ALBERT R. MANN LIBRARY New York STATE COLLEGES OF AGRICULTURE AND HoME ECONOMICS AT CORNELL UNIVERSITY DATE DUE GAYLORD 4 556 The Natural History of Animals Cornell University The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003696584 WY ONILONYLSNOD MYOM LY (yaald YOLSVD) SHIAVAS BEAVERS (Castor fiber) The Beaver has a wide distribution in the temperate parts of Europe and North America, though its numbers are rapidly diminishing on account of the merciless way in which it has been hunted down for the sake of its pelt. This furnished the material from which top-hats and the like were originally made. Originally dwelling in a simple burrow scooped. out in a river- bank, the animal has gradually evolved into a skilled architect, capable of constructing a dam across the course of a stream, the framework consisting of the trunks and branches of trees felled by the powerful incisor teeth. Upon the dam rounded dwellings or “lodges” of mud are constructed, with openings well below the water level. The broad flattened tail is a swimming organ, and is not used as a trowel as often stated, The Natural History of Animals The Animal Life of the World in its various Aspects and Relations BY J. R. AINSWORTH DAVIS, ma. TRINITY COLLEGE, CAMBRIDGE PROFESSOR IN THE UNIVERSITY OF WALES, AND PROFESSOR OF ZOOLOGY AND GEOLOGY IN UNIVERSITY COLLEGE, ABERYSTWYTH HALF-VOL. VII LONDON THE GRESHAM PUBLISHING COMPANY 34 SOUTHAMPTON STREET, STRAND 1904 KA Tm dey CONTENTS HALF-VOL. VII NERVOUS SYSTEM AND SENSE-ORGANS CHAPTER LVII.—GENERAL PRINCIPLES—NERVOUS SYSTEMS OF BACKBONELESS ANIMALS (INVERTEBRATA) AND BACK- BONED ANIMALS (VERTEBRATA) GENERAL PRINCIPLES—Properties of Protoplasm: Sensitiveness and Spontaneity. Stimuli: Reaction of Protozoa to Stimuli NERVOUS SYSTEMS OF BACKBONELESS ANIMALS (INVERTEBRATA) Division of Labour in Cell-Communities NERVOUS SYSTEMS OF ZOOPHYTES (CCELENTERATA)—Ectoderm and Endoderm: Nerve-Cells and Nerve-Fibres, Neurons; Sea-Anemones; Jelly-Fishes - NERVOUS SYSTEMS OF SEGMENTED WORMS (ANNELIDA)—Nerve-Ring and Nerve-Cord in Earth-Worm, &c.; Head and Brain; Reflex Action; Visceral Nervous System; Comparison with Vemertine Worms (Nemertea) NERVOUS SYSTEMS OF JOINTED-LIMBED INVERTEBRATES (ARTHROPODA)— Comparison with Annelids - Nervous Systems of Crustaceans (Crustacea)—Apus, Crayfish, and Crab - Nervous Systems of Atr-breathing Arthropods (Tracheata)—Peripatus, Centipedes and Millipedes, Scorpions and Spiders, Insects - NERVOUS SYSTEMS OF MOLLUSCS (MOLLUSCA)—Mail-Shells (CAz#on), Snails and Slugs (Gastropoda), Bivalves (Lamellibranchia), Tusk-Shells (Scaphopoda), Head-footed Molluscs (Cephalopoda) ° - - - NERVOUS SYSTEMS OF BACKBONED ANIMALS (VERTEBRATA) Brain, Spinal Cord, Sympathetic Nervous System, Spinal Nerves, Cranial Nerves ; Development of Nervous System - THE BRAIN OF VERTEBRATES—Development and Structure; Increase of Com- plexity in ascending the Scale - v Page 16 19 20 vi CONTENTS CHAPTER LVIII—SENSE-ORGANS OF BACKBONELESS ANI- MALS (INVERTEBRATA) AND BACKBONED ANIMALS (VERTE- BRATA) The Nature and Uses of Sense-Organs; Organic Sensations; Special Senses - - ToucH—General Nature: Haptic and Thermal Sensations; Tactile Organs of Zoophytes, Annelids, Arthropods, and Vertebrates. Pacinian Bodies and the Pressure-Sense. Organs of Active Touch - - - TasTE—General Nature: Gustatory Organs of Annelids, Insects, Molluscs, and Vertebrates; Taste-Buds - - SMELL—General Nature: Distance-Senses (Telesthetic Senses); Olfactory Organs of Arthropods, Molluscs, and Vertebrates - - - - - BALANCE AND HEARING—General Nature a = Z Balancing Organs of Jelly-Fishes (Hydrozoa)—Otocysts and Tentaculo- cysts and their Uses - - Balancing Organs in Segmented Worms (Annelida)—Earth- HE, Oto- cysts of Lug-Worms (Avenicola) - Balancing Organs in Molluscs (Mollusca)—Otocysts of Bivalves (Lamedit- branchia), Snails and Slugs (Gastropoda), and Head-footed Molluscs (Cephalopoda) Organs of Balance and Hearing in Crustaceans (Crustacea)—Otocysts of Lobsters, Prawns, Shrimps, and Crabs; Otocysts in Tail of Opossum Shrimp (JZyszs); Sound-producing Crustaceans—Rock-Lobster (Pa/z- nurus), Musical Strand-Crab (Ocypoda macrocera), Squeaker Crab (Psopheticus stridulans) - Organs of Balance and Hearing in Insects ({nsecta)—Chordotonal Organs: Musical Organs of Grasshoppers, Green Grasshoppers, and Crickets Organs of Balance and Hearing in Backboned Animals (Vertebrata)— Otocyst in the Brain of Larval Ascidians (Uvochorda); Structure and Uses of the Auditory Organs of Aquatic- and Land-Vertebrates - SIGHT—Shin-Seeing (Dermatoptic Vision) of Earth-Worms and Bivalve Molluscs - Direction-E-yes—Euglena, Jelly-Fishes (Hydrozoa), Star-Fishes and Sea- Urchins (Echinodermata), Segmented Worms (Annelida), Arrow- Worms (Chetognatha), and Limpet (Patella) - - Picture-Eyes—Compound Eyes of Crustaceans and Insects. Camera Eyes of Annelids, Arthropods, and Molluscs. Camera Eyes of Verte- brates—Structure and Development; Visual Organs of the Double- Eyes (Anableps) ; Pineal Eyes of certain Reptiles - - ANIMAL INSTINCT AND INTELLIGENCE CHAPTER LIX.—GENERAL PRINCIPLES—INSTINCT AND IN- TELLIGENCE IN HIGHER BACKBONELESS ANIMALS (INVERTEBRATA) AND BACKBONED ANIMALS (VERTEBRATA) GENERAL PRINCIPLES—Reflex Action, Instinct, Intelligence, Reason - Page 24 25 29 30 32 33 34 34 35 40 43 49 CONTENTS INSTINCT AND INTELLIGENCE IN HIGHER INVERTEBRATES (INVERTEBRATA) INSTINCT AND INTELLIGENCE IN INSECTS (INSECTA)—Mason-Bee (Chalicodoma muraria), Nesting Instincts and Homing Faculty; Nesting Instincts of Solitary Wasps; Egg-laying Instincts of Butterflies and Moths - - - - - INSTINCT AND INTELLIGENCE IN MOoLLuscs (MOLLUSCA)—Intelligence of Octopus; Homing Instinct of Limpet (Paze//a) and Garden Snail (Helix aspersa) INSTINCT AND INTELLIGENCE IN VERTEBRATES (VERTEBRATA) WARNING COLORATION—Intelligence of Young Chicks - - - - - - NEsT-BUILDING IN BIRDS—Eider-Duck, House-Swallow, House-Martin, Palm Swift - - - - MIGRATION OF BIRDS—Golden Plover, White Stork, Cuckoo - - - - ASSOCIATION OF ORGANISMS—THE WEB OF LIFE CHAPTER LX.—ASSOCIATION OF ORGANISMS—GENERAL PRINCIPLES—ANIMALS AND PLANTS GENERAL PRINCIPLES—Modern Tendency to Over-Specialization; Nature Study; Complex Character of Relations between different Organisms - - PLANTS AND ANIMALS Classification of Plants: Leaf-Green (Chlorophyll - - - - PLANTS AND ANIMALS IN RELATION TO AIR—Breathing; Feeding of Green Plants; Bacteria; Mutualism (Symdzoszs) of certain Plants - RELATIONS BETWEEN THE NUTRITION OF PLANTS AND ANIMALS—Plants as the Food of Animals; Plant-Food derived from Animals. Carnivorous Plants, —Butterwort (Pinguzcula); Sundews (Droserace@)—Sundew (Drosera), Venus’ Fly-Trap (Dzonea); Pitcher-Plants—Nepenthes, Sarracenia; Aquatic Car- nivorous Plants—Water Fly-Trap (Addrovandia), Bladder-Worts (Utricularia) MESSMATES OR COMMENSALS (COMMENSALISM)—Epiphytic Orchids; a Liver- Wort (Frullania dilatata) associated with a Wheel-Animalcule (Calidina symbiotica); Sea-Weed and Associated Animals; Ant-Plants; Sloths and Algze MUTUALISTS (SYMBIOSIS)—Lichens; Bacteria of Stomach (Sarcima ventricult); Ray-Animalcules (Radiolaria) and Yellow Cells; Sea-Anemones and Algzee- PARASITES (PARASITISM)—Clover-Dodder (Cuscuéa), Potato-Fungus (Phytophthora); Fly-Mould (Zmpusa), Silkworm-Mould (Cordceds), Bacteria of Disease; Turnip-Disease produced by a Fungus-Animal (Plasmodtophora), Plant-Galls DEFENCES OF PLANTS AND ANIMALS AGAINST ONE ANOTHER—Colourless Corpuscles and defensive Proteids (Antitoxins) of Animals; Spines, Thorns, Prickles, &c., of Plants; Raphides of Arum-“ Lily” (Richardia), &c.; Poisonous Substances, Latex, &c., of Plants; Ant-Plants (Myrmecophilous Plants), Plants protected by Mites - - - - - - vii Page 53 56 58 59 61 63 64 65 68 74 75 76 79 viil CONTENTS POLLINATION OF FLOWERS BY ANIMALS—Structure of Flowers; Nature and Results of Pollination. Flowers Pollinated by Insects and their Characteristics —Orchids, Honeysuckles. Flowers Pollinated by Snails and Slugs—Golden Saxifrage (Chrysosplenium). Flowers Pollinated by Humming-Birds and Sun- Birds. Mammals as Agents of Pollination. Prevention of Self-Pollination - DEFENCES OF FLOWERS AGAINST UNBIDDEN GUESTS te DISPERSAL OF PLANTS BY ANIMALS - CHAPTER LXI.—ASSOCIATION OF ORGANISMS—COLONIAL ANIMALS COLONIAL ANIMALCULES (PROTOZOA)—Epistylis, Codosiga, Volvox, Protero- spongia. Origin of Many-celled Animals (4@e¢azoa) from Colonial Animalcules COLONIAL SPONGES (PORIFERA)—Crumb-of-Bread Sponge (Halichondria panicea) COLONIAL ZOOPHYTES (CQELENTERATA)—Colonial Sea- Flowers (Anthozoa)— Compound Corals, Sea-Pens (Peznatula). Colonial Hydrotds (Hydrozoa)— Hydroid Zoophytes ; Aglaophenia—Conipound Jelly-Fishes (Szphonophora) COLONIAL Moss-POLYPES (POLYZOA) COLONIAL ASCIDIANS (UROCHORDA)—Botryllus, Salps (Sa/ga), Fire-Cylinder (Pyrosoma) CHAPTER LXII.—ASSOCIATION OF ORGANISMS—SOCIAL BACKBONELESS ANIMALS (INVERTEBRATA) Advantages of the Social or Gregarious Habit - SOCIAL INSECTS (INSEcTA) SocIAL MEMBRANE-WINGED INSECTS (HYMENOPTERA)—Social Bees—Halictus, Humble-Bees (Bombus), Honey-Bee (Apis melliifica). Social Wasps (Vespide) —vVespa Germanica, Hornet (V. cvadvo). Ants—Polyrhachis, CEcophylla, Wood-Ant (Formica rufa), Black Garden-Ant (ZLaszus niger), Foraging-Ants (Eciton) SoclAL NET-WINGED INSECTS (NEUROPTERA)—Termites or White-“ Ants ”— Yellow-necked Termite (Calotermes flavicollis), Light-shunning Termite (Zermes lucifugus), Warrior Termite (Z. bellicosus) SociaL FLIES (DIPTERA)—Social Larvee of the Army-Worm (Scéara militaris) CHAPTER LXIII.—ASSOCIATION OF ORGANISMS—SOCIAL BACKBONED ANIMALS (VERTEBRATA) SociaL FISHES (PISCES)—Herring (Clupea harengus), Salmon (Salmo salar), Eel (Anguilla vulgaris), Predaceous Forms SocIAL BIRDS (AVES)—Rook (Corvus frugilegus). Recognition Marks in Birds SocIAL MAMMALS (MAMMALIA)—Beneficial Nature of the Habit in various forms. Prairie-Marmot (Cynomys Ludovictanus) and its Allies. True Marmots (Arctomys). Beaver (Castor). Recognition Marks and Odours of Mammals Page 83 go 95 99 Iol IOI 104 105 107 108 128 129 134 CONTENTS CHAPTER LXIV.—ASSOCIATION OF ORGANISMS—COURTSHIP AND MATING The Law of Battle and the Law of Beauty COURTSHIP AND MATING OF MAMMALS (Mamma Lia) THE Law oF BATTLE—Red Deer (Cervus elaphus) THE Law oF BEAUTY—Lion, Goat, Mandrill (Patio mormon), Red Deer (Cervus elaphus), American Howling Monkeys (ycetes) COURTSHIP AND MATING OF BIRDS (AvEs) THE Law oF BATTLE—Ostrich, Sand-Pipers, Ruffs, Chaffinches, &c. THE Law oF BEAUTY—Males of ordinary Farmyard Birds. Pheasants. Peacock (Pavo cristatus). Scarlet Tragopan (Cerzornis satyrus). Singing Birds. Bustards (Ozzs). Musk-Duck (Cazvina) COURTSHIP AND MATING OF REPTILES (REPTILIA) THE Law oF BaTTLE—Chameleons. American Alligator (Alligator Mississippt- ensts) - THE Law oF BEAUTY—Throat-Pouch of Indian Lizard (Sz¢ama). Scent-Glands of Crocodiles, Snakes, and Lizards COURTSHIP AND MATING OF AMPHIBIANS (AmpuHIBia) THE Law oF BATTLE—Frogs and Toads THE Law oF BEauTy—Great Crested Newt (AZolge cristatus), Edible Frog (Rana esculenta) COURTSHIP AND MATING OF FISHES (PIscEgs) THE Law OF BATTLE—Salmon (Salmo salar), Three-spined Stickleback (Gaster- osteus aculeatus) THE Law oF BreautTy—Dragonet or Golden Skulpin (Callzonymus lyra), an Indian Flat-Fish (Arnoglossus macrolophus) COURTSHIP AND MATING OF INSECTS (INSEcTA) THE Law oF BATTLE—Beetles, Saw-Flies, &c. THE Law oF BEAUTY—Butterflies. Musical Organs. Assembling of Male Moths THE FINDING OF MATES—Olfactory Organs, Eyes, Luminous Organs - COURTSHIP AND MATING OF SPIDERS (ARANEID&) Hunting Spiders (AZ¢zd@) COURTSHIP AND MATING OF CRUSTACEANS (CRUSTACEA) Indian Fiddler Crab (Gelasimus annulipes) 145 146 148 151 152 152 154 157 159 160 163 166 168 x CONTENTS CHAPTER LXV.—ASSOCIATION OF ORGANISMS—MESS- MATES OR COMMENSALS (CoMMENSALISM) FISHES (PISCES) AS MESSMATES—Fierasfers and Sea-Cucumbers. Perch-like Fishes (Amphiprion) and Giant Sea-Anemones (Discosoma) of Australia. Indian Rock-Perch (M/inous inermis) and Encrusting Zoophyte (Stylactis minot) - . MOLLuscs (MOLLUSCA) AS MESSMATES—Indian Sea-Snail (Pleurotoma symbiotes) and Colonial Sea-Anemones (Zfizoanthus). Bivalve Molluscs associated with Sea-Urchins and Crustaceans - - INSECTS (INSECTA) AS MESSMATES—Humble-Bees (Bombus) and Lodger-Bees (Pstthyrus). Ants and Aphides. Slave-making Ants. Ants’-Nest Insects (Myrmecophilous Insects) and other Guests of Ants - CRABS (BRACHYURA) AS MESSMATES—Buffoon-Crab (Dorippe facchino) and Sea-Anemone (Cancrisocia expansa). WHermit-Crabs (Eupagurus, &c.) and Cloak-Anemones (Adamsia), Hydroid Zoophytes (Hydractinia), and Colonial Sea-Anemones (Zfzzoanthus). Sponge-Crabs (Dromid@). Hermit-Crab (Eupagurus) and Sponge (Suberites) SIPHON-WORMS (GEPHYREA) AS MESSMATES—Aspidosiphon and Cup-Corals — - CHAPTER LXVI.—ASSOCIATION OF ORGANISMS—PARASITES Ectoparasites and Endoparasites. Origin of Parasitism, and Degeneration resulting from the Habit. Fecundity of Parasites. Brood Parasitism BIRDS (AVES) AS BROOD-PARASITES—Cuckoo (Cuculus canorus). Cow-Birds (Molobrus) FISHES (PISCES) AS PARASITES—Lampreys and Hags (Cyclostomata) MoLuuscs (MOLLUSCA) AS PARASITES—Parasitic Sea-Snails (Zhyca, Stilaster, Entocolax, Entoconcha). WLarve (Glochidia) of Freshwater Mussels - INSECTS (INSECTA) AS PARASITES—Parasitic Bugs (Hemiptera)—Bed-Bug (Cimex lectularius), Reduvius, Lice - Parasitic Flies (Diptera)—Gnats, Midges, &c., Forest-Fly (Aippobosca eguina), Deer-Fly (Liptoptena cervi), Swallow-Fly (Stenopteryx hirundinis), Sheep-“ Tick” (Melophagus ovis), Bee-“ Louse” (Braula ceca), Nycteribia, Horse-Bot ca abel eguz), Sheep-Bot (@strus ovis), Fleas - - Beetles (Coleoptera) as Parasites and Brood-Parasites—Parasites of Bees and Wasps (Stylofide), Oil-Beetles (Sz¢arzs and Meloé) Parasitic Membrane-winged Insects (ffymenoptera)—\chneumon- Flies (Polynema, Microgaster, Pimpla, Pteromalus, Agriotypus, Sia Thalessa, Leucospis) SPIDER-LIKE ANIMALS (ARACHNIDA) AS PARASITES— Ticks and Mites (Acarina) —Ticks (Lxodes), Mange- or Itch-Mites (Dermatophagus, Dermatocoptes, Sar- coptes), Hair-Mites (Szmonea). Tongue-Worms (Linguatula) : Page 170 172 175 179 182 184 186 188 188 189 190 192 194 195 CONTENTS xi Page CRUSTACEANS (CRUSTACEA) AS PARASITES—Parasitic Fork-footed Crustaceans (Copepoda)—Fish-“ Lice” (Argulus, Achtheres, Lern@a) - - 196 Parasitic Barnacles (Cirripedia)—Sacculina— - - - 197 SEGMENTED WoRMS (ANNELIDA) AS PaRASITES—Parasitic Bristle- Worms (Che- topoda)—Myzostoma, Branchiobdella - - 199 Parasitic Leeches (Discophora)—F ish-Leeches (Pscicola) - 200 FLUKES (TREMATODA) AS PARASITES—Fish-Flukes (Octobothrhium), Udonella, Frog-Fluke (Polystomum), Diplozoén, Distomum macrostomum 200 TAPE-WORMS (CESTODA) AS PaRaSITES—Archigetes, Common Tape-Worm (Tenia solium), Armed Tape-Worm (Zetrarhynchus) and Formation of Pearls 203 THREAD-WORMS (NEMATHELMIA) AS PARASITES—Thorn-headed Worms (Echino- rhynchide) - 205 ANIMALCULES (PROTOZOA) AS PARASITES—Gregarines (.SAorezoa)—Cockroach Gregarine (Clepsidrina blattarum) - - - 206 UTILITARIAN ZOOLOGY CHAPTER LXVIIL—ANIMAL FRIENDS—ANIMALS AS A SOURCE OF FOOD—DOMESTICATED MAMMALS Evolution of Human Civilization. Food of Savages - - 208 WILD ANIMALS AS A SOURCE OF FOOD MAMMALS (MAMMALIA)—Spiny Ant-Eater (Zchzdua) and Duck-Bill (Ornitho- rhynchus). Pouched Mammals (dZarsupialia)—Kangaroo. Hoofed Mammals (Ungulata). Elephants (Proboscidea). Gnawing Mammals (Rodentia)— Hares - and Rabbits. Flesh-Eaters (Carndvora)—Jaguar (Fels onca), Lion (£. leo), Seals (Phocid@). Sea-Cows (Szrenta)—Manatees (Manatus) and Dugongs (Halicore). Whales, &c. (Cetacea) - 211 Birps (AVES)—Plovers’ Eggs, Edible Birds’ Nests - 214 REPTILES (REPTILIA)—Turtles (Chelonza)—Green Turtle (Chelone mydas). Lizards (Lacertilia)—Iguanas ({guanide), Water-Lizards (Varanide). Crocodiles, &c. (Crocodilia), Snakes (Ophidia) - - - 214 AMPHIBIANS (AMPHIBIA)—Edible Frog (Rana esculenta) 214 FISHES (PISCES) - - 214 MoLLuscs (MOLLUSCA)—/ead-footed Molluscs (Cephalopoda)—Cuttle- Fishes, Squids, and Octopods. Swazls and Slugs (Gastropoda)—Periwinkle (Lztforina), Whelk (Buccznum), Limpet (Patella), Ormer or Sea-Ear (Yalzofzs), Sea-Hare (Aplysia), Land-Snails (Helix). Bivalves (Lamellibranchia)—Oyster (Ostrea), Cockle (Cardzum), Mussel (Mytilus), American Clams (Mya, Mactra, Venus), Razor-Shells (So/ez), Piddocks (Pholas), Date-Shells (Lzthodomus), Ark-Shells (Arca). Primitive Molluscs (Amphineura)—Mail-Shells (Chon) 214 * INsEcrs (INSECTA)—Honey of Bees (Agzs, &c.). Locusts, Ants, Termites, Cicadas, Manna of Scale-Insects (Cocczd@), Kungu Cake 215 xii CONTENTS CENTIPEDES AND MILLIPEDES (MYRIAPODA)—Centipedes (Scolopendra) CRUSTACEANS (CRUSTACEA)—Crab (Cancer, &c.), Lobster (Homarus), Prawn (Palemon, &c.), Shrimp (Crangon, &c.), Barnacles (Lepas, Pollicipes, Balanus) BRISTLE-WORMS (CHATOPODA)—Palolo Worm (Palolo viridis) - HEDGEHOG-SKINNED ANIMALS (ECHINODERMATA)—Roe of Sea-Urchins (£chz- noidea). Sea-Cucumbers (Holothurotdea)—Béche-de-mer or Trepang ZOOPHYTES (CCELENTERATA)—Sea-Anemones (cul de mu/let) - DOMESTICATION OF ANIMALS Origin of Domestication and its Influence upon the Evolution of Civilization DOMESTICATED MAMMALS (MAMMALIA) AND THEIR USES—The Dog (Canis Jamiliaris). The Cat (Felis domestica). Oxen (Bos) and Buffaloes (Budbalus). The Sheep (Ovzs aries). The Goat (Capra hircus). Camels (Camelus). Llamas and Alpacas (Lama lama and L. facos). The Pig (Sus scrofa). The Horse (Zguus caballus). The Ass (Eguus asinus). Mules and Zebra-Mules. Elephants (EZlefhas). Rabbits and Hares (Lepus cuniculus and L. timidus). The Fat Dormouse or Loir (AZyoxus glis) DOMESTICATED BIRDS (AVES)—The Fowl (Gallus domesticus). The Duck (Anas boschas). The Goose (Anser domesticus). The Turkey (Afeleagris gallopavo). The Guinea-Fowl (Mumzda meleagris). The Pigeon (Columba livia). The African Ostrich (S¢truthio camelus) DOMESTICATED INSECTS (INSECTA)—The Honey-Bee (Afzs melliifica). The Silk- worm Moth (Bombyx mort). The Cochineal Insect (Coccus cact?) Page 215 216 216 217 217 217 220 246 251 LIST OF ILLUSTRATIONS HALF-VOL. VII COLOURED PLATES BEAVERS (Castor fiber) CONSTRUCTING Dam. From a Drawing by Wilhelm Kiihnert ........00. 0000s cc seeeeceeueeesuaes Frontispiece WHITE STORKS (Ciconta alba) ASSEMBLING FOR MIGRATION. From a Drawing by Wilhelm Kithnert........ 0... cece cece sees cece eee sence een e ene eaeene PRAIRIE Docs (Cynomys Ludovicianus). From a Drawing by Friedrich Specht .........0. 000 cscs cece cece eee e ee ee ee en ee eeeeteeees LLAMAS (Lama lama) CARRYING GOODS IN THE ANDES. From a Drawing by Friedrich Specht........ 0... cece cece cece ee ceee eee ee en sne teen sete cene BLACK-AND-WHITE ILLUSTRATIONS N.B.—Figs. 1123, 1124, 1125, 1129, 1130, and 1131 are from Alcock’s A Naturalist in Indian Seas, by the courtesy of Mr. John Murray. Page Proteus Animalcules (Ameba) showing Central Nervous System of a Centipede effect of stimuli (after Verworn) 4 (Zithobius) (after Vogt and Yung) A Typical Neuron (after Stohr) 6 | Central Nervous Systems of Termite Neurons of Jelly-Fishes and Sea-Anemone (Termes), Water-Beetle (Dytiscus), and (after R. and O. Hertwig) 7 Blow-Fly (fusca) (after Lespés, Gegen- Central Nervous System of Earth-Worm baur, and Blanchard) (after Leuckart) 8 | Central Nervous System of Mail-Shell Structure of Nerve-Cord of Earth-Worm (Chiton) (after B. Haller) (after Retzius) g | Central Nervous System of River-Snail Diagram of a Simple Reflex Action 9 (Paludina) (after Von Jhering) Central Nervous Systems of Annelids (after Central Nervous Systems of Pond-Snail Quatrefages) 10 (Limneus) and Garden-Snail (Helix) Structure of a Nemertine Worm II (partly after Lacaze-Duthiers) Central Nervous System of Apus (after Central Nervous System of Cuttle-Fish Lankester and Pelseneer) 12 (Sepia) (after Bourne) Central Nervous System of Crayfish (after Development of Central Nervous System Vogt and Yung) 13 in a Vertebrate Central Nervous System of Crab (after Development of Vertebrate Brain (after Gegenbaur) - 14 Huxley and Wiedersheim) Central Nervous System of Peripatus (after Brains of Trout, Frog, and Dog (after Balfour) 15 Ecker and Wiedersheim) _- xiii PAGE 62 134 232 Page 15 16 17 17 18 18 20 2I 21 xiv LIST OF ILLUSTRATIONS Cerebral Hemispheres of Man Complex Neuron from Cortex of Human Brain (after Stohr) Tactile Organs (after Korotneff and Vej- dovsky) Tactile Organs of Insects (after Von Rath and Jobert) - Organs of Touch of Frog and Duck (after Merkel and Carriére) Touch-Corpuscle from Finger Tip of Man A Pacinian Corpuscle A Deep-Sea Fish (Zretmophorus) (after Giglioli) - Taste-Organs of a Wasp (after Will) - Taste-Bud from the Tongue of a Rabbit (after Engelmann) Olfactory Organs of Crayfish, Millipede, and Wasp (after Leydig and Hauser) Tip of Optic Tentacle of Garden-Snail in section Diagram of a Comb-gilled Snail (after Lang) - Olfactory Cells from an Amphibian (Pro- teus) (after Stricker) Tentaculocysts of Jelly-Fishes (after Heckel) - Ceatral Nervous System of Lob-Worm (Arenicola piscatorum) (after Vogt and Yung) - Otocyst of a Heteropod (Pterotrachea) (after Claus) Otocysts of Limpet (Fate//a) Otocyst of Lobster (Homarus) - Opossum Shrimp (d@yszs) (after Ger- steecker) - Chordotonal Organs (after Graber) Stridulating Organs and Ears of Grasshop- pers (after Landois, Graber, and Fischer) Body of an Ascidian Tadpole in section (after Kowalevsky) Diagram of the Membranous Labyrinth of a Lower Vertebrate (after Wiedersheim) Euglena (partly after Franzé) Rhopalia of a Jelly-Fish (after Heeckel) Eye-Cup of a Star-Fish Eye-Spot of Nais (after Carriére) Direction-Eyes of « Leech (Hzrudo), a Limpet (Patella), and an Arrow-Worm (Sagit/a) (after Whitman, Carri¢re, and O. Hertwig) Head of Male Honey-Bee (AZis mellifica) Mosaic Vision (after Johannes Miiller) Sections through the Compound Eye of an Earwig (Forficuda), and the Camera Eyes of a Spider (Zpeira diadema), and a Marine Annelid (Alcéofe) (after Carriére, Grenacher, and Heckel) Page 22 44 Page Diagrammatic Sections through Camera Eyes of Cephalopods (after Lang) - 45 Cross Section through the Head of a Tad- pole (from Selenka) = - 46 Pineal Eye of a Tuatara (Hatteria) (after Spencer) - 47 YounG ORANG-UTANS (Sima satyrus) 52 A Limpet (Patel/a vulgata) leaving its Scar at Ebb-tide - - 57 Nest of the Eider-duck (Somateria mollis- sima) (photo. by R. A. L. van Someren) 60 Relations between Animals and Plants (partly after Scott-Elliot) 65 The Sundew (Drosera) (from Kerner) 69 Venus’ Fly-Trap (Dion@a muscipula) (from Kerner) 70 PITCHER-PLANTS (Wepenthes destillatoria) 72 Bladderworts (Utricularia) (from Kerner) 73 Traps of Bladderwort (U¢ricelarza) (from Kerner) 74 Liverwort (Frzllania dilatata), showing cups in which a Rotifer (Ca/idina sym- biotica) lives (from Kerner) 75 Cross-section through a Lichen (Col/ema) (from Kerner) 76 A Ray-Animalcule (Avachnocorys circum- texta) (after Heeckel) 77 A House-Fly (fusca domestica) killed by Fly-Mould (Zmfusa musce) (from Ker- ner) - a 77 Bacteria (from Kerner) 78 Insect-Galls on Leaf of Oak i ipseus (from Kerner) 79 Raphides (from Kerner) 80 Ant-sheltering Acacia (4. spherocephala) (after Schimper) 81 A Saw-Wort (Serratula lycopifolia) de- fended by Ants (Formica exsecta) 82 Diagrammatic Section through a Simple Flower - 84 The Nottingham Catch Fly (Sz/ene nztans) visited by a moth (Dzanthecia albi- macula) (from Kerner) - 86 Cross-pollinated Flowers (from Kerner) 87 Pollination of Monkey Musk (Afimulus luteus) (from Kerner) 90 A Teasel (Dipsacus lactniatus), showing Water-cups (from Kerner) 92 Snowdrop (Galanthus nivalis) (from Ker- ner) 93 Flower of Plumbago Europea (from Ker- ner) 93 Section through Flower of a Honeysuckle (Lonicera alpigena) (from Kerner) 94 Fruit of Linnea borealis (from Kerner) 97 Fruits of Goose-grass (Galium aparine) (from Kerner) : 93 LIST OF ILLUSTRATIONS Hooked Fruits of Avens (Gez nee (from Kerner) Colonial Animalcules (after Greeff, Stein, and Saville Kent) - Small Colony of a Coral (Astroides caly- cularts) Small Part of a Colony of Aglaophenia (from Graber) Diagram of a Colony of Compound Jelly- Fish (Siphonophora) (after Claus) A Compound Jelly-Fish (Stephaléa corona) (after Heeckel) - Parts of Colonies of Moss-Polypes (after von Nordmann and Busk) - Fire-Cylinder (Pyvosoma) and Botryllus (after Carpenter and Herdman) Nest of a Social Wasp (Vespa Germanica) (after Janet) - - Horse Ants (ormica rufa) collecting Food and Building- Materials - - Ant (Alyrmica rubra) ‘‘ Milking ” an Aphis (Aphis sambucz) (from Selenka) Light-Shunning Termite ( Zermes lucifugzus) (after Grassi) Section through Mound of Warrior Termite (Zermes bellicosus) (after Houssay) Royal Cell of Warrior Termite (Zermes bellicosus) (after Smeathman) : Part of a Shoal of Herrings (C/upea haren- gus) CRESTED PENGUINS OR ROCK-HOPPERS (Eudyptes chrysocome) A Rookery Lesser Tern (Sterna minuta) and Ringed Plover (4 gialitis hiaticola) Male Ptarmigan (Lagopus mutus) in Winter Plumage - Hartebeest (Bubalis caama) Foot of Sheep ( Ovzs arzes) dissected to show scent-gland (after Cossar Ewart) Male Mandrill (Papio mormon) Cock Chaffinches Fighting - THE HORNED PHEASANT OR TRAGOPAN (Certornis Satyrus) - Male Australian Bustard (O¢zs australis) (after Murie) - Owen’s Chameleon (Chamealeo Owen) Male Lizard (Sz¢axa) with Throat-pouch Great Crested Newt (M@olge cristatus) Head of Adult Male Salmon (Salmo salar) Gemmeous Dragonet (Cal/zonymus lyra) A Tropical Beetle (Chzasognathus Grantiz) Orange-Tip (Azthocharis cardamines) and Cabbage-White Butterflies (Pieris bras- stc@) - “ Assembling” of Oak Eggar Moths (Zaso- campa quercus) : Page 98 100 102 103 104, 104. 105 106 112 117 119 123 125 126 129 130 131 133 134 141 142 145 147 148 150 151 152 153 155 158 159 161 163 The Emperor Moth (Saturnia carpint) Horizontal Sections through the Heads of a Male and Female Mayfy ( (Cloé fuscata) (after Zimmer) A South American Gisg vein (Phengodes Hieronymz) (after Haase) Courtship Attitudes of Male Spiders (after Dr. and Mrs. Peckham) Indian Fiddler Crab (Gelasimus annulipes) (from Alcock) Indian Rock Perch (A@inous inermis) with Commensal Polypes (Stylact’s minot) (from Alcock) An Indian Sea-Snail (Plearotoma symbiotes) with Commensal Sea-Anemones (Z#z20- anthus) (from Alcock) - Slave Raid of Amazon Ants (Polyergus ru- fescens) Scenes in Ant Life (after Wasman and Janet) - Buffoon-Crab (Dorippe facchino) with Com- mensal Sea-Anemone (Cancrisocia ex- pansa) (from Selenka) Anderson’s Blanket-Crab (Chlenopagurus Andersonz) with Commensal Sea-Ane- mones (Zfzz0anthus) (from Alcock) Andaman Sponge-Crab ( Cryptodromia pile- zfera) .with Commensal Sponge (from Alcock) A Cup-Coral (Heteropsammia Michelini), showing the dwelling of its Commensal Siphon-Worm (Aspidosiphon corallicola) (from Alcock) Young Cuckoo (Cuculus canorus) ejecting a Fledgling Meadow-Pipit from the Nest (from Blackburn) - Sucker of a Lamprey (Petromyzon) - Parasitic Cap-Shell (Zhyca ectoconcha) at- tached to the Skin of a Star- Fish (Linckia muitiforis) (after Sarasin) Swallow-Fly (Stexopteryx hirundinis) and Sheep-‘‘ Tick” (A@elophagus ovis) - Horse-Bot (Gastrophilus eguz) (from Ritzema Bos) A Bee-Parasite (Stylops aterrimus) (from Selenka) Stages of Sitarzs humeralis (from Selenka) The Yellow-legged Ichneumon-Fly (4Zcro- gaster glomeratus) (after Ritzema Bos) A Female Ichneumon-Fly ( 7alessa) using her Ovipositor (after Riley) A Tick (Zxodes) Hair-Mite (Szmonea folliculorum) and Itch- Mite (Sarcoptes scabez) (after Leuckart) Carp-“‘Louse” (Argus) - Perch-‘‘ Louse” (Achtheres percarum) Sprat-‘‘ Louse” (ZLerze@a) - XV Page 164 165 165 167 168 171 172 176 179 179 181 182 182 xvl LIST OF ILLUSTRATIONS Page Sacculina (after Boas and Delage) 198 Myzostoma (after Von Graff) 199 Fish-Leeches (P2sczcola) (after B. Hofer) 200 Pollack-Fluke (Octobothrium pollachit) 201 Diplozodn paradoxum~—- 202 Distomum macrostomum (after Loos and Vosseler) 203 A Simple Tape-Worm (Archigetes Sieboldt) (from Vosseler) 204 A Fish Tape-Worm ( Zetrarhynchus) (after Pintner) 204 Thorn - headed Worm ( Gigantorhynchus gigas) (from Ritzema Bos) 205 Cockroach Gregarine (Clepsidrina blat- tarunt) (after Biitschli) 206 Reindeer (Rangifer tarandus) - 219 Greyhound - - 221 Dachshund 221 Types of Ancient Egyptian Dogs 222 Fallow Cat (Felis maniculata) - 223 Hungarian Oxen 224 Dwarf Zebus of Ceylon 225 BRITISH WILD CATTLE (Bos primigenius) 226 The Mouflon (Ovzs musimon) 227 Fibres of Wool 228 Angora Goats 229 Bactrian Camels (Camelus Bactrianus) 231 Alpacas (Lama pacos) 232 Prehistoric Hog-Maned Horses 234 Przewalsky’s Horse (Zguus Preewalskiz) 234 Head of ‘‘Matopo”, Prof. Cossar Ewart’s Zebra (Zguus Burchell), and of a Nor- wegian Pony (from Cossar Ewart) Arabian Horse A Clydesdale A Shetland Pony Egyptian Sculpture - ‘Sir John”, one of Prof. Cossar Ewart’s Zebra Hybrids (from Cossar Ewart) Indian Elephant (Zlephas Indicus) lifting Timber Angora Rabbit : The Fat Dormouse or Loir (AZyoxzs glis) - Game Fowls Grey Lag Geese (Amser cinereus) Guinea-Fowls (Vumida meleagris) Ostriches (Struthio camelus) on a South African Farm Honey-Bee (Apzs mellifica) Hind-leg of a Worker Bee showing Pol- len-Basket Under side of a Worker Bee, showing Plates of Wax Extended Mouth-parts of a Worker Bee Honey-Comb and Brood (after Benton) Cowan Hive in Longitudinal Section (from Cowan) Silkworm Moth (Bomdbyx mor?) (after Riley) Nopal (Ofzntéa) and Cochineal Insect (Coccus cacti) NATURAL HISTORY NERVOUS SYSTEM AND SENSE- ORGANS CHAPTER LVII GENERAL PRINCIPLES—NERVOUS SYSTEMS OF INVERTEBRATES AND VERTEBRATES GENERAL PRINCIPLES Some of the properties of living matter or protoplasm have already been pretty fully considered, in sections which may be regarded as expansions of part of the brief sketch of Human Physiology given in vol. i, pp. 24-59. We have seen that protoplasm is a very complex and eminently unstable substance, which is continually breaking down into simpler compounds, with the result that stored or potential energy is transformed into actual or kinetic energy, without which movement and other life-manifestations would be impossible. The breaking- down process ultimately results in the formation of waste pro- ducts, which being physiologically useless are cast out of the body. One such product is carbonic acid gas or carbon dioxide, and a primary object of Breathing or Respiration is to get rid of this. But Breathing also includes the taking in of free oxygen, without which the breaking down of the complex body-substance would not take place at the rate necessary for liberating the energy required. We have also seen that the gradual wasting of the body associated with the breaking-down process requires to be made good; hence the necessity for Food, which is built up into fresh protoplasm. In cases where Growth is taking P Vor. IV. 1 95 at 2 NERVOUS SYSTEM AND SENSE-ORGANS place the food taken in must obviously be larger in amount than when it is merely a question of compensating for waste. By a process of over-growth with subsequent separation from the parent-body new individuals are developed, capable of leading independent existences, and ultimately giving rise to a further generation in their turn. Another very characteristic property of protoplasm is Contractility, z.e. spontaneous change of shape. Hence all the various kinds of Animal Movement, without which food could not be secured, enemies escaped, or unfavour- able surroundings quitted. The present section is an expansion of the last part of the brief sketch of Human Physiology already mentioned, ze. the part headed Nervous System and Sense Organs. What these are, and why they should exist, cannot be understood without reference to another fundamental property of protoplasm, which we may broadly term Sensitiveness and Spontaneity, there being, unfortunately, no briefer way of putting it. The surroundings of an animal are constantly changing; all sorts of external agents are continually acting upon it to varying extents; and life wholly depends upon successful adjustment or adaptation to this per- petually altering Environment. Alternations of day and night, succession of seasons, tidal flow and ebb, variations of food- supply, the diminution or increase in number of enemies, may be taken as examples of changes which have much to do with the preservation or extinction of old species and the evolution of new ones. That protoplasm is sezsztzve means that it is not inert to its surroundings, but reacts, in ways which tend to the preservation of life, to the influences which are constantly affect- ing it. If, when you are not looking, someone touches your hand with a red-hot poker, the member thus treated is drawn back without the exercise of will-power, and immediately after a painful sensation is experienced. This practically illustrates the fact that human protoplasm is sensitive to one external agent, ze. heat, and the usefulness of reaction is sufficiently obvious. If animals were not sensitive to heat many of them would very quickly perish in an untimely manner. And a little consideration will make it apparent that Sensitiveness to a great variety of external agents is absolutely necessary to existence. All actions, however, are not the direct results of external agents acting for the time being. Protoplasm is sfon- GENERAL PRINCIPLES 3 taneous, i.e. it performs actions which find their starting-point within the body itself, as in the case of many voluntary human actions. Any change in the surroundings which brings the sensitive- ness of an organism into play is technically known as a stimulus (L. s¢émulus, an ox-goad), and stimuli may broadly be classified as mechanical, chemical, thermal, photic, and electrical. The corresponding stimulating agents are pressure, change in chemi- cal nature of the surroundings, heat, light, and electricity, which are scientifically defined as different forms of energy, or, to use the old expression, “force”. Protoplasm, like every other kind of matter, may be regarded as made up of excessively minute particles or molecules, much too small to be seen with even the most powerful microscope, which are in a state of constant vibration, throbbing, or to-and-fro movement. The pendulum affords a simple example of vibratory movement. It may further be said that every sort of stimulus is of the nature of a vibra- tion, e.g. in a sound-wave transmitted through air the particles of air move in a particular way and at a rate depending upon the pitch of the sound. All the changes that take place in living matter result from modifications in the movement of its molecules, but we are profoundly ignorant of what exactly takes place when, say, a muscle-fibre contracts or an impulse passes along a nerve. The adjustment to surroundings that is necessary for the maintenance of life results from these molecular changes in the body, which take place in response to the action of pressure, heat, light, &c., these themselves being of a vibratory nature, as has already been stated. So far as an animal is “sensitive” to its surroundings it is comparable to a complex musical instrument capable of playing all sorts of tunes with all kinds of variations, in response to external influences of different kind. The reaction of an animal to its environment at any given moment depends upon how external agents are acting upon it at that moment: it is they which ‘‘call the tune”. If the supposed musical instrument could also play tunes of its own accord, independently of the direct action of the surround- ings, such tunes might be taken to represent the “spontaneous” actions of an animal. That Sensitiveness and Spontaneity, as above defined, are essential properties of living matter, may best be realized by 4 NERVOUS SYSTEM AND SENSE-ORGANS studying a very simple organism, such as the Proteus Animal- cule (Ameéa), which is a particle of comparatively pure proto- if plasm (fig. 1006). That this creature is sensitive to mechanical stimuli is easily proved by tapping the glass slide on which one of the body (pseudopods) by which creeping is effected will be drawn in, and the animal will assume a spherical form (fig. 1006, 4). A good example of chemical stimu- A 1 (8) very weak caustic potash to the species of Amceba (4. max) is B moving along, the reaction consisting in this case of the protrusion of long pointed pseudopods (fig. 1006, 8B). The same kind of Amceba reacts in a marked way to changes of tem- perature (fig. 1006, c). At freezing C x o° C. to 35°C. it moves about with \ ever-increasing activity; above this 40 and at about 4o’C. the animal has 25 assumed a spherical form, and dies >? )* in a condition of ‘“heat-stiffening ” <—— or coagulation. This illustrates very Fig. 1006. — Proteus Animalcules (Amada), oe eee eres stimulus has only a certain range of action, the range in this case being between oC. and 40°C., which are known as the smznzmum Between these two points is an optimum one (35° C. in this instance), at which the stimulus exerts its greatest effect by way of promoting activity. Light does not appear to affect taking in of food, which process goes on most actively at night. If a constant current of electricity is passed through the body of an Amceba which is protruding pseudopods in all directions is crawling about under the microscope. The protruding lobes lation is afforded by the addition of @0O water in which the slug-shaped | : point (0° C.) it is spherical and inert; as the temperature increases from 1 a ae the activity gradually diminishes; D Boe much enlarged, showing effect of various stimuli. well the fact that any particular and maximum points of heat-stimulation for this particular animal. the creeping movements of Ameceba, but is said to check the it will begin to creep against the current, and all those pseudo- NERVOUS SYSTEMS OF INVERTEBRATES 5 pods will be drawn in which are not at the front end for the time being (fig. 1006 D). Since a hungry Amceba creeps actively about for a long time we are probably justified in concluding that some of its movements are spontaneous, and these are probably initiated by chemical changes which take place within its body, and may be called internal stimuli. NERVOUS SYSTEMS OF INVERTEBRATES The Ameeba, like most other animalcules, is a single cell or structural unit, which has to discharge all the functions of life, and does not exhibit the principle of division of labour to the same extent as animals belonging to the higher groups, which are collectively termed Metazoa, as contrasted with the Ani- malcules or Protozoa. Every member of the former group is made up of more or less numerous cells, and may therefore be styled a cell-community. It is clear that in such a case ad- vantageous adjustment to the surroundings is best secured on the principle of division of labour, by which the vital activities are shared among the members of the community. Evolution on these lines has resulted in the development of Digestive Organs, Respiratory Organs, Organs of Movement, &c., the complexity of which is very great in some of the higher groups of animals. Hence the need for some means of central control, some way of correlating the diverse parts of the body, and at the same time of adjusting the body to its environment. These duties are discharged by the Nervous System, with the aid of Sense Organs, which keep it in touch with external agents. The Sensitiveness and Spontaneity of a Metazoon, in fact, are more or less centred in the Nervous System and Sense Organs, and this is true to an increasing extent as we consider animals higher and higher in the scale. At the same time it must not be forgotten that every cell in the body is endowed with ad/ the primary properties of protoplasm, though cells specialize as it were in different directions, according to the nature of the organs of which they form a part. Nervous Systems or ZoopHyTES (C@LENTERATA). — The members of this primitive group, comprising Freshwater Polypes (Hydra), Hydroid Zoophytes, Jelly-Fish, Sea-Anemones, and 6 NERVOUS SYSTEM AND SENSE-ORGANS Corals, correspond to a fairly early stage in the evolution of the Metazoa from simple colonies of Protozoa, and furnish us with some idea of the way in which nervous systems have arisen. Reduced to its simplest terms the body of such an animal is practically a living stomach, and is made up of two layers of cells—an inner one (endoderm) and an outer one (ectoderm). We are here more especially concerned with the outer layer. It would appear that the nervous system was first evolved in the interest of adjustment to the surroundings, and it is not there- fore surprising to find that it has come into existence by modi- fication of some of the cells making up the ectoderm, since this layer immediately adjoins the outer world. The same is also true for the essential parts of all the sense-organs. These specialized nervous elements are known as nerve-cells or ganglion cells, which constitute so many centres of cor- relation and control. A typical cell of the kind is star-shaped and possesses a large nucleus, but it may also be round or ovoid. In Verte- Seater ateed brates, of which our knowledge is considerable, Neuron, much enlarged it is usual for a nerve-cell to be prolonged N., Nucleus; .F., nerve- . ° . fibre (cut short). into a number of branching prolongations, and one nerve-fibre, which may be of very great length (fig. 1007). It is convenient to speak of the cell with its extensions as a xeuyon, and investigation will probably show that the neurons of Invertebrates are broadly similar to those of Vertebrates, though in many instances our knowledge is here very incomplete. It is clear that the nerve-cells need to be in communication with the environment, the parts which they control or correlate, and other similar cells with which they co- operate. This is provided for by the slender extensions of the cell-body, which constitute lines of communication. There is reason to think that the branching extensions are paths of con- duction ¢o the cell, while the nerve fibre is a similar path from the cell. It was formerly believed that the neurons in a nervous system are united together by their processes into a complicated net-work or plexus, but it is now known that in Vertebrates at any rate this is not so, though the slender extensions of one neuron are closely adjacent to those of others. The same thing is probably usually true for Invertebrates. NERVOUS SYSTEMS OF INVERTEBRATES 7 Although neurons belong to the external cell-layer they do not remain at the surface of the body, but sink as it were more or less inwards.to take up a more sheltered position, leaving to sense-cells the duty of reception of stimuli from external agents. In some Jelly-Fish we find cells of the external layer which are beginning to sink in to constitute neurons, while others have actually done so and acquired at the same time a more specialized shape (fig. 1008, a). A nervous system of very primitive kind is found in a Sea- Anemone, which, leading as it does a sluggish life fixed to some firm object, does not require any very ela- borate correlating mechanism. There is here a delicate continuous nerve-layer underlying the ectodermal cells that directly adjoin the exterior, and made up of innumerable neurons of which the extensions run in all directions (fig. 1008, B). Even in this case, however, there is a certain amount of centralization, for the nerve-layer is thicker in the upper side of the body where the mouth is placed, and in the tentacles which fringe this region. The free-swimming Jelly-Fish, having more * ‘ Fig. 1008.