, 26, 27, 40 Ookinete, 85, 86 Oostegites, 349; 352, 355, 359 Ootheca, 397 Ootype, 185 Oozooid, 600; 603, 604 Opalina, 99; 6, 24, 38, 100 INDEX 633 O. ranarum, 7, 22, 99 Opalinidae, see Prociliata Opening (Aperture), Atrial, see At- rial opening; Excretory, see Ex- cretory organs, Renal openings; Genital, see Generative organs; of Mantle cavity, see Mantle. See also Anus, Mouth, Oscula, Ostia, Pneumostome, Pores Operculum, 237 Ophiocoma, 564 Ophioglypha, 562 Ophiopluteus, 556 Ophiothrix, 564 Ophiura, 562, 564 Ophiuroidea, 562; 547 Ophryocystis, 89 O. mesnili, 90 Opisthobranchiata, 492; 479, 493 Opisthosoma, 271; 443, 448, 454, 456,458,465. See also M-esosovadi, Metasoma Opisthoteuthis, 525; 513 Optic lobes of Crustacea, 303 Oral aspect (side, or surface), 124 ; of Echinodermata, 547 Oral cone, 129; 130, 135, 138 Oral disc, 159 Oral siphon, 592 Oral valves, 578 Ornithodorus moubata, 465 Orthoceras, 524; 524, 528 Orthoptera, 409 ; 388 Oscarella, 1 19 Oscula, of Porifera, no, 112, 113; of Radiolaria, 71, 76 Osphradia, 490, 492 Ossicles, of Echinodermata, 550; Adambulacral, 558; Ambulacral, 558, 562, 567; Basal, 579; Bra- chial, 580; Centrodorsal, 578, 579; Cirrhal, 580 ; Infero-marginal, 558 ; of Holothuroidea, 572, 573 ; Pin- nulary, 580; Radial, 580; Rosette, 579; Supero-marginal, 558. See also Auriculae, Skeletal plates Ossicles, System of, in Asteroidea, 558 ; in Crinoidea, 579 ; in Echino- idea (plates), 464; in Holothuro- idea (calcareous ring), 573 ; in Ophiuroidea, 562 Ostia, 279; 112 Ostracoda, 329; 291, 295, 297, 309, 3i4> 316 Ostrea, 510; 504, 507 O. edulis, 473, 510, 511 Otocyst, see Statocysts Otoplana, 189 Ova, see Eggs Ovarian lamella, 340 Ovarioles, 397 Ovary, see Generative organs Ovicell, 535 Oviducts, see Generative organs Ovigerous frenum, 340 Ovigerous legs, 466 Ovipositor, of Insecta*, 387, 409, 432 ; of Phalangida, 465 Ovotestis of Helix, 486 Pachytylus migratorius, 410 Paedogamy, 78 Paedogenesis, 396 Paired limbs, see Limbs Palaeonemertini, 208 Palaeophonus, 452 Palinura, 362 Palinurus, 373 Pallial arteries of Lamellibranchiata, 505 Pallial budding, 599 Pallial circulation of Lamellibran- chiata, 506 Pallial gills, 490 Pallial sinus, 540 Palliovisceral cords, 474 Palmella, 42 Palolo worm, see Leodice viridis Palps, Labial, see Labial palps; Mandibular, see Mandibular palps ; of Acarina, see Pedipalps ; of Poly- chaeta*, 231, 235 Paludicola, 189 Paludina, 492 ; 470, 478, 490 P. vivipara, 478 Pancreatic tissue, 518 Pandalus, 300 Pafidorina, 51 ; 40, 52 Panorpa coniftiimis, 426 Pansporoblasts, 93 Panthalis, 233; 231 Pantopoda, 466 Papillae of mouth of Ascaris, 218 Parabasal body, 1 1 Paractinopoda, see Synaptida Paragaster, 1 1 o Paragnatha, 303 634 INDEX Paramecium, 102 and n.; 8, 10, 11, 13, 16, 17, 23, 24, 29, 37, 38, 97, 99, 108 P. aurelia, 3 1 P. caudatum, 17, 29, 35 Paramitoses, 19 Paranehalia, 350 Paraoesophageal (Circumoesopha- geal) connectives, see Nervous system Parapodia, of Aplysia, 494 ; of Poly- chaeta, 229, 231-40 Parasitic castration, by Isopoda, 357 ; by Rhizocephala, 344 Parasitic habits, of Acarina, 441, 445 ; of Anoplura, 424; of Aphani- ptera, 441 ; of Ciliata, 99, 100, 102, 104, 105 ; of Cirripedia, 343-7 ; of Copepoda, 334-8 ; of Crustacea, 295; of Cyamtis, 360; of Dino- flagellata, 49 ; of Diptera, 434, 435 ; of Enta?noeba, 64; of Hemiptera, 419 ; of Hirudinea, 267 ; oiHistrio- bdella, 263 ; of Hymenoptera, 434 ; of Isopoda, 356-8 ; of Mallophaga, 423 ; of Nematoda, 214-27 ; of Pen- tastomida, 469 ; oi Plasmodiophora, 82; of Platyhelminthes, 174, 190- 204; of Polymastigina, 61-3; of Protozoa, 38 ; of Sporozoa, 82-95 > of Trypanosomidae, 56-9 Parazoa, see Porifera Parenchyma, 181; 174, 191, 198 Parenchymatous tissue, see Paren- chyma Parthenogenesis, of Crustacea*, 314, 324,329,331,419; oflnsecta, 394, 419, 424; of Protozoa, 28; of Rotifera, 213 Parthenogonidia, 53 Patella, 490; 473, 478, 480, 488, 489 P. coerulea, 473 Peachia, 164; 165 Pecten, 5og; 504, 507, 510, 523 P. tnaximus, 509, 509 P. opercularis, 509 P. tenuicostatiis , 510 Pectines, 450, 453 Pectinibranchiata, see Monotocardia Pedal cords of Amphineura, 474 Pedalioji, 213 Pedicellariae, 550; Crossed, 558; of Echinoidea, 566 ; Gemmiform, 566, 566; Ophiocephalous, 566, 566; of Asteroidea, 558; of Echi- nodermata, 566; Tridactyle, 566, 566; Trifoliate, 566, 566; Un- crossed, of Asteroidea, 558 Pedicellina, 