—a, Stages in the complex adjustments to effect with the eM- Bwolution of the Neurons of a ; : lly-Fish, enlarged. 1, Part vironment than their fixed relatives, possess, JP Ninctavcr in a Sex as might be anticipated, a more centralized a showing Neurons, en- nervous system. It is true that there is here also a continuous nerve-layer in the deeper part of the ectoderm, but part of this is concentrated into what may be called a central nervous system. This may be either in the form of a double nerve-ring placed near the edge of the umbrella, or it may con- sist of small masses of neurons placed at regular intervals in the same region. Nervous SysTEMS oF SEGMENTED Worms (ANNELIDA).—The members of this group are comparatively complex in structure, and possess a well-defined nervous system, that conforms to the two-sided or bilateral symmetry of the body. The primi- tive nerve-layer in the ectoderm is retained more or less, but it is largely superseded by the central nervous system, which consists of a nerve-ring surrounding the front end of the diges- tive tube, and a double nerve-cord running along near the under 8 NERVOUS SYSTEM AND SENSE-ORGANS side of the body (fig. 1009). In the lower Annelids this system is closely connected with the ectoderm or outer layer of the skin, but in the more specialized members of the group it has sunk within the muscular layers of the body-wall, where it is much better protected. Connected with these central organs are a large number of slender nerves, that come into intimate relation with the various organs of the body, and are made up of excessively minute nerve-fibres which are prolongations of the nerve-cells. On the upper side of the nerve-ring are two little swellings, that may be regarded as an incipient brain or chief central organ, and are technically known as ganglia. A ganglion is a thickened part of a nerve-cord, where nerve-cells are concentrated as a result of evolution along centralizing lines. The ventral cord swells into a pair of ganglia in each segment of the trunk, for which they act as controlling organs. In such a form as the Earth-Worm the ganglia Fig. 1009.—Front Part of the Central’ 2° NOt very distinct, and nerve-cells are Nervous System of an Earth-Worm,en- Scattered throughout the whole of the larged. H.L., Head-lobe; WV.R., side of 2 nerve-ring: V7 visceral or sympatheic Central nervous system, but in the free- aie Siete ais living Bristle-Worms and Leeches con- centration of nerve-cells has taken place to a much greater extent, and the ganglia are clearly marked. The relations of these active forms to their surroundings are com- paratively complex; hence greater concentration of nerve-cells with increased efficiency of the nervous system. The complex nature of the neurons will be gathered from fig. 1010, which represents a few of them in part of the ventral cord of an Earth-Worm. The front end of a bilaterally symmetrical animal, such as a segmented worm, is more subject to the action of external agents than the rest of the body, and becomes specialized into a head, in which the most important part of the nervous system, /e. the brain, and the chief organs of sense are located. Even in a segmented worm we are justified in considering the brain as the highest part of the nervous system, because it is the chief centre of correlation and administration. Voluntary action, con- sciousness or awareness of existence, sensation, and intelligence, so far as these exist in so lowly an animal, are dependent upon NERVOUS SYSTEMS OF INVERTEBRATES 9 it. That this is so in the common earth-worm we know from the fact that the mole stores up these unfortunate creatures as a sort of living larder, having pre- viously bitten off the front ends of their bodies, and consequently re- moved such brains as they possess. This does not destroy life, but pre- vents the victims from crawling away. The ventral nerve-cord is subordi- nate to the brain, but exerts a con- siderable amount of independent con- trol, each pair of ganglia dominating the ring or segment to which it be- longs. To these collections of nerve- cells are due what are technically known as reflex actions, which are quite independent of will. We may instructively consider one common sort of reflex action which manifests itself in muscular movement. If the skin of one of the segments is stimu- lated mechanically, chemically, or otherwise, some amount of contrac- tion in the muscle of the body-wall Fig. roro.—Diagram of part of the Ventral Cord of an Earth-Worm, showing a few Neurons, enlarged. The two arrows (on the right) indicate the direction of nerve-impulses. 1, Nerve-roots: z, afferent nerve-fibres; 3, efferent nerve-fibres: 4, a neuron, of which the branches extend through three segments immediately follows. For the performance of this or any other reflex action three nervous elements are requisite: (1) a nerve-centre consisting of one or more, usually of several, nerve-cells, which in the latter case co-operate with one an- other; (2) one or more nerve-fibres consti- tuting an afferent tract carrying impulses Zo the nerve-centre from sensitive ectodermal cells which have been acted upon by the Muscle — mechanical, chemical, or other stimulus; Fig. 1or1.—Diagram of a Simple Reflex Action. S., Stimulus; S.O., and (3) one or more nerve-fibres forming sense-organ; affiz., afferent nerve; NV.C., nerve-centre; ef., efferent an efferent tract carrying impulses from nerve. Direction of nerve-impulses the nerve-centre to the executive structures indicated by arrows. which perform the reflex action, these being muscle-fibres in the case supposed (fig. ror). Even in ourselves many actions are of reflex nature, e.g. the involuntary withdrawal of the hand from a red-hot substance as described on an earlier page (p. 2). fe) NERVOUS SYSTEM AND SENSE-ORGANS It has already been stated that one of the duties of a nervous system is to correlate the organs of the body itself, and even in an earth-worm there is a special arrangement for controlling the digestive organs, and consisting of nerves which run from the sides of the nerve-ring to the gut, and branch out in a complex way, the branches swelling here and there into extremely minute ganglia. This arrangement is called the wesceral nervous system, and, like the ventral cord, is subject to the control of the brain. Before leaving segmented worms one feature in the nerve- cord is deserving of notice. It is distinctly of double nature, and in some cases its two longitudinal halves are widely separated (fig. 1012). In the evolution of this type of nervous system it is probable that each side of the body de- veloped and was regulated by its own longi- tudinal nerve-cord, and this is actually the arrangement found in the curious unseg- mented forms known as Nemertine Worms. Hl) Though these constitute a special group PS ventrai Al quite distinct from Annelids, they are de- scended from common ancestors, some of ' the primitive characters of which they have Fig. ror2.—Front Part of Cen» probably retained, one being the possession tral Nervous System in two Marine : . . Annelids, enlarged. In A the two Of a Strong lateral nerve on either side, in- Lath theneiein ce stead of a double ventral cord (fg. tora). ee ae Such an arrangement is not a desirable one, for it means imperfect correlation between right and left sides of the body. The ventral cord of an Annelid has quite likely been derived from lateral cords of the kind, which have migrated downwards and come into more or less intimate relation with one another in the interests of centralization. NERVOUS SYSTEMS OF JOINTED-LIMBED INVERTEBRATES (ARTHRO- popA).—There can be little doubt that the members of this huge group have sprung from ancestors which resembled Annelids in many respects. But they have specialized in various ways, partly as the result of centralizing tendencies which have resulted in increased complexity of structure, associated with very perfect adjustment to surroundings. The body, instead of being greatly elongated and made up of a large number of rings or segments, is comparatively short, and composed of relatively few segments. A Brain B NERVOUS SYSTEMS OF INVERTEBRATES II In the higher members of the group, ¢.g. lobsters and insects, the segments in front have fused into a well-developed head, followed by a thorax, to constitute which other segments have coalesced, while this is succeeded by an abdomen, where the amount of union of segments varies greatly in different cases. These three suc- cessive regions of the body differ greatly from one another as to size and shape, and may undergo further fusion. Thus, in a A PROBOSCIS PORE __PROBOSCIS IN SHEATH mye = — SS LO C2] = . ST SSS (Le 7 i ce BRAIN MOUTH cut LATERACNERVE HEADSLIT\\ pennanee ——— Za — DORSAL NERVE Yr PROBOSCIS SHEATH Vy 7 MMM : ( oe Y/ ROBOSCIS ‘B. ROBOSCIS PORE LATERAL NERVE Tl. _—-NERVE RING BRAIN DORSAL NERVE LATERAL NERVE PROBOSCIS IN SHEATH Za LATERALNERVE Fig. 1013.—Diagrams to illustrate Structure of a Nemertine Worm, represented as a transparent object A., Side view; B., front end, seen from above; C., cross section. Lobster, head and thorax are welded together, and in a Spider not only is this so, but the abdominal segments have closely united into a rounded mass. The nervous system of an Arthropod, like that of an Annelid, consists of nerve-ring and double ventral nerve-cord, but the ganglia are better developed, and in the higher members of the group they are more or less fused together into larger nerve- masses, just as the segments to which they belong are similarly united. There is, in other words, an increasing amount of cen- tralization in the nervous system as we pass from lower to higher forms in any subdivision of the Arthropods. And this is clearly 12 NERVOUS SYSTEM AND SENSE-ORGANS advantageous in regard to correlation of the different parts of the body, and adaptation to the environment. It may also be noted that while in many lower Arthropods the two halves of the ventral cord are more or less separate, they are intimately united together in higher forms. Nervous Systems of Crustaceans (Crustacea). — Successive stages of fusion in the nervous system may be illustrated by comparison of Apus, Crayfish, and Crab. In the first of these, which is one of the lower forms, there is a nerve-ring with clearly-marked brain, and a ventral cord of which the two halves are widely separate (fig. 1014). The brain of a typical Annelid, such as the Sea-Centipede (Nereis), is lodged in a head-lobe (prosto- mium) that forms the front of the head and overhangs the mouth, and it supplies with nerves the eyes and feelers which are borne upon this lobe (fig. 1014). The brain of Apus is placed in a corresponding position, and is in the main equivalent to that of Nereis, though probably not entirely so. A Fig. 1014.—Front Part of the Crustacean possesses two pairs of feelers Central Nervous System of AP, (antennules and antennz) situated in front 6, Fics wice guts ame Of the mouth, but most likely their original through nerve-ring: Gy1-G4, position was behind that aperture, and they ganglia of one half of ventral cord; . : Opn, Ants, Antz, Md, Mx1, have shifted forwards into a position more Mzx.2, Th/.1, nerves to eyes, anten- nules, antennz, mandibles, firstand Suitable for the work they have to perform, second maxillz, and first thoracic a - feet; V., visceral nervous system. Dy Way of exploring the surroundings. These two pairs of feelers belong to two segments of the head, each of which is provided with a cor- responding pair of ganglia. In Apus those of the segment to which the antenne belong are the first pair of the ventral cord (see figure), but the nerve for each front feeler or antennule arises from the side of the nerve-ring, and can be traced into the brain. This is intelligible if we suppose that organ to be equivalent to the brain of an Annelid, plus the ganglia supplying the antennules, which have shifted forwards along the sides of the nerve-ring. If this view be correct, a certain amount of fusion and centralization has taken place at the front end of the nervous system in Apus, as compared with an Annelid. But it is here necessary to state that NERVOUS SYSTEMS OF INVERTEBRATES 13 some authorities hold a different view as to the antennules, be- lieving that these have always been situated in front of the mouth, and are in reality outgrowths from the head-lobe. If so they are comparable to the sensitive palps on the head of a Nereis, and the brain of Apus is strictly equivalent to the brain of an Annelid. We shall assume here the truth of the first view, as the balance of evidence is in its favour. The body of Apus is made up of a com- paratively large number of segments, while in Crayfish and Crab, as in all members of the highly-organized group (Decapoda) to which they belong, there are relatively few, ze. twenty, so far as can be definitely made out. Five belong to the Head, eight to the Thorax, and seven to the Abdomen, each with a pair of ganglia and, except the last, provided with a pair of limbs. The nervous system of the Crayfish has undergone a certain amount of fusion and centralization (fig. 1015). The brain is larger and more complex than that of Apus, and it supplies not only the first but also the second feelers, the ganglia corresponding to which have shifted along the nerve-ring. Even greater fusion has taken place at the front end of the ventral cord, where there is a large ventral ganglion, which has resulted from the _ union of the last three pairs of head-ganglia ga or ee Raa (supplying the three pairs of jaws), and the _ G., Gullet (in cross sec- é < . . tion); Bx, brain; W.R., side three first pairs of thoracic ganglia (supplying of neering; 7.G., ventral the three pairs of foot-jaws). It is interesting Sipe es al to notice that the third thoracic ganglia are erst, sensi ot Sh caught as it were in the act of uniting with 7%? anterior and posterior those in front of them. The last five pairs of thoracic ganglia (supplying pincers and walking-legs) are clearly defined, although by reference to the figure it will be seen that the last two are beginning to unite, while just in front of this the doubleness of the cord is practically demonstrated by the fact that its two halves diverge, for the passage of an artery which runs vertically downwards from the heart to supply the ventral region of the body. The first five pairs of abdominal 14 NERVOUS SYSTEM AND SENSE-ORGANS ganglia are quite distinct, and smaller than those of the thoracic region, in correspondence with the relatively small size of their segments. But the last two pairs have united into a somewhat larger nerve-mass, which supplies the last two segments of the body, that include the large tail-fin. It has been shown by ex- periment that the brain of the Crayfish is the dominating centre of the nervous system, while the remaining nerve-masses are centres of reflex action for the segments which they supply. Turning now to the Crab, in which the head and thorax are relatively short and broad, and the abdomen insignificant, the brain is comparable to that of a Crayfish, but all the ganglia of the short ventral cord have fused together into a single mass, placed near the under side of the thorax, and perforated by the artery which runs down from the heart (fig. 1016). In all the three Crustaceans described there is a visceral nervous system, the roots of which are indicated in the figures. Nervous Systems of Atr-breathing Arthro- Fig. r0x6.—Central Ner- Pods (Tvacheata).—Comparison of Peripatus, foes Pye Ot dewano Myriapods, Arachnids, and Insects will show or eset ew, that the same lines are followed as in Crus- seen behind) with momerous taceans. In the less-specialized forms, where the body is elongated and there has been -but little fusion between segments, the nervous system is very like that of an Annelid. But in cases where the body is compara- tively short and much fusion has taken place, the nervous system is concentrated to a corresponding degree. We have already had occasion to see that Peripatus is the most primitive of all living air-breathing Arthropods, and has the closest affinity to Annelids. This view is fully borne out by examination of the nervous system. The upper side of the nerve- ring is swollen into a relatively large brain, and the two halves of the ventral cord are widely separate, though united by numerous transverse bands of nerve-fibres. The outer part of each cord, through its entire extent, contains numerous nerve-cells, and these are not aggregated into well-marked ganglia (fig. 1017). Myriapods, such as ordinary Centipedes and Millipedes, are rather more specialized than Peripatus, and possess a well-marked NERVOUS SYSTEMS OF INVERTEBRATES 15 head, although there is no distinction between thorax and ab- domen. The central nervous system consists of the usual nerve- ring and double ventral cord, and well-developed ganglia are present, between which the two halves of the cord commonly remain distinct (fig. 1018). In Centipedes there is a certain amount of fusion between the ganglia at the front end of the cord, the region from which spring the nerves of the three pairs of jaws, and also those of the poison-claws. Fig. 1017. Dissection of Peripatus from Fig. 1018.—Dissection of a Centipede (Lithobius) the upper side, to show Central Nervous from above, enlarged System. 1.a., Intestinal aperture. In regard to Arachnids, it will be sufficient for our present purpose to remark that the relation between the nervous system of an elongated form, such.as a Scorpion, with that of a shortened form, such as a Spider or Mite, is much like that existing between a Crayfish and a Crab (p. 14). For in a Scorpion many of the pairs of ganglia of the ventral cord remain distinct, though there is a good deal of fusion between those at its front end, while in a Spider or Mite all the ganglia of the cord have consolidated into a single nerve-mass. Among the Insects, again, we find the same principles exem- 16 NERVOUS SYSTEM AND SENSE-ORGANS plified. Some of the simpler forms possess a nervous system very much like that of a short Centipede, and from this condition all degrees of fusion and concentration are found, the maximum being reached where all the ganglia of the ventral cord have united into a single nerve-mass, precisely as in Crabs and Spiders. Three such stages, as exemplified by a Termite, a Water-Beetle, and a Fly, are represented in fig. 1019. In those insects which begin life as larvae, it commonly happens that in this early stage of life the nervous system is simpler than in the adult, exhibiting less fusion and concentration. This is exemplified by comparison of a cater- pillar with the butterfly or moth “4 which it becomes, or a bee-grub: with an adult bee. Cases are known, however, where the ner- vous system is condensed both in larva and adult, eg. the House-Fly and its allies (AZus- cide). A curious reversal of the ordinary state of things is found in the Ant-Lion (AZyr- meleo), for here the nervous. Fig. 1or9.—.Central Nervous Systems of a Termite < (Termes, a), a Water-Beetle (Dytiseus, 8), and a Blow. System of the relatively short Cinta and, squat Jarve, is mare con- centrated than that of the elon- gated adult. That this should be so is probably not merely due to difference in shape, for the complex habits of the rapacious larva involve elaborate adjustments to the surroundings, which need an efficient and centralized nervous system for their proper performance (see vol. ii, p. 111). So far as we know, the life of the adult is relatively simple. It remains to be added that all the air-breathing Arthropods. possess a visceral nervous system, which may attain considerable complexity, and takes origin from the nerve-ring. Nervous Systems or Mottuscs (Motiusca).—The least con- centrated type of nervous system is found, as might be expected, among some of the Primitive Molluscs (Amphineura). The central nervous system of a Mail-Shell (Chzton), for instance, consists of a nerve-ring from which four thick nerves run back (fig. 1020). Two of these are pedal cords, that traverse the substance of the muscular foot, while the others are /ateral cords y NERVOUS SYSTEMS OF INVERTEBRATES 17 placed at a higher level, and uniting with one another behind above the intestine. The nerve-cells are distributed pretty uni- formly throughout both ring and cords, in the course of which are no distinct ganglia. The pharynx with its rasping organ receives branches from the nerve- ring, which do swell into small ganglia, and this is also the case with a pair of nerves running from the lateral cords to the under side of the stomach (see figure). In this sluggish animal digestion is the dominant function, and that is possibly why the only distinct ganglia in the ner- im) A/G / i i i MY y We «wid ( L Fig. 1020.—A Mail-Shell (Chiton) dis- vous system are related to the digestive _ sected from above, to show Central Ner- organs. The visceral nervous system con- vous System M., Mouth; /.A., intestinal aperture; sists in this case of (1) the nerves which 2-4. nerverings P.C., pedal cord; 2.C., lateral cord; Sz., stomach nerve passing run from the nerve-ring to the pharynx, — back to pair of gastric ganglia; V7, part (2) the lateral cords and their branches. of Visceral nervous system. Passing from a simple form like the Mail-Shell to those which are more specialized, we shall find that as we ascend the scale to higher and higher types the nervous system becomes more and more centralized, in the same sort of way as in Arthropods. The nerve-cells are no longer scattered throughout the central ner- vous system, but are collected into definite ganglia, of which the most important are thickenings of the nerve-ring. This is very well seen in Snails and Slugs (Gastropoda), a vast number of which present a similar arrangement to that represented in fig. 1021 for the River-Snail (Padvadina). In the middle of the figure is seen the nerve-ring, which is thickened into three distinct pairs of ganglia —(1) brain-ganglia above, (2) side - ganglia laterally (dotted in the figure), and (3) foot- ganglia below. The brain-ganglia, as shown Fig. t021.—Central Nervous System of a River-Snail (Palu- dina), enlarged. See text. The circles shaded in the centre and connected with the pedal ganglia are the so-called ‘‘ears” (OZo- cysts) at the top of the figure, give origin to a cord that supplies the pharynx, and swells into a pair of small ganglia from which nerves run to the pharynx. This is part of the visceral nervous system, the rest of it consisting of a nerve-loop by means of which the VoL. IV. 96 18 NERVOUS SYSTEM AND SENSE-ORGANS two side-ganglia are connected together, and in the course of which are three ganglia sup Fig. 1022.—Central Nervous Systems of Pond- Snail (Lzsnceus, a) and Garden-Snail (Hedéx, B), diagrammatic and enlarged Br.G., Brain ganglia; Ped.G., pedal ganglia; S.G., side ganglia (dotted). Above are seen the small buccal ganglia connected by pharyngeal nerves (PA.) with the brain ganglia, while below, the short nerve-loop with its ganglia is represented (darkly shaded). plying many of the organs of the body. As in all Gastropods, the upper part of the body of the River- Snail has been subjected to a sort of twisting process, as the spiral shell suggests, and this has affected the nerve-loop, making it 8-shaped, as shown in the figure. This well- specialized central nervous system is associated with the presence of a clearly defined head, while just the contrary is the case in a Mail-Shell. Centralization has taken place to a still greater extent in some of the Gastropods, e.g. in the Pond-Snail (Limneus) and Garden-Snail (Helix aspersa, fig. 1022), where the nerve-loop, which here has not been inflenced by the twisting of the body, is so short that its Oop. GN, “Mantle G. “--""Mantle.N. Nerve-loop------- Fig. 1023.— Central Nervous System of Cuttle-Fish (Sepia), seen from the right side three ganglia are closely approxi- mated to one another, and also to the foot-ganglia and side-ganglia of the nerve-ring. Both in Bivalve Molluscs (La- mellibranchia) and Tusk - Shells (Scaphopoda) the nervous system follows the type described for Gastropods, but is less concen- trated, and the brain-ganglia are relatively small, which may be correlated with the absence of any definite head in the former group, and its imperfect develop- ment in the latter. Among the Head-Footed Mol- G., Ganglia; WV., nerve; P%.G., pharyngeal or luscs (Cephalopoda) various de- buccal ganglia; Osfh.N., osphradial nerve of water- testing organ (Osphradium). this in the Pearly Nautilus, type, while in the active Cut grees of concentration are found, there being the least amount of which is a primitive and isolated tle-Fishes, Squids, and Octopi cen- NERVOUS SYSTEMS OF BACKBONED ANIMALS 19 tralization is at a maximum.’ The Common Cuttle-Fish (Sepza officinalis, fig. 1023) possesses a nerve-ring of which the ganglia are exceedingly large and closely connected. In one respect the nerve-ring is less complex than that of the Garden-Snail, for it here includes two only of the three ganglia of the nerve-loop, which is long, distinct, and, like the body, not twisted. The nerve-ring of Cephalopods is more or less enclosed in a gristly case, serving as a sort of skull. NERVOUS SYSTEMS OF BACKBONED ANIMALS (VERTEBRATA) The nervous system attains its maximum complexity in back- boned animals, especially in the highest Mammals. The chief part of the central organs consists of a tube, which is placed near the upper side of the body, and in all but the lowest members of the group is sheltered within a skull and backbone. The front end of this nerve-tube is usually swollen into a brain, which is the chief organ of correlation and adjustment, while the rest of it is known as the spinal cord or spinal marrow. The central structures also include a visceral, or, as it is here usually called, a sympathetic nervous system, which where best developed consists of a couple of cords running longitudinally near the under side of the backbone, and swelling at intervals into sympathetic ganglia. Besides these there are outlying ganglia of similar nature in close connection with some of the internal organs, and connected with the cords just mentioned. The body of a Vertebrate is undoubtedly made up of rings or segments, and although this is not at first sight apparent, the serial arrangement of certain structures shows it to be the case. We find, for example, that a regular succession of spinal nerves is given off from the spinal cord, one pair to each segment. From the brain arise from 10 to 12 pairs of cranial nerves, the number of which, however, does not tell us how many segments © are fused to form the head. The number would be a guide if cranial nerves were precisely equivalent to spinal nerves, but this does not appear to be the case. While on the one hand some of them are complex, and equivalent to more than one pair of spinal nerves, others are only comparable to bits of such nerves, so to speak. The sympathetic system is closely 20 NERVOUS SYSTEM AND SENSE-ORGANS connected with the brain and spinal cord, to which it is sub- ordinate, and its nerves branch out in the organs of digestion, circulation, &c. A few further details have already been given with regard to the nervous system of Man (see vol. i, p. 49). It was stated at the commencement of this section that the essential elements of the nervous system, z.¢. the neurons, are derived from the ectoderm or outer cell-layer. Considering that brain and spinal cord are far removed from the surface, while the body is traversed in all directions by nerves, it seems very difficult to believe such a statement, but the study of development shows that there is no doubt at all about the matter. At a comparatively early stage in the de- velopment of an embryo part of the ectoderm covering the upper sur- face thickens into a nerve- Fig. 1024.—Development of Central Nervous System in a plate, which sinks below Vertebrate Embryo, diagrammatic the surface, and at the a, Upper side of embryo, showing folding-up of the nerve-plate; . B, ¢ and D, stages in folding-up of nerve-plate, as seen in cross) Same time folds up to io ices eOnstiuite the dere tube The details for the Lance- let have already been given (vol. iii, p. 345), but in that animal the nerve-plate sinks below the surface before it is completely folded into a tube, while in average cases the two processes go on simultaneously, as will be gathered from fig. 1024. The walls of the nerve-tube thicken, and by a process of unequal growth the spinal cord and the various regions of the brain come into existence. The rest of the nervous system grows out from the nerve-tube, e.g. the spinal nerves grow out from the spinal cord to the parts of the body which they supply. It therefore follows that these and the other nerves, as well as the sympathetic system, are really zxgrowths from the ectoderm or outer cell-layer, although in the adult they are far removed from the surface. Tue Brarn OF VERTEBRATES.—At first sight the brains of Fishes, Front NERVOUS SYSTEMS OF BACKBONED ANIMALS 21 Amphibians, Reptiles, Birds, and Mammals look so extremely unlike that comparison seems hopeless, but such an idea is soon dispelled by a study of de velopment, which is the key to the whole matter. What ” CON a m —_ takes place is broadly as fol- Fo OM. HL Sp.C. lows (fig. 1025). The front FE. M. H. end of the nerve-tube grows Pn. | Olf. = C.H. i T,B OL cb rapidly, and divides into three os successive swellings, which TIC SS lan PN a“ we may term the Fore-, Mid-, cr and Hind-~- Brains. These / ee ersallgmmoaaony) oe ‘ YYyyf Z UE ALLIED LT Ta three original swellings are 9, 7. nn Py, converted into the central Fig. 1025.—Development of Vertebrate Brain, as seen in part or axis of the adult longitudinal section, diagrammatic brain, the front part of which a, Brain and spinal cord at early stage; B, brain at later 7 : . : stage enclosed in brain-case, the floor of which is shaded with is called the ’Twixt-Brain,and oblique lines; 7, 7, #., S#.C., fore-, mid-, and hind-brains, 4 and spinal cord; 7. B., ’twixt-brain; C.H., cerebral hemispheres; the hind par t the Medulla OUf,, olfactory lobe projecting into nasal capsule; Pz. and Py., : pineal and pituitary bodies; O.Z., optic lobes; J7.0., medulla Oblongata or Spinal Bulb oblongata; Cé., cerebellum; WV. W., notochord; 11, optic nerve. (continuous behind with the Spinal Cord), while the roof of the middle section is thickened Fig. 1026.—Brains of Trout (a), Frog (B), and Dog (c), seen from above, and drawn same length OUf,, Olfactory lobes; C.H., cerebral hemispheres (CZ. is a cleft between them in B); Fs; “twixt-brain ; Px., pineal body; O.Z., optic lobes; C2., cerebellum; J7.0., medulla oblongata; V., V., WV., cranial nerves; 7., 7., spinal nerves. into a pair of swellings known as Optic Lobes, each of which, in Mammals only, is divided into two smaller projections by a 22 NERVOUS SYSTEM AND SENSE-ORGANS transverse groove. The differences between various classes of Vertebrates mainly depend upon the relative size and structure of certain outgrowths from the axis, the position of which will be realized by reference to fig. 1026. From the ’twixt-brain two lobes grow out, which become the Cerebral Hemispheres (re- presented in some Fishes by a single lobe), from the front end of which spring Olfactory Lobes, related to the organs of smell. An unpaired outgrowth, the Cerebellum, arises from the upper side of the hind-brain. In Birds and Mammals the Cerebral Hemispheres and Cerebellum are of such great relative size that they largely overlap and conceal the central axis. That the brain should be made up of so many parts is a result of the division of physiological labour, these different parts sharing between them the work that has to be done. The most respon- sible duties are vested in the Cerebral Hemispheres, to which all the other regions are sub- ordinate. The other regions of the brain, the spinal cord, and the sympathetic system, all Fig. 1027.—Cerebral Hemispheres of Man, seen from above. aa, Cleft between hemispheres; », B, convolu- have important shares in the tions. work of the nervous system, but all are subsidiary to the cerebral hemispheres, which exercise supreme control over the body at large, and are the chief centres of correlation and adjustment. And besides this, consciousness, sensation, will, and intelligence are dependent upon them. As we ascend the scale among the Vertebrates we shall find the hemispheres getting relatively larger and more complex, as the expression of a centralizing tendency (fig. 1026). There is also a great deal of division of labour between the parts. of the hemispheres themselves, and their highest duties ap- pear to be discharged by what is known as the cerebral cortex, an external layer of nerve-cells. In all the higher Mammals the extent of this cortex is more or less increased by the presence of winding furrows, resulting from a process NERVOUS SYSTEMS OF BACKBONED ANIMALS 23 of folding or convolution. These attain their maximum com- plication in the human subject, where also the hemispheres are of relatively enormous size (fig. 1027). The amount of convolu- tion is related to the intelligence of the particular species, but hasty deductions must be avoided, since they are also propor- tionate to the bulk of the body. Some of the most brilliant advances in modern surgery are due to the edie adi the cerebral cortex is divided into nerve-centres, some of which are NG a concerned with sight, hearing, ce ue “ and other special sensations, while NK ay i others again control definite sec- tions of the muscular system. We But so far it has not been found ay We possible to locate the higher Sy aS 2b ae Ne ST \ mental functions, such, for ex- . SS a Sts. ample, as memory. The Cere- bellum also possesses a very Fig. 1028.