536 Pedicidiis hiimanus, 424 ; 423 Pedipalpi, 442, 467 Pedipalps, 442, 448, 449, 452, 458, 460, 461, 462 Peduncle, see Stalk Pelagia, 157 Pelagothuria, 577 ; 576 Pelagothurida, 577 Pellicle, 8 ; 44, 98, 99 Pelmatozoa, 582; 581 Pelornyxa, 65 P. pal us tr is, 66 Pen of Loligo, 525 Penaeidea, 362 Penaeus, 312, 316, 372 Penis, see Generative organs Pennaria, 142 ; 141 Pennatula, 161, 162 Pennatulacea, 161 Pentacrinus, 582 Pentastomida, 469 ; 467 Pentremites , 582 Peptonephridia, 256 Peracarida, 352; 300, 349 Peraneftia, 48; 12, 15, 16, 41 P. trichophoriim, 47 Perforate shells, 69 Peribranchial cavities, 595 Pericardial sinus, see Pericardium Pericardium, of Arthropoda, 279 ; of Cephalopoda, 518; of Ciona, 596, 598 ; of Crustacea, 3 1 1 ; of Entero- pneusta, 588; of Insecta, 391; of Mollusca, 470 ; of Snail, 482 Perichaetine, 254 Perihaemal coelom of Enteropneu- sta, 585 Perihaemal system of Echinoder- mata*, 550; 561, 563, 567, 580 Periostracum, of Brachiopoda, 537 ; of Mollusca, 472 Peripatiis, 281 ; 121 n., 270, 277, 283, 284, 285, 467, 468 P. capensis, 282, 283, 285, 286 P. edwardsi, 283 Peripharyngeal band, 592 Periplaneta, 409 Peripneustic larva, 439 Periproct, 564 Peripsocus pliaepterus, 415 Peripylaea, 71 Perisarc, 134 Perisomatic cavity, 345 Peristome, of Ciliata, 97 ; of Echi- noidea, 564 Peristomial cirri, 232 Peristomium, 232; 235, 237 Peritoneum, 121, 229, 244 Peritricha, 104; 97, 98 Peritrophic membrane, of Crustacea, 309; of Insecta, 390 Perivisceral cavity, 122; oi Acantho- bdella, 267 ; of Arthropoda, 279 ; of Chaetopoda, 229 ; of Ciona, 596 ; of Echinodermata, 550; of Mol- lusca, 470; of Rotifera, 209. See also Coelom, Haemocoele Perla maxima, 414 Pernicious malaria, 88 Perradial, 152 Petrohius maritimiis, 407 Phaenoserphus viator, 435 Phaeococcus, 46 Phaeodaria, see Tripylaea Phaeodium, 76 Phalangida, 465 Phalera hucephala, 393 Pharynx, see Alimentary canal Phascolosoma, 269 Pheretima, 256 Philodinidae, 212 Phlehotomiis, 58 Phobotaxis, 33 Pholas, 511 Phoronidea, 545 Phoronis, 546; 545, 546 Phorozooids, 608 Phosphorescent Protozoa, 16 Photogenic organs of Insecta, 391 Photosynthesis, see Holophytic nutri- tion Phoxichilidiiim femoratum, 467 Phragmocone, 524 Phryganea, 427 ; 426 Phylactolaemata, 536; 532 Phyllohiiis urticae, 401 Phyllobranchiae, 366 Phylloceras heterophyllum, 528 Phyllopoda, 318 Phyllopodium, 300; 298 Phylloxhra vastatrix, 420 Physalia, 145 ; 147 Phytomastigina, 41 ; 40, 54, 56 NDEX • 635 Pieris rapae, 403 Pigments, Blood, see Chlorocruorin, Haemocyanin, Haemoglobin ; of Crustacea, 307; of Haemospori- dia, 85; of Lepidoptera, 429; of Phytomastigina, 41. See also Chromatophores, Melanins Pilema, 156; 157 Pilidiiim larva, 207; 205, 208 Pinnate tentacles, 157, 573, 577 Pinnulary ossicles, 580 Pinnules, 578 Piroplasma, 88, 465 Placenta, of Onychophora, 285 ; of Salpida, 604; of Scorpionidea, 452 Placocystis, 582 Plagiostomum lefnani, 189 Planaria, 180 P. alpina, 176, 183, 187 P. lugubris, 189 P. torva, 176 Planorbis, 267, 495 Planula, 131 ; i, 125 Plasmodia, 7, 82 Plasmodiophora, 82 Plasmodium, 31 Plasmodium, 86; 33, 38, 88 P . falciparum , 88 P. malariae, 88 P. vivax, 87, 88 Plasmodroma, 39 Plasmogamy, 26 n. Plasmotomy, 24, 78, 93, 100 Plastin, 19 Plastogamy, 31 ; 26 n. Platyctenea, 173 Platyhedra gossypiella, 429 ; 428 Platyhelminthes, 174; 2, 121 Plecoptera, 414; 393 Pleodorina, 52 P. calif ornica, 6 P. illinoiensis, 6 Pleopods, 348. See also Abdoininal limbs of Crustacea Plesiocaris vagicollis, 400 Pleurohrachia, 173 P. pileus, 171 Pleurobranchiae, 311 ; 361 Pleuron, 295 Pleuropodite, see Precoxa Plicate canals, 506 Plumatella, 530, 536, 546 P.fungosa, 531 636 INDEX Plwnularia, 142; 141 Pluteus, 556; 556 Pneumatophore, 144 Pneumostome, of Arachnida, 446 ; of Pulmonata*, 482, 496 Podia, see Tube feet Podical plates, 386 Podobranchiae, 361 Podocoryne, 142 Podocyrtis schojnbiirgki, 75 Podophrya, 107 Podura aqiiatica, 409 Pole capsules, 93 Polian vesicles, 560; 563, 567, 574 Poly cells nigra, 189 Polychaeta, 230; 229 Poly cirrus, 230 Polycladida, 190 Polyclinum, 603 Polydisc strobilization, 157 Polyembryony, of Hymenoptera, 396 ; of Polyzoa, 535 Polyenergid nuclei, 20 Polygordius, 261 ; 248, 250, 252, 287 Polykrikos, 49 ; 6, 50 P. schwarsi, 50 Polymastigina, 60 Polyp, 129; 130, 131, 132 Polypide, 530 Polyplacophora, 474 Polyps, 133, 135, 136, 137, 142, 143, 157-70 Polystomella, 71 ; 26, 33, 35, 63, 70, 74 P. crispa, 18, 73 Polystomum, 191; 192, 193, 194 P. integerrimum, 152 Polythalamia, 69 ; 67, 68 Polytoma, 50; 15, 24, 29, 35, 40, 41 P. uvella, 18, 25 Polytricha, 102 Polyzoa, 530; 2 Pomatoceros , 230; 231, 234 P. triqueter, 238 Pontohdella, 267 Porcellana, 374 Pore plate of Radiolaria, 71 Pore-rhombs, 582 Pores, Collar, see Collar pores; Dorsal, see Dorsal pores ; of Pori- fera, no; Proboscis, see Proboscis pores; Water, 582. See also Mad- reporite Porifera, no; i, 117 Porites, 168; 169 Porocytes, in Poromya, 504 Porthetria dispar, 429 Portuguese man-of-war, see Physalia Posterior aorta, of Araneida, 458; of Carcifius, 370; of Helix, 484; of Lamellibranchiata, 505 ; of Scor- pionidea, 450; of Sepia, 51S Posterior interradius of Echinoidea, 570 Posterolateral arms of Plutei, 556 Posterolateral edge of Carcinus, 365 Postsegmental region of Crustacea, 295 Preantennae, 282; 272 Prebranchial zone, 592 Precheliceral segment, 443 ; 444 Precoxa, 298 Pregenital segment of Arachnida, 443 ; 444 Preischiopodite (Preischium), 298, 299, 300 Preoral lobe of Echinodermata, 556 Preoral region, of Annelida, 228 ; see also Prostomium; of Arthropoda, 271; of Crustacea, 295, 296; of Echinodermata, see Preoral lobe; of Enteropneusta, see Proboscis. See also Presegmental region, Preoral somites Preoral somites, of Arachnida, 443 ; of Arthropoda, 271 ; of Crustacea, 296 ; of Myriapoda, 377, 379 ; of Onychophora, 283 Presegmental region of Crustacea, 295 Primary body cavity, see Haemocoele Primary embryo, 535 Prismatic layer, of Brachiopoda, 537 ; of Mollusca, 472 Probasipodite, 298 Proboscis, of Acarina, 462, 465 ; of Bonellia, 268 ; of Buccinum, 492 ; of Chaetopoda, 231; of Entero- pneusta, 585; of Nemertea, 205; of Pantopoda, 466, 467 ; of Rhyn- chobdellidae, 263 ; of Suctorial Copepoda, 334, 335, 338 Proboscis cavity, of Balanoglossus, 585, 588; of Chordata, 583 Proboscis complex of Balanoglossus, 587 Proboscis pore oi Balanoglossus, 585 INDEX 637 Proboscis sheath, of Buccinum, 492 ; of Nemertea, 205; of Rhyncho- bdellidae, 263 Probuds, 606 Procerebrum, 274, 305 n. Procerodes lohata, 189 Prociliata (Opalinidae), 99 ; 21, 22, 28 Proctodaeum, 121; of Arthropoda, 278; of Crustacea, 307; of tro- chosphere, 250 Proepipodites, 299; 300, 311, 317, 320, 328 Progressive feeding, 434 n. Proliferating region, of Syllidae, 246 ; of Tapeworms (neck), 199 Proostracum, 524 Propodite, 300 Prorodon, 100 P. teres, 103 Prosobranchiata, see Streptoneura Prosoma(" Cephalothorax "of Arach- nida), 271, 443, 449, 452, 454, 458, 461, 465 Prosopyles, 112 Prostate glands, of Oligochaeta, 254, 259 ; of Sepia, 522 Prostomium, of Annelida, 228; of Chaetopoda, 229, 231, 235, 238, 251, 258; of Gephyrea, 268; of Hirudinea, 263 Protaspis larva, 288, 289 Proteolepas, 343 Protephemeroptera, 404 Protobranchiata, 507; 501, 504, 505 Protocerebrum, 274, 303 Protochordata, 583 Protoclypeastroida, 571 Protococcaceae, 43 Protoconch, 525 Protodonata, 404 Protodriliis, 261 P. chaetifer, 261 Protohymenoptera, 405 Protomerite, 90 Protomonadina, 56 Protoplasm of Protozoa, 7 Protopodite, 298; 288, 333, 340 Prototroch, of Cyphonautes, 535; of Pilidium, 207; of trochosphere, 250 Protozoa, 3 ; i Protozoa and Metazoa, Connection between, 39 Protura, 409 Proventriculus, of Carcinus, 368; of Earthworms, see Gizzard ; of In- secta, 388; of Ga?nmarus, 359; of Ligta, 355 Pseudocolonies of Protozoa, 7 Pseudonavicellae, 90 Pseudopodia, 11; of Amoebina, 63, 64; of Foraminifera, 67, 68, 69; of Heliozoa, 78 ; oiMastigamoeba, 56 ; of Mycetozoa, 81 ; of Radiolaria, 71 Pseudopodiospores, see Amoebulae Pseudotracheae, 436 Pseudotransverse fission, 24; 25, 40 Pseudovelum, 154 Psocoptera, 415 Pterobranchia, 585 Pteropods, 494 Pterostichiis ^ 401, 435 Pterotrachea, 492 ; 490 Pterygota, 409 Pterygotus, 454 ; 453 P. osiliensis, 453 Pulex irritans, 442 Pulmonata, 495 ; 481 Pulvillus, 385 Pupa, 399; 336, 431, 557 "Pupa" of Holothuroidea, 557 Pupae, Coarctate, 440 ; 402 ; Exarate, 402 ; Obtect, 402 Puparium, 402 Pure lines, 30 Purpura, 489 Pycnogonum littorale, 467 Pygidium, 287 Pyloric caeca, of Asteroidea, 560; of Insecta, 389 Pyloric chamber, 307 Pyloric sac, 560 Pyrenoids, 41 ; 50, 51 Pyriform organ, 535 Pyrosoma, 607; 599, 603, 604, 605 Pyrosomatida, 604; 601 Quadrant, 248 Quartan ague, 88 Quartettes, 248 Rachiglossa, 489 Radial" blood vessel", 553 ;56i, 567, 