—Complex Neuron from Cortex of Human $ 5 Brain, greatly enlarged complicated cortex. As might be expected, the minute structure of the brain in a higher Mammal is most remarkably complex. Details would be out of place here, but fig. 1028, which represents one of the most specialized neurons from the cortex, will suggest the elaboration which exists, bearing in mind that the number of neurons in the brain is enormous. And it is particularly interesting to know that, as recent investigations have proved, these ultimate elements of the nervous system maintain themselves during the entire life of the animal. There are not successive crops of nerve-cells as once supposed. Were this the case, indeed, such things as memory would be almost unintelligible. CHAPTER LVIII SENSE-ORGANS OF INVERTEBRATES AND VERTEBRATES Sense-organs are the intermediaries between the nervous system and the environment, and essentially consist of ecto- derm cells (end-organs) capable of being influenced by external agents or stimuli. The stimulation of a sense-organ may be immediately followed by a reflex action, or it may lead to a voluntary action, and it is commonly associated, in higher animals at least, with what is technically termed a sensation, 2.e. an awareness of something in the surroundings. Supposing that in ourselves a light is suddenly flashed in the eyes when it is night. The eye is first affected, then the optic nerve, and then some of the nerve-cells in the brain. It is not till these last are brought into operation that we ‘see a light”, and by comparison with past experiences are put into possession of a piece of informa- tion about our surroundings. It must be added, that besides special sensations, such as those of hearing, sight, &c., there are others of obscurer nature, which tell us something about the state of the body itself, and are known as organtc sensations. Such are feelings of hunger, discomfort, &c., which, though of great importance for the well-being of the body, since they often guide to actions, e.g. feeding, which conduce to its welfare, will not be considered here, since they are not related to special sense-organs. Nor will reference be made to the ‘“ muscular sense”, by which muscular efforts are gauged. It will be convenient to place the subject-matter of the present chapter under the time-honoured headings of Touch, Taste, Smell, Hearing, and Sight, for it is by means of sen- sations which can be broadly classified in this way that we derive most of our knowledge of surroundings. But many of the lower animals possess sense-organs of which we can only conjecture the use, and the stimulation of which must result in 24 TOUCH 25 sensations of which we can form little if any idea. And even when with reasonable certainty we can correlate sense-organs possessed by such animals with some of our own, it by no means follows that the vange of a given sensation is the same for one of them as for ourselves. As regards hearing, for example, there is reason to think that some animals can hear sounds which are pitched much higher than any by which we are affected; nor is this very surprising when we reflect that the range of hearing is not the same in all human beings. Many persons, for example, cannot hear the high and piercing sounds made by Bats. These remarks are made as a warning against applying the results of human physiology to lower animals with too great assurance. TOUCH Undoubtedly the most primitive of all the senses is that of Touch, and we may broadly state that the skin is the Tactile Organ, remembering that its outer layer, commonly known as the epidermis, is no other than the ectoderm or outer cell-layer of the embryo. We must also include here the cellular lining of the mouth-cavity and, when such exist, the nasal cavities, WGA Mis oN Th Rrain Fig. 1029.—Tactile Organs. a, Cells from the ectoderm of a Sea-Anemone (Actizia); 7., a touch-cell, with outer end produced into a stiff process; S¢., stinging cell, with sensitive trigger-hair (77.); GZ, glandular cell. 3B, Head of a Freshwater Annelid (Bokemilla comata), seen from above, and showing epidermis in optical section, enlarged; T., tactile processes of some of the epidermic cells, which are continuous internally with nerve-fibres. since these have been developed as in-pushings of the ectoderm. The external agents of stimuli which by their action upon the skin evoke sensations of touch are of two sorts. There are, in the first place, mechanical agents, such as contact or pressure, and, in the second place, heat-rays. The sensations which result are respectively known as haptzc and thermal. 26 NERVOUS SYSTEM AND SENSE-ORGANS Single epidermal cells or groups of such cells are specialized for the reception of stimuli leading to sensations of touch, but in such forms as Ccelenterates and Annelids many scattered cells of the kind probably minister to other senses besides that of touch. And it must be re- membered that even the special sense- cells of hearing and sight are derived from the skin, which is in fact the primitive sense-organ. Cells which are regarded as tactile, from some of the lowest animals, are represented in fig. 1029. The firm external covering with which the bodies of Arthropods are clothed is naturally a hindrance to the Fig. 1030.— Tactile Organs of Insects, é 2 7 a greatly enlarged. On the right is a group of reception of stimuli by the underlying such srucures, and on the left asingleone, enidermis. The difficulty is got over by the existence of little pores in the hard investment. Under each pore is an enlarged sense-cell, placed at the base of a stiff tactile bristle, with which external bodies come into contact (fig. 1030). In aquatic Vertebrates the sense-cells of the skin are in direct contact with the surrounding medium, although they are not infrequently protected by being lodged in pits, grooves, or canals which open at intervals to the ex- terior. But in terrestrial Verte- brates there are special end-organs of touch which have sunk below the epidermis, though they remain suf- Fig. 1031.—Organs of Touch - A, Small piece of the skin of a Frog, in vertical ficiently near to the surface to be section, enlarged; £/., epidermis; 7.C., touch-cor- stimulated when the body comes into puscles; .V.F., nerve-fibres. 3B, Touch-corpuscle from the bill of a Duck, much enlarged; S.C., sense- contact with surrounding objects. cells; V.F,, nerve-fibre; S%., fibrous sheath. Such are the groups of touch-cor- puscles which are to be found in the skin of the Frog, and around the edge of the Duck’s bill (fig. 1031). The latter animal feeds upon small worms, &c., which live in the mud that is strained through its bill, and such special arrangements are clearly necessary to aid in the discrimination between what is edible and what is not. Another example is afforded by the TOUCH 27 numerous touch-corpuscles which underlie the little ridges seen on the tips of our fingers and thumbs (fig. 1032). Birds, and Mammals there are also curious struc- tures known as Pacinian bodies (fig. 1033), in which the ending of a nerve is surrounded by a series of layers arranged almost like the coats of an onion. There is reason to think that these are very sensitive to slight pressures. They abound, for instance, in the wing-membranes of Bats, and it is well known that these creatures can easily steer their way in the dark through a veritable maze of obstacles, such as that afforded by a series of strings running in various directions. Pacinian bodies are also found connected with tendons, ligaments, and various internal organs. In Reptiles, aa ane Fig. 1032. — Touch- Corpuscle from Finger Tip of Man, in section, greatly enlarged The use of these is probably to apprise the central nervous system of variations in pressure and tension which take place as between the different parts of the body itself. We are still very much in the dark as to how far there do or do not exist special end-organs which are affected by variations in temperature. It is known that definite spots in the human skin are sensitive to such variations, but there do not appear to be any special sense-organs in these spots. Some of the sensory nerve-fibres terminate in the skin by dividing into a number of little branches which do not YH $i become continuous with modified epidermal NS cells, and it has been suggested that these “free nerve endings” are related to the H, Fig. 1033.—A Pacinian Corpuscle temp erature sense. in Longitudinal Section, enlarged. A While the entire external surface of the sets (ea ines oy sheat , enters the base o body 1s sensitive to contact, pressur e, and corpuscle, loses its sheath, traverses a central core (7), and ends in an changes in temperature, this is 10. ANY cesar expansion Ga). ‘The cor puscle is mostly made up of numerous cases insufficient to enable the requisite ree ee ebrous layers le, 4). adjustments to the environment to be brought about. And we accordingly find that in many animals organs of active touch have been evolved, which explore the 28 NERVOUS SYSTEM AND SENSE-ORGANS surroundings, and help to detect the presence of food, or to give warning of danger. Such are the tentacles of Jelly-Fish and Sea-Anemones, the slender outgrowths on the head of a Sea- Centipede, the two pairs of antenne on the head of a Cray- fish, the single pair on the head of an Insect, and the tentacles on the heads of Snails and Slugs. The “whiskers” of a Cat or Rabbit belong to the same class of structures. They are stiff Fig. 1034.—A Deep-Sea Fish (Zretmophorus) with its Pelvic Fins drawn out into long Tactile Organs hairs, at the base of each of which a touch-corpuscle is to be found. Such organs of active touch may either from the first have done duty as sensory organs, or may have originally been evolved in the interests of some other function. The former is probably true for the feelers of a Sea-Centipede or Insect, but the large feelers of a Crayfish (and very likely the small ones too) were probably jaws at an earlier stage, having later on been shifted in front of the mouth, and modified in shape and structure to do duty as sense-organs. There can be no doubt that the paired fins of Fishes were originally evolved in TASTE 29 relation to swimming, but it sometimes happens that they have been transformed into tactile organs, as in the deep-sea form (Evetmophorus) represented in fig. 1034. Snakes employ their tongues as tactile organs, as also do Woodpeckers and Ant- eaters. This, however, is pro- bably only an extension of the original duties, for the primary use of the tongue seems to be that of a tactile organ in re- lation to the mouth-cavity. TASTE Sensations of Taste are pri- marily important because they assist in the selection of suit- able food. The stimulus is a _ Fig. 1035.—Taste-Organs of a Wasp. a, Under side of chemical one, and consists of 5 suueutep,euyone substances in solution. We know but little about the gustatory organs of lower forms, but as these show a preference for certain kinds of food it is pro- bably correct to assume that such organs are present. In the Earth-Worm, for example, groups of modified epidermal cells in the neighbourhood of the mouth are very likely related to taste. Certain regions of the mouth-parts of some Insects are studded with minute pits, beneath each of which is a sense-cell, drawn out ex- ternally into a short bristle, and continuous with a nerve-fibre internally. They are present, for example, in Bees and Wasps, and are almost aren certainly of a gustatory nature (fig. 1035). Ce I ea aee Cuttle-Fishes and many Snails possess a enlarged. The bud’ contains slender taste-cells, the exter- sense-organ on the floor of the pharynx, below jarends of which project into the front end of the rasping-ribbon. It pro- 23M Pitconinuouswith the bably has to do with taste. In Lung-Fishes, Amphibians, Reptiles, Birds, and Mammals the organs of taste consist of groups of sense-cells in the lining of the mouth-cavity, and since this cavity is developed as an 30 NERVOUS SYSTEM AND SENSE-ORGANS in-pushing from the exterior the cells in question are of ecto- dermic nature. The largest amount of specialization takes place in Mammals, where the “taste-buds”, as the group of cells are called, are associated with small projections or papilla of the surface of the tongue (fig. 1036). SMELL Many of the lower animals can undoubtedly smel: as well as taste, though to definitely associate this sense with special cells or groups of cells is not at present possible. Our know- ledge is more complete in the case of Arthropods, Molluscs, and Vertebrates, where experiments lead to results of more de- groups of olfactory setze on its under side, enlarged; 4, an olfactory seta, further enlarged. , Tip of feeler of a Milli- pede, greatly enlarged, showing olfactory cylinders among the ordinary tactile bristles. cv, Two olfactory cones from feeler of a Wasp, in section, greatly enlarged. finite kind. In all cases the stimulus is of a gaseous nature, and in aquatic animals the gases that are smelt are dissolved in the surrounding water. The sense of smell is obviously of great importance as regards adjustment to the environment. By its means food is in many cases detected, while it often enables animals to recognize friends or foes, even when these are at a considerable distance. This is, of course, due to the nature of the stimulus. Since Smell, Hearing, Sight, and the Temperature Sense are able to give information about objects which are more or less far away, they may be grouped together as Distance-Senses (teleesthetic senses), and are in marked contrast to Touch (so far as haptic sensations are concerned), which only conveys knowledge regarding things that actually come into contact with the skin. SMELL 31 There is naturally a tendency for olfactory organs to be developed at the front end of the body, where they can be most usefully em- ployed, and they are commonly to be found on the feelers of Arthropods. In the Crayfish, for example, the small first feelers (antennules) bear groups of flat- tened bristles which undoubtedly have to do with smell, and similar structures are present on the antennz of Millipedes and Insects (fig. 1037). Land-Snails and Slugs, among the Molluscs, are known to be endowed with a keen sense of smell. In the common Garden-Snail (Helix aspersa) some of the epidermic cells at the tips of the long eye-bearing tentacles are believed to minister to this function (fig. 1038), though experiments have been made which appear to show that olfactory cells are elsewhere present. Among aquatic Molluscs what is known as a water- testing organ (osphradium) is usually present in the neigh- bourhood of the breathing organs (fig. 1039). This is generally considered to be of olfactory nature. In Vertebrates the sense- cells related to smell form part of the lining of the cavities of Fig. 1038.—Tip of Optic Tentacle of Garden- Snail, in diagrammatic longitu- dinal section, enlarged. The tentacular nerve (Tent. N.) gives off an optic nerve (Op...) to the eye, and then expands into a ganglion (Gz.) which sends fibres to an olfactory patch (O//) of cells on the tip of the tentacle. the nose, and since these are developed as pits in the ex- ternal surface, such cells must necessarily be of ectodermic character. When the sense of Fig. 1039.—Diagram of a Comb-gilled Snail, seen from above. The roof of mantle-cavity and overlying shell sup- posed transparent 1, Mouth; 2, brain ganglion; 24, nerve-cord connecting side ganglion (abovg) with foot ganglion (below); 3, one of the three ganglia on the twisted nerve-loop; 4, gill; 4, os- phradium; 5, opening of intestine; 6, heart in pericardium; 8, a gland (purple-gland in Purpura); 9, siphon; 10, ro, foot; 11, operculum. 32 NERVOUS SYSTEM AND SENSE-ORGANS smell is keen the nasal cavities are large and complex, and folds project into them which increase the surface over which olfactory cells are distributed. These cells are frequently of the shape represented in fig. 1040, from which it will be seen that from the outer end a number of slender processes project into the nasal cavity. In some Fishes, such as the ordinary bony forms, the originally single nostril of each half of the nose is divided into two apertures, which respectively serve for the entry and exit of water, that appears to flow continuously through the nasal cavity. There can be no doubt that many fishes possess a very keen sense of smell, and the experiments of Bate- son have proved that some of them (e.g. Dog-Fish, Conger-Eel, and Sole) are mainly guided by this i in their search for food. This being so, the noc- wcney Cals froma, turnal habits of many species is readily intelligible, eatin eile and the sense of smell must also be very useful in water of such depth that the light is dim. In Vertebrates which live on land the courses taken by the food which is swallowed and the air that is breathed are more or less distinct. Each nasal cavity, in fact, opens at the back into the digestive tube, and the natural way of breathing is “through the nose”. This is clearly to the advantage of the sense of smell, for the air which passes over the olfactory cells is constantly being renewed, and the incoming current is con- tinually bringing with it gaseous matter capable of being smelt. An inward flow is greatly promoted by the act of “sniffing”, as we know from our own experience. BALANCE AND HEARING There are certain sensory structures among the Invertebrates which though often classified as Auditory Organs have probably nothing to do with hearing in the ordinary sense, but are con- cerned with advantageous adjustment of the body as regards its position in space. This is of the greatest importance in reference to the maintenance of balance and the direction of movement. They are stimulated by vibrations in the surrounding medium, water or air as the case may be, and there can be little doubt that they have furnished the material from which undoubted BALANCE AND HEARING 33 organs of hearing have been evolved. Indeed the auditory organs still retain, in ourselves for instance, the old function side by side with the new. : BaLancinc OrcGans or JELLY-Fisu (Hyprozoa).—Jelly-Fish are often provided with balancing organs placed at regular in- tervals round the edge of the umbrella. In the simplest case these are little pits lined by specialized sense-cells, from each of which a bristle projects. Within the pit one or more calcareous particles (otoliths) are found, and these also have been derived from the ectoderm. In many species the mouths of these pits close up, converting them into little sacs (otocysts) which lie close to the surface. Other kinds, again, possess short balancing-tentacles (tentaculo- cysts), evolved no doubt from some of the ordinary sort (fig. 1041). In such instances the otoliths are derived from the entoderm cells which make up the inner part of the tentacle. «@ wx Pag Though these different or- Wig: doar Temnacaloeyaus off Jallprishy entarsea gans may be constructed in 4, Of Solmaris coronantha; 2, of Polyxenia cyanostylis, various ways they are affected by the same sort of stimulus. Their sensory cells are jolted by movements in the surrounding water and by the swimming move- ments of the animals themselves, and the otoliths appear to intensify the action, as it were. The sense-cells are closely con- nected with the nervous system, and this again with muscle- fibres. We have present, in fact, the necessary machinery for muscular reflex actions (see p. 9), under which may be included the checking or stopping of swimming movements actually in progress. One of the most obvious uses of the sense-organs described appears to be that of enabling their possessors to keep well below the surface of the water during rough weather, for crea- tures of such flabby and delicate structure are quite unfitted to withstand the buffets of the waves. Supposing that on a stormy day a jelly-fish is swimming obliquely upwards. When it comes sufficiently near the surface for the balancing organs to be stimulated with a certain degree of vigour by the swing VoL. IV. 97 34 NERVOUS SYSTEM AND SENSE-ORGANS of the water, reflex modification of the swimming movements will take place, and the upward course will be altered into a downward one. BaLancinc OrcGaNns IN SEGMENTED Worms (ANNELIDA).— Members of this group commonly react very quickly to jolting or agitation of the surrounding medium, and this may lead to movements promoting escape from danger. Earth-Worms, for example, when partly protruding from their burrows, will often draw back with extreme rapidity on the approach of a heavy footstep. The skin is no doubt the sense-organ in this case, but we are ignorant as to details. A few Annelids, however, have a pair of otocysts in the front part of the body, as, ¢.g., the Common Lob-Worm (Avenzcola pusca- torum.), where they are closely connected with the brain (fig. 1042). They have undoubtedly been evolved from pits in the ectoderm like the similar sacs found in some of the jelly- fish, and three stages in this evolution are Fig. 1042 Front Part of Permanently retained in three kinds of Lob- Central Nervous Systemof Lob- Worm. In one of these (4. Claparedzz) there Worm (Arenicola piscatorum), enlarged. g, Gullet (in cross iS simply a pair of depressions on the head, Ticose dudes cacags-. 1G aROLneE (A. prscatorum) otocysts which are ae eae canals teextenor stil] in communication with the exterior, and in a third (4. Grudzz) closed otocysts. The otoliths of the second species are minute sand grains taken in from the exterior, while those of the third are calcareous par- ticles secreted by the ectoderm. BaLtancinG Orcans In Mo ttuscs (Mottusca).—Most Mol- luscs possess a pair of otocysts, developed as pits in the ectoderm, which become closed and travel inwards to the neighbourhood of the foot. They are attached to the foot-ganglia,; although their nerve-supply is derived from the brain (see fig. 1021, p. 17). It occasionally happens in Bivalves that the communication with the exterior is retained. The lining of these organs partly con- sists of sense-cells provided with stiff processes, and one or more calcareous otoliths are present. The otocysts of Cuttle-Fishes and their allies are lodged in the gristly case which surrounds the thickened nerve-ring, pretty much as in backboned animals the corresponding organs are sheltered in gristly or bony capsules that form part of the wall BALANCE AND HEARING 35 of the brain-case. And it is definitely known that in Molluscs of this kind maintenance of equilibrium and adjustment of the swimming movements are seriously interfered with if the oto- cysts are injured, which leaves little doubt as to the use of these organs. The otocysts of some of the free-swim- ming Sea-Snails (Heteropods) are particu- larly large and well-developed (fig. 1043), and are undoubtedly related to balance and steering. The majority of Snails and Slugs, however, are adapted to a creeping mode of life, the organ of locomotion being the mus- AS cular flat-soled foot, which is also concerned __ Fig. 1043.—Otocyst of a Hetero- with maintaining the balance of the body. faged. ‘The large ptoitn i seza Since the otocysts are presumably related ese rece nice is lined below by sensory cells, pro- to both these uses, it is not surprising to eee ae Farha el find them placed close to the upper surface _ * lined by cells beating long cilia of the foot, by the slightest movement of ~ — which they must therefore be affected, and H. J. Fleure has described an interesting arrangement in the Limpet and Sea- Ear which probably conduces to this. In the two forms men- tioned each otocyst is connected with the foot by a fibrous band, and there is a similar bond between the two otocysts (fig. 1044). These organs are thus kept “in touch” with the foot and with one another, and, being also moored by their nerves to the foot-ganglia, are kept steady, which seems desirable when their functions are considered. Orcans oF BaLancE AND HEARING IN Crustaceans (Crustacea). — Such higher forms as Lobsters, Prawns, Shrimps, and z asec aes Crabs are provided with otocysts lodged in cross section through Otocysts the bases of the small feelers or antennules, {0%)gn Poot of # Timpsh or These organs arise, as in cases already de- scribed, as pits in the ectoderm, and they usually, though not always, remain open through life. In a Lobster, for example, they are lined by a thin horny membrane continuous with the hard covering of the body, and studded with delicate bristles, at the bases of which are sense-cells (fig. 1045). The otoliths are sand grains which have been taken in from the exterior. 36 NERVOUS SYSTEM AND SENSE-ORGANS That the otocysts are concerned with equilibrium and adjustment of movement has been definitely proved by experiments upon the Prawn (Palemon). When this creature moults it sheds not only the defensive armour of the body but also the lining of the otocysts, getting rid at the same time of the sand grains. which serve as otoliths. Under ordinary circumstances _ these would be replaced by a fresh supply of the same material, but Sats the specimens experimented upon Fig. 1045.—Otocyst of Lobster (a) in longitudinal sec- ; : : tion, enlarged and diagrammatic; sensory bristlesareseen Were Only provided with iron proentng in fs cavity, which cntina nomerue lings, some of which in due course were introduced into the otocysts. It was then found possible by means of a magnet to move the particles in various ways, and as a result of this the Prawns could be induced to assume all sorts of positions, under the impression, so to speak, that they were falling over in this or that direction, which they would have been if the shifting of the otoliths had been produced by ordinary causes. ANTENNULE Fig. 1046.—Opossum Shrimp (J/yszs), cnlarged. One of the otocysts (Ear) is seen in the tail {n one Crustacean, the Opossum Shrimp (JZyszs), the otocysts, in this case closed, are lodged in the flaps of the tail-fin, but why they should have this position is not known (fig. 1046). It is generally assumed that animals which are endowed with a voice or its equivalent also possess powers of hearing, at least if the voice is used for the benefit of one another. Since some BALANCE AND HEARING 37 of the higher Crustaceans are able to emit sounds, it is quite possible that their otocysts are beginning to acquire a new use, ze. that of serving as auditory organs. The Rock-Lobster (Palinurus), for example, makes a creaking noise by moving the basal joints of the large feelers, which then rub against their sockets. An unpleasant sound of similar nature can be produced by twisting a glass stopper in the neck of its bottle. A more specialized case is that of the Musical Strand-Crab ({Ocypoda macrocera), which has been described by Alcock (in A Naturahst in Indian Seas). In this animal the inner side of the large nippers is provided with a ridge or scraper placed near the base of the limb, and a rasp-like ridge or key-board on the fixed joint of the claw. By drawing the scraper over the key- board a sort of chirping sound is produced, not ks . . 2 Fig. 1047.—Chordotonal Organs. On the right is shown part of unlike the one with which an abdominal segment of the larva of a Gnat (Corethra plumicor- nis), seen as a transparent object, enlarged. In the centre is the our native gr asshoppers nerve-cord (darkly shaded) with the ganglion (gz.) of the segment; ss é.nz., longitudinal muscles; ch.z., ch.g., ch.d., and ch., chordotonal have made us familiar. nerve, ganglion, ligament, and organ; ¢.4., branched tactile bristles. The same zoologist speaks 9p ete st ara he canal wenn wi oth of the Squeaker Crab (Psopheticus stridulans) of the Andaman Sea as making a dismal noise by rubbing a spine which projects from the base of its nippers against a rough knob near the eye-socket. Orcans oF BALANCE AND HearinG In Insects (INsECTA).— A variety of organs situated in different parts of the body are probably connected with balance or hearing, or both. Among those which are most likely concerned with equilibrium and move- ment are certain peculiar structures (chordotonal organs) that are especially characteristic of aquatic larve, though not limited to these. Gnat larve, for example, possess such organs, one of which is represented in fig. 1047. It consists essentially of a group of rod-shaped cells contained in a tube that opens to the exterior. Many insects make sounds which are doubtless heard by 38 NERVOUS SYSTEM AND SENSE-ORGANS their fellows, a well-known instance being afforded by Grass- hoppers and Crickets. A Grasshopper possesses a chirping arrangement something like that of the Musical Strand-Crab (p. 37). The wing- cover is provided with a sharp edge or scraper which is rubbed along a key-board placed on the inner edge of the thigh of the hind-leg (fig. 1048). The chirping sounds audible to our own ears are pro- duced by the male insect, but the females of some species are also provided with these “stridulating” organs, which nd doubt make sounds that can be heard and ap- preciated by the opposite sex. These o-4 324 sound-producing insects also posses’ what eee we may feel justified in calling ‘ears ”. eee pe munopes: (Stents), On either side of the first ring of the showing beaded key-board (dotted line 7 onright), enlarged; B,fivebeadsofsame, abdomen there is a membrane compar- snr bea kya of ele able to a drum-head (fig. 1048) stretched hopper (Acridium), to show ear; wings OVEr an air-space, and closely connected rata eae atdominal cigaa ® with sensory arrangements somewhat like those already described for a gnat-larva. The ears of Green Grasshoppers and Crickets are situated in the shins of the fore-legs, just below the knee. OrGANS OF BaLaNcE AND HeEarinc IN BackBoNED ANIMALS (VERTEBRATA).— The tadpole larve of Sea- Squirts pos- sess remarkable sense-organs formed by specialization of part of the wall of the brain, and projecting into its cavity. One of these is of the nature of an otocyst, and is pro- Pere: bably a balanci fi Fig. 1049.—Body of an Ascidian Tadpole, in longitudinal y a ba ancing organ ( g- section, enlarged. The tail is cut short; the dorsal nerve-tube IO. ) (z.2.) swells into a brain, into which project a balancing organ 49). (6.) and an eye (¢.); @., atrial cavity; c., groups of embryonic From Fishes onwards we cells; g., gut; #., mouth; ., notochord; /., adhesive papilla. find undoubted ears, similar, broadly speaking, to the essential parts of our own organs of hearing, and there can be no doubt that these also have to do with equilibrium and movement. If we trace the development of the SIGHT 39 ear we shall find that it begins as a pit in the skin, and by closure of the mouth of this a vesicle is produced, which if it underwent no further modification would be called an otocyst. As it is, however, a very complex shape is assumed, the final result being known as the membranous labyrinth, or internal ear (fig. 1050). This sometimes, as in a Skate or Dog-Fish, remains in communication with the exterior throughout life. It is sig- nificant that in Fishes the auditory pit arises in close connection with the “lateral line”, which is a groove or tube containing groups of sense-cells belonging to the skin. And this suggests that the ear is no more than a bit of this line which has sunk beneath the surface and become specialized as regards struc- ture and function. It is extremely pro- bable that the lateral line of Fishes and Amphibian larve has to do with main- tenance of balance and direction of move- ments, and if so, the fact that the ear has to do with these functions is quite intelli- gible. We know so little about the Mee eens division of physiological labour between SS ee the different parts of the complex laby- presenting the outer part of the original q . ° . ingrowth; a.s., Z.s., and 4., anterior ver- rinth that a discussion of details would ticat, posterior vertical, and horizontal be out of place here. But experiments misty cls: » wide: se, se have shown that the semicircular canals *:* ‘nd olher dotted patches), groups have some connection with movement and equilibrium, and it is interesting to note that they lie in three planes which are mutually at right angles. It is also certain that the labyrinth is the sense-organ of hearing proper. In land-verte- brates there are more or less perfect arrangements for conducting air-waves from the exterior to the deeply-seated and well-protected internal ear. This has already been sufficiently illustrated by the brief account of the auditory organs of Man given elsewhere (see vol. i, p. 56). SIGHT Sxin-SeEInc.—The simplest kind of sight is literally that of “seeing without eyes”, and it amounts to no more than the power of distinguishing between light and darkness, or detecting 40 NERVOUS SYSTEM AND SENSE-ORGANS sudden variations in the amount of illumination. But even this limited sort of vision may be of the greatest importance to its possessor, since it often gives valuable information about the surroundings. In such skin-seeing (dermatoptzc vision) it is usual to find colouring-matter or pigment in or below the epidermis, which localizes the action of light-rays upon sensitive cells in this layer. This is the case, for instance, in Earth-Worms, the safety of which must often depend upon their avoidance of light. A further and more interesting illustration is afforded by many of the bivalve Molluscs which live in sand or mud, and which feed and breathe by means of two tubes, the siphons, which project from the hinder end of the body (vol. ii, p. 249). Such animals are often found hidden in their bur- rows with only the extreme tips of the siphons projecting. But even though thus concealed they would more frequently fall victims than they do to octopi and fishes, or, in the case of those which live between tide-marks, to strand-haunting birds, Fig. 105t.—A, Euglena viridis, enlarged: # were they not provided with some flagellum; 7., mouth; V., nucleus; £.v., pulsating Suto ee OF Seen We provi? of such enemies. Warning is often given by the siphons themselves, which are commonly pigmented and sensitive to changes in light-intensity. And experiments on specimens kept in aquaria have shown that the fully-extended siphons are rapidly drawn in if a shadow is suddenly cast upon them, an event that would happen under natural conditions on the approach of a voracious fish or too inquisitive bird. Eyes.—Localization and improvement of the powers of sight have led to the evolution of definite visual organs or eyes, though many of the lower Invertebrates have more or less retained the old faculty of diffuse skin-seeing. The simplest organs of the kind are known as eye-spots, and their presence is marked by dense pigment. These are possessed even by some Animalcules, e.g. by a little green creature (Zuglena viridis) which often swarms in stagnant water (fig. 1051). The eye-spot in this case is marked by the presence of a tiny patch of red colouring-matter on which rest several little lenses that serve to concentrate the light. In some of the Jelly-Fish the margin of the umbrella bears SIGHT 41 a number of compound sense-organs (rhopalia) derived from tentacles, and having to do with balance and adjustment of move- ments, sight, and possibly smell. Their visual part consists of a group of pigmented ectoderm cells, upon which a lens may rest (fig. 1052). Examination of a Common Star-Fish (Uvaster rubens) will * Hitec | Fig. 1052.—Rhopalia of Pericolga guadrigata, seen from various points of view, enlarged. The otocyst, containing numerous otoliths, is seen in the lower part of a, B, and c; the rounded pigmented eye, with clear, central, refracting portion, is indicated in a, c, and D. reveal the presence of a bright-red spot at the tip of each arm, borne upon an unpaired tube-foot. This is undoubtedly an eye, and microscopic examination shows that it is made up of a multi- tude of little cups, each of which is lined with elongated cells, some of which are sensory, while others contain pigment (fig. 1053). Sea -Urchins possess a circlet of somewhat similar eyes placed near the upper pole of the body. In some of these animals each of the minute cups may be provided with refracting structures, which presumably concentrate the light. Jelly-Fish, Star-Fish, and Sea-Urchins are ‘3 radially symmetrical animals, and their eyes are _Fig. 1053.—An Eye-Cup . . . of a Star-Fish, greatly en- correspondingly disposed. But in “Worms”, targed, in section. The Arthropods, Molluscs, and Vertebrates, where a ae the body is bilaterally symmetrical, and there is a more or less well-developed head, the eyes are usually situated upon this, as being the most useful position. But eyes may be present elsewhere, especially in some of the Planarian Worms, and certain Bivalve Molluscs. The visual organs so far described may be called Drrection- Eves, as they can do no more than detect the direction from which the light-rays which influence them are coming. Eyes of 42 NERVOUS SYSTEM AND SENSE-ORGANS the sort are present in many Worms and Molluscs, and some of them are less complex than those of a Jelly-Fish or Star-Fish. Nothing, for example, could be much simpler than the eye-spots @ on the head of the common freshwater worm ; Nais. Each of these is simply an enlarged epidermal cell, along one side of which are several much smaller cells containing pig- ment (fig. 1054). We may take as examples of greater complication the eyes of a Leech, a Limpet, and an Arrow-Worm, the nature of which is sufficiently indicated in fig. 1055. ‘ They essentially consist of a group of visual _. % or retinal cells, associated with pigment and Eyegpernta Fechwater Avena refracting structures. Those of the Leech ened og oe’ are particularly interesting, because they closely resemble in structure certain organs of touch which are present in the skin of the same animal, differ- ing from these, however, in being larger, surrounded with pigment, and limited to the front end of the body. It is, in fact, a case of tactile organs which are acquiring a new function. The simple Fig. 1055.