575, 580 Radial fission, 24; 25, 53 Radial nerve, Aboral, 580; Ecto- neural, 554; 567, 570 Radial ossicles, 580 638 NDEX Radial perihaemal vessel, 550; 561, 563, 567, 580 Radial spiral cleavage, 248 Radial symmetry, 557; of Coelente- rata, 123, 125, 130, 159; of Echino- dermata, 123, 546, 557 Radial water vessel, see Water vascular system Radii of Echinodermata, 547 ; 570, 573 Radiolaria, 71 ; 36, 38 Radula, 484 ; 47 1 , 476, 477, 489, 490, 492, 494, 495, 496, 499, 519 Radula sac, 484 Receptaculum seminis, see Sperma- theca Rectal caeca, 560 Rectum, see Alimentary canal Reduction division, 23, 32, 83 Regeneration, in Crustacea, 303 ; in Turbellaria, 188 Regular sea urchins, see Endocyclica Relation of Protozoa to the Environ- ment, 35 Relicts, 352 Renal openings (apertures, papi liar) of Mollusca, 47 1 , 476, 483 , 490, 517 Renopericardial openings (aper- tures, canals), 483; 499, 518 Repeated fission of Protozoa*, 24; 40, 48, 51 Reproduction, of Metazoa, see Asexual reproduction. Sexual re- production; of Protozoa, see Fis- sion Reproductive aperture, see Genera- tive organs Reproductive organs, see Generative organs Reserve materials, 15 Respiration, of Arthropoda, 278 ; of Crustacea, 3 1 1 ; of Cyclops, 334 ; of Lamellibranchiata, 503 ; of Proto- zoa, 16; of Tubicolous Polychaeta, 235. See also Respiratory move- ments. Respiratory organs Respiratory movements, of Aphro- dite, 233; of Arachnida, 445; of Branchiopoda, 387; of Carcimis, 366 ; of Crustacea, 3 1 1 ; of Cyclops, 334; of Insecta, 392; of Mysis, 353; of Pulmonata, 482; of Tubicolous Polychaeta, 237 ; of Tnhifex, 260 Respiratory organs, of Arachnida, 445 ; of Arthropoda, 278 ; of Branchiopoda, 317; of Chaeto- poda, 230 ; of Crustacea, 311; of Echinodermata, 552; of Holothu- roidea, 575 ; of Lamellibranchiata, 503 ; of Ligia, 355. See also Gills, Lungs, Tracheae Respiratory trees, 553, 575 Resting eggs, 213. See also Winter eggs Resting phase of Phytomastigina, 42 Rete mirabile, 575 Retinaculum, 429 Retinulae, 274, 277 Retractor muscle(s), of penis, 486 ; of proboscis, 205; of stomach; 560; of tentacles, 573 Retral processes, 71 Retropharyngeal band, 594 Rhahdammina , 7 1 ; 8 R. ahyssorum, 9 Rhabdites, 178 Rhabditis, 215; 216, 217, 220, 221 Rhabditoid larva, 221 Rhabdocoelida, 189 Rhabdoliths, 46 Rhabdom, 277 ; 274 Rhabdomeres, 274; 276 Rhabdopleura , 590; 583, 585 R. normani, 589 Rhipicephalus, 465 Rhipidoglossa, 489 Rhizocephala, 343 Rhizochrysis, 44 ; 46 Rhizocrinus, 582; 581 Rhizomastigina, 56 ; 63 Rhizoplasts, 11 Rhizopoda, see Sarcodina Rhizopodia, 11 Rhizostoma, 156 Rhizostomeae, 156 Rhombifera, 582 Rhyacophila, 427 Rhynchobdellidae, 267; 263, 264 Rhynchocoel, 205 Rhynchodaeum, 207 Rhynchodemus terrestris, 190 Ring canal of Polyzoa, 530 Rings (Nervous, Water vascular, etc.) of Echinodermata, 547; 550. See also Nervous system. Water vascular system, etc. Ripe proglottis, 199; 201 INDEX 639 Ripple-marking, 528 Rods of eyes of Arthropoda, 274 Rosette ossicle, 579 Rostellum, 199 Rostrum, of Cephalopoda, 524; of Crustacea*, 297, 331, 349, 365, 372 Rotifer, 212 Rotifera, 209; 2 Rotulae, 567 Royal pair, 412; 414 Sahella, 231 Saccocirrus, 261 ; 228, 262 Saccidina, 343 ; 344, 345, 346 Sagitta, 545; 121, 545. See also Chaetognatha S. hipiinctata, 544, 544 5. hexaptera, 543 Salivary glands, of Arachnida*, 450, 458, 462, 467; of Helix, 485; of Hirudinea, 263 ; of Insecta, 388 ; of Lithobius, 378 ; of Onychophora, 284; of Sepia, 519 Salpa, 607; 173, 599, 603 aS. democratica-miicronata, 604, 605 Salpida, 607; 600, 603, 604 Saltatoria, 409; 410 Sao hirsuta, 289 Saprophytic nutrition, 14 n.; of Phytomastigina*, 40, 41, 44, 46, 48, 49, 50; of Protozoa, 15, 37 Saprophytic Protozoa, see Sapro- phytic nutrition Saprozoic, see Saprophytic Sarcocystis, 95 S. lindemanni, 96 Sarcodina, 63 ; 38, 39, 43 Sarcophaga, 390 Sarcosporidia, 95 Sarsia, 145 ; 146 Saxicava, 511 Scallops, 509 Scalpellum, 341 ; 343 S. vulgare, 342 Scaphites, 529 Scaphopoda, 495 Scent scales, 429 Schellackia, 86; 85 Schistocephalus gasterostei, 202 Schistosoma, 196 Schizocystis, 88 ; 89 Schizogony, 83 ; 32, 86, 88, 89 Schizogregarinaria, 88 Schizopoda, 348 Schizozoites, 83; 32, 86, 88 Sclerotium, 81 Scolex, 199 Scolopendra, 272 n., 378 Scorpio, 452 S. swammerdami, 449 Scorpionidea, 448 Scutigera, 378 Scutum, 338 Scyphistoma, 155 Scyphomedusae, 150; 132 Scyphozoa, see Scyphomedusae ScytomonaSy see Copromonas Secondary body cavity, see Coelom Secondary embryos, 536 Segmental organs, 242 ; 240. See also Coelomoducts, Nephridia Segmentation, 124; of Annelida, 228, 229; of Arthropoda, 271; of Cestoda, 199; 124; of Chordata, 583 ; of Vertebrata, 124, 584; sug- gested by certain organs in Mol- lusca, 476, 527 Segmentation of the ovum, see Cleavage Segments of the body, see Somites Seminal groove, of Oligochaeta, 256 ; of Opisthobranchiata, 494 Seminal vesicles (Vesiculae semi- nales), of Insecta, 397; of Oligo- chaeta, 253, 259 Sense organs, of Araneida, 461 ; of ascidian tadpole, 598; of Chae- topoda, 230, 231, 235, 258; of Coelenterata, 128, 13.6, 139, 139, 143, 150; of Crustacea, 305; of Echinodermata, 554 ; of Hirudinea, 265 ; of Insecta, 383 ; of Mollusca, 475, 482, 509, 522, 523 ; of Myria- poda, 375 ; of Nemertea, 207 ; of Onychophora, 280; of Platyhel- minthes, 174; of Protozoa, 14; of Rotifera, 212. See also Eyes Sepia, 522; 513, 514, 515, 519, 520, 521, 523, 523, 524, 524, 525, 527 S. officinalis, 513 ; 516, 517, 518, 519 Sepiola, 513, 525 Septa, of Polychaeta, 251 ; of shell of Tetrabranchiata, 526, 527, 528 ; of Zoantharia, 167 Septibranchiata, 504 Sergestes, 316 S. arcticus, 299 Serpula intestinalis , 255 640 INDEX Sertularia, 142; 141 Sexual congress, see Sexual distinc- tion and sexual behaviour, Mutual fertilization Sexual distinction and sexual be- haviour, of Arachnida, 452, 457, 461, 465; of Archiannelida, 261, 263; of Balanoglossus , 589; of Bonellia, 268 ; of Cephalopoda, 523 ; of Coelenterata, 1 3 1 ; of Crus- tacea, 314, 318, 324, 329, 332, 334, 335, 336, 337, 34,i, 343, 344, 355, 359, 368; of Echinodermata, 553 ; of Insecta, 394, 412, 413, 415, 420; of Myriapoda, 379, 381; of Nematoda, 220 ; of Nemertea, 205 ; of Onychophora, 285 ; of Panto- poda, 467 ; of Polychaeta, 246 ; of Protozoa, 28 ; of Rotifera, 212. See also Generative organs Sexual reproduction, of Metazoa, see Generative organs ; of Protozoa, 26. See also Life history Shape, see Body Shell of Crustacea, see Carapace Shell of Echinoidea, see Corona Shell glands, of Crustacea, see Maxillary glands ; of Platyhelmin- thes, 185 Shell ligament, 499 Shell types of Foraminifera, Arena- ceous, 69; Imperforate, 69; Per- forate, 69 Shells, of Brachiopoda, 537 ; of Crustacea, see Carapace; of Fora- minifera*, 9, 67, 68, 69, 70, 70, 71 ; of Mollusca*, 471, 472, 474, 475, 476, 477, 479, 480, 490, 491, 492, 494, 509, 510, 511, 515, 520, 525, 527-9 ; of Protozoa, 8 Shield-shaped tentacles, 573 Sialis, 425 S. lutaria, 425 Sicula, 148 Sida, 325 Silicoflagellata, 46 Silicoflagellidae, see Silicoflagellata Silver fish, see Lepisma saccharina Simocephaliis , 325 5'. sima^ 326 Sinus system of Hirudinea, 265 Sinuses, Haemal, of Arachnida*, 450, 459 ; of Arthropoda, 279 ; of Crustacea, 312; of Helix, 483; of Lamellibranchiata, 506 ; oiLumhri- culiis, 260 ; oi Pomatoceros , etc., 230 Siphon, of Echinoidea, 569; of Gasteropoda, 482 Siphonoglyphes, 159 Siphonophora, 133; 144, 146, 147 Siphonozooids, 162 Siphons of Lamellibranchiata, 501 Siphuncle, 526; 525 Sipunculoidea, 269 Sipunculus, 269, 269 Size of Protozoa, 4 Skeletal plates, of Echinoidea, 564; of Ophiuroidea, 562 Skeletogenous layer, no Skeleton, External, see Corals, Cu- ticle, Perisarc, Shell; Internal, see Internal skeleton Skull of Cephalopoda, 520 Slimonia, 452; 453 Small intestine of Insecta, 390 Social life of Hymenoptera, 434 Soldiers of Isoptera, 412 ; 414 Solenia, 160 Solenocyte, 242 ; 23 Solidago, 396 Somatoblast, 250 Somites (body segments). Series of, in Arthropoda, 271, 272-3; in Crustacea, 292-3 ; in Polychaeta*, 231, 232, 235, 237, 238. See also Tagmata Somites, Mesoblastic, see Mesoderm segments Spatangoida, 571; 570 Spatangus, 571 Sperm sacs, see Seminal vesicles Sperm vesicle of Chaetognatha, 544 Spermatheca (Receptaculum semi- nis), of Helix, 486 ; of Insecta, 397 ; of Nematoda, 218; of Platyhel- minthes, 185 Spermathecal ducts of Gasteropoda, 486 Spermatic atrium, 259 Spermatophores, of Crustacea, 314; of Helix, 486; of Peripatus, 285; of Sepia, 522 Spermatozoa, of Crustacea, 314; of Hirudinea, 266 ; of Nematoda, 218 Spermiducal glands of Oligochaeta, 254 Sphaeractinomyxon, 95 Sphaerella, see Haematococcus INDEX 641 Sphaerophrya, 108; 107 S. sol, loi Spherularia, 226 ; 226 Spicules, of Alcyonaria*, 160, 161, 163 ; of Porifera, in, 113, 115; of Radiolaria*, 