—Direction-Eyes of a Leech (Hzrudo, a), a Limpet (Patella, 8), and an Arrow-Worm (Sagitta, c), in section, and enlarged to various scales In a the elongated eye is placed below a transparent patch of the epidermis (eg.); it is enclosed in a pigmented sheath (Zg.), and consists of an external layer of large refracting cells (7.c.), surrounding a core of slender sense-cells (s.¢.), which are continuous with nerve-cells (z.c.), and these again with nerve-fibres (v.). B is an open cup, lined by a thickened retina (ve¢.) with clear refracting part externally, and dark pigment between its cells; #v., nerve. In c there are three lenses imbedded in pigment (Zg.), external to which are retinal cells (ve¢.c.), that contain refracting rodlets (vd.) in their inner ends. eye-cups, of which one is to be found at the base of each tentacle in a Limpet, are interesting for quite a different reason. For they are almost certainly to be regarded as degenerate structures, which have been greatly simplified as a result of adaptation to the mode of life characteristic of their possessor. The activity of a Limpet is practically limited to feeding excursions in the vicinity SIGHT 43 of its home, and the eyes are under the shadow of the large conical shell. Under such circumstances complex visual organs are unnecessary. Picrure-Eves.—The development of refracting structures in direction-eyes has led to the possibility of further specialization in vision, and has resulted in what we may call Picture-Eyes, capable of giving more or less definite information about the form and colour of external objects. Two kinds of these may be distinguished, ze. Com- _ pound Eyes and Camera Eyes. Bee (apie edifice) nace Compound Eyes are characteristic of a ircca’ Theancsna ave seen iront, larged. The antennz are seen in front, great many Arthropods, such as Lobsters 2rd tce spall simple eyes neat ases, but the most conspicuous struc- and Crabs, where they are placed at the _ tuesare theenormous compound eyes, with their minute hexagonal facets. end of stalks, and Insects, where they are in the form of two large projections on the head (fig. 1056). Examination with a lens shows that such an eye is covered by a transparent patch of the hard covering of the body, which is divided into a multitude of minute square or polygonal areas, commonly known as facets. These may be exceedingly numerous, as will be seen from the following calculations made by Leeuwen- hoek more than a century ago:— house-fly, 4000; gadfly, 7000; goat- Sess moth, 11,000; death’s- head moth, lll 12,000; swallow-tail butterfly, 17,000; Aig: a rn dragon-fly, 20,000; a small beetle SR Va ae (Mordella), 25,000. It was origi- ) y a te nally believed that these elaborate és 7 structures were aggregates of simple __ Fis. 10s7—Diagram of a Compound Eye, in 7 5 section, enlarged, to illustrate theory of ‘mosaic eyes, acting independently; and they vision”. Numerous radiating visual pyramids “ d ” are indicated, each consisting of external refract- were therefore called compoun ing structures (*., 7.) and internal groups of retinal ‘ : cells (7z.). On the right side part of the pigment eyes, a rather misleading term. Sec- of several pyramids is inserted. The course of 1 light-rays from an external object is indicated for tions through such eyes (fig. 1057) Hzhttey: foe a ee eve have demonstrated that each facet ‘5 the outer end or base of a very slender visual pyramid (ommatidium), the external part of which consists of various refracting structures, while internally is a group of sensitive visual cells connected with nerve- fibres. Adjacent pyramids are optically separated from one another by means of pigment. Comparison of various compound eyes shows that there are 44 NERVOUS SYSTEM AND SENSE-ORGANS great differences in detail, and much has yet to be learnt re- garding the exact structure and use of the numerous parts which are present. The most plausible explanation which has yet been given of the mode of action of this sort of eye is that of ‘‘ mosaic vision”. According to this a visual pyramid is only stimulated by light-rays which exactly correspond in direction with its long axis, and numerous pyramids co-operate so as to enable the shape and colour of surrounding objects to be perceived (fig. 1057). \ i / “A i Xt lk Fig. 1058.—Sections through the Compound Eye of an Earwig (Fozjicula, a), and the Camera Eyes of a Spider (Epeira diadema, 8), and a Marine Annelid (A dczoge, c), enlarged In a numerous radiating visual pyramids are seen, ending externally in the facets of the thickened cuticle, and con- nected internally with nerve-branches; one of the pigmented zones is indicated. In B the cuticle is thickened into a rounded lens, and behind this is a transparent layer, upon which abut the retinal cells, continuous with nerve-fibres; each retinal cell contains a refracting rodlet. c is a vesicle, of which the external part is thickened into a spheroidal lens, while the rest constitutes a retina, consisting of an internal refracting layer, separated by pigment from the external sensitive part, into which nerve-fibres are seen running. Camera Eyes are found in Annelids, Arthropods, Molluscs, and Vertebrates. Just as in a photographer's camera a picture of external objects is imaged on a sensitive plate by means of a lens, so also in a camera eye do we find refracting structures which focus light-rays on a retina, or layer of sensitive visual cells. Scattering of light is prevented in the former case by a blackened lining, in the latter by a layer of pigment. One of the two exceptionally large eyes present on the head of a marine Bristle-Worm (A /czofe) is represented in fig. 1058. A Sea-Centipede (Verezs) possesses four smaller and less complex eyes of similar kind on the upper side of its head-lobe, and in some of the tube-inhabiting Bristle-Worms (e.g. Branchiomma and SIGHT 45 Dasychone) there are eyes of elaborate nature on the gill-filaments of the head. Spiders, among Arthropods, have a group of simple eyes (ocelli) on the top of the head. These are constructed on the camera principle, though they differ in detail from those of Alciope (fig. 1058). The spherical shape of the lens and its closeness to the retina suggest that only near objects can be seen with any degree of distinctness. A great many Insects possess ocelli in addition to the two large compound eyes. In Bees, for instance, there are three of these arranged in a triangle on the top of the head. In this and similar cases it is extremely probable that the compound Fig. 1059.—Diagrammatic Sections through Camera Eyes of Cephalopods A, Eye of Nautilus; c., internal cavity; ~ and s., refracting and sensitive layers of retina; 2.c., layer of nerve-cells; 2.f., nerve-fibres. B-D, Stages in development of eye of Cuttle-Fish (Sefza); in B the epidermis (¢/.) has folded in to produce a vesicle (v., v.); in c a fold (2.4) is growing out to form the iris; D is the adult eye; 7, protective external fold; z., iris; 2, 2., outer and inner parts of lens; ~. and s., refracting and sensitive layers of retina. eyes are used for seeing things at a distance, while the ocelli are used at close range. As focussing arrangements are entirely absent this would certainly be a great convenience. The most familiar example of camera eyes among Molluscs is afforded by the Garden-Snail (Hex asfersa, fig. 1038), where they are placed near the tips of the long front tentacles. It is extremely short-sighted, as we might expect, in view of the fact that the lens is practically spherical and very close to the retina. The Pearly Nautilus possesses eyes which are constructed on the “pinhole camera” principle. There is no lens, and sea-water is admitted by a minute hole into the large internal cavity (fig. 1059). Large and complex eyes are found in the rapacious Squids and Cuttle-Fishes, and some idea of their structure and mode of de- velopment will be gathered from fig. 1059. A few of the Bivalve Molluscs possess numerous complex camera eyes situated on the edges of the mantle-flaps, as in the Scallops (Pectex), where they 46 NERVOUS SYSTEM AND SENSE-ORGANS are bright-red in colour. Their presence is possibly in relation to the fact that some species are active swimmers. Before speaking of the camera eyes of Vertebrates, it may be well to mention certain simpler visual structures which are found in some of the most primitive members of that group. In the tadpole larva of a Sea-Squirt there is a simple cup-like direction- eye formed by thickening of the wall of the brain, and projecting into that organ (see fig. 1049, p. 38). Since the larva is transparent light-rays are able to reach it. The adult condition results from a remarkable series of modifications (see vol. iii, p. 421), which include simplification of the nervous sys- tem with loss of the brain-eye and brain- otocyst. The only compensation for this loss of vision consists in the appearance of a circlet of pigmented eye-spots round the openings by which currents of sea- water enter and leave the body. The visual organs of the transparent Lancelet (Amphzoxus) are of even simpler kind. The so-called “eye” is merely a Fig. 1060.—Diagrammatic Cross Sec- . 5 tion through the Head of a Tadpole,to Geeply- pigmented spot in the extreme Ilustrateche Develepmentof she Syes, front end af the netve-tubée, and there enlarged. , Fore-brain; 7. and 71., retina and its external pigment-layer; i¢ in addition a series of similar but %., lens-pit; 2, lens; @., an artery; 72., mouth cavity; 7.,a nerve; ##., pharynx; smaller spots in the floor of the nerve- py» pituitary body. ] : tube behind the head-region. The facts just mentioned prepare us for the statement that the ordinary camera eyes of Fishes and still higher Vertebrates are partly derived from the brain, and in this they differ from the camera eyes of Invertebrates, which are of epidermic nature. Two stages in the development of the Vertebrate eye are represented in fig. 1060. From either side of the fore-brain of the embryo an optic vesicle grows out towards the ectoderm, in which a corres- ponding pit makes its appearance. The end of the vesicle becomes as it were pushed in to form a double-walled optic cup, of which the inner and thicker layer is destined to produce the greater part of the retina, or sensitive eye-screen, while the outermost pigmented layer of this is derived from the outer part of the cup. The external ectodermic pit closes, and is pinched off as a vesicle, which lies in the optic cup (see right-hand side of figure), and ultimately thickens into the lens. The stalk of the optic cup becomes the SIGHT 47 optic nerve. Since the brain itself is of ectodermic origin (see p. 20), it is clear that the parts of the eye so far mentioned are all derived from ectoderm. The rest of the eyeball, including its two outer coats and refracting contents (see vol. i, p. 57), are formed from the middle embryonic layer (mesoderm). This curious kind of development clearly suggests that in the remote ancestors of Vertebrates the eyes were internal projections from the brain, and received their light through the transparent tissues external to them, as is still the case in the single eye of the tadpole of a Sea- Squirt. The free ends of the visual cells (rods and cones) were directed towards the cavity of the brain. As in the course of evolution the brain became more and more complex, an opaque skull was developed for its pro- tection, and the brain-eyes, having their supply of light thus cut off, were obliged, so to speak, to grow outwards. Subse- quently they were improved into camera eyes by the development of a lens. Further improvements consisted in the evolution of eye-muscles, eyelids, and =) Li complex focussing arrangements. The fig. :06r.—Section through the visual ‘cells (rods and cones) of the Ver- Ei" s Saee eae tebrate eye present the remarkable pecu-_ Jens 7.7» retina: 4, blood-vessel liarity of pointing away from the light, one result of the manner in which the retina is developed. In Vertebrates, such as Fishes, which have to see under water, the lens of the eye is spheroidal, and one mark of the aquatic ancestry of the Amphibia is the possession of a lens of similar shape. But thoroughgoing land Vertebrates have lost this primi- tive character, for in them the lens is more or less flattened and biconvex, as an adaptation to seeing in air. An extremely interesting and remarkable arrangement is found in certain bony fishes known as Double-Eyes (Azad/eps), native to the coasts and estuaries of tropical America. The name has been given because either eye, as seen from the exterior, is marked off into upper and lower halves by a dark transverse band. Dis- section shows that the upper half of the lens is biconvex, and the lower half spheroidal. And since these fishes habitually swim at the surface, with only the lower part of the eye immersed, we can 48 NERVOUS SYSTEM AND SENSE-ORGANS only conclude that this half can see clearly in water, while the upper half has been so modified that distinct vision in air has also become possible. Some of the Reptiles possess a more or less degenerate third or pineal eye on the top of the head (fig. 1061). It is connected with the roof of the ’twixt-brain. There seems good reason to believe that the ancestral Vertebrates had at least one visual organ in this position, probably serving as a means of detecting enemies attacking from above, a contingency to which aquatic forms are peculiarly liable. We may perhaps compare it with the internal brain-eye of the Ascidian tadpole, which also is unpaired and dorsal. ANIMAL INSTINCT AND INTELLIGENCE CHAPTER LIX GENERAL PRINCIPLES—INSTINCT AND INTELLIGENCE IN HIGHER INVERTEBRATES AND VERTEBRATES GENERAL PRINCIPLES Having briefly surveyed the salient facts regarding the Ner- vous System and Sense-Organs we naturally pass on to the consideration of those higher manifestations of life known as Instinct and Intelligence, which play a very important part in the adjustment of animals to their surroundings. To do anything like full justice to the subject at least half a volume would be required, and it is only possible here to attempt a brief summary of general principles, adding to this a few typical illustrations. Many other examples, however, will be found in other parts of this book. As regards the present section, the writer wishes to ac- knowledge his great indebtedness to the works of Principal Lloyd Morgan, 2.¢. Habet and [ustinct, Animal Life and Intelligence, and Animal Behaviour, to which are referred those readers who wish further information on this branch of zoology. Something has already been said about Reflex Actions (see p. 9), which are comparatively simple responses to external stimuli. In very lowly animals, such as Animalcules (Protozoa), these, together with equally simple spontaneous actions, are sufficient to meet all the contingencies of existence. So apparently pur- poseful, however, are many of these actions, that some observers are inclined to ascribe mental powers to such forms. Either to prove or to disprove such a view is impossible, for we have no direct knowledge of the mind of any animal save Man, and can only make more or less probable guesses about other forms. We may feel pretty sure, however, that the evolution of the nervous Vou. IV. 49 98 50 ANIMAL INSTINCT AND INTELLIGENCE system through increasingly complex stages has been associated with a corresponding evolution of mind, and there is considerable justification for doubting whether animals devoid of a nervous system, or possessed of a very imperfect one, are endowed with more than a dim consciousness or awareness of existence, or are capable of ‘manifesting either Instinct or Intelligence. An animal which inherits the power of performing more or less complex actions helping to adjust it to its surroundifigs, independ- ently of experience or instruction, is said to display Instinct, and such actions may be termed instinctive. They differ from Reflex Actions in being more elaborate, and many of them are partly or entirely spontaneous. But our knowledge is at present too in- complete to enable us to draw the line between actions which are of reflex character and those which are instinctive. It is only when dealing with the higher Invertebrates and the Vertebrates that we can use the latter term with any degree of certainty. The Birch-Weevil (see vol. iii, p. 394), for instance, certainly displays instinct when she constructs an elaborate leaf-funnel for the re- ception of her eggs. This very complicated piece of work is per- formed, so far as we know, with unerring certainty and without previous experience. Nor can the weevil have more than a hazy knowledge of the purpose of her work, which is probably done quite mechanically. An animal is said to show Intelligence when it profits by ex- perience, accommodating its actions to the exigencies of changed or changing surrounding. There is an inherited basis to such actions; it is the controlling power which makes them intelligent. The difference between Instinct and Intelligence is explained with admirable lucidity in the following passage from Lloyd Morgan (in Animal Behaviour):—“ Such an animal as a newly-hatched bird or an insect just set free from the chrysalis is a going concern, a living creature. It is the bearer of wonderfully complex auto- matic machinery, capable, under the initiating influence of stimuli, of performing instinctive acts. But if this were all, we should have no more than a cunningly-wrought and self-developing automatic machine. What the creature does instinctively at first it would do always, perhaps a little more smoothly as the organic mechanism settled down to its work—just as a steam-engine goes more smoothly when it has been running for a while; but otherwise the action would continue unchanged. Instinctive behaviour would GENERAL PRINCIPLES 51 remain unmodified throughout life. The chick, however, or the imago insect, is something more than this. It affords evidence of the accommodation of behaviour to varying circumstances. It so acts as to lead us to infer that there are centres of intelligent control through the action of which the automatic behaviour can be modified in accordance with the results of experience. When, for example, a young chick walks towards and pecks at a lady-bird, the like of which he has never before seen, the behaviour may be purely instinctive; and so, too, when he similarly seizes a wasp- larva. . . . But when, after a few trials, the chick leaves lady-birds unmolested while he seizes wasp-larvee with increased energy, he affords evidence of selection based on individual experience. And such selection implies intelligence in almost its simplest expression. We may say, therefore, that, whereas instinctive behaviour is prior to individual experience, intelligent behaviour is the outcome and product of such experience. This distinction is presumably clear enough; and it is one that is based on the facts of obser- vation. But we must not fail to notice that, though the logical distinction is quite clear, the acquired modifications of behaviour, which we speak of as intelligence, presuppose congenital [.. in- herited] modes of response which are guided to finer issues. We may say then, that where these congenital modes of response take the form of instinctive behaviour, there is supplied a general plan of action which intelligence particularizes in such a manner as to produce accommodation to the conditions of existence.” The quotation just given implies, what is no doubt true, that in the course of mental evolution Instinct does not decome Intelligence, but is gradually replaced by it, z.e. inherited specialized behaviour is replaced more or less by self-specialized behaviour. The larger the amount of such replacement the greater the intelligence. And this enables us to understand the peculiar helplessness of the young of higher Mammals, especially those of our own species. The complex instincts of lower forms have been lost, and it takes a long time to learn how to act intelligently. The remark does not apply to all helpless young, for in some of these, e.g. in nest- ling birds, such instincts are only deferred. The influence of strongly-developed parental affection is noticeable in both cases. To interpret the action of animals with any likelihood of accuracy it is necessary to avoid two extreme views of opposite kind. One of these ascribes almost human attributes to even the 52 ANIMAL INSTINCT AND INTELLIGENCE lowest animals; it is a case of interpreting the observed in terms of the observer. The other and older view regards Man as the only intelligent animal, all the others being simply living machines worked by Instinct and Reflex Action. There has been in the past a great dearth of patient unbiassed observation on living animals, but the number of competent investigators is now fortunately increasing, and the results already obtained clearly point to the conclusion that extreme opinions in either direction are inadmissible. The difference between Instinct and Intelligence may also be realized by taking some metaphorical illustration. Let us then compare the successful adjustments of an animal to its environ- ment to the effective shots of a rifleman aiming at a series of targets. And let us also suppose that a certain minimum score is necessary for the maintenance of a bare existence, while marriage is only permitted as the reward of a good score. The shooting of such a rifleman would be comparable to the actions of an animal actuated by pure Instinct, if he were provided with a series of loaded rifles previously sighted and adjusted in such a way that he would merely have to press the triggers to mechanically secure a large number of points—a sort of ‘ you-press-the-button-and- we-do-the-rest” arrangement. If the targets remained fixed the privileges attached to success would be easily secured. But the actions of life have to bring about adjustments to surroundings which are constantly altering, and this may be represented in the illustration by substituting moving targets for stationary ones. The purely “instinctive rifleman” would do pretty well if his targets moved but slightly, though bull’s-eyes would be infrequent, and his total would be smaller. But with increasing movement the percentage of hits would dwindle till first of all the prize of matri- mony would be denied him, and finally the score would be so small that even bare existence would not be permitted. Our illustration can easily be modified to represent the gradual replacement of Instinct by Intelligence. By endowing our im- aginary rifleman with increasing capacity to adjust his rifles, so as to secure a reasonable score with shifting targets, we make his shooting more and more intelligent, less and less instinctive. And were he simply given the loaded rifles, and left to learn the art of marksmanship for himself, success would require a high degree of intelligence. The loaded rifles would represent the gift of inheri- YOUNG ORANG-UTANS (Sima satyrus The Orang-utan, a man-like ape native to Sumatra and Borneo, is characterized by the great relative length of its arms, a pecu- liarity associated with purely arboreal habits. The long hair is of a reddish colour. Orangs are undoubtedly very intelligent, and the rounded intellectual-looking forehead gives a very human appearance. The mental powers of the Gorilla and Chimpanzee, however, are more considerable, though the powerful brow-ridges which these forms possess greatly detract from their personal appearance. The Orang builds a kind of stick-nest in the fork of a tree as a temporary shelter, from which it does not sally forth to feed until the day is well advanced. Small family parties are commonly found associated together, though the male appears to lead a soli- tary life during a large part of the year. The young are as helpless as those of the human species, and those which have been brought up in captivity present many similar traits. Their wants are ex- pressed by loud lamentations, and they protest loudly if their food is not to their taste. They also greatly appreciate being nursed and “cuddled”. Unlike human infants, however, they are eager to be washed and combed. (SNYALYS VIWIS) SNVLQ-ONVYO OSNNOA IN HIGHER INVERTEBRATES 53 tance; but were there no profiting by experience most of the shots would go wide. Parental care might be here symbolized by sup- posing the raw beginner protected and instructed by an expert shot until the necessary experience had been acquired. We do not know how far down in the scale of animal life some sort of consciousness exists, but the dawn of intelligence is marked by the appearance of what Lloyd Morgan calls “effective con- sciousness ”, z.é. a realization of existence which enables more or less successful adjustments to a changing environment. In ourselves we find Intelligence reinforced by Reason, the “ideational stage” in mental evolution, where actions depend upon motive, instead of being due to mere impulse dictating certain sorts of behaviour “on the spur of the moment”. It involves appreciation of abstract ideas with powers of reflection and deliber- ation, leading us to trace the relations between cause and effect, and to construct ideals of existence by which our conduct is more or less regulated. The dim beginnings of Reason are probably to be found among the higher animals, but the body of facts with which we are at present acquainted is far too small to justify positive statements or wide-sweeping generalizations. INSTINCT AND INTELLIGENCE IN HIGHER INVERTE- BRATES (INVERTEBRATA) The most instructive cases so far investigated are to be found among Insects (Insecta) and Molluscs (Mollusca), and it will be enough for our present purpose to briefly describe a few of these. InsTINCT AND INTELLIGENCE 1N InsEcTs (INSEcTA).—A good example of the stereotyped nature of complex instincts is given by Fabre (in Souvenirs entomologigues) in his account of one of the Mason-Bees (Chalicodoma muraria) native to South Europe. The female makes a nest consisting of nine or ten cells placed one on top of the other, using cement made of a mixture of earth and saliva, to which little stones are added. After each cell is built it is stored with honey and pollen, after which an egg is laid in it, anda roof is added. The entire series is then thickly covered with cement till the nest assumes a hemispherical form. The three operations of building, storing, and egg-laying which take place in regard to each cell follow one another with automatic regularity, and there is no harking back to an earlier stage. For conditions 54 ANIMAL INSTINCT AND INTELLIGENCE artificially imposed with a view to altering the order do not succeed in this, as they would do if the actions were of very intelligent kind. For example, a nest with fully-constructed top-cell partly stored was substituted for the original nest in which the uppermost cell had only been commenced. The bee did not apparently detect the imposture, and proceeded to raise the walls of the substituted cell till it was one-third greater than the normal height. In another experiment a bee had completed the construction of a cell, and was preparing to store it, when another nest with an in- complete top-chamber was substituted. On her return with honey and pollen she appeared greatly puzzled at the change, and finally deposited her load in the nest of a neighbour. The result of another similar experiment was somewhat different, for the bee removed the roof of the last complete cell and stored this a second time, afterwards laying a second egg in it. The last two experi- ments seem to prove the existence of a certain infusion of intelli- gence, as shown by the attempts to meet the altered circumstances, though these attempts were not of very satisfactory kind. It is somewhat remarkable that this bee is apparently unable to recognize its own nest, though we must not forget that its visual powers are of different kind from our own, but it has a well-marked memory for localities, returning to the spot selected for building- purposes from considerable distances. Fabre also showed that individuals removed as far as four kilometres from their nests, into what was probably unknown country to them, were able to find their way home. Quite a number of animals are endowed with a strong “homing faculty” of this kind, but how far this may be due to a “locality sense” which cannot be explained by applying the known principles of human physiology, it is as yet impossible to say. In this particular instance, even if we were to assume that the bees had some previous acquaintance with the distant place to which they were taken, we should still be quite unable to explain exactly ow they got home. Locality-memory, however, would seem to imply some amount of intelligence. Readers de- siring further details of the fascinating observations and experi- ments by Fabre on Mason-Bees and many other insects are referred to the original work, or the translation of the same which has recently appeared. Suggestions have more than once been made in the course of this book as to the kind of investigation which amateur naturalists IN HIGHER INVERTEBRATES 55 might profitably attempt. The habits of Insects and other higher Invertebrates offer an inexhaustible and intensely-interesting field to multitudes of such workers. Accurate observations recorded with scrupulous exactness are here badly needed, and those who enlarge our knowledge in this direction are contributing to the advance of two branches of knowledge, zoology and the science of mind (psychology), not to mention sociology and education, both of which are intimately connected with the latter. It is indispensable that observations on instinct and intelligence, should be made with a perfectly open mind, and not with the object of collecting material for the support of this or that view. And it is peculiarly necessary to remember that the mental standard of human beings can only be applied with many reser- vations in explanation of the actions of animals, especially when dealing with creatures like Insects which, though of highly com- plex structure, have specialized on lines of their own. A series of observations made in this spirit, and which are not only of the utmost value but of absorbing interest, have been recorded by Dr. and Mrs. Peckham (Ox the Habits and Instincts of the Solitary Wasps). These insects have attracted much attention on account of their habit of storing up caterpillars, flies, spiders, &c., for the benefit of their progeny, the victims having previously been stung (see vol. iii, p. 391). Instincts of very complex nature are here involved, but the zoologists just mentioned have shown that these instincts are not so stereotyped as commonly supposed, there being a certain amount of adaptability to circumstances, which is strong presumptive proof of some degree of intelligence. Pure instinct is manifested by the fact that any particular species of these wasps is always found to select the same kind of prey, and, for a given species, there is so much uniformity in the mode of nest-construc- tion, the way of disabling the victims, the manner of taking them into the nest, &c., that instinct is undoubtedly the dominant factor. But, except in regard to the kind of prey, there is a sufficient amount of adjustment to varying circumstances to warrant the conclusion that intelligence also plays some part in the complex series of operations. It appears, for example, that the prey is not uniformly stung in the nerve-cord, as once believed, and it may be killed instead of paralysed by its injuries, proving in either case suitable food for the larve. This certainly discounts the view that this part of the series of actions is stereotyped by instinct. 56 ANIMAL INSTINCT AND INTELLIGENCE And a convincing proof of the power of profiting by experience which constitutes intelligence is given in a letter of Dr. Peckham’s, quoted by Lloyd Morgan (in Anzmal Behaviour), in reference to a species (Sphex ichneumonea) which preys upon grasshoppers, and after leaving them a short time while she makes an excursion into the nest, returns and drags them in by their feelers. One individual, being several times thwarted in her storing work by removal of the victim to a short distance when she was in the nest, soon learnt the inadvisability of losing sight of her booty, and either at once dragged it into the hole or, straddling over it, substituted pushing for pulling. One of the most remarkable points about the nesting-instinct in so many solitary insects is the elaborate provision made for the welfare of offspring which will never be seen, and which commonly require food of quite different nature from that taken by the adult. The parent would seem to be urged on by irresistible impulses, and can hardly be supposed to realize the meaning of its work, except perhaps in a very dim sort of way. Butterflies and Moths illustrate the food-question very clearly. It is true that they do not construct and store nests, like the solitary wasps just mentioned, but they instinctively lay their eggs on special sorts of plant, upon the leaves of which their voracious offspring are destined to feed. A Peacock Butterfly (Vanessa Jo), for example, selects a nettle for the purpose, but her own food consists of nectar drawn from the recesses of flowers by means of suctorial mouth-parts, differing greatly from the powerful biting jaws of the leaf-eating caterpillar. It is almost impossible to believe that remembrance of her own larval days guides to the choice of a suitable place for egg-laying, for the caterpillar is converted into the adult by a series of revolu- tionary changes which amount to reconstruction. INSTINCT AND INTELLIGENCE IN Mottuscs (Mottusca).—Com- paratively few observations have been made upon the members of this group, some of which are very highly organized. Several good illustrations of both instinct and intelligence have, however, been recorded. The Octopus is one of the highest Molluscs, and appears to be a very intelligent creature. Schneider saw a young one seize a hermit-crab and then let it go, being stung by the zoophytes covering its shell. For some time at least this individual was observed to avoid hermit-crabs, having learnt to associate them IN HIGHER INVERTEBRATES 57 with painful sensations. Other Octopi manifested still greater intelligence, for they pulled hermits out of their shells, taking care not to touch the zoophytes, realizing apparently that these were the stinging element. More remarkable than this is an observation made by Madame Jeannette Power. This lady on one occasion saw an Octopus, that held a stone by one of its arms, watching a large bivalve (Pzxua) of which the shell was beginning to open. When this operation was complete the Octopus quickly inserted fda Fig. 1062.—A Limpet (Patella vulgata) leaving its Scar at Ebb-tide the stone between the valves so as to prevent them from coming together again, and then proceeded to make a meal of the helpless bivalve. Some of the Gastropods possess a well-marked “ homing instinct”, a particularly good example of this being afforded by the Common Limpet (Patella vulgata, fig. 1062). As elsewhere described (see vol. ii, p. 197) this creature lives on a particular spot, which in course of time becomes a more or less well- marked “scar”, to which it can hold so firmly as to defy waves and tide. From this home it wanders out to feed when un- covered by the water, and also when well covered. From such excursions, which may extend to a distance of several feet, it later on returns to settle down again on the scar, the surface 58 ANIMAL INSTINCT AND INTELLIGENCE traversed being often very irregular and covered with acorn- barnacles. When the animal gets back to the scar it of course arrives wrong way on, so to speak, and it quickly shuffles round so as to get into the proper position. A memory of locality certainly exists, and this would seem to imply intelligence. In the course of time a Limpet acquires a very accurate knowledge of the topography of a fair-sized area around its home, and if picked up when on the crawl and placed within this area is able to get home, though the time taken varies considerably. Exactly how it gets home we do not know. The simple cup-like eyes cannot render assistance, nor can we very well suppose that the otocysts help to guide it. Experiments appear to demonstrate that the animal does not smell its way back, and we are there- fore reduced to touch, or to a “locality sense”, or to both. The most obvious organs of touch are the two large tentacles on the head, with which the Limpet constantly touches the rock as it crawls, and it is no doubt by means of these that a good deal of the topographical knowledge is acquired. But as it can get home without the aid of these organs there must be some other organs of guidance. The edge of the mantle-flap is pro- vided with a very large number of small tentacles which can be stretched out and actively moved, as they are sometimes, if not always, when the animal is adjusting itself on its scar. These perhaps have something to do with the matter, and so may still other sense-organs, but further investigation is required. The problem here to be solved, like most of those connected with locality-knowledge, is of a particularly baffling kind, though not to be regarded as insoluble. The Garden-Snail (Helix aspersa) is another Mollusc possessed of a “homing instinct ” INSTINCT AND INTELLIGENCE IN VERTEBRATES (VERTEBRATA) There is here an almost unlimited amount of material which might be discussed, but a few examples must suffice. Warninc CoLoration.