71, 75, 76 Spines, of Echinodermata, 550; of Echinoidea, 566; of Ophiuroidea, 562 ; of Starfish, 558 Spinnerets, 458 ; 460, 461 Spiracles, of Arthropoda, see Stig- mata; of Blastoidea, 582 Spirochaeta, 465 Spirochona, 107; 38 S. gemmipara, 108 Spirographis, 231 Spirostomum, 102; 35 S. ambiguum, 103 Spirula, 524; 5 13, 5^4 Spirulirostra, 524; 524 Sponges, see Porifera Spongilla, 119 S. lacustrisy 119 Spongillidae, 116, 117 Spongin, 113 Spongioplasm, 8 Sporangium, 82 Spore cases, 33 ; 88, 89, 92, 93 Spores, 33 ; 8, 48, 64, 75, 82, 85, 88, 89, 91, 92, 93, 94 Sporoblasts, 82; 33, 85, 86, 93 Sporocyst, 33; 18, 85, 91 Sporont, 31, 32, 87 Sporozoa, 82; 4, 38, 39 Sporozoites, 31; 82, 85, 86, 88, 89, 90, 92, 93 Spumellaria, see Peripylaea Squilla, 351 S. mantis, 351 Staggers, see Gid Stalk, of Brachiopoda, 537 ; of Cri- noidea, 550, 578, 582; of Entero- pneusta, 585; of Lepas, 338; of Pelmatozoa, 582; of Protozoa*^, 4, 60, 81,104, io7> io8> T09;of Ptero- branchia, 595, 596 Stalked gland organ, 186 Staphylocystis , 202 Statoblasts of Polyzoa, 532 Statocysts (Otocysts), of Calypto- blastea, 136; of Crustacea, 306; of Turbellaria, 176 Statolith of ascidian tadpole, 598 Stauromedusae, 150; 152 Stegomyia, 224 Stenopodium, 298 Stentor, 102; 13, 97, 98 S. coeruleiis, 18, 103 Stephanoceros, 213 Stereum, 82 Sterna, of Arachnida*, 450, 453, 465; of Crustacea, 295, 367; of Myriapoda*, 378, 381 Sternites, see Sterna Stewart's organs, 569 Stigma of Odonata, 415 Stigmata (Spiracles), of Arachnida, 447, 462, 465 ; of Insecta, 391, 392, 393, 439 ; of Myriapoda, 378, 381 ; of Onychophora, 284 Stigmata of Tunicata, 592, 594, 595 Stimuli, Effect of, on Protozoa, 33 Stipe, 148 Stolon, of Alcyonaria, 160 ; of Hydra- tuba, 155 ;of Hydrozoa, ^eeHydro- rhiza; of Tunicata, 599, 599, 605 Stomach, see Alimentary canal Stomatopoda, 350; 298, 316, 349 Stomodaeum (Fore gut), 121; of Alcyonaria, 159; of Arthropoda, 278 ; of Ciona, 592 ; of Crustacea*, 307, 320, 340 ; of Ctenophora, 172 ; of Insecta, 387; of Nematoda, 218; of Onychophora, 284; of Rotifera, 212; of Tricladida (pha- rynx), 181 ; of trochosphere, 250; of Zoantharia, 164 Stonecanal, 551,552, 560,564,570,580 Streptoneura, 489; 478 Strobilization, of Aurelia, 155, 157; of Cestoda, 24, 199 Stromatocystis , 582 Stronibus, 490 Strongyloid larva, 221 Strongyloides stercoralis, 222 Structureless lamella (Mesogloea), 125, 126, 128, 129, 130, 135, 140, 152, 156, 158, 159, 160, 161, 162, 163 Stylaria, 258; 253, 259 S. proboscidea, 258 Stylets, of Insecta, 417 ; of Nemertea, 205 ; 206 Stylommatophora, 495 Stylonichia, 104; 37 S. mytilus, 105 Stylops, 442 Subchela, 303 41 642 INDEX Subchelate limbs, 350, 359, 372, 443, 458 Subdermal cavities, 113 Subgenital pits, 152 Subimago, 420 Subneural gland of Timicata, 592 Suboesophageal ganglion, see Gan- glion, Suboesophageal Subtentacular canals, 580 Subumbral pit, 152 Subumbrellar cavity, 135, 138 Subumbrellar ectoderm, of Medusa, 135 ; of trochosphere, 250 Subumbrellar musculature, 135 Suctoria, 107; 4, 15 Sulcus, 48 Summer eggs of Cladocera, 329; of Mesostomum, 188 Superlinguae, 407 Supero-marginal ossicles, 558 Superposition image, 278 Supraoesophageal ganglia, see Gan- glion, Cerebral Surface, of Ciliata, 98 ; of Protozoa, 8, 16. See also Ectoplasm Suture line of ammonoid shell, 529 Swarm spores, 33 Swarming of Polychaeta, 246 Sycon, 117; 112, 113 S. raphanus, 119 Sycon grade, 113 53;///^?, 235; 231, 233, 245 S. ramosa, 246 ; 245 Symbiosis, 38, 42, 46, 48, 104, 170, 188,413 Symmetry, 123, 557; of Actinozoa, 159; of Echinodermata, 547, 548, 557; of Metazoa, 124; of Protozoa, 4. See also Bilateral symmetry, Radial symmetry Sympathetic system, of Crustacea, 305; of Insecta, 398 Symplasts, 6 Synagoga, 347 S. mira, 347 Synalpheus, 346 Synapta, sii; 573, 57^ Synaptida, 577 Syncarida, 351 ; 349 Syncytia, 6, 93, 95. See also Plas- modia Syngamy, 26; 32; of Ciliophora*, 26 n., 28, 29, 30, 100, 107, 108; of Dinoflagellata, 49 ; of Mastigo- phora, 40; of Sarcodina*, 68, 75 78, 79, 82; of Sporozoa*, 83, 85, 86, 88, 89, 90, 93 ; of Volvocina*, 26, 40, 50, 52 ; of Zoomastigina, 56 Syracosphaera, 46 S. pulchra, 45 Syringopora, 163 Syzygy, 83 and n.; 85, 88, 89, 90 Tahanus, 436 Tachardia lacca, 420 Tactile organs, see Sense organs Taenia, 198, 204 T. coenurus, 202 T. echinococcus, 202 T. serrata, 202 T. solium, 200, 201 Taenioglossa, 490 Tae?iiothrips inconsequens, 