—A large number of animals pos- sessed of noxious properties advertise their objectionable nature by means of bright though somewhat crude colours, and simple but striking patterns, the net result of which is to render them extremely conspicuous (see vol. ii, p. 301). Such are the IN VERTEBRATES 59 striped “blazer” of the Wasp, and the spotted jacket of the Lady-Bird. Unless very hard pressed by hunger it appears that the foes of animals so coloured and marked give them a wide berth. But without careful observation and experiment it would remain an open question whether this resulted from Instinct or Intelligence, or a mixture of the two. The cases which have so far been properly investigated appear to prove that Intelligence here comes into play, and that a young animal has to learn from experience that some things are good to eat and others not. The thorough and long-continued researches of Lloyd Morgan upon artificially - hatched chicks definitely prove that they at least have to acquire such useful knowledge for themselves. He thus describes (in Animal Behaviour) how some of his chicks learnt that alternate bands of black and orange, as possessed by the caterpillars of the Cinnabar Moth, are associated with disagreeable sensations:—‘“ The following experiment was made with young chicks. Stripes of orange and black paper were pasted beneath glass slips, and on them meal moistened with quinine was placed. On other plain slips meal moistened with water was provided. The young birds soon learnt to avoid the bitter meal, and then would not touch plain meal if it was offered on the banded slip. And these birds, save in two instances, refused to touch cinnabar caterpillars, which were new to their experience. They did not, like other birds, have to learn by particular trials that these caterpillars are unpleasant. Their experience had already been gained through the banded glass slips; or so it seemed. I have also found that young birds who had learnt to avoid cinnabar caterpillars left wasps untouched.” Nest-BurLpinc 1n Birps.—There can be no reasonable doubt that in its main features the nest-building of birds is a matter of instinct. One of the best proofs of this is afforded by cases where individuals kept in captivity from the time of hatching, under conditions which excluded the possibility of instruction or imitation, have nevertheless constructed nests of the kind proper to their species. Further experiments, however, are much to be desired, especially on birds which indulge in architecture of such characteristic kind as to be quite unmistakable. It would, of course, be necessary to make the nesting conditions in such cases as natural as possible. Other instincts, tending to the benefit 60 ANIMAL INSTINCT AND INTELLIGENCE of the eggs or young, are often associated with that for nest- building. Of this the Eider-Duck (Somaterta mollissima, fig. 1063) may be taken as an example. Egg-laying and _ building are here not consecutive acts, but the former takes place at intervals during the latter, in a somewhat variable fashion. Three succes- sive stages are shown in the illus- tration, which is taken from photo- graphs by Mr. R. A. L. Van Someren. The first represents four eggs resting in the incom- plete nest, and the second (on a larger scale) the complete down- lined nest with its full complement of eggs. The third figure shows the same nest during the tem- porary absence of the mother-bird, and illustrates an interesting as- sociated instinct. Before leaving her duties she had pulled the down over the eggs, so as to cover them completely, an act dis- tinctly conducive to their welfare. For, snugly tucked up under their ‘eider-down quilt”, they were not only kept warm, but also, as the figure clearly proves, effectively screened from observation. But although nest-building is almost certainly instinctive in the main, it is subject to modification in individual cases in ways which vouch for the intelligence of the %. builders. And such modification ee “ane affect the style, materials, and place of construction. Often- quoted illustrations are those of the House-Swallow and House- Martin, which have taken advantage of the evolution of human civilization so far as concerned with domestic architecture. IN VERTEBRATES 61 This change of habit, of course, took place in the remote past, but the following very interesting modern example of pre- cisely similar kind is given by Headley (in Zhe Structure and Life of Birds):—“ The Palm Swift in Jamaica till 1854 always built in palms. But in Spanish Town, when two cocoa-nut palms were blown down, they drove out the Swallows from the Piazza of the House of Assembly and built between the angles formed by the beams and joists.” Of other such cases Newton thus writes (in A Dictionary of Birds):—“ But though in a general way the dictates of hereditary instinct are rigidly observed by Birds, in many species a remarkable degree of elasticity is exhibited: or the rule of habit is rudely broken. Thus, the noble Falcon, whose ordinary eyry is on the beetling cliff, will for the convenience of procuring prey condescend to lay its eggs on the ground in a marsh, or appropriate the nest of some other bird in a tree. The Golden Eagle, too, remark- ably adapts itself to circumstances, now rearing its young on a precipitous ledge, now on the arm of an ancient monarch of the forest, and again on a treeless plain, making a humble home amid grass and herbage. Herons also show the same versa- tility, and will breed according to circumstances in an open fen, on sea-banks, or (as is most usual) on lofty trees. Such changes are easy to understand. The instinct of finding food for the family is predominant, and where most food is, there will the feeders be gathered together. This explains, in all likelihood, the associated bands of Ospreys or Fish-Hawks, which in North America breed (or used to breed) in large companies where sustenance is plentiful, though in the Old World the same species brooks not the society of aught but its mate.” Micration or Brrps.—Nothing can be more familiar than the fact that innumerable species of birds undertake periodic journeys, often of extreme length, from one region to another, and at the same time nothing in the entire realm of natural history is more mysterious. Broadly speaking, the same migrant species has its own line of travel between its two places of residence. The Golden Plovers, for example, of the northern part of North America, fly south to the north of South America via the Bermudas and Antilles. The paths of a number of species are more or less coincident, in many cases, to form what is known as a “ migration route”, and some of these routes 62 ANIMAL INSTINCT AND INTELLIGENCE have been determined with some approach to accuracy. A vast number of facts concerning migration have already been collected, and these receive large additions every year, so that in the course of time we may expect to have a fairly complete knowledge of the movements of many migratory birds. To discover how they find their way is a much more difficult problem, especially in cases where there can have been no previous experience. Many of them, eg. the Common White Stork (Czconza alba), “assemble” before migration, as if to practise their powers of flight, and the writer once saw the roof-ridges in the street of a midland town “lined” with hundreds of swallows at 6 a.m. one morning, less than two hours after which all had disappeared. It has been suggested that the old birds impart geographical knowledge to the young, and also that the migrant flocks are ‘personally con- ducted” by experienced leaders. But many well-ascertained facts militate strongly against such views, at any rate for certain species. Some young birds go off by themselves, even the first time they migrate, and this may take place under conditions which pre- clude the possibility of their having previously acquired informa- tion from their elders. The Common Cuckoo (Cuculus canorus), for example, winters in Africa, and, as everyone knows, its off- spring are reared by other birds. The old Cuckoos have all left this country by the end of August, and the young ones take their departure later. In such a case we cannot doubt the ex- istence of a “ migratory instinct”, but how far this is modified by intelligence has yet to be determined. We are equally igno- rant as to the sense-organs which are the agents of the instinct, and its possible modifications by intelligence, even more than we are in the cases of insects and molluscs which possess a keen sense of locality. We ourselves are not entirely devoid of a faculty of the kind, and it appears to be comparatively well developed in savage races. It is to be hoped that extended observation and experiment will some day throw more light upon the subject, till when it will be wise to suspend our judgment, and remain in a critical attitude, rather than indulge in prema- ture generalization. It is scarcely necessary to add that there is room for a host of unprejudiced observers in this field of work. WHITE STORKS (Ciconta alba) ASSEMBLING FOR MIGRATION The seasonal Migration of ‘many birds is a phenomenon familiar to all, and one which, in spite of much research, is still but little understood. Food-supply has no doubt much to do with it, but the reasons for migration are the least mysterious part of the matter. How birds are able to find their way over vast stretches of land and sea to regions suitable for their purposes is at present quite beyond our comprehension. In most cases it appears that the young birds are the first to depart on what must be for them an unknown journey, which greatly adds to the difficulty of the problem to be solved. One of the best-known migratory birds is the White Stoxk, the rough stick-nests of which are such common objects on roofs and chimneys in Holland, Denmark, and North Germany. The locality-sense is strongly developed, for year after year a nest is tenanted by the same pair of birds. They arrive in spring, leaving again in late summer, by which time the young are well grown. Before their departure they “assemble” in large numbers on the meadows, and fly away in troops, some of which have been estimated to include as many as five thousand indi- viduals. They winter in Africa, some of them getting as far south as Cape Colony. NOILVHYDIN. HOF ONIIGWASSY (vaqv YINOD!ID) SMYOLS JLIHM ASSOCIATION OF ORGANISMS— THE WEB OF LIFE CHAPTER LX ASSOCIATION OF ORGANISMS—GENERAL PRINCIPLES— ANIMALS AND PLANTS GENERAL PRINCIPLES The study of natural science during the last half-century has advanced so rapidly that it is no longer possible for one man to grapple seriously even with a single subject, and there is an ever- increasing tendency towards specialization. No doubt the sum of our knowledge is thereby constantly being increased at a rate which would otherwise be impossible, but there is another side to the question. For extreme specialization is somewhat apt to lead to a neglect of general principles, and to a more or less complete loss of the sense of proportion. To be unable to see the wood on account of the trees is bad enough, but to have one’s vision restricted to a single tree, or perhaps a single branch, is very much worse. In no department of knowledge is the cramping tendency of special- ization more apparent than in natural history. There seemed at one time a chance of establishing a science of Biology, designed to deal with both plants and animals, but this has now been merged into botany on the one hand and zoology on the other, and many of the important relations that exist between plants and animals are not given the prominence which they undoubtedly deserve. This cannot altogether be helped, but even under existing circum- stances it is both desirable and possible that work of specialist kind should be preceded by studies of a wider and more general nature. This is one of the aims of the new subject of Nature- Study, so far as biology and geology are concerned, another object being to foster that intelligent interest in and accurate observation 63 64 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE of natural objects upon which much of the future happiness of our embryo citizens will depend. If properly taught as a connected whole, and not as a string of isolated facts (to be “learnt ”), this subject ought to fare better than physiography and general ele- mentary science, which, though designed with the laudable inten- tion of giving a broad foundation in non-biological science, are now somewhat discredited. The relations which bind together the innumerable plants and animals now living on the globe are so numerous, and often so complex, that from this point of view the world has been com- pared to a spider’s-web of elaborate texture, in which all the threads are directly or indirectly connected, so that when one is touched the entire structure is thrown into vibration. It is not within the scope of this work to deal with the com- plex relations which link together the members of the vegetable kingdom, and students who desire information of this kind are referred to the English edition of Schimper’s Plant Geography, to Kerner von Marilaun’s altogether admirable book Zhe Natural History of Plants, and also to Scott Elliot's Mature Studzes, which gives an excellent account of the leading facts and principles in small compass. But it may be well to attempt here a brief description of the salient features which mark the relations exist- ing between plants and animals. This is much more fully dealt with by the authors just mentioned. PLANTS AND ANIMALS It may not be superfluous to remark here that the vegetable world is divided into the following great groups, beginning with the highest:—1. Seep PrLants (Spermaphyta), including most of the large and obvious forms, such as ordinary forest-trees and the inhabitants of our flower-gardens. 2. FERN-LIKE PLANTS (Pteridophyta), comprising not only ferns, but also horse-tails, club-mosses, &c. 3. Mosses anp Liverworts (Bryophyta). 4. Lower Prants (Thallophyta), in which the body is not divided into stem, root, and leaf, or such a division is only incipient. Multitudes of Thallophytes are minute or microscopic, and in any case they may broadly be assigned to one of three sub-groups: (2) Alge, embracing brown, green, and red sea-weeds (with a smaller number of freshwater weeds), with a host of smaller PLANTS AND ANIMALS 65 forms living not only in water but in most damp places; (4) /ungz, including toad-stools, moulds, mildews, the microscopic yeast-plants, and the still smaller bacteria; and (c) Lichens, which are intimately connected communities of alge and fungi. All these plants, except fungi (and a few seed-plants), contain leaf-green or chlorophyll, a substance of great biological import- ance, as elsewhere explained. It is convenient to distinguish forms which possess it as “green plants”, though the chlorophyll AIR R R CO, O co, O co / FF) ,2 > s 5 (0 oN NITROG@s N \ \ FooP DEATH & DEG, ey Bg VUE CY: YY WY) YM: / Wi Uy ee MAY GLE Ss : WL GLI Ni By rai fe 3 Ye Leth NITRATES LLY // NAT RIT 9 Yi yy CO YY; iif iy Mi MIM OM IMT a v4 4 Gi MM EMMI MUTE Fig. 1004.—Relations between Animals and Plants: arrows indicate the taking in or giving out of various substances Both green plant and animal take in oxygen (0) and give out carbon dioxide (cog) in the course of respira- tion (rR). The animal feeds on plants, and by nitrogenous excretion and ultimate death adds to the store of organic matter in the soil. The green plant in the course of feeding (F) takes in carbon dioxide (co2) from the air, returning oxygen (0), and also takes up water with dissolved salts from the soil; its dead parts contribute to the store of organic matter in the soil, The groups of bacteria 8,-B3, respectively produce ammonia com- pounds, convert these into nitrites, and these again into nitrates. The bacteria By, and the tubercle-fungi B;, fix the free nitrogen (N) of the air, with production of nitrates. The bacteria Bg, in the absence of oxygen, decompose organic matter with liberation of free nitrogen (N). may be obscured by the presence of other pigments, as, for example, in brown and red sea-weeds. RELATIONS BETWEEN ORGANISMS AND THE CONSTITUENTS OF THE ATMOSPHERE (fig. 1064).—In considering this question it must not be forgotten that the gases which are mixed together in air are also found dissolved in both fresh and salt water, and the relations between these dissolved gases and aquatic organisms are pretty much the same as those subsisting between ordinary air and land organisms. The most important of these gases are carbon dioxide (carbonic acid gas, CO,), oxygen, and nitrogen. It has already been explained in the section on BREATHING (vol. ii, p. 379) that plants and animals respire in the same way, taking in oxygen to facilitate the breaking-down processes which continually go VoL. IV. 99 66 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE on in the body, and giving out carbon dioxide as a product of waste. It is quite a mistake to suppose that plants ‘‘ breathe in carbon dioxide and breathe out oxygen”, as often supposed. If there were not some compensating arrangement, it is clear that the amount of oxygen in air would rapidly diminish and the quantity of carbon dioxide rapidly increase. But it appears that this is not so, for the composition of air remains practically the same, at least for very long periods of time, though probably in earlier periods of the earth’s history its constituent gases were in different proportions from what is now the case. This constancy of composition at the present day is intelligible when we re- member how green plants feed, and of what their food consists. Such plants, in fact, bridge over in a fashion the gulf between non-living and living matter. For their food consists of carbon dioxide (obtained from the air), and water in which are dissolved certain substances of simple kind, foremost among these being nitrogen-containing compounds known as nitrates, of which salt- petre is a well-known example. In any sort of tree, shrub, or herb the carbon dioxide is taken in by the leaves (and the green part of the stem), while the roots absorb from the soil a watery solution of nitrates, &c., so dilute as to be comparable to the ordinary drinking water of most districts. These simple con- stituents of the food are converted step by step into the living substance of the plant by the agency of that substance itself. The first step in this series of up-building processes takes place in the leaves (and the green parts of the stem), and consists in a reaction between carbon dioxide and water, giving rise to a sub- stance which is more complex in nature than either of them. This can only go on in daylight and in the presence of chloro- phyll, which in some way not clearly understood enables the living substance associated with it to press the energy of sunlight into its service for the purpose of building up a comparatively complex substance from simple ones. And this first step in the manufac- ture of living matter is accompanied by the liberation of free oxygen into the surrounding air as a by-product. For the carbon dioxide and water, which are the raw materials in this work, contain more oxygen than is required for the purpose, and the surplus passes away to the exterior. It therefore follows that green plants in the course of their feeding (1) take carbon dioxide From the air, and (2) give out free oxygen Zo the air. And these PLANTS AND ANIMALS 67 gases are respectively taken in and given out in such proportions that the amount of carbon dioxide in the air does not rapidly grow larger, and the amount of free oxygen rapidly get smaller, as would undoubtedly be the case if the results of breathing were not compensated. The action and reaction between organisms and the air also involve chemical processes which have to do with nitrogen, and in which a leading part is played by various bacteria which live in the soil, Green plants get the nitrogen which they require for feeding purposes in the form of dissolved nitrates, which are derived from more than one source. It is a familiar fact that ordinary earth or soil, such as is to be found in a garden, is more or less dark in colour, largely as the result of the presence of organic matter. This partly consists of the remains of organisms which have died and decayed, and partly of substances derived from the nitrogen-containing excreta of animals. The rotting, decomposition, or breaking down of such materials is the result of chemical changes brought about by certain bacteria in the presence of oxygen, with production of ammonia compounds. Another set of bacteria convert these compounds into salts known as nitrites, from which nitrates are then produced by the action of still another group of bacteria. The nitrates serve as food to green plants, which in their turn are devoured by animals. We thus see that by the death and decay of organisms material is produced which helps to build up the bodies of new generations. This, however, is not the only source of nitrates in the soil, for what are known as xztrzfying bacteria are there present, which possess the remarkable power of abstracting free nitrogen from the air, and causing it to enter into combination. There is another arrangement by which, in leguminous and a few other plants, the same end is attained. If, say, a pea- or bean-plant is dug up, and the earth washed away from its roots, these will be found to bear a number of rounded thickenings. Within each such “root-tubercle” live a number of microscopic fungi (possibly bacteria) that appropriate the free nitrogen of the air which circu- lates in the soil, employing it to build up nitrates. We have here a striking example of Mutualism (symbiosis), ze. the intimate association of two organisms for their common benefit. The leguminous plant has a supply of nitrates ready to hand, while the tubercle-fungus is sheltered, and no doubt nourished. 68 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE It is clear that by the action of nitrifying bacteria and tubercle- fungi the nitrogen of the air is steadily diminished, but here again we find a means of compensation. For there are certain denztrzfy- ing bacteria, which, in the absence of oxygen, act upon decaying organic matter in such a way that free nitrogen is liberated. RELATIONS BETWEEN THE NUTRITION OF PLANTS AND ANIMALS. —A little reflection will show that animals are entirely dependent upon plants in the matter of food. This is obviously so as regards purely vegetarian animals, while carnivorous forms are indirectly dependent upon the vegetable world. Many flesh- eaters feed entirely upon vegetarians, but if they prey upon other flesh-eaters, and these again upon still other carnivorous creatures, and so on, we get to plants in the end. Plants, considered as food for animals, have been concerned in the evolution of burrowing, climbing, parachuting, and flying forms, especially the last three. (For details, see vol. iii, pp. 291, 281, 292.) On the other hand, animals contribute to the store of plant food. For, as we have already seen (pp. 65-67), they breathe out carbon dioxide, which green plants take up, while their nitro- genous excreta and dead bodies are partly converted into nitrates, which the same plants are able to use for the purposes of nutrition. There are also a number of Carnivorous Piants, which do not altogether rely upon simple substances as food, but are provided with “traps” for the capture and digestion of insects or other small creatures. One of Darwin’s most interesting books (Lusectivorous Plants) is devoted to these forms, some of which are native to our own country, while several others may be seen in botanic gardens. One of the simplest kinds of arrangement is seen in the Butterwort (Pzmguzcula), that is often to be found growing in damp places among our mountains. The pale-green slippery leaves are arranged in a rosette, from the centre of which violet flowers grow up. Small flies alighting on the leaves are held fast by a sticky fluid, secreted by a multitude of little knobbed hairs which project from the surface. The edges of the leaves then curl over the prey, and there is an increased exudation of the fluid in question, which acts very much like gastric juice, converting the flesh of the booty into a soluble form that is then absorbed as food. The widely distributed members of the Sundew family (Dvoseracee) exhibit greater specialization in PLANTS AND ANIMALS 69 relation to the catching of insects than the Butterworts, though the means employed are essentially the same. Our native forms, the Sundews (species of Drosera), are fairly common in marshy places, and are often found growing side by side with the Butter- wort. Here, again, the leaves are arranged in a rosette, from the centre of which rises a stem bearing a number of small flowers. The end of each leaf is thickly studded with long reddish “tentacles”, shaped something like pins, upon the heads of which are little drops of sticky fluid that glisten like dew (fig. 1065). Should an unfortunate insect alight on one or more of these tentacles it sticks fast, other Fig. 1065.—The Sundew (Drosera). 1, Tip of a tentacle, greatly enlarged, showing viscid secretion. 2, 3, 4, Leaves, enlargéd, showing tentacles fully extended, partly approximated, and entirely approximated. tentacles bend towards it, there is an increased flow of the diges- tive juice, and the final result is the same as in a Butterwort. Venus’ Fly-Trap (Dzonza muscipula), growing on marshy ground in the east of the United States, is a near relative of our Sundews, but its “traps” are much more elaborate (fig. 1066). The end of each leaf is divided into halves which can move towards each other almost as on a hinge, while their edges are fringed with bristles. The upper side of each half is studded with small violet hairs, which secrete a digestive fluid, and three large sensitive hairs project from its centre. If an insect happens to visit one of these leaves, and touches one or more of the six sensitive hairs, the result is somewhat dramatic. For the halves of the leaf close rapidly together, the bristles on their edges inter- 70 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE locking, so that a very perfect trap is constituted, in which there is plenty of room for an average victim, since as the two halves move together they become concave towards each other. As in the other cases, digestion and absorption complete the tragedy. Among the most notorious of carnivorous forms are the widely- distributed Pitcher Plants, in which the leaves are hollowed into structures which may be described as a combination of lure, pit- Fig. 1066.—Venus’ Fly-Trap (Dionwa muscipula) fall, and stomach. Such are the species of Mefenthes, which range from Madagascar through South Asia and the East Indies to the Philippines. In this case the lidded pitchers look some- thing like hot-water jugs, and are attractively coloured. The way in which they serve their purpose is thus described by Kerner (in Zhe Natural History of Plants):— The bright pitchers of Mefenthes, visible from afar, are sought, just as flowers are, by insects, and probably by other winged creatures as well; and this occurs all the more because there is a copious secretion PLANTS AND ANIMALS 71 of honey by the epidermal cells upon the under surface of the lid, and on the rim round the mouth of each pitcher. The swollen and often delicately-fluted rim, in particular, drips and glitters with the sugary juice, and it would be permissible in this connection to speak of a honeyed mouth and sweet lips in the most literal sense of the words. Animals which suck honey from the lips of Mefenthes pitchers wander, as they do so, only too readily upon the interior surface of the orifice. But the inner face is smooth and precipitous, and rendered so slippery by a bluish coating of wax that not a few of the alighted guests slip down to the bottom of the pitcher and fall into the liquid there collected. Many of them perish in a short time; others try to save themselves by climbing up the internal face of the pitcher, but they always slip again on the polished, wax-coated zone, and tumble back once more to the bottom.” In some species the inwardly bent rim of the pitcher is fringed with sharp teeth which curve downwards and facilitate entry but forbid exit. Another very interesting Pitcher-Plant (Sarracenia variolarts), native to the marshes of Alabama, Carolina, and Florida, presents arrangements of somewhat different kind. It possesses a rosette of elongated hollow leaves, of which the ends bend sharply over like hoods. The narrow opening of a pitcher is just under the hood, from which a little flap hangs down. Allurement by colour is not wanting, for though most of each pitcher is green, its hooded top is veined with red, and there are purple blotches here and there. In this region, too, there are numerous trans- lucent patches between the veins, which from inside the pitcher must look like openings or ‘ windows ’. As in Mepenthes, honey is provided on the inner surface of the hood and round the margin of the aperture, from which a sugary ridge runs right down to the ground, serving as an attractive but fatal pathway to many wingless insects, especially ants. The pitcher is a pitfall of the deadliest kind, for its interior is clothed with slippery overlapping scales, of which the narrow pointed ends are directed downwards, so that insects, once imprisoned, are quite unable to climb out again. And if a winged insect tries to fly out it naturally makes for the apparent windows in the hood, for the actual opening faces downwards and is veiled in darkness, and in most cases falls back exhausted into the putrid 72 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE fluid which fills the lower part of its prison. The unfortunate victims are not digested, as in Mefenthes, but either drown or starve, after which their bodies decompose to form a sort of liquid manure, parts of which are no doubt absorbed as food. Yet, strange to say, a few flies, and a small moth, regularly lay their eggs in the decomposing mass contained in these pitchers, and possess climbing-irons, so to speak, which enable them to get out again with the greatest ease. One such form is a species of Blow-Fly (Sarcophaga Sarraceni@), own cousin to the speckled nuisance (S. carnarza) that lays her eggs in our meat, and to which we give the same name. Each foot of this fly is possessed of a long and sharp claw, which can be pushed between the scales of the pitcher, and firmly fixed into the under- lying tissue. The maggots which hatch out of her eggs feed on the putrefying substances surrounding them until they are full grown, when they easily get out of the pitcher, not by climbing, which would be impossible in their case, but by the simple device of eating a hole in the wall. Once outside, they enter the ground, and there pass into the motionless pupa stage, from which the adult fly later on emerges. The small moth (Xanthoptera semicrocea) for which the Sarracenia pitchers have no terrors is adapted for climbing in much the same way as the Blow- Fly. For each of the second legs possesses a pair of long sharp spines at the end of its shin (tibia), while two pairs of such spines are similarly situated on each of the hind-legs. The caterpillars do not, like the fly-maggots, eat their way out of the pitcher, but climb out, though in quite a different way from their mother. Their solution of the problem is equally effective, for they spin a web of silken threads over the slippery scales, and thus secure the necessary foothold. All the carnivorous forms so far mentioned, though they live in marshy places, are land plants, more or less perfectly adapted for the capture of insects and other small terrestrial animals. Some of them, however, have aquatic relatives, which are to be found floating in ditches and ponds, where they prey chiefly upon small crustaceans, such as water-fleas, mussel-shrimps, and copepods, though the larve of gnats and other insects are also among their victims, besides which they catch large numbers of the minute motile plants known as Diatoms. The floating habit conduces to success in this matter, for small crustaceans, &c., are PITCHER-PLANTS (MNepenthes) 4 The plate represents a typical species (Wepenthes destillatoria) of a group of pitcher-plants which ranges from Madagascar through south and south-east Asia to the East Indies, Philippines, and tropical Australia. They live in damp forest-regions, at the side of pools, in the shallow water of which their seeds germinate. The leaves are modified in a remarkable manner for the purpose of’ catching and digesting flying-insects. The attached end of the leaf-stalk is broadened into a green expansion, followed by a tendril-like section, while the end of the stalk swells into a pitcher, which is overhung by a lid representing the blade of the leaf. In- sects are attracted by the bright colours of the pitchers, and the ‘ nectar which is abundantly secreted around their openings and on the under side of their lids. But the inner side of the pitcher is as slippery as glass, and any insect that steps upon it quickly slides down into the contained fluid, which partly consists of a powerful digestive juice that reduces to solution the nutritious parts of the victim. The “peptonized insect-extract” thus prepared is absorbed by the lining of the pitcher, and constitutes a highly nutritious and stimulating food. PITCHER-PLANTS (NEPENTHES DESTILLATORIA) AT THE EDGE OF A TROPICAL POOL PLANTS AND ANIMALS 73 most abundant at or near the surface. Among these aquatic carnivores are certain small cousins (species of Addrovandia) of Venus’ Fly-Trap, which are specialized in much the same way. They are native to South and Central Europe, India, and Aus- tralia. The Bladderworts (species of U¢ricularia, fig. 1067) are widely- distributed ditch-plants, closely related to the Butterworts, and Fig. 1067.—Bladderworts ( Utricularia) represented in the British flora. They feed in part upon small aquatic organisms, and catch their prey in little bladder-like traps formed by modification of parts of the feathery leaves (fig. 1068). Each of these snares is not unlike a large water-flea in shape, and the resemblance is greatly increased by the presence of two branching bristles at the free end. Here, too, is placed the small opening into the bladder, guarded by a little transparent flap serving as a door, which opens inwards with the greatest ease, but prevents exit. Why little creatures should be attracted to 74 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE these traps is by no means clear, but minute crustaceans, &c., are fond of prying into holes and corners in search of food, while some of them may make use of the two branching bristles as a place of refuge from their enemies. And as the two bristles act as guides to the mouth of the trap the result is often tragic. Slimy hairs grow in this dangerous neighbourhood, which pos- sibly have attractions to offer, while the little transparent door Fig. 1068.—Traps of Bladderwort (U¢ricudaria), enlarged. 1, External view. 2, Longitudinal section. 3, Three absorptive branched hairs from interior. 1 and 2, X 4; 3) X 250. must look like a spot of light, and perhaps acts as a lure. It is at any rate certain that victims are numerous, and their decom- posed remains are absorbed by curious branched hairs which line the trap. The prey is apparently not digested as in Sundews and Butterworts. The remaining topics dealt with in this chapter also more or less involve questions of nutrition, but are placed under separate headings for the sake of clearness. ASSOCIATION OF PLANTS AND ANIMALS AS MESSMATES, MUTUALISTS, AND PARASITES CoMMENSALISM.—Two associated organisms are known as Messmates or Commensals when they live together to the benefit of one or both; the union, however, not being of so intimate a nature as to be essential to the life of either. The term commen- salism was coined to express such relations as existing between different animals, but there appears to be no reason why its mean- ing should not be extended to cover cases where two plants, or a plant and an animal, are similarly related. As an instance of the former we may take those tropical Orchids which regularly live upon trees, and are on that account said to be Epiphytes (Gk. MESSMATES, MUTUALISTS, AND PARASITES 75 eft, upon; phyton, a plant). The advantage to the Orchids is obvious, though they do not absorb the sap of the plants upon which they live. These last, however, apparently derive no benefit from this one-sided arrangement. There are numerous cases of commen- salism between plants and animals in which the latter alone are benefited. In one of the Liverworts (/rullania dilatata, fig. 1069) which grow on tree-trunks there are little cup-like outgrowths on the under sides of the leaves, serving as the abodes of a species of Wheel-Animalcule (Ca/idina symbiotica). Marine plants often bear animals as mess- mates, which do them no harm. Ona kind of brown sea-weed (Fucus serratus), for ex- ample, are frequently to be seen the little spiral tubes of a sort of Annelid (SAzvo0rb7s), which no doubt secures an increased supply of nutriment and dissolved oxygen by being moved about in the water when the tide is up. The Australian Sea-Horse (Phy/lop- teryx egues) also benefits by its association with the sea-weeds to which it bears a resemblance (see vol. ii, p. 296). Some instances are known of commen- salism between a plant and an animal, in which both derive advantage from the as- sociation. Ant-plants illustrate such an arrangement (see p. 81), and so do the Sloths of South America, in which minute . alge live in the grooves of the fluted eo ee hairs, For these aleve are provided with oe ee ne nail a sheltered home, and at the same time — dravins is of one cup withits Router give a greenish tint to the hairs, the Sloths being thereby rendered less conspicuous to their enemies. Mutuatism (Symbiosis).—Organisms living together as Mutu- alists are very intimately associated for mutual benefit. Mutualism between two plants is well illustrated by leguminous forms and the minute fungi which live in the tubercles on their roots. And every Lichen may be regarded as a joint-stock community, con- 76 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE sisting as it does of an alga and a fungus closely interwoven (fig. 1070). Animals and plants may also be associated in an intimate way. It appears, for example, that the process of digestion in many animals is aided by certain bacteria which always live in their internal organs, as, eg., Sarvcina ventricui in the human stomach (fig. 1073). Bacteria of the sort are provided with a sheltered home and abundant food. Among the Animalcules (Protozoa) a well-known example is afforded by some of the Radiolaria, which always contain so-called ‘yellow cells”, that are regarded as a kind of alga (fig. 1071). These cells are not only shel- tered, but also absorb carbon dioxide, water, and salts from the fluids of the Radiolarian, which in its turn is pro- vided with abundant free oxygen for breathing purposes, and possibly bene- fits in other ways. A somewhat similar association between some Sea - Ane- mones and minute alge has been de- scribed. It is, however, possible that “yellow cells” and “alge” are not ; plants at all, but specialized parts of the Fig. 1070. —Cross-section through a Lichen : (Collema), showing the colourless threads of Ray-Animalcules and Sea-Anemones the Fungus, and the dark necklace-like fila- ments of the Alga. 450. themselves. ParasITISM.—An organism is known as a parasite when it feeds upon the substance of another or- ganism, to the serious or fatal detriment of this unwilling “host”. An ectoparasite lives on the outside of its host; an exdoparasite within it. Many plants prey upon other plants in one way or the other. Clover-Dodder (Cuscuta), for example, is ectoparasitic upon Clover, while various fungi live as endoparasites within higher plants, eg. Potato-Fungus (Phytophthora infestans) within the tissues of the Potato plant. A large number of plants are known which are endoparasitic with regard to animals. In autumn many dead flies will be seen adhering to various objects by a sort of fluffy halo which sur- rounds them. These have been killed by the Fly-Mould (Am- pusa musc@, fig. 1072), the delicate threads of which branch in MESSMATES, MUTUALISTS, AND PARASITES 17 all directions through their tissues. Other fungi attack various caterpillars, e.g. the silk-worm disease known as “ muscardine” is due to the Silk-worm Mould (a species of Cordiceps). A number Fig. 1071.—A Ray-Animalcule (Avachnocorys circumtexta) with yellow cells (2), much enlarged of skin-diseases, such as ringworm and “ barbers’ rash” are caused by parasitic plants of somewhat similar nature. But the most notable, and at the same time the smallest, of the endoparasitic plants which attack animals are certain kinds of bacteria, which may literally swarm within the body, and give rise to a host of diseases, such as relapsing fever, typhoid, leprosy, Asiatic cholera, tuberculosis, diphtheria, anthrax, lock-jaw, and bubonic plague. Some idea of the small size of _ bacteria will be gathered from fig. 1073, or from Coy a Nea statements that make some appeal to the imagi- 5% Fy Mout (@mewe nation. It is said, for example, that 250,000,000 individuals of the species associated with bubonic plague could be crowded into the small space of a square inch. A number more than six times as great as the population of the United Kingdom at the last census. 78 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE In dealing with those animals that feed upon plants it is impossible to draw any clear line between vegetarians and para- sites. We shall, however, be justified in applying the latter name to a number of small forms which live, generally for part of their lives only, within the tissues of plants, one consequence being the formation of certain abnormal growths. Some of these will be dealt with later on in connection with the subject of agricul- J tural pests. The clubbing of turnip-roots (‘‘finger-and-toe” or Fig. 1073.—Bacteria. 1, The “‘blood portent ’ (Micrococcus prodigiosus); 2, gelatinous stage of the same. 3, Bacteria which produce acetic acid (Bacterium aceti); 4, the same on larger scale. 5, Bacteria of Asiatic cholera (Spivochete cholere asiatice); 6, the same on larger scale. 7, Anthrax bacilli (Bacillus anthracis) with red blood-corpuscles; 8, the same on larger scale. 9, Bacteria of relapsing fever (Spivrochete Obermeieriy and red blood-corpuscles. 10, Symbiotic bacteria (Sarcina ventriculi) from human stomach. 4, 2, 3, 5, 7s and 9, X 300. 10, X 800. 4, 6, and 8, X 2000. “anbury ”), for example, is caused by one of the Fungus-Animals (Plasmodiophora brassice), which interferes with the nutrition of the plant, causing it to grow in an unusual way. And it not infrequently happens that cereals and some other cultivated forms are attacked by small Eel-Worms, the presence of which has a stunting or distorting effect. Most persons have noticed the curious local outgrowths known as “galls” that are common upon some plants, and tempt com- parison with the tumours and cancers of animals. They are due to the attacks of Gall-Flies, small forms belonging to the order of Membrane-winged Insects (Hymenoptera). The female gall- MESSMATES, MUTUALISTS, AND PARASITES 19 fly punctures a bud, or leaf, or stem, by means of her sharp ovipositor, and lays an egg in the incision. The injury is trifling, but sets up irritation, probably caused by some secretion, and the result is an abnormal growth. Some of the different galls to be seen on oak-leaves are represented in fig. 1074. Other examples are furnished by the familiar “oak-apples”, and the “ bedeguars ” of rose-bushes. A particular species of gall-fly always selects the same sort of plant, and attacks the same region, the resulting gall being of definite size, shape, and colour. A remarkable case is cited below (see p. 81), where the gall benefits the plant on which it is found. DEFENCES OF PLANTS AND ANIMALS AGAINST ONE ANOTHER.—A good deal of space has already been devoted (vol. ii, p. 275) to the innumerable devices by which various animals are more or less protected in reference to carnivorous forms; but animals are also liable to be attacked by plants, especially by microscopic but deadly bacteria that induce many sorts of disease, particularly those of infectious or contagious nature. One important function of the white or colourless cor- puscles which live in lymph or blood : appears to be to repel the attacks of Fiz Sica it as dangerous “germs” of the sort (see vol. iii, p. 4). The principle involved in vaccination or inocu- lation is related to the fact that animals which have been pur- posely subjected to the influence of a disease-germ that has been weakened by artificial methods (or to the action of a related but less dangerous kind of germ), are thereby rendered able to resist more or less successfully the onslaughts of the same sort of germ in its more virulent form. Another important application of pre- ventive (and curative) medicine has resulted from the discovery that some animals are protected from particular disease-germs by means of complex substances (defensive proteids or anti-toxins) contained in their blood. The best-known example is afforded by diphtheria, which can be warded off, or combated if con- 80 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE tracted, by means of an anti-toxin extracted from the blood of the horse. Turning to the other side of the question, we find that numerous plants are protected by various means against vege- tarian animals. Many species, for example, more or less suc- cessfully ward off the attacks of browsing forms by mechanical devices. Of this nature are the thorns, spines, prickles, and stinging hairs with which painful experience has made most of us more or less familiar. Good illustrative cases are such com- mon forms as gorse, blackthorn, holly, thistle, and stinging-nettle. This is not, however, the only use of sharp-pointed outgrowths, for not a few forms, ¢.g. the bramble, are “hook-climbers”. It is particularly noticeable that ripening fruits are often mechani- cally guarded, the prickly husks of horse- chestnut being a case in point. Quite a number of herbaceous plants contain in their soft tissues bundles of _ exceedingly sharp needle-like crystals Fig. 1075.—Cell from Leaf of Vir- ; : ginia Creeper (Ampelopsis), containing (raphides, fig. 1075) which protect them a bundle of needle-crystals (raphides) : : of oxalate of lime; highly magnifed against the ravages of slugs and snails, as experiment has shown. Many such crystals are to be found, for example, in the leaf-stalks of the Arum-“ Lily” (Richardia 4 thiopica). Many plants are protected by chemical means, ze. by the formation within their tissues of substances which are poisonous or nauseous, or otherwise detrimental to the well-being of would- be consumers. Forms such as Foxglove, Aconite, Monkshood, Hemlock, and Yew no doubt ward off to a great extent the attacks of browsing animals in this way. Fruits and seeds are often thus protected, and may be either simply nauseous (especially when unripe) or else contain active poisons, as in the case of the seeds of Laburnum and Strychnos nux-vomica. The attacks of both large and small animals are checked in many cases by means of a sticky fluid known as J/atex, which flows from an injured part, and when fresh has a milky appearance. Common examples among British plants are the Spurges (Euphorbia), Poppy, Greater Celandine, and Dandelion. This secretion hardens when dry, and forms a protective coat over the wound. Some tropical trees produce a kind of latex which, in the solidified condition, is known to us as india-rubber. It is almost MESSMATES, MUTUALISTS, AND PARASITES 81 certainly to be regarded as a means of defence against wood- boring insects, especially beetles. The many varieties of resin and gum are also of protective nature. We have already had occasion to note (vol. ii, p. 301) that quite a number of animals advertise their disagreeable properties by conspicuous colours or by other means. Such warning colora- tion also appears to be present in certain plants, as, for instance, in some of the poisonous toad-stools, which are of glaring and repulsive appearance. And here also it would seem that, as among animals, cases of Mimicry are to be found, for some harmless toad-stools closely resemble their poisonous brethren, and thus gain some amount of protection. The highly desirable Boletus edulis, for ex- ample, is very liable to be mistaken for its virulently poisonous cousin (2. Satanas). Another set of plants contain aro- matic or fragrant essential oils which, though pleasant enough to our own sense of smell, act as deterrents to many animals. Such are sage, mint, lavender, and many spice-producing forms. There is still another means of defence, and one which is more M- 4. sors ” FBO Fig. 1081.—Pollination of Monkey Musk (Af2#e2lus luteus). 1, External view of flower; 2, same in longitudinal section, with open stigma; 3, ditto with closed stigma. 4, Pollen is deposited on the lower lip of the stigma by a proboscis passing in the direction of the arrow; 5, the stigma has closed, and the proboscis passing on opens the closed anthers and becomes covered with pollen; 6, proboscis being withdrawn in direction of arrow, but does not deposit pollen on the stigma as this is closed. 1, 2, and 3, natural size; 4, 5, and 6, somewhat enlarged. a Foxglove flower, for example, is wonderfully adapted to cross- pollination by humble-bees, but after a time the purple corolla falls off, and in so doing drags the stamens attached to it over the stigma, so that this is self-pollinated, and if it has not already received foreign pollen, the egg-cells will be self-fertilized. Full details of this sort of arrangement will be found in Kerner and other botanical works, but would be out of place here. It is perhaps desirable to add that some authorities believe the im- portance of cross-pollination to have been somewhat exaggerated. DerFENCES OF FLOWERS AGAINST UNBIDDEN GuEsts.—The animals which are most serviceable as agents of cross-pollination are those capable of carrying pollen from one plant to another, though interchange between flowers on the same plant is beneficial to a lesser degree, and is often the chief kind of crossing which takes place in cases where a considerable number of small flowers MESSMATES, MUTUALISTS, AND PARASITES 91 are crowded together. The most specialized plants lay themselves out, so to speak, to attract insects with well-marked powers of flight, or in some cases birds, and it may be mammals. These are the ‘‘bidden guests”. But there are also “ unbidden guests” with a liking for nectar, especially wingless insects, such as ants. In many cases these undesirable visitors simply rob the flowers with- out conferring any benefit. There are also numerous snails and slugs which eat up flowers altogether if they get the chance. It is for all reasons desirable that these useless and dangerous visitors should be kept away. Ogle, in the preface to his translation of a work by Kerner (Unézdden Guests), thus presents the matter :— “Now Nature, who at first sight often appears a prodigal, is always found, on closer examination, to be the most rigid of econo- mists. If no insects are to be allured, she gives . . . no nectar; she cuts off the bright petals and suppresses the attractive odours. Nor even when a bait is wanted will she give it one minute sooner than is necessary. The brilliancy, the scent, and the nectar are only furnished when the flower is ready for its guests and requires their presence; just as a thrifty housewife lights her candles when the first guest is at the door. The mature bud is furnished with no such attractions. Still more, even when the flower is mature, when its pollen is ready for transference or its stigma for pollination, when all the allurements are consequently displayed and insects invited to the feast, she still shows her economy. Guests might come who were not of sufficient importance, and the banquet be wasted on them; for it is only when insects have a certain shape, size, or weight that she requires their visits, and can use them profitably for her purposes. She requires, moreover, that they should make their entrance by the main portal, which she has specially adapted to suit their and her requirements. All insignificant and unremunera- tive visitors, all such, moreover, as would creep in by the back entrance, must be kept out... .” It will be convenient to first describe the chief ways by which wingless insects are kept away from the flowers. One rather curious device is found in some of the Balsams, e.g. in a species native to the Himalayan region (/mfatzens tricornis) which has been carefully studied. Here, as in many other plants, there are little expansions (stipules) at the bases of the leaves, where they join the stem. Of each pair of stipules one is transformed into a nectar-secreting gland (nectary) in the form of a thick curved plate, 92 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE so situated that insects creeping up the stem are sure to find it. As soon as the Balsam begins to open its flowers nectar is abun- dantly secreted by these nectaries, and being both abundant and more accessible than that in the flowers, unwelcome guests are diverted from these. Regarding this sweet fluid and its use Kerner makes the following remarks:—“ The honey-loving ants lick it up eagerly, and are content not to stray farther upwards. Actual observation shows that the flowers of J/mpatiens tricorntis are free from ants, whilst these stipular nectaries are much fre- quented by them. Their pres- ence in. the flowers is very un- desirable, since they could readily get at the honey there without touching the pollen or stigma. And more than this, they would not only pilfer the honey, but they would also drive away those winged insects for which the honey is prepared—the welcome guests that pollinate the flowers. We are justified on the facts in regarding this diversion of the unbidden guests as an indirect protection of the floral honey.” Insects are not infrequently prevented from reaching the flowers by means of a watery barrier. This is obviously so in the case of water-lilies and other aquatic plants, and may also be observed in some land forms. In Teasels, for example, the leaves are arranged in pairs, the bases of each pair being united together to form a sort of cup, in which water collects, so as completely to surround the stem (fig. 1082). Slippery surfaces are often present, on which creeping insects can find no foothold. There is sometimes a smooth coating of wax, as in certain Willows, where the catkin-bearing twigs are thus protected. Another interesting case is that of the Snowdrop Fig. 1082.—A Teasel (Dzpsacus laciniatus), showing water-cups formed by fusion of bases of leaves MESSMATES, MUTUALISTS, AND PARASITES 93 (Galanthus nivalis, fig. 1083). Here the smooth flower-stalk is bent over, so that the flower hangs down, and a creeping insect trying to reach it is almost certain to fall when it reaches the sharp bend. The nectar secreted in the green grooves of the petals is intended for flying insects. If one of these approaches from’ below it will first touch the stigma, effecting polli- nation if it has previously visited another snowdrop. And in getting the nectar it is sure to jolt the stamens, causing a shower of pollen to fall on its back, ready for transfer to other blossoms. A more drastic method of dealing with creeping insects is found in many plants which exude a sticky fluid, espe- _Fig: 1083.—Flower of Snowdrop ; : . (Galanthus nivalis) cially in the neighbourhood of the flowers, or actually upon them. Sometimes this is of the nature of latex, as in the Lettuce (Lactuca sativa), where the flower-heads are surrounded by overlapping scales from which the milky secretion readily escapes, quickly coagulating into a sticky mass that catches and smothers ants and the like. The flower-stalks of Catch-Flies (see p. 86) and various other forms are covered by a glutinous layer, to which the bodies of trespassers are often found ad- hering. But more commonly the secretion is poured out by variously-situated glan- dular hairs. In Plumbago, for instance (fig. 1084), they are borne on the calyx. It would appear that in some instances the captured insects are used as food, after the J fashion of the carnivorous plants already Fig. 1084.—Flower of Plumbago : Europea (enlarged), showing glan- described (see p. 68). dioline Unis on Vhesealpe So far we have dealt with the exclusion of wingless insects, but in the case of large flowers evolved in relation to bees, wasps, butterflies, &c., small-winged insects are equally undesirable visitors, since they steal nectar without effecting cross-pollination. Such forms are altogether excluded, or else made effective by arrangements in ‘the flowers, which Kerner thus describes in general terms (in Zhe Natural Ffistory of Plants):—* Peculiar folds and cushions, walls and gratings, 94 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE brushes and thickets of hairs are present, guarding the entrance and rendering access difficult, whilst still allowing it. Large and powerful animals find these obstacles no hindrance, and readily brush them aside; small ones, however, cannot do this, but have to climb over or circumvent the obstacles. And in many cases this enforced divergence by small insects from the direct path brings about the desired result. For, in circumventing these folds and barricades and hairs, they are unconsciously led past the anthers and stigmas, contact with which is unavoidable. Thus, what would otherwise be useless visitants become welcome guests. They are conducted indirectly to the honey by these curious struc- tures, which may, in a sense, be termed ‘path-finders’.” Cursory examination of such flowers as Foxglove or Pansy will show the presence of barricades of the kind mentioned (see also fig. 1085). Path-finders for the guidance of invited guests are often present in the form of conspicuous colour-streaks, which converge towards the source Fig. 1085.—Section through Flower of a Of nectar. Pansy, Azalea, and Pelar- prontie taht of hair eaiea "= = Gonium are particularly good examples of this. The defences and other arrangements which have been evolved in various connections by plants and animals are never completely successful, and with changed surroundings are apt to fail. This applies not only to ‘mice and men”, but also to flowers. Kerner states, for example, that the flowers of some 300 European plants are systematically robbed by humble-bees, which take a short cut to the nectar by biting through the calyx or corolla. The result may be disastrous, for in some of these plants but few seeds are produced, so that they are becoming rare, and in course of time will probably die out altogether. Certain Alpine Catch-Flies (Szdene Pumito and S. Efizabethe) are in this evil case. Kerner suggests that such plants date back to a time when there were no, or but few, humble-bees in the region where they now grow, and that they have since failed to evolve means of defence against the new kind of attack. Wingless enemies of soft-bodied character, especially snails MESSMATES, MUTUALISTS, AND PARASITES 95 and slugs, are not kept off by smooth surfaces or sticky secretions. But such creatures are easily baffled by prickles, bristles, thorns, and other sharp structures, and these are often found in the neigh- bourhood of the flowers. DISPERSAL OF PLants By ANIMALS.—Since the large majority of plants are fixed, means of dispersal are clearly a necessity, as otherwise they would have to struggle for existence with their own offspring. And it is only when numerous individuals of a species are placed in favourable surroundings that the species has any chance of escaping extinction. It is not therefore surpris- ing to find that there are almost innumerable ways by which dis- persal is effected. Sometimes the plant itself is the agent, sending out creeping stems above or below ground, or ejecting its fruits, seeds, or spores to a distance by explosive or elastic mechanisms. Currents of air and water are also of great importance in this connection. But we are here only concerned with the chief ways in which animals are pressed into the service of plants for this purpose, or it may be render assistance of more casual kind. Many of the small plants which float in ponds, such as Duck- weeds (Lemna) and various algz, must often cling to the legs of water-birds, and get carried bodily from place to place. And it is noticeable that the buds of somewhat larger aquatic plants, such as Frog-bit and Bladderwort ({vdrocharis and Utricularia), possess a slimy covering by means of which they readily adhere to the plumage of such birds. Among marine plants a curious means of transit is exemplified by various sea-weeds which certain crabs plant on their backs to make themselves inconspicuous (see vol. ii, p. 287). On the decease of such a crab his little “‘ garden” goes on growing, unless perchance he has been swallowed whole by some predaceous form. A good many land-plants propagate by means of “ offshoots ”, ze. specialized branches, &c., which grow into new individuals, and cases have been noted where animals assist in the dispersal of such offshoots. Some of the rounded Mexican Cacti (species of Mammillaria), for example, produce little spherical shoots studded with barbed bristles, and which are very readily detached from the parent plant. They readily cling to the coats of various mammals and may thus be carried for a considerable distance. Dispersal of Seeds and Fruits by Animals.—As already explained (p. 85), a seed may be regarded as a matured ovule, 96 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE in which is contained a dormant plantlet, that has resulted from the fertilization of an egg-cell. The fertilizing process stimulates the growth of various parts external to the ovules, leading to the production of what may broadly be called a “fruit”, which for our present purpose may be considered as a seed-carrier. A cherry or plum, for example, is a fruit, within which is a single seed—the “stone”. A long account of the different kinds of fruit would be out of place here, but it may be well to add that many are hard and dry, e.g. hazel-nuts (of which the “kernels” are the seeds), poppy-‘‘ heads”, and the so-called “seeds” of Sun- flower or Carrot. The dispersal of seeds in many plants results from the fact that a considerable number of animals are fruit-eaters. And in such cases the seeds being protected by hard coats often escape digestion. It would appear that the attractive colours and palatable qualities of numerous fruits have been evolved with direct reference to this. While still unripe such fruits are incon- spicuous and more or less nauseous, but become extremely conspicuous by the time they are ready for consumption, thus advertising their desirable properties as articles of diet. Though monkeys and other fruit-eating mammals no doubt largely assist in plant dispersal, birds seem to play a more important part in the matter. Kerner made a large number of experiments which tend to prove this. He found, for example, that the hard-coated seeds of stone-fruits and berries passed quite uninjured through the bodies of ravens and jackdaws; also that the blackbird, song- thrush, rock-thrush, and robin, which eagerly devour fleshy fruits, throw up the seeds if these are large, as in Barberry and Privet. The fate of small seeds swallowed by the last four birds is thus described by him (in Zhe Natural History of Plants):—“ Of the fruits and seeds which passed through the intestine of one or other of these birds, 75 per cent germinated in the case of the blackbird, 85 per cent in the case of the thrush, 88 per cent in the case of the rock-thrush, and 80 per cent in the case of the robin. . . . From these experiments it is evident that the dispersal of edible fruits through the agency of thrushes and blackbirds is not, as was formerly supposed, an exceptional phenomenon obtain- ing in the mistletoe only, but one that may take place in the case of many other plants, and other observations prove that, as a matter of fact, it does take place.” MESSMATES, MUTUALISTS, AND PARASITES 97 Some animals store-up seeds and fruit for future use, and as for various reasons many of these escape being eaten, the storing habit undoubtedly promotes dispersal. Squirrels, jays, and many ants may be cited in illustration. The case of ants is peculiarly interesting. According to Kerner’s observations the seeds which prove attractive to these little creatures are those which, although smooth, possess a little rough outgrowth technically known as a “caruncle”, as in Violet, Greater Celandine, Snowdrop, Peri- winkle, and some Spurges. It is only this caruncle which is eaten, the rest of the seed being left untouched, and capable of germination. Besides the seeds and fruits which specially appeal to the appetites of animals, there are many others which become attached to their bodies, and are thus effectively dispersed. This may take place without any special adaptations to clinging, as in the case of the floating seeds of many aquatic plants, which adhere to the plumage of birds, or where moist earth containing seeds sticks to the feet of birds or other animals. There are, however, a large number of fruits and seeds which are either sticky or else studded 5. oe _-Fruitof Liv. with hooks, their chances of transport by animals 22 éoreatis (xs) stodded with glandular hairs being thus greatly increased. Stickiness results in many cases from exposure to moisture, as in the seeds of Meadow Saffron (Colchicum) which have often been observed adhering to the feet of sheep, cattle, and horses. A somewhat more special- ized case is afforded by fruits which owe their viscidity to the presence of glandular outgrowths, e.g. Linnea borealis (fig. 1086). A firmer means of attachment is found in seeds and fruits provided with hooks, and its efficiency would seem to be proved by the fact that about ten per cent of Flowering Plants are pro- vided with such arrangements. They have apparently been evolved, at least in many cases, in relation to the hairy coats of Mammals, for they are particularly characteristic of plants of low stature, with which such animals are likely to come into contact. Many examples are found among the members of our native flora, as everyone who knows the country must have observed. The little globular fruits of the Goosegrass or Cleavers (Galium aparine, fig. 1087) are studded with little recurved bristles which prove very effective holdfasts, and the “burrs” Vou. IV. 101 98 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE of Burdock (Arctium mazus, fig. 1088) cling with great tenacity to sheep and other animals. Each burr consists of a number of fruits enclosed by a great many narrow scales, each one of which is bent into a hook at its tip, A different but equally effective arrangement is present in Avens (Geum urbanum, fig. 1088). The group of fruits is not surrounded by clinging scales, but each is provided with a long hook. In some foreign fruits the hold- fasts are of formidable character, and cause much pain to the unfortunate animals which unwillingly promote dispersal. A well-known instance is that of the Harpoon-Plant (Har- pagophytum) of South Africa, the large fruits te which are covered with stout radiating pro- with Hooks: a few of the hooks, jections provided with powerful hooks. They are the source of much inconvenience to such animals as antelopes and lions, being said to sometimes cause the death of the latter. Dispersal of Spores by Animals.—Fleshy fungi are eaten by various insects that swallow vast numbers of the minute spores by which such plants propagate, these passing uninjured through their bodies. In some cases flies are attracted by a sweet fluid (as in Ergot, Claviceps purpurea), or by evil-smelling moisture that exudes on the spore-producing surface (as in the Stinkhorn, Phallus impudz- Fig. 1088.—Group of Hooked Fruits of Avens cus). Earth-Worms and other bur- flee ai Or iene sass payee oe soe 0 aoubt help “tp the Fruits of Burdock (Avctivm majus) surrounded isperse the spores of underground by hooked scales. : fungi, such as truffles. The last- named plants are also eagerly sought and devoured by pigs, with similar results. The dissemination by animals of disease-produc- ing bacteria is too notorious to require emphasizing. CHAPTER LXI ASSOCIATION OF ANIMALS—COLONIES Having considered the chief sorts of relation which exist between animals and plants, we have now to deal with the asso- ciation of individual animals, whether of the same or different species. In the sections on Food and Defences (volume ii) one kind of connection has been treated at considerable length, ze. that which links carnivorous (and to some extent omnivorous) forms with their prey, and we have seen that the bodily structure of both attackers and attacked have been more or less perfectly adapted to the exigencies of attack and defence. Another chapter of the same story will engage our attention rather later on, when Animal Parasites receive consideration, but it will be convenient in the meantime to enter into some particulars regarding other kinds of relation. Animals of the same species may be associated together in three chief ways, conveniently described under the headings of Colonial Animals, Social Animals, and Courtship and Mating of Animals. COLONIAL ANIMALS CoLoniaL ANIMALCULES (PRoTOzOA).—The minute and lowly creatures known as Animalcules are distinguished from animals higher in the scale by the fact that they are single cells or units of structure, z.e. they are unicellular. They propagate, as a rule, by splitting (fission) or budding (gemmation), and in a number of species the new individuals which thus come into existence remain connected together, forming a colony (fig. 1089). The members of such a colony are usually all alike, each of them performing all the duties of life for itself, and species for which this is true have therefore been described as “ physiologically unicellular”. Most of them are fixed, as, for instance, in Epistylis 99 100 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE (fig. 1089), which is closely allied to the common Bell-Animalcule (Vorticella), a non-colonial form. And another colonial form (Codosiga), represented in the same figure, also has solitary relatives. In cases where a number of units are associated together it is clearly advantageous that there should be a “ divi- sion of labour”, on similar principles to those which have increased Fig. 1089.—Colonial Animalcules, enlarged to various scales. a, Efistylis; c, c, small individuals conjugating with large ones; d, d, d, d, individuals undergoing fission. 3, Codosiga. c, Proterospongia. the output and improved the quality of manufactured articles. There are certain colonial Animalcules which exhibit an early stage in division of “physiological” labour. Of this an excellent example has already been given (see vol. iii, p. 333) in Volvox, where some members of the colony become specialized in con- nection with propagation by means of eggs. As another instance we may take Proterospongia (fig. 1089), which consists of nu- merous individuals imbedded in a gelatinous substance. Some of these are charged with the duty of securing food for the colony, and the projecting end of each such individual is surrounded by a COLONIAL ANIMALS 101 “collar” from the centre of which springs a thread (flagellum) which executes lashing movements. Within the jelly are other amceba-like individuals, which divide actively, some of the pro- ducts of division serving to increase the size of the colony, while others are probably liberated to found fresh communities. Pro- terospongia is of special interest, as it suggests the way in which Sponges have possibly been evolved from simpler animals, for it is characteristic of Sponges that the spaces within their bodies should be more or less lined with ‘collar cells” that strikingly resemble the collar-provided individuals of the colonial Animalcule just described. All forms higher in the scale than the Protozoa are collectively known as Many-celled Animals or Metazoa. In any one of these, e.g. a Zoophyte, a Worm, or a Mollusc, the body is a more or less complex community of cells, exemplifying in various degree the principle of division of physiological labour, with accom- panying specialization. And there can be little doubt that these cell-communities have been gradually evolved from colonial Pro- tozoa. This view has been discussed to some extent in an earlier section (see vol. iii, p. 333). CotoniaL SponcEs (PorIFERA).—In this group of animals the colonial condition is the rule, a colony being produced by the budding or incomplete fission of an original individual. Some- times the members of a community are fairly distinct (see vol. iii, p. 343), but in other cases it is difficult or even impossible to say where one ends and others begin. The absence of sharp boundary-lines between adjacent individuals is well exemplified by a very common British species, the Crumb-of-bread Sponge (Halichondria panicea), which may be seen as an encrustation of light-brown colour on rocks near low-tide mark. CotontaL ZOOPHYTES (C@LENTERATA), —Vegetative propaga- tion by means of budding or fission is very characteristic of members of this large group, and the buds or fission-products commonly remain united together to form colonies, of which the members are usually clearly marked off from one another. They are united together by what may be termed a “common flesh” (ccenosarc), and their digestive cavities all communicate with a more or less complex system of canals by which this is traversed. It therefore follows that food taken in and digested by one in- dividual may benefit other members of the same community, a 102 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE fact which has had much to do with the course of evolution in certain species. Reference should be made to vol. i, pp. 474-481, where some of the Colonial Zoophytes are figured and described, and also to vol. iii, pp. 327-328, for a brief account of the life- histories of such colonies. We are here only concerned with the characteristic features of colonial life, and it will be convenient to consider separately the two sub-groups of Sea-Flowers (Azthozoa) and Hydroids (//ydrozoa). Colonial Sea-Flowers (Anthozoa or Actinozoa).—To students of the British fauna the most familiar Sea- Flowers are the solitary forms known as Sea-Anemones, which abound on our shores. But in war- (Oa g mer parts of the globe SHR ie en fi Sy ~..,. Sorals are equally A abundant, and these may be either solitary or colonial. The former, or cup-corals, may be compared to anemones, but the lower part of the body is supported by a limy skeleton, while the latter may be re- garded as colonies of cup-corals, and present wide variations in shape, according to the mode of growth. In the majority of cases the members of the colony are all alike (fig. 1090), but this is not invariably the case. For in some of the Eight-rayed Sea-Flowers (Octac- tinia), e.g. the Sea-Pen (Pennatula), some of them are devoid of tentacles, and participate neither in active feeding nor in the production of egg-cells. Their special duty appears to be that of promoting breathing by setting up currents of sea-water which circulate through the fleshy substance of the colony. Ciliary action is the agency employed. [fydrowds (ff1ydrozoa).—The branching or encrusting colonies known as Hydroid Zoophytes exemplify division of labour more or less. As we have elsewhere seen, some of the individuals are specially concerned with egg-propagation, and these may be liberated as little free-swimming jelly-fish or medusz (see vol. iti, Fig. rogo.—Small Colony of a Coral (A strofdes calycularis) COLONIAL ANIMALS 103 ‘ p. 350). This, however, is not the only possibility, as will be seen by reference to fig. 1091, which represents a small part of a species of hydroid (Ag¢aophenza). In addition to the ordinary members of the colony, each provided with mouth and tentacles, there are two kinds of small mouthless individuals. One of these is in the form of a slender thread, which can be stretched out to some little distance, and is possessed of a thickened sticky tip. It acts as a food-catcher, ensnaring small animals to be swallowed and digested by its larger fel- lows for the benefit of the community. The other kind of mouthless indivi- dual is somewhat stouter, and richly provided at its free end with “ batteries” of stinging-cells, capable of dealing effectively with larger prey, or warding off the attacks of enemies. When these fighting in- dividuals ate called into #, Ordinary individual; 4, c, food-catchers, between them another action, the other members is seen capturing a crustacean larva, d, fighting individual; é, diges- : tive cavity of colony; _/, outer layer of body; g, horny investment. of the colony can be with- drawn into the little cups that surround their bases, being thus out of harm’s way. An extreme case of division of labour is presented by the free- swimming colonies of Hydroids known as Compound Jelly-Fish (Siphonophora), which have probably been evolved from simple medusz by a process of budding (see vol. i, p. 481). The shape of the colony depends upon the way in which this process has been effected. Sometimes the buds have arisen from the “umbrella” of the original medusa, or they may have grown from the walls of the mouth-bearing “handle”. The chief kinds of individual that have been thus produced are represented diagrammatically in fig. 1092. The umbrella of the original medusa loses its function as a swimming organ and becomes a float, while (in the case repre- sented) the handle, of which part only is shown, carries a variety of members which contribute in various ways to the common weal of the community. Some are swimming-bells which, by alternately opening and closing, effect propulsion through the water. Others Fig. 1091.