424 Tagmata, 271; of Crustacea, 296, 333, 349- 'S'ee also Abdomen, Cephalothorax, Head, Mesosoma, Metasoma,Opisthosoma,Prosoma, ■ Pygidium, Thorax, Trunk 1 Tail, 585 ; 545 ; of Chaetognatha, 542, 544, 545 ; of Chordata, 585 ; of Tunicata, 597, 598, 600, 606 Tail fan, 348; 351, 352, 357, 362 Tanaidacea, 354 Tanais, 354 Tardigrada, 467 Tarsus, 385 ; 408, 424, 441 Tealia, 170 Tectibranchiata, 494 Teeth, of Echinoidea, 567 ; of Ophiu- roidea, 564 Tegenaria guyonii, 460 Tegmen, 579 Tegmentum, 475 Tegmina, 409 Telosporidia, 82 ; Reduction division of, 23, 32 Telson, 295 ; 290, 292-3 ; of Arach- nida, 445, 452 ; of Crustacea*, 295, 320, 324, 332, 347, 351, 355, 358; oi Lithobius, 378 Teninocephala , 190 T. nii?ior, 191 | Temnocephalea, 190 1 Tentacles, of Actinozoa, 157, 158, 163; of Ciona, 592; of Coelen- terate polyp and medusa, 129, 130, 131, see also Tentaculocysts ; INDEX 643 Tentacles (cont.) of Ctenophora, 172, 173; of Gas- teropoda, 471, 477, 482, 494, 495 ; of Holothuroidea*, 573, 577; of Hydrozoa*, 132, 133, 135, 138, 139, 143, 145, 147; of Nautilus, 527; of Polychaeta, 230, 231, 235, 237; of Polyzoa, 530; of Scypho- medusae, 151, 152, 155; of Suc- toria*, 15, 107, 108, 109; of Turbellaria, 176 Tentaculata, 173 Tentaculocysts, 154 Terebella, 231 Terebratula, 537, 542 T. seniiglohosa, 538 Teredo, 511 ; 512 Terga (Tergites), 295, 297, 373, 378, 381,449 Terga of Lepas, 338 Tergo-sternal muscles, 392 Termes, 414 Terminal tentacle of Echinodermata, 554, 564 Terricola, 189 Tertian ague, 88 Test of Tunicata, 592; 597, 598, 600, 602 Testacella, 496 ; 495 Testes, see Generative organs. Go- nads Testicardines, 542 Tetrabothriata, 204 Tetrabranchiata, 525; 513 Tetragraptus, 148; 149, 150 T. denticulatus , 150 T. hicksi, 150 T. similis, 149 Tetrastemma, 208 Textrix denticulata, 460 Thalassicolla, 75 ; 76 T. pelagica, 36 Thaliacea, 603 ; 599, 600, 601 Theca, 167 Thecoidea, 582 Theridium, 444 Thompsonia, 345 ; 346 Thoracic limbs, of Crustacea*, 292- 3, 294, 298, 300, 301, 303, 333, 348, 349, 350, 351, 355; of Euca- rida, 300; of Peracarida, 300. See also Legs, Maxillipeds Thoracic membrane, 237 Thoracica, 338 Thorax, of Arthropoda, 27 1 ; of Crustacea*, 271, 294, 296, 316, 319, 331, 347, 349, 351, 355, 358; of Insecta*, 385, 415 ;of /w/m^, 381 ; of Polychaeta, 237 "Thorax" of Tunicata, 596, 598 Thysanoptera, 424 Thysanozoon, 190 Thysanura, 406 Tibia, 385 Ticks, 465 Tiedemann's bodies, 560, 569 Tin tinnidium , 1 04 T. inquilifiwn, 1 01 Tintinnina, 102 Tipula, 92 Tocophrya quadripartita, loi Todarodes Sagittarius, 525 Tomoceros, 408 Topotaxis, 33 Tornaria larva, 590; 588 Torsion of Gasteropoda, 477 Toxiglossa, 490 Trabeculae, of Ciona, 594; of Crinoidea, 580 Tracheae, 278; of Arachnida*, 445, 447, 448, 459, 462, 465 ; of Insecta, 391 ; of Myriapoda, 378, 381 ; of Onychophora, 284; of Woodlice, 279. See also Stigmata, Tracheal gills Tracheal gills, 394, 414, 417, 421, 422, 425, 427 Trachelomonas, Flagellum of, 12 Tracheoles, 139 Trachomedusae, 143; 132, 154 Trachylina, 133; 142, 143 Transverse fission of Protozoa, 24 Trematoda, 190; 174 Triaenophorus nodulosus, 204 Triarthrus hecki, 288 Triatoma, 59 Trichinella spiralis, 222 Trichobranchiae, 366 Trichodina, 104 T. pediculus, loi Trichoinonas , 60; 14 T. muris, 60 Trichonympha, 62 ; 63 T. campanida, 62 Trichoptera, 426; 393 Trichosphaerium, 69; 67 Tricladida, 189; 186 Trilobita, 287 644 INDEX Trilobite stage of Xiphosura, 457 Triploblastic animals, i ; 1 26 Triploblastica, see Triploblastic ani- mals Tripylaea, 71 Tritocerebrum, 274, 305 Triungulin, 430 Trochal disc, 211 Trochanter, 385 Trochocystis, 582 Trochodiscus lotigispinus, 75 Trochosphere larva, 250; i, 2; of Mollusca, 472, 507 ; of Polychaeta, 230, 247, 250 ; of Polyzoa, 532, 534 Trochostoma, 577 ; 576 Trochus, 211 Trochus, 478 Trophallaxis, 434 Trophi, 212 Trophochromatin, 22 Trophozoite, 83 Trunk, 271 ; of Arthropoda, 271 ; of Crustacea*, 291, 294, 296, 297, 322, 325; of Trilobita, 287 "Trunk" of ascidian tadpole, 597 Trunk ganglion, 597 Trunk limbs, of Crustacea*, 290, 291, 293, 298, 317, 318, 324, 325, 328, 329, 331; of Onychophora, 283. See also Abdominal limbs, Thora- cic limbs Trunk segments of Polychaeta, 23 1 Trypanosoma, 58 T. brucei, 57 T. cruzif 59 T. equiperdum, 59 T. gambiense, 59 T. lewisi, 59 T. rhodesiense, 59 Trypanosomidae, 56 Trypanosyllis, 246 T. gemmipara, 245 Tryphaena pronuba, 428, 433 Tube feet (Podia), 546 ; 550, 563, 564, 566, 571, 573, 579 Tubifex, 259 ; 260 Tubipora, 162; 163 Tubularia, 137; 133, 138, 139, 145 Tunicata, 590; 584 Turbellaria, 188; 172, 173, 174 Tylenchus devastatrix, 2,2.