—Small part of a Colony of Agdaophenia, enlarged 104 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE are transformed into fishing-lines that catch food, and at the same time are so well provided with stinging-cells as to effectively keep off enemies. There are also digestive individuals, which devour and digest the animals caught by the fishing-lines. Some mem- bers are reduced to tentacles, ministering to the sense of touch (and possibly smell); others again are in the form of protective plates, covering and sheltering adjacent in- dividuals. And there are also egg-produc- ing members of the colony which may be liberated as little meduse, thus promoting dispersal. In different species there are considerable variations in detail, and the actual arrangements in one case (Stephalia corona) are shown in fig. 1093. CotoniaL Moss- Potyres (PoLyzoa). —With a single exception the mem- bers of this group produce colonies by budding, after the fashion of Hydroid _Fig. 1og2.—Diagram of various in Z oophytes, for dividuals in a colony of Compound : P Jelly-Fish (Siphonophora), central which they are Fig. 1093.—A Compound Jelly- Fish cavity of colony indicated in black. E : (Stephatia corona), reduced. 2, Float; Ft, Float; s, swimming-bell; 2, 2, SOMetimes MIS- 5, swimming-bell; g, gas-conducting indi- Ggetve divin Z2s rnc taken, though in Winticsmaldueaeindiul ma aula Acai ome fo reality much higher ic," ° ml Meme in the scale. Some of the members of such a colony may be greatly specialized for various purposes (fig. 1094), e.g. they may be modified into rounded receptacles (ovicells) in which the eggs develop till the time of hatching. In certain species there are bird’s-head individuals, which execute vigorous snapping movements, the object of which is extremely doubtful. Cleanliness is possibly promoted, or per- haps the attacks of small parasitic forms may thus be warded off. It has also been noticed that the powerful jaws often succeed in catching little worms, crustaceans, &c., apparently holding them tenaciously till they die and decompose. The suggestion has been made that the decayed fragments of these victims are carried by COLONIAL ANIMALS 105 ciliary action into the mouths of the unmodified members of the colony, thus serving as food. But there is no definite proof that such is the case. Individual Moss-Polypes may also undergo still greater modification into whip-like threads that actively lash about in all directions. Cleanliness and defence have here again been suggested as the ends to be served, and cases have been observed where the action is so vig- orous as to move the entire colony about. That the surrounding water should be thoroughly stirred up is probably advantageous with refer- ence both to feeding and breathing. The only thing, however, that we definitely know about these curious structures is, that they have been evolved from bird’s-head individuals by suppres- sion of the “head”, and prolongation of the “lower jaw ” into a slender filament. CotontaL Tunicates (UrocHorpa).—The formation of colonies is clearly related to powers of increase by means of budding or fission, and consequently all the members of certain animal groups devoid of such powers, e.g. Arthropods and Molluscs, are non-colonial. This is also true for the vast majority of Backboned Animals, the most notable exception being afforded by many species of the lowly and degenerate forms known as Sea-Squirts, Tunicates, or Ascidians. Most of these are fixed to some firm object when _ Fig. 1094.—Parts of Col- adult, and their sedentary life has no doubt had i’ pay na 3 much to do with the degeneration they have ieee a perenes undergone (see vol. iii, p. 421). A good many red m lemscssof a0 or Tunicates are non-colonial or “solitary”, but he i re others bud to produce colonies of various shape. In such species the individual members may be borne on a creep- ing stem and clearly marked off from one another, much as in a hydroid zoophyte, or the association may be much more intimate. In the latter case the individuals are sunk within a sort of common body (like the ccenosarc of colonial corals), and there is a contin- uous protective investment or common test. A good instance is afforded by Botryllus (fig. 1095), to be found at low tide on our coasts as a sort of bluish encrustation on sea-weeds and stones. It 106 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE presents at regular intervals a number of flower-like markings, each of which is made up of a circlet of individuals, with their mouths near the tips of the “petals”. A small hole in the centre of the flower leads out of a cavity (‘‘common cloaca”) into which are discharged the waste products of the members of the group. The surface population of the sea is also partly made up of colonial Tunicates. The Salps, for instance, present two stages in their life-history, one of which propagates by budding, the other by eggs (see vol. iii, p. 422). A large number of the latter stage are connected to- gether when young into ‘‘chains ”, which may be regarded as temporary colonies. These ultimately break up into their constituent members. A notable example of a permanent free- swimming Tunicate colony is afforded by the ‘Fire - Cylinder (Pyrosoma, fig. 1095), Fig. 1095.—a, Fire-Cylinder (Pyrosoma) in side view, the small rounded . . areas are the mouths of members of the colony. 3B, Open end of same. abundant in the Medi- q Sell ley of Botryllus, showing circlets of individuals. », Two ci torranean and else- where, and giving off a bright phosphorescent light, possibly as a means of protection. The colony is shaped like a hollow cylinder which, in one large species (Pyrvosoma gigantea), may be as much as 5 feet long, and possesses a contracted aperture at one end, the other being closed. The external surface is covered with pointed projections of the firm test. The small but very numerous individuals are imbedded transversely in the wall of the cylinder, their mouths being ex- ternal. The large central cavity receives all the products of waste, and is comparable to the common cloaca of a Botryllus circlet. The size of the colony is augmented by budding, and eggs are also produced, which develop into minute colonies that are liberated into the surrounding water, there to grow to their full size. CHAPTER LXII ASSOCIATION OF ORGANISMS—SOCIAL BACKBONELESS ANIMALS (INVERTEBRATA). . Many. animals are social or gregarious, and in such cases division of labour between the members of a community is more or less perfectly exemplified. The mere fact that many individuals of the same species live together in the same place does not entitle them to be termed social, in the sense here intended, unless there is some sort of co-operation which benefits the animals living together. It would hardly be justifiable, for example, to describe oysters, cockles, or star-fishes, as colonial animals. But even here the species may be benefited, e.g. weakly individuals are weeded out in the keen struggle for existence, so that the stock becomes increasingly healthy and strong. And from such casual kinds of association communities of very complex kind have gradually been evolved, the benefits to be derived from division of labour between individuals giving various species a greatly improved chance of survival in the competition with other species. But here a qualification must be made. For in the world of organisms, by the irony of fate, an extreme penalty attaches to elaborate special- ization resulting from adjustment to the exigencies of a certain set of conditions. The surroundings of animals (and plants) are con- stantly changing, and if these alter suddenly, as they are liable to do, a well adapted species may be unable to adjust itself with sufficient rapidity to the new order of things, and hence be doomed to extinction, while simpler but more plastic forms may survive. Many lowly organisms have endured through countless ages, while others of more complex kind have quickly succumbed to rapid alterations of their environment at a time when their continued dominance seemed most certain. Innumerable in- stances of this far-reaching principle are to be drawn from the geological record, which preserves for us the past history of the globe. 107 108 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE The most interesting cases of the social habit are to be found among Insects and Backboned Animals, which it will be well to consider separately. SOCIAL INSECTS (INSECTA) Some of the most remarkable facts in natural history have been made known by those who have studied the complex com- munities existing among various species of Membrane- Winged Insects (Hymenoptera) and Net-winged Insects (Neuroptera). The extent to which division of labour is carried varies greatly in different cases, so that we are able to get some notion of the successive stages which have marked the evolution of the social habit. SoctaL Memprane-WINGED Insects (HyMENopTERA).— The most salient point distinguishing highly organized communities of Bees, Wasps, and Ants is the presence of a varying number of “castes” or kinds of individual. In the Honey-Bee (Ages medli- fica), for instance, a hive contains not only males (drones) and an egg-producing female (queen), but also numerous “ workers”, which are a second kind of female, having nothing to do with the production of eggs, but, as their name indicates, labouring for the benefit of the republic. And in other cases there may be more than three castes, as we shall see in the sequel. Worker honey- bees differ markedly in size and structure from the queen, as the result of a long course of evolution, and it will be desirable to begin with simpler communities, where such sharp distinctions do not exist. Bres.—Some account has already been given of Carpenter- Bees, Mason-Bees, and Leaf-cutting Bees, solitary forms in which the female not only lays eggs but also makes and stores a nest (see vol. iii, p. 390). These and many other non-colonial insects exhibit very elaborate adaptations to their surroundings, and it would be a mistake to consider them as necessarily lower in the scale than colonial forms, which have evolved on entirely different lines. Here, as in all other cases, success in the struggle for existence may be attained in widely different ways. From the purely solitary life led by the bees above-mentioned, we pass to a curious case described by Fabre (from his observa- tions in South France), where a certain amount of co-operation is SOCIAL INSECTS 109 associated with a large amount of independence. The case is that of certain rather small “short-tongued ” bees (species of Hadictus), which are represented in the British fauna. There are here no workers, but by the united labours of a number of females a branching underground passage is dug out at night, there being a single opening to the exterior, and close to this an enlargement or “hall” for the greater convenience of individuals wishing to pass one another. Within this underground home each female makes her own particular nest, consisting of ovoid waterproof cells, and attends to her domestic duties after the fashion of solitary species. A sentinel is said to be posted at the common opening of the burrow, so that some understanding would seem to be arrived at in the matter of mounting and relieving guard. But, apart from this, the individuals living together have no more social organization than the different families occupying a dwelling made up of a set of ‘‘flats”, who use a common stair and the same street door. If the said families had constructed these by their joint efforts the analogy would be more complete. The last example is a sort of side-branch in social evolution; for a comparatively simple case in the direct series of adaptations we may turn to the large insects familiarly known as Humble-Bees (species of Bombus), which are well represented in our own country, and live above or under the ground in communities which endure for a single year only. They exemplify the beginning of the caste-system, for in addition to males there are three varieties of the opposite sex, z.e. queens, small females, and workers, which in appearance and structure resemble one another pretty closely. We do not find the same marked differ- ences that exist between the queen and worker in honey-bees, while the power of egg-laying is not restricted to the queen, though she is the mother of most of the members of the com- munity. The habits of several species have been closely observed, and the succession of events is somewhat as follows. A queen which has survived the winter begins her work as foundress of a society when the spring is well advanced, and food in the form of nectar and pollen is abundant. Selecting a sheltered spot, on the surface or below the ground according to the species, she successively constructs two or three large waxen cells, the material for which is derived (as in social bees generally) from a number of small glands that open on the under side of her abdomen: a Ilo ASSOCIATION OF ORGANISMS—THE WEB OF LIFE proportion of pollen being added. When a cell is of full size it is lined with a mixture of pollen and honey, several eggs are laid in it, and a roof is put on. After several days’ rest the next cell is made, and stocked in the same way. About the time that the second cell is completed duties of another kind are added to the tale of work. For, meanwhile, the eggs first laid have hatched out, and thé bee-grubs, having exhausted their scanty store of provisions, require feeding. To do this their mother bites a hole in the enclosing cell, and supplies honey from her mouth as required. Here, and in other cases, the ‘‘honey” is not the same thing as the “nectar” found in flowers. A bee swallows the latter, taking it into a crop or “honey-bag”, into which the gullet dilates. Within this receptacle it undergoes a kind of fermentation by which it is converted into honey. So far the life-history is much like that of a solitary form, all the work being done by the mother. But in the next stage division of labour begins to play an important part. The full-fed grubs spin silken cocoons, and pass into the quiescent or pupa stage, from which they emerge as “workers”. By gnawing away the wax the queen assists their escape from the enclosing cell. As workers become numerous they justify their name by under- taking the labours of building and storing, ultimately enabling the queen to devote herself entirely to egg-laying. For each egg a separate cell is constructed. As the community increases in size small females may be produced, and towards autumn larger “drone cells” are made, and still larger “queen cells”. It is stated that these are not stored with food, the corresponding grubs being from the first assiduously nursed by the workers. By the time that drones and queens are mature the community has attained its full size, and may consist of from 300 to 400 individuals, under favourable circumstances. The pairing of the young queens in the course of a nuptial flight constitutes the climax of the year’s drama, for as winter approaches the temporary community becomes disintegrated. All the workers and drones perish, together with many of the queens, but some of the latter live through the winter in a torpid state to found fresh societies the following spring. It should be added to this account that when the community is in full working order special unclosed cells are made, to be stored with honey or pollen for general use. These “honey tubs” and “pollen tubs” serve as a larder, which SOCIAL INSECTS III is constantly being replenished during fine weather, to be drawn upon when it is wet. Old brood-cells may be enlarged for the same purpose, but are never put to their original use a second time. For one species of Humble-Bee (Boméus ruderatus) a remark- able arrangement has been described. It is said that in every nest one bee is told off as a “trumpeter”. This individual sounds veverllé at from 3 to 4 a.m., rousing her fellows to the labours of the day, and if removed is replaced by another. The habit of storing food, existing to some extent in Humble-Bees, is carried much further in the Honey-Bee (Afzs mellifica) and its numerous relatives, and has probably had much to do with the evolution of the complex social life which these exhibit. It enables a community to live on through the unfavourable season of the year, thus becoming permanent, and this continuity has rendered possible division of labour to a greater degree, being at the same time associated with well-marked differences between the castes, so far as queen and workers are concerned. The former is of comparatively large size, and her only duty is to produce eggs, while the varied labours of the hive fall to the lot of her smaller sisters. The community is only temporary as regards the drones, none of which survive the winter, but are replaced by a fresh set which hatch out the following year. A few further details regarding the Honey-Bee will be given in a later section. The Social Wasps (Vesféde) live in communities which, so far as at present known, exist for one season only, as in Humble- Bees, to which they present a further resemblance in the fact that the workers are not markedly unlike the queen, and are more or less capable of laying eggs. The building-material, however, is not wax but a sort of paper, made by chewing woody matter and mixing it with a fluid secreted by certain glands of the mouth-region. We may take to illustrate the annual cycle one of three British species (Vespa Germanica, fig. 1096) in which the nest is constructed underground. The foundress queen begins work in spring, making a small number of cells in the place which is to be the top of the nest, and depositing an egg in each. The cells are neither stored nor closed. Her next task consists in feeding the grubs as they hatch out, first with honey or fruit- juice, and later with the bodies of insects, especially flies. By 112 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE means of her strong jaws she removes the hard parts of the prey, chewing up the rest into a kind of mince-meat adapted to the tender digestions of her offspring. When they have reached their full size the grubs spin cocoons and pass into the pupa stage, from which they emerge as worker wasps about four weeks after the date when the eggs were laid. Without loss of time these take over the work of building and nursing, and even feed their mother, who soon has noth- ing to do but lay eggs in the cells as they are con- structed. The complete nest consists of a series of combs, connected by little pillars, and the building operations are carried on from above downwards. Each comb is made up of a large number of roughly hexagonal cells, the mouths of which are directed down- wards. Towards the end of summer cells of larger size are made, in which queens and drones are Fig. 1096.—Section through Nest of a Social Wasp (Vespa reared, but many of the Germanica), rather less than 14 natural size ‘ e.g., Entrance gallery; s.g., side galleries; 1-7, combs connected latter oe brought Wp ia the by pillars (7 above main figure shows arrangement of the three ordinary small cells. After cells of the youngest comb); exv., papery envelope of nest; 7, root é to which first foundation is attached (other roots with secondary Mating has taken place the sesohnet eit a.n., part of an ant’s nest; 77 2, larve of community is soon broken up; most of the insects die, but some of the queens survive the winter, and found the com- munities of the following year. Wasps appear to be extremely sensitive to cold, and it is perhaps partly for this reason that the nest is surrounded with a covering made up of layers of paper (see fig. 1096). The nests of many species of social Wasp are suspended from plants, while the Hornet (Vespa crabro) prefers to build in a hollow tree. There is a large amount of variation as to size, shape, and durability, while in some cases earth is added to the ordinary building material. SOCIAL INSECTS 113 The social habits of Ants are even more complex than those of Bees and Wasps, and some account has already been given of the way in which the members of certain species procure and store food (see vol. ii, pp. 103 and 206). There is no more fascinating department in the whole realm of natural history than the study of ant life, for these little creatures live in a wonder- land which is all their own. The elaboration of some of their communities is very considerable, and the welfare of the individual is rigorously subordinated to the interests of the species. Some of the more salient points are thus ably summarized by Sharp (in The Cambridge Natural ffistory):—‘ Observation has revealed most remarkable phenomena in the lives of these insects. Indeed, we can scarcely avoid the conclusion that they have acquired in many respects the art of living together in societies more perfectly than our own species has, and that they have anticipated us in the acquisition of some of the industries and arts that greatly facilitate social life. The lives of individual ants extend over a considerable number of years—in the case of certain species at any rate,—so that the competence of the individual may be developed to a considerable extent by exercise; and one genera- tion may communicate to a younger one by example the arts of living by which it has itself profited. The prolonged life of ants, their existence in the perfect state at all seasons, and the highly social life they lead are facts of the greatest biological importance, and are those that we should expect to be accompanied by greater and wider competence than is usually exhibited by Insects. There can indeed be little doubt that ants are really not only the ‘highest’ structurally or mechanically of all insects, but also the most efficient. There is an American saying that the ant is ruler of Brazil. We must add a word of qualification; the competence of the ant is not like that of man. It is devoted to the welfare of the species rather than to that of the individual, which is, as it were, sacrificed or specialized for the benefit of the community. The distinctions between the sexes in their powers or capacities are astonishing, and those between the various forms of one sex are also great. The difference between different species is extreme; we have, in fact, the most imperfect forms of social evolution coexisting, even locally, with the most evolute. These facts render it extremely difficult for us to appreciate the ant; the limitations of efficiency displayed by the individual being in some VoL. IV. 102 114 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE cases extreme, while observation seems to elicit contradictory facts. About two thousand species are already known, and it is pretty certain that the number will reach at least five thousand.” The caste-system is well-marked among Ants, and there are at least three sorts of individual, males, queens, and workers or modified females. The two former are commonly winged, and pairing usually takes place in the air. After this has been accom- plished the males soon die, while the females cast, or it may be bite off, their wings, and enter upon their further duties. In some species the male alone possesses wings, while it more rarely happens that the contrary is true. Cases are also known where both kinds of male exist in the same species, the female being winged, or both kinds of female are associated with winged males. There may also be distinct castes of workers, one consisting of large individuals (workers major), and the other or others of small ones (workers minor). In many cases, too, there is a caste of “soldiers” distinguished by the great size of their mandibles. Like workers they are modified females. To all intents and pur- poses, indeed, the societies of Ants, like those of Bees and Wasps, are female republics. The “queen”, it is true, has all her wants attended to by the workers, but does not actively direct the affairs of the community, having no special authority. The care bestowed upon her, indeed, would appear to be simply in recognition of the fact that she is necessary for the continuance of the society. In ant-societies there may be more than one queen. There appears to be no doubt that these insects are able to communicate certain kinds of information to one another. Indeed, without some power of communication there would be endless confusion in a large com- munity. As it is, we find that foraging expeditions, warfare, and the complex economy of the nest are all carried out in an orderly fashion. In human societies even republics require some sort of government for the direction of individual efforts, but this often appears not to be the case here. The armies of our native species, for example, so far as we know, are entirely made up of rank and file, without officers and non-commissioned officers. Yet such an army often seems to conduct its campaigns strategically, and deals very effectively with tactical problems which arise after it has taken the field. How this is possible we are not yet able to say, for our own mental powers have been evolved on very different lines. There is certainly a basis of instinct, z.e. inherited SOCIAL INSECTS II5 capability of doing certain things on impulse when appropriate occasions present themselves; but since individual ants profit largely by experience, we may also say without hesitation that many of their actions are intelligent. There can be little doubt that young workers receive a practical education in their duties, learning by example if not by precept. If so, we have a very convincing proof of marked intelligence. “Where, as in some tropical ants, there are numerous castes, the mental life of the community is probably more complex, but comparatively few observations have been made on this difficult subject. The early stages in the formation of societies have been ob- served in some species, and are probably substantially the same in all. A foundress queen lays her first batch of eggs, and carefully tends the larvee when they hatch out, until they pass into the pupa stage, from which they emerge as workers, who at once concern themselves with the industrial work of the young community. The queen is therefore soon able, as in the ordinary social wasps and bees, to restrict herself solely to the duty of egg-laying. One important point in the domestic economy of all ant-societies may here be mentioned. Special cells of paper or wax are not con- structed for the reception of eggs, as in Bees and Wasps, but these are deposited in chambers, variously situated, according to the species. It is further to be noted that the larva may or may not spin a cocoon before passing into the pupa state. When a cocoon is made it is removed by the workers at the proper time, so as to facilitate the escape of the perfect insect. There is a large amount of variation as to the number of individuals contained in an ant-society. This is very large in most of the kinds which have been carefully studied, and it is naturally so in cases where the social life is very complex. Simple instances are afforded by some of the Indian Ants (species of Polyrhachis), where a single queen and less than a dozen workers live together in a little one-chambered dwelling that looks almost like a minia- ture bird’s-nest, and is constructed of a papery substance with a lining of silk. These small homes are found on leaves, and are commonly so placed or made as to be inconspicuous. Another sort of Asiatic Ant (Zcophylla smaragdina) lives in larger com- munities upon foliage, of which the leaves are converted into dwellings in a very remarkable manner. The workers roll them up and fix their edges together by means of a viscid fluid derived 116 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE from the silk-glands opening near the mouths of the larve. A worker engaged in this task holds a larva firmly in her jaws, and holds it to the required spot, using it in fact as a living gum-bottle. Some of the leading features in the communal life of a large society of the industrious insects under discussion may be learnt from the study of our largest native species, the reddish-coloured Wood-Ant or Horse-Ant (Formica rufa). It abounds in the fir- woods of our southern counties, where the large “ant-hills” which it constructs are conspicuous objects. The winged males and females are not far short of half an inch in length, and there are two kinds of worker, which are respectively about one-fourth, and from one-fifth to one-sixth of an inch long. The nest may be nearly three feet high and some eighteen feet round at its base, and is made up of fir-needles, together with all sorts of plant fragments. The vicinity of the nest is trodden down into a number of ‘“ant-roads”, which are the scene of much busy going and coming. The larger workers are principally concerned, when outside the nest, with collecting building materials, while an important duty of the smaller workers is to collect the “honey- dew” of aphides, insects which are often picturesquely described as “ant-cows”. The substance in question is a sugary fluid that exudes in considerable amount from the intestines of these little creatures, and is eagerly swallowed by the workers, a great deal of it passing into their dilated crops. Having filled themselves up with this desirable food, the workers hurry back to the nest, and obligingly distribute some of their store for the benefit of the larve, and their adult friends who have meanwhile been engaged with the internal economy of the nest. There are no special receptacles corresponding to the honey-tubs of humble-bees or comb-cells of ordinary bees, for storage of what is not immediately needed. Indeed none of our native ants indulge in the luxury of a larder, and remain in a torpid condition during the winter. The food is by no means limited to honey-dew, but is of very mixed nature, for caterpillars, various adult insects, and miscellaneous vegetable matter all figure in the bill of fare. There is a constant return of foraging parties to the central home (fig. 1097). The ant-hill is literally riddled with labyrinthine galleries expanding at intervals into rounded chambers, and for some depth the underlying ground is mined with passages continuous with those above. It is easier to destroy an ant-hill than to get any SOCIAL INSECTS 117 clear idea of its internal economy, but J. G. Wood (in Susects at Home) thus describes a very ingenious device by which he was enabled to gain some knowledge of the kind:—“I have, however, succeeded in obtaining an excellent view into the interior of a Wood-Ants’ nest, though it was but a short one. Accompanied by my friend Mr. H. J. B. Hancock, I was visiting some remark- ably fine Wood-Ants’ nests near Bagshot. We took with us a large piece of plate-glass, placed it edgewise on the top of an ant- hill, and, standing one at each side, cut the nest completely in two, i oe me leaving the glass almost wholly buried in it. After the expiration of a few weeks, during which time the Ants could repair damages, we returned to the spot, and, with a spade, removed one side of the nest as far as the glass, which then served as a window through which we could look into the nest. It was really a wonderful sight. The ant-hill was honey-combed into passages and cells, in all of which the inhabitants were hurriedly running about, being alarmed at the unwonted admission of light into their dwellings. In some of the chambers the pupe were treasured, and these chambers were continually entered by Ants, which picked up the helpless pupz and carried them to other parts of the nest where the unwelcome light had not shown itself. Un- fortunately, this view lasted only a short time.” 118 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE The most important and arduous duty of the workers is to look after the eggs, larvae, and pups, and these are distributed through the nest with due regard to variations of moisture and temperature, since both of these affect development. The queens are carefully tended, and their eggs are carried off to suitable chambers. From time to time these are carefully licked, and it is also said that they are smeared with nutritious fluid that is absorbed by the embryos. When the larve hatch out they are fed with great assiduity and their toilet requirements attended to. The full-grown larve spin cocoons, within which they become pupe (the so-called ‘ants’ eggs”), which also receive unremitting attention. The workers bite away the enclosing cocoons when the perfect insects are ready to come out. Some of them are workers, others winged males and females which fly about in swarms. After mating, the large majority of the swarming in- dividuals perish, but some of the females survive to found fresh communities, or sometimes to be taken into existing nests. The stings of Wood-Ants are not sufficiently well developed to be of use, but their poison-bags contain formic acid, which can be squirted to a considerable distance, and is an effective defence. This particular acid, as its name indicates (L. formzca, an ant), was first known as a product of insect-life. The strong mandibles of the workers are also weapons of no despicable character. These ants co-operate for offence and defence, and Lord Avebury (in Ants, Bees, and |Wasps) thus describes their tactics, and those of a related species:—‘formica rufa, the common Horse Ant, attacks in serried masses, seldom sending out detachments, while single ants scarcely ever make individual attacks. They rarely pursue a flying foe, but give no quarter, killing as many enemies as pos- sible, and never hesitating, with this object, to sacrifice themselves for the common good. Formica exsecta is a delicate, but very active, species. They also advance in serried masses, but in close quarters they bite right and left, dancing about to avoid being bitten themselves. When fighting with larger species they spring on to their backs, and then seize them by the neck or by an antenna. They also have the instinct of acting together, three or four seizing an enemy at once, and then pulling different ways, so that she on her part cannot get at any one of her foes. One of them then jumps on her back and cuts, or rather saws, off her head. In battles between this ant and the much SOCIAL INSECTS 119 larger /. pratensis, many of the /. exsectas may be seen on the backs of the 7. pratensis, sawing off their heads from behind.” Such practices would be greatly deprecated in human warfare. Some of the most remarkable features in ant-life have refer- ence to the use they make of aphides (fig. 1098), and some species, instead of merely sallying forth to collect honey-dew, in the way described above for the Wood-Ant, have advanced to the pastoral stage of social life, and may be described as cattle-keepers. This is well illustrated by our native species of Lasius. The common little Black Garden-Ant (Zaszus niger), which lives in elaborate underground dwellings, is particularly partial to aphides which live on twigs and leaves, moving them to convenient places for “ milk- ing” operations, and carrying their eggs into its sheltered home for the inclement winter season. The small Yellow Ant (Laszus flavus), another underground species, goes further than this, for Lord Avebury states that four or five distinct kinds of aphis are found in some numbers Fig. 1098—Ant (Myrmica rubra} “ Milking” % 7 . : an Aphis (Axis sambuci) in its nest, belonging to root-feeding species. The same observer made some most interesting obser- vations (on captive specimens) of the way in which (like Z. xzger) these ants tend another sort of aphis, which is not a root-feeder, and he gives the following summary of the facts (in Ants, Bees, and Wasps):—‘ Here are aphides, not living in the ants’ nests, but outside, on the leaf-stalks of plants. The eggs are laid early in October on the food-plant of the insect. They are of no direct use to the ants, yet they are not left where they are laid, exposed to the severity of the weather and to innumerable dangers, but brought into the nests by the ants, and tended by them with the utmost care through the long winter months until the follow- ing March, when the young ones are brought out and again placed on the young shoots of the daisy. This seems to me a most remarkable case of prudence. Our ants may not perhaps lay up food for the winter; but they do more, for they keep during six months the eggs which will enable them to procure food during the following summer, a case of prudence unexampled in the animal kingdom.” It should be added that after carrying the young aphides to the appropriate food-plant the ants wall them in with earth, and the enclosures thus made may be almost 120 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE called “ cattle-pens.” Some remarks will be made in a subsequent chapter on ants as slave-owners, and on the beetles, &c., which live in their nests. The caste-system is carried to an extreme in one of the common Foraging-Ants (Zczton hamata, see vol. ii, p. 104) of Tropical America, a carnivorous species which possesses a power- ful sting. Besides winged males and wingless females there are “soldiers” with enormous jaws, large workers, and two sizes of small worker. These ants and those of allied species have no permanent abode, but wander about from place to place after the fashion of armies. After carrying on offensive operations for some time they construct temporary quarters, where they cultivate the domestic virtues and bring up their offspring. Belt gives the following account of ants of the sort in regard to this matter (in A Naturalist in Nuaragua):—“ They make their temporary habitations in hollow trees and sometimes underneath large fallen trunks that offer suitable hollows. A nest that I came across in the latter situation was open at one side. The ants were clus- tered together in a dense mass, like a great swarm of bees, hanging from the roof, but reaching to the ground below. Their innumerable long legs looked like brown threads binding together the mass, which must have been at least a cubic yard in bulk, and contained hundreds of thousands of individuals, although many columns were outside, some bringing in the pupz of ants, others the legs and dissected bodies of various insects. I was surprised to see in this living nest tubular passages leading down to the centre of the mass, kept open, just as if it had been formed of inorganic materials. Down these holes the ants who were bringing in booty passed with their prey. I thrust a long stick down to the centre of the cluster, and brought out clinging to it many ants holding larve and pupe.” Ants as agriculturalists and mushroom-growers have been dealt with in an earlier section (see vol. 1, p. 207). Soca Net-Wincep Insects (NEUROPTERA).—The interesting social insects known as Termites live in complex communities somewhat resembling those of Ants, with which, under the name of ‘White Ants”, these forms are often confounded. The resem- blance, however, is very superficial, while the differences are pro- found. Termites are not invested in strong plate-armour, their exo-skeleton being comparatively thin, nor do they possess poison- SOCIAL INSECTS 121 glands or stings. But the jaws of the individuals which do the work of the community are very powerful, as in Ants. Most species shun the light and are pale in colour (hence the name “White” Ants), and in such cases only the kings and queens possess eyes, the other castes being blind. There are, however, leaf-cutting Termites in South Africa which move about in open daylight. Eyes are here present in all castes. There is no marked metamorphosis, for the young do not hatch out of the egg as helpless grubs, but as active nymphs, which attain their full size after several moults. The number of castes varies greatly in the different species, and matters are complicated by the presence of nymphs in various stages of development. But in all cases which have been investigated the just-hatched nymphs are to all appearance alike, and it is probable that their subse- quent fate depends upon the nature of the food, the matter being more or less regulated by the mature inhabitants of the nest. The same thing is, indeed, largely true for social Bees and Wasps. In the Honey-Bee, for example, the grubs destined to become queens are fed differently from their fellows. Only the fully mature queens and kings (as the full-blown males are here usually termed) are provided with wings, both pairs being of equal size, the arrangement of veins being quite unlike that characteristic of Bees and Wasps, as might be antici- pated from the fact that Termites belong to an entirely different order. Near the base of each wing there is a weak place, facili- tating detachment after the first and only flight has taken place. Queens and kings swarm from the nest much as in Ants, and associate themselves in pairs. The vast majority fall a victim to insectivorous birds and other animals, but enough survive to secure the formation of fresh societies, at least in some cases. Nor do we find that speedy death is the necessary sequel to mating for a king termite, as in the case of drone bees, for a nest commonly contains, in some species at any rate, a royal couple, both of whom are carefully tended for the term of their natural lives. Of other kinds of individual, soldiers are always found and generally workers, some of both these castes being modified females and others modified males. A Termite society is not, like those of Bees and Wasps, a female republic. The only two known species of European Termite have been carefully studied, in Sicily, by Grassi and Sandias, whose chief 122 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE results will be briefly summarized. The societies of some of the African forms are still more complicated, but here our know- ledge is in many respects very incomplete, though it may prove interesting to give a few details. Rather more than 100 species of Termite have been so far described, and these are probably only a tithe of those which actually exist. They abound in the tropical and warmer temperate regions of the world. The Yellow-Necked Termite (Calotermes flavicollis) of the Mediterranean littoral is of peculiar interest, for its communi- ties are small (under 1000 individuals), and the habits are com- paratively simple. The home is a hollow within a dead or decaying tree, and the architectural operations are limited to increasing the size of the hollow as may be necessary, and making partitions or the like with waste matter ejected from the intestine, saliva being employed as a cement. Within this simple home are found a king and queen, together with a number of soldiers and nymphs. There are no workers. The soldiers are distinguished, as in Termites generally, by the possession of huge heads and formidable jaws. The habits as regards food are somewhat remarkable, and promote sanitation of the nest in an unusual degree. Wood is the staple diet, but it is a substance very difficult of digestion, and the pellets which are voided from the intestine are eaten again and again, until their nutritive properties are exhausted, when they are either employed as building materials, heaped up in remote parts of the nest, or dropped outside. Partly digested food may also be ejected from the crop, suggesting the arrangement found in other social insects as regards sweet substances. The salivary secretion is also highly nutritious, and not a mere digestive juice. All the cast skins are used as food, and burial rites are simple, the bodies of deceased friends augmenting the bill of fare. The young nymphs are fed at first on saliva, from which they are promoted to material ejected from the crop and intestinal pellets, wood pure and simple being eaten more or less at a still later stage. A grim sort of fate attends the soldiers, for their huge jaws would appear to cut them off from the most abundant items in the dietary, and they are driven to cannibalism. Not only do they devour the dead, but shorten the sufferings of the sick and dying by eating them alive. It is supposed that they are in a state of permanent hunger, and SOCIAL INSECTS 123 may be well excused for sometimes doing a little private slaughter among their healthy relatives, as they are said to be apt to do when excited. It is on the whole a good thing that human social life has evolved on rather different lines, in spite of the horrors of war and other matters which an intelligent Termite would deprecate. Winged queens and kings swarm from the nest at certain seasons of the year, and any pair fortunate enough to escape the appetites of birds or other foes is capable of starting a fresh society. The Light-shunning Termite (Termes lucifugus) is the second and only other European species. realize the advan- below which is seen a frame of comb. tages of a machine which enables us to give them empty comb, and thus save them the labour of comb-building ”. To supply comb-foundation is the next best thing to giving empty combs. This consists of thin plates of wax, which have been passed between suitably-embossed rollers, so that the “foundations” of the cells are laid, and there are also projecting ridges of wax, furnishing enough material for the completion of the cells, save that required to cover them. Foundation is made with either small cells suitable for worker-brood (or storage), or with larger cells adapted for drone-brood. It is possible, by supplying one or other kind as desired, to regulate within certain limits the number of workers and drones produced in the hive. DOMESTICATED INSECTS 259 There unfortunately appear to be no means of ascertaining how far British apiculture is profitable. Honey to the value of £30,349 was imported into this country in 1903. Tue Sitx-Worm Mora (Bompyx mort, &c., fig. 1189).—The most important and best-known kind of Silk- Worm Moth is the one (Bombyx mort) of which the caterpillar or ‘ silk-worm” feeds upon the leaves of the mulberry- tree. The life-history is sufficiently familiar. From the egg a minute larva hatches out which is full grown in about five weeks, during which time it casts its skin several times. At the end of this period the silk- worm spins a cocoon, which consists of two long threads, the hardened secretion of two large glands that open on the under-lip. The material known as “cat-gut” is made from the secretion of the silk-glands, which are removed from the caterpillar and subjected to suitable treatment. The culture of silk-worms is generally supposed to have been first practised in China, the first allusion to it dating back to 2640 B.c., according to Chinese re- cords. Thence the industry spread through Korea into Japan, and also into India, Persia, and Central Asia. Its introduction into Europe is ascribed to the Emperor Justinian, who is said to have induced a couple of Persian monks to undertake a journey to China with the view of surreptitiously obtaining eggs. These worthies are stated to have been successful in their mission, reaching Con- stantinople with a supply of eggs (concealed in bamboos) in the year 550 A.D. To this source the silk-industry of Southern Europe Fig. 1189.—Silk-Worm Moth (Bombyx mori). a», Caterpillar (silk-“‘ worm”); B, female moth; c, cocoon, 260 UTILITARIAN ZOOLOGY may be traced, and to France and Italy, in particular, it is now of great importance. In the former country about 137,500 cwts. of raw silk (worth £1,080,000) is produced annually, while the Italian yield in 1902 was 823,718 cwts. (worth Zo, 855 057); Of late years the Chinese have engaged in the culture of the Oak Silk-Moth (Saturnia Pernyt), of which the larve feed on oak-leaves. The silk is coarser and less valuable than the ordinary kind, but possesses the merit of greater strength. An allied species (S. yama-maz) is cultivated in Japan. How far silk is im- portant to Britain may be gathered from the fact that in 1902 we im- ported 1,252,848 lbs. of raw. silk worth 4728,020, and silk goods to the value of £14,321,541. THe CocHINEAL INSECT ( Coccus Fig. r190.—Nopal (Opuntia coccinellifera\) and Cochineal Insects (Coccus CACTI, fig. II go). cact), enlarged, female to left, male to right —The colouring - matter known as cochineal, as also (to some extent) the pig- ments known as carmine and lake, are derived from a species of bug native to Mexico, which feeds upon the Nopal (Opuzn/za coccinellifera), a plant of the cactus sort. The culture of this insect dates back to the times of the ancient Mexicans, and is now of some importance in Central America. The insect and its food- plant have also been successfully introduced into the Canary Islands, Algeria, Java, and Australia. The colouring-matter is obtained from the dried bodies of the female insects, which are ground and extracted. It requires about 70,000 of them to produce a pound of cochineal. The introduction of cheap aniline dyes has caused this industry to decline, while carmine and lake can now be manufactured chemically. THE GRESHAM PUBLISHING COMPANY 34 SOUTHAMPTON STREET, STRAND, LONDON, W.C. * * * &€ * KF eK A NEW CENTURY: A NEW ENCYCLOPEDIA The New Popular A LIBRARY IN ITSELF. A General Dic- tionary of Arts, Sciences, Literature, Biography, Encyclopedi a. and History. Edited by CHARLES ANNANDALE, M.A., LL.D., Editor of Ogilvie’s “Imperial Dic- tionary of the English Language”. Profusely illustrated. In 14 handsome volumes, super- royal 8vo, in Roxburgh library binding, 125. 6d. per volume, net. A New Century demands a New Encyclopedia. As time advances, knowledge increases. To sum up that knowledge a new Encyclopedia is required, and everyone ought to possess that new Encyclopedia. ' The Gresham Publishing Company, having acquired exclusive control of the well-known POPULAR ENCYCLOPEDIA, are now issuing a New and Revised Edition of that famous authoritative work of reference. 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The Household A Family Guide to the Preservation of Health and to the Domestic Treatment of Ailments and Disease. cH By J. M‘GREGOR-ROBERTSON, M.B. C.M. (Hon.). With Physician. an Introduction by Professor M‘KENDRICK, M.D., LL.D., F.R.S., Glasgow University. Illustrated by about 400 figures in the text, and a Series of Engraved Plates. In 4 divisions, super-royal 8vo, cloth, at 9s. net each; also in 1 volume, Roxburgh binding, £1, 135. net; 2 volumes, ditto, £1, 17s. net. One aim of this book is to supply in as plain language as can be used some knowledge of what science has to say as to the body which we inhabit ; the second aim is to give reliable assistance in the domestic treatment of simple ailments. The bodily ills to which young and old are liable are considered more fully than is usual in popular works. The first portion of the book treats of ihe human body in health, and the various changes produced by discase. This part has been divided into sections, each section being devoted to one set of organs. For example, the bones and joints are considered in one section, the nervous system in another, the digestive organs in a third, and so on. The first half of each section describes the particular organs in their healthy condition, and the second half discusses the diseases to which they are liable. By this method the healthy and diseased states of each part of the body are placed in relationship to, and mutually explain, one another. This section, moreover, contains special chapters on the MANAGEMENT OF CHILDREN IN HEALTH, the DISEASES OF CHILDHOOD, and the DISEASES OF WOMEN. The second portion of the book is devoted to HYGIENE, or the conditions of health as regards Foon, DRINK, CLOTHING, EXERCISE, &c., and the rules to be observed for the promotion of health, both of individuals and communities. 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A History of the From the Earliest to the Latest Times. By Rev. THOMAS THOMSON and CHARLES ANNAN- Scottish People DALE, M.A, LL.D. With 40 Original Designs by W. H. MARGETSON, ALFRED PEARSE, WALTER PAGET, GORDON BROWNE, and other eminent artists. In 6 divisional volumes, super-royal 8vo, cloth elegant, 8s. 6@. net each. The main features may be stated as follows: It is a full and aetailed History of Scotland from the Earliest Times to the Latest. It is a History of the Scottish People, their manners, customs, and modes of living at thc various successive periods. it is a History of Religion and Ecclesiastical Affairs in Scotland. It is a History of Scotland’s progress in Commerce, Industry, Arts, Science, and Literature. It is illustrated by a series of original designs reproduced in facsimile from drawings by eminent artists. Prospectus of any Book. post free, The Gresham Publishing Company. 3 NEW EDITION, REVISED AND GREATLY AUGMENTED. Ogilvie’s OF THE ENGLISH LANGUAGE. A complete Encyclopedic Lexicon, Literary, Etymological, Scientific, Technological, and Pro- 1 ‘ : nouncing. Edited by CHARLES ANNAN- Imperial Dictionary DALE, M.A., LL.D. Illustrated by above three thousand engravings on wood, besides a splendid series of full-page plates, many of which are coloured. This edition of the IMPERIAL DICTIONARY is beautifully printed on paper of imperial size, specially made for the Work. It is issued in eight Divisional Volumes of a handy size for reference, bound in cloth, with a fine design on side, at Ios. net each volume. The reception accorded by the press and the public to this new edition of the IMPERIAL DICTIONARY has been such as to show that the care and labour bestowed upon it have met with due recognition, and to prove that it will continue fully to maintain its established position as a standard lexicon of the English language, and as a work of the highest utility far the purposes of general reference and everyday requirement. d DISTINCTIVE POINTS. To sum up the chief points of this edition— I. It is the latest revised dictionary, and has a supplement of many thousand new words. II. It contains more words, exclusive of compound and obsolete words, than any other English dictionary. III. The pronunciation is explained on a plan which is simplicity itself. S IV. It gives the pronunciation and the meaning of the word as recognized co-day. V. It has more illustrations than any other English dictionary. e VI. It has full-page plates (coloured and otherwise), which are an outstanding feature of the work No other English dictionary contains full-page plates. VII. It has clear type, beautifully printed on fine paper, and is substantially and elegantly bound. VIII. It has a specially prepared Supplement issued with each volume, and not, as is usually the case, relegated to the end of the complete work. IX. It has a very full Appendix, probably the best and finest given with any dictionary in the world. X. It is sold on a plan at once acceptable and convenient, within the reach of all, and the price is very moderate. “The IMPERIAL DICTIONARY”, says the S¢. James’ Gazette, “is a work which fairly deserves the epithet of monumental. It is really what it professes to be—‘a complete encyclopeedic lexicon, literary, scientific, and technological’. 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The Gresham Publishing Company. - 1 SELECTIONS FROM THE WORKS OF THE CHIEF The Cabinet of POETS, ORATORS, AND PROSE WRITERS OF IRELAND. Edited by CHARLES A. READ, F.R.H.S., Irish Literature: and KATHARINE TYNAN HINKSON. In 4 vol- umes, super-royal 8vo, cloth extra, gilt edges, price 8s. 6d. net each. As there is an Irish Nationality, so there is an Irish Literature, in which that Nationality has found expression. And what a magnifi- cent Literature it is! how brilliant the roll of Irish writers from Swift, Steele, and Goldsmith, to Sheridan, Grattan, Moore, and the score of gifted men and women who are identified with the present renaissance of Irish letters! THE CABINET OF IRISH LITERATURE is an attempt to give every Irishman, every Irish household, a share in the priceless treasures with which the literary genius of the race has enriched mankind. It brings within the compass of a single great work a representative selection of all that is best in irish Literature. Not, be it said, the old Irish Literature in the old Irish tongue; of that most is unknown to the people itself, though a good deal of it, in translation, wil -be found in this book. By Irish Literature is meant the Literature read and understood by the Irishmen of the present day—the expression of the ideas they really feel, of the life they truly live, in Mayo, in Limerick, in Cork, and in Derry. It is a selection made without fear or favour, free from any bias, religious, political, or social. Merit is the only passport to its pages. Coulson Kernahan It gives, in brief pointed biographical notices, the life-history of every great Irish writer. We are enabled to realize the personality of the man as well as appreciate the qualities of his work. A It deals not only with the past but with the present; and it is the only work that brings home to us by illustrative extracts the great revival in Irish Literature that is now taking place. W. B. Yeats, Douglas Hyde, George Bernard Shaw, Nora Hopper, Seumas MacManus, Richard Ashe King, George Egerton, Moira O'Neill, are a few among the many scores of modern writers whose works are represented in the CABINET. No Irishman who is proud of his nationality can afford to be without a work which thus focuses the whole intellectual activity of the race. To the younger generation it will be an inexhaustible source of in- spiration, a priceless influence in forming their taste, in moulding their character—in a word, in perpetuating those qualities which now, as in the past, are associated with the name of Irishman. The CABINET was originally planned by Mr. Charles Anderson Read, but this accomplished Irish poet and novelist did not live to see the fruition of his hopes. His work was completed by Mr. T. P. O'Connor, under whose auspices the first edition was issued. Now, after the lapse of nearly a quarter of a century, the time has come for a new edition of this monumental work, which shall take due account of the extraordinary activity in Irish ‘etters during the intervening years. Under the able editorship of Miss Katharine Tynan (Mrs. Katharine Tynan Hinkson), herself one of Ireland's most distinguished writers, the work has been thoroughly revised and brought down to the present hour. In its get-up it is all that a book of its great importance should be. The illustrations are many and of the highest artistic value. Some of the most eminent black-and-white artists of the day, including John HH. Bacon, Charles M. Sheldon, W. Rainey, R.I., G. P. Jacomb- Hood, R.I., and W. H. Margetson, have been commissioned to illus- trate typical scenes from the masterpieces of our literature, and these drawings, rendered by the latest processes of photographic reproduc- tion, and printed on specially prepared paper, add an unique charm to the work. The CABINET is further embellished with a large num- ber of ‘photographs of the most eminent Irish writers; and the cover design, in gold upon green cloth, is the work of Talwin Morris, the well-known designer. F, Frankfort Moore Prospectus of any Book post free. The Gresham Publishing Company. 5 The Book of the Home. with artistic design, price £2, 25. net. AN ENCYCLOPADIA OF ALL MATTERS RELATING TO THE HOUSE AND HOUSEHOLD MANAGEMENT. Produced under the general editorship of H. C. DAVIDSON, assisted by over one hundred specialists. Copiously illustrated by coloured and black-and-white plates and engravings in the text. In 4 volumes, super-royal 8vo, cloth, Also in 8 divisional volumes, cloth, price 5s. net each. THE Book or THE Homt is intended to form a complete work of reference on all subjects connected with household management. No efforts have been spared to ensure that every matter bearing upon the Home and Home Life shall receive full and sufficient treatment, and that the information given shall be reliable and in the best sense of the phrase up-to-date, A few among over one hundred specialists who have contributed to the work: Mrs. Apa S. BALLIN, Editor of Baby—the Mother's Magazine, and of Womanhood. Miss BERTHA BANNER, Training Teacher of Sewing and Dressmaking at the Liverpool Technical College for Women. Mr. A. Buiack, C.E., Architect, Author of First Principles of Building. Mrs. Davipson, Author of Dainties, Daughters can do for themselves, &c. Miss J. ForsTER, Principal of the Cheshire County Council Dairy Institute. Mrs. H. R. HAwEIs (the late), Author of The Art of Decoration, The Art of Beauty, &c. Miss HELENA HEAD, Principal of the Liverpool Girls’ School for Secondary Education in Domestic Science, and Author of the Manual of Hlousewifery, Mrs. A. Hopcson, Home Decorator to The Lady. Mr. R.@KEITH JoHNsTON, Author of Household What our Miss GERTRUDE J. KiNG, Secretary to the Society for Promoting the Employment of Women. Miss E. E. Mann, Head Teacher at the Liverpool Training School of Cookery. Colonel M. MoorE-Lang, Contributor to the Field and other agricultural papers. Mrs. C. S. PEEL, Dress and Household Editor of Hearth and Home, and Author of The New flome. Miss. B. SIBTHORPE POOLEY, Lecturer to the Liver- - pool Ladies’ Sanitary Association. Miss RANKIN, Head Teacher of Laundry Work at the Liverpool Technical College for Women. Miss FLORENCE STACPOOLE, Lecturer to the National Health Society and the Councils of Technical Education, and Author of Handbook of House- keeping for Small Incomes, &c. Mr. DAvip TOLLEMACHE, Iate editor of The Chey Difficulties and How to overcome Them. and Connoisseur. The contents of THE Book OF THE HOME may be grouped under four heads. The first deals with all matters concerning the House—from the choice of its site to the least of its internal decorations. The householder is instructed in the laws regarding landlord and tenant, and counselled in the important matters of sanitation and ventilation, heating and lighting, and the stocking and management of the garden. The housekeeper is advised as to furnishing, everything necessary for the comfort and adornment of a well-equipped house being described in detail, hints being also given regarding removals, painting and papering, artistic decoration, arrangement of linen and store cupboards, &c. In the second the daily routine of the Household is considered—the duties ot the servants, their wages, their leisure and pleasures, the management of the kitchen, laundry, and store-room. Plain and fancy cooking receive due attention, recipes being given of a large variety of dishes, and suggestions made for breakfast, lunch, afternoon-tea, dinner, and supper. A number of menus are added suitable for the different seasons. Invalid cookery also has its special section. In the third are discussed the legal and customary duties, and the occupations and pastimes, of Master and Mistress, the former being instructed as regards insurance and the making of a will, and the smaller matters of carving, the care of the wine-cellar, and the inspection of garden and stables, while the latter is advised as to account-keeping, payments, shopping, and innumerable other matters connected with her duties as Mistress. Other subjects treated under this head are dress, home occupations, visiting and entertaining, and indoor and outdoor amusements, In the fourth sound, systematic, and practical advice is given as to the management, in healt!: and sickness, and the education, of children, and also on such important subjects as occupations for boys and girls, the ceremonies necessary on the coming out of a daughter, and the preparations and formalities necessary before and after a marriage. THE BoOK OF THE HOME will thus be at once an indispensable ally to the young bride and the novice in housekeeping, and a valuable work of reference to the more experienced. Prospectus of any Book post free. 6 The Gresham Publishing Company. s The Animal Life of the World in its The Natural History various Aspects and Relations. By J. R. AINSWORTH Davis, M.A., of Trinity College, Cambridge, s e . > a they of Animals: and of University College, Aberystwyth. Profusely illus- trated with full-page colour and black-and-white plates, and engravings in the text, by eminent animal artists. In 8 half-volumes, cloth extra, price 7s. net each. While the sum of human knowledge is gigantic now as compared with what it was a hundred years ago, in the department. of Natural History the books upon which the great majority of us must depend have undergone practically no change. The general Natural History still follows the lines adopted by Goldsmith in his famous and delightful Earth and Animated Nature. That is to say, they are little more than classified catalogues of animals, taking up in succession the various groups and individuals, and describing them one after another, each as standing by itself. This is not what the intelligent reader of the present day requires. He must be put in a position to take a comprehensive grasp of the subject; he demands a competent guide, not a directory, however accurate. It is with this end in view that THE NATURAL HIsTORY OF ANIMALS has been compiled. It treats this great subject on essentially modern lines, giving an accurate and vivid account of the habits, relationships, mutual interdependence, adaptation to environment, &c., of the living animals of the world. It is needless to say that the production of such a work demanded a man who has devoted his life to the study of biology and zoology, and who at the same time is a gifted writer and expounder. This rare combination has been found in the person of Prof. J. R. AINSWORTH DAvis, M.A., of Trinity College, Cambridge, and of University College, Aberystwyth, the author of the present work. Prof. DAvis is well known to naturalists as an ardent worker in Natural History, particularly in the field of marine zoology. He isa very distinguished graduate of Trinity College, Cambridge, the chief scientific school in Britain, perhaps in the world, and has done a great deal of literary work, both scientific and in other directions. Briefly, the object of Prof. Davis’s work is to give in a readable form and in non-technical language a general survey of the whole animal world from the stand-point of modern science—and the work may fairly claim to be a Natural History on a new plan, the first comprehensive work in English of its own special kind. Formerly Natural History had much the character of a miscellaneous aggregate of disconnected facts, but hardly any fact or feature connected with any animal can now be considerea as isolated from others; and animals as a whole must be looked upon as interrelated in ‘the most surprising manner both with one another and with their surroundings. Every household library should contain a Bible, a Dictionary, an Encyclopedia, and a work on Natural History. This is the ‘irreducible minimum"; other books we may have, these we must. For THE NATURAL HISTORY OF ANIMALS it may fairly be claimed that it has a better title than any other work to become the Natural History for the Household. It is a work in which the adult reader will find a never-failing mine of information, while the younger members of the family will delight in its wealth of illustration, and its store of interesting and suggestive anecdote. To teachers THE NATURAL HISTORY OF ANIMALS may be regarded as indispensable. More than usual attention has of late been directed to the important subject of Nature-study; and in this respect the appearance of Prof. Davis’s work could scarcely have been more fitly timed. In the domain of Natural History it is pre-eminently the book for the purpose. Its clear and orderly arrangement of facts, its masterly grasp of general principles, its comprehensiveness of scope and simplicity of style, combined with the most absolute scientific accuracy, render this work an invaluable book of reference for those who aspire to teach Nature-study on up-to-date principles. The Illustrations, as befits a work of such importance, are on the most lavish scale. A large number are in colour, reproductions, by the latest processes of colour engraving, of exquisite pictures by the most eminent animal draughtsmen. In illustrating the work talent has been sought wherever it was to be found ; and the list of artists is representative of several nationalities. A large number of the designs are the work of Mr. A. FAIRFAX MUCKLEY, who is probably unsurpassed in the capacity to depict living creatures with absolute fidelity to detail without sacrificing the general artistic effect. FRIEDRICH SPECHT, one of the most eminent German animal painters of the past century, is represented in THE NATURAL HISTORY OF ANIMALS by many of his best designs in colour and_ black-and-white. W. KUHNERT, another German artist whose work is universally admired; and M. A. KOEKKOEK, the talented Dutch painter, are also among those who have assisted in the embellishment of the work. An important feature is the series of diagrammatic designs showing the structure of certain typical animals, specially drawn under the direction of Prof. Davis. Prospectus of any Book post free. The Gresham Publishing Company. 7 The Modern Carpenter, A Complete Guide to Current Practice. Prepared under the * editorship of G. LisTER SUTCLIFFE, Architect, Asso- Joi ner 9 and ciate of the Royal Institute of British Architects, Mem- : ber of the Sanitary Institute, editor and joint-author of = ° 3) Cabinet-Maker ¢ “Modern House-Construction”, author of “Concrete: Its Nature and Uses”, &c. With contributions from many specialists. Illustrated by a series of about Ioo separately-printed plates and 1000 figures in the text. In 8 divisional volumes, super-royal quarto, handsomely bound in cloth, with cover design by Mr. TALWIN Morris, price 75. 6@. net each. In complete sets only. In preparing THE MODERN CARPENTER the editor has had the great advantage of working upon the basis of Newlands's Carpenter and Joiner's Assistant, which for nearly half a century has been accepted as a standard authority on the subjects of which it treats, and for many years has been recommended by the Royal Institute of British Architects as a text-book for the examination of that society. And yet in the present work it has been possible to preserve only a very small part of Newlands's treatise, invaluable though this has been to two generations of craftsmen. While the fundamental features of arrangement and method which distinguish this famous work have been retained, the matter has had to be entirely rewritten, and many new sections have been added, on subjects not touched upon in the older work, with which the carpenter of the present day requires to be familiar. In the new book, indeed, the old foundations that have stood the test of half a century of practical use have been retained, but the superstructure is wholly new. The lesson to be learned from this fact is not far to seek. It is that the modern carpenter requires a far wider expert knowledge than sufficed his predecessor. The development of wood-working machinery, the introduction of new kinds of timber, improvements in the design of structures, the more thorough testing of timbers, and progress in the various industries with which Carpentry, Joinery, and Cabinet-making are intimately allied, have all helped to render the craft more complex. The carpenter of the present day has no use for the old ‘‘rule of thumb" methods; his calling is both an art and a science, and knowledge, knowledge, and again knowledge is the primary condition of success. The editor of THE MODERN CARPENTER, Mr. G. Lister Sutcliffe, Associate of the Royal Institute of Architects, needs no introduction to practical men; his name is already well known not only through his professional position in the architectural world, but through his editorship of A/odern House- Construction, a work which, although issued only a few years ago, has already become a standard book of reference. Mr. SUTCLIFFE's large experience has enabled him to enlist the services of a highly- qualified staff of experts, whose special knowledge, acquired through long years of practical work, is now placed at the disposal of every member of the craft. The first condition in selecting the contri- butors to the work was that they should be practical men, not only possessing the indispensable knowledge, but having the ability to impart it. The result is that within the eight divisional-volumes of this work we have a treatise on every branch of the craft, distinguished by four outstanding qualities :— It is (rt) complete, (2) clear, (3) practical, and (4) up-to-date. An idea of the scope of THE MODERN CARPENTER may be gathered from the fact that while its predecessor, The Carpenter and Joiner's Assistant, comprised only eight sections, the new work includes no fewer than sixteen. A glance at these will show that the work covers the whole field; it is a complete encyclopzedia upon every subject that bears upon the everyday work of the practical man. I, Styles of Architecture. IX. Staircases and Handrailing. il. Woods: Their Characteristics and Uses. X. Air-tight Case-Making. II. Wood-working Tools and Machinery. XI. Cabinet-Making. IV. Drawing and Drawing Instruments. XII. Wood-Carving. V. Practical Geometry. XIII. Shop Management, VI. Strength of Timber and Timber Framing. XIV. Estimating. VII. Carpentry. XV. Building Law. VIII. Joinery and Ironmongery. XVI. Index, Glossary, &c. The Illustrations are not the least of the many notable features of this great undertaking, The work is embellished in the first place with about 100 full-page plates, reproduced, some in colours, by the most approved processes of mechanical engraving, and printed on specially-prepared paper. In addition to this unique collection there are no fewer than 1000 diagrams and designs in the body of the work. No trouble or expense has indeed been spared to procure illustrations where these could elucidate the text. ; Prospectus of any Book post free, 8 The Gresham Publishing Company. Charles Dickens’ The Imperial Edition of the Novels of CHARLES DICKENS, in 16 volumes, large square 8vo, cloth Novels extra, gilt top, price 4s. 6d. net each volume. e An Ideal Issue. ONE NOVEL, ONE VOLUME. Despite varying lengths, the paper, &c., is so adjusted that each volume is uniform in thickness and size. The Cheapest Edition. The price of each volume is 45. 6d. net, making the edition the cheapest of the best editions. Sumptuously Bound. The cloth is of the finest and is im- perial red in colour. The embellishments (produced in gold) are an appropriate design of national arms and imperial em- blems by the eminent designer, Talwin Morris. Illustrations a Unique Feature. Every picture drawn spe- cially at enormous cost for this “Imperial” edition by the best known and most celebrated Artists of to-day. George Gissing’s Masterly Study. A literary character study, the work of this great authority, forms one of the volumes of this issue, and is illustrated with pictures of some of the quaint old hostelries and places made famous by Dickens, and is altogether an invaluable addition to this issue. Presentation Portrait. To every subscriber to this edition will be presented with the last volume a magnificent Photo- gravure of Charles Dickens. It is printed on the finest plate paper, 22 inches by 30 inches, and has been specially engraved for this edition. A List of the Novels. The following is a list of the volumes in the Imperial Edition:— The Pickwick Papers. Oliver Twist. Nicholas Nickleby. Martin Chuzzlewit. The Old Curiosity Shop Barnaby Rudge. David Copperfield. Bleak House. Sketches by Boz. . Hard Times and Master Humphrey’s Clock. ip Xi Christmas Books. i" Dombey and Son. Little Dorrit. A Tale of Two Cities. Great Expectations. Charles Dickens: A Critical Study. By GrorGE GissInc. Prospectus of any Book post free.