^ T. dispaYy 225 T. tritici, 225 Tympana, 411 Typhlosole, 254 Tyroglyphus, 462 T. siro, 463 Umbo of Brachiopoda, 537 Umbrella of trochosphere, 250 Umbrellar surfaces, etc., of Medusae, see Exumbrellar, Subumbrel,lar Uncini, 237 Uncoiling of Gasteropoda, 529 Undulating membranes, of ciliates, 14,97,102; of flagellates, 13,58, 61 Uniramous limbs of Crustacea, 301 Urochorda, see Tunicata Uropods,348; 353,355,356,362,373 Urosome, 333 Uterus, of Cestoda, 186, 199; of Chirocephalus, 321 ; of Cyclops, 334 ; of Paliidina, 492 ; of Platyhel- minthes, 186; oi Rhabditis,2iS; of Rhabdocoelida, 186; of Scorpion- idea, 452; of Trematoda, 186 Vacuolaria, 48 Vacuoles, 8; Contractile, 16, 17, 43, 46, 64, 75, 78, 81, ICO, 104; Food, 15, 60; Gas, 68; Hydrostatic, of ectoplasm, 35,71, 75, 78 ; of Dino- flagellata, 48 Vagina, see Generative organs Vahlkampfia, 64 Valves of shell, of Brachiopoda, 537 ; of Conchostraca, 317; of Lamelli- branchiata, 472, 509, 510, 511; of Lepas, 339 Vanadis, 243 Vas deferens, see Generative organs Vasa efferentia, of Insecta, 397; of Platyhelminthes, 185 Vascular system, 122; of Anostraca, 311; of Araneida, 458 ; of Arthro- poda, 279 ; of Balanoglossus, 587 ; of Carcinus, 370; of Chaetopoda, 230; of Chilopoda, 378; of Ciona, 596; of Crustacea, 311 ; of Diplo- poda, 381; of Insecta, 390; of Lamellibranchiata, 505 ; of Lernan- thropus, 314; of Limulus, 457; of Malacostraca, 312; of Nemertea, 205, 207; of Scorpionidea, 450 "Vascular" system of Echinoder- mata, see Lacunar system "Vascular" system of Scyphomedu- sae, 152 " Vascular " tissue of Echinodermata, see Lacunar tissue Vegetative phase, 31 Vegetative pole, 248 Vein, Abdominal, 519; Afferent branchial, 506; Branchial, 519; Efferent branchial, 507; Genital, 519; Ink sac, 519; Longitudinal, of kidney, 507; Pulmonary, 482. See also Circulus venosus, Sinuses, Vena cava Velella, 145; 147 Veliger, 474 Velum, of Medusae, 135, 154; of Rotifera, 211 ; of Veliger larva, 474 Vena cava, 518 Ventral, see Dorsal and ventral Ventral blood vessel, of Balanoglossus, 588; of Chaetopoda, 230, 240; of Rhynchobdellidae, 265 Ventral "blood vessels" of Echino- dermata, 553, 567, 575 Ventral cirrus, 23 1 Ventral mesenteries, of Alcyonaria, 159; of Polychaeta, 251 Ventral midline of Nematoda, 214 Ventral plate of trochosphere, 250 Ventral plates of Ophiuroidea, 562 Ventral siphon, 501 Ventral tube of Collembola, 408 Ventricle, see Heart Venus' Girdle, see Cestus Veneris Vermes, 174 Vertebrae of Ophiuroidea, 562 Vertebrata, i, 2, 583, 584, 585 Vesiculae seminales, see Generative organs Vesicular nuclei, 19 Vespa, 433 V. crabro, 431 Vestibulata, 102 Vestibule, 15 ; 97, See also Gullet Vibracula, 533 Visceral clefts, 583 Visceral hump, 471, 472, 477, 481, 482, 490, 5i5> 525 Visceral mass, 344 Visceral nerves, see Sympathetic system Vitellarium, of Platyhelminthes, 185 ; of Rotifera, 212 Vitelline ducts, 185 INDEX 645 Vitrellae, 277 Volvocina, 49; 38, 44; reduction division of, 23, 32; syngamy of, 26, 40 Volvox, 52 ; 27, 33,35, 39, 4°, 54, 55 V. aureus, 53 ; 54," 55 V.globator, 54; 55 Vorticella, 104; 4, 5, 16, 29, 97 Waldheimia, 537 ; 538, 540, 542 Water pore of Echinodermata, 552 Water vascular ring, see Water vascular system Water vascular system, 550; 551, 552, 560, 564, 569, 580 Wings, 385; 403 Wire worm, see lulus terrestris W^orkers, of Isoptera, 412; 413, 414; of Wasps, 434 Xanthophyll, 41 Xanthoplasts, 41 Xenocoeloma, 337 Xenopsylla cheopis, 442 Xestobium rufovillosum, 431 Xiphosura, 454 Xylophaga, 511 Yellow cells of Chaetopoda*, 229, 240 "Yellow cells ", Symbiotic, see Zoo- xanthellae Yolk gland, see Vitellarium Young, see Life history Yungia, 190 Zoaea larva, 316; 349, 351,361,370, 372, 372, 373 Zoantharia, 163 Zones of fission, 259 Zoochlorellae, 43 Zooecium, 530 Zooids,of Coelenterata,seeHydranth, Polyp ; of Polyzoa, see Polypide ; of Protozoa, 4 ; 6 ; of Rhabdopleura, 590; of Tunicata, 599; of Volvo- cina, 6, 51-4 Zoomastigina, 54; 40, 56 Zooxanthellae, 43 ; 36, 46, 75, 76 Zygoptera, 417 Zygote, 31; 32, 40, 48, 52, 78, 82, 83, 85, 86, 88, 89, 90, 93, 100, 108 CAMBRIDGE : PRINTED BY W. LEWIS, M.A. AT THE UNIVERSITY PRESS *> -:^ ; "V ^ ^' .i¥^