Chapter I: Part 1

AGRICULTURAL ZOOLOGY

BY

DR. J. RITZEMA BOS,

LECTURER IN THE ROYAL AGRICULTURAL COLLEGE, WAGENINGEN, HOLLAND.

WITH AN INTRODUCTION BY

ELEANOR A. ORMEROD, F.R.MET.S., F.R.M.S., ETC.,

FORMERLY HON. CONSULTING ENTOMOLOGIST TO THE ROYAL AGRICULTURAL SOCIETY
OF ENGLAND.

TRANSLATED BY

J. R. AINSWORTH DAVIS, B.A. (TRIN. COLL. CAMB.), F.C.P.,

PROFESSOR IN THE UNIVERSITY OF WALES, AND PROFESSOR OF BIOLOGY AND
GEOLOGY IN THE SCIENTIFIC AND AGRICULTURAL DEPARTMENTS OF THE UNIVERSITY
COLLEGE OF WALES.

WITH 149 ILLUSTRATIONS.

LONDON: CHAPMAN & HALL, LD.
1894

AUTHOR’S PREFACE.

The present volume of the Thaer Library was undertaken with the intention of providing agricultural colleges with a condensed review of the entire animal kingdom, but treating in greater detail the animals harmful or helpful to agriculture. I have, however, omitted all reference to the domesticated farm animals, as in all such institutions these are treated of, not by the zoologist, but by the lecturer on stock-breeding. Although the book is not allowed to exceed a certain size, I have taken great pains to make it intelligible, and venture to hope that it may be found suitable for the _private use of the practical farmer_. To the farmer who wishes more exhaustive information, and desires a reference book on the animal foes of agriculture, stock-breeding, horticulture, fruit-tree culture, and forestry, I venture to point out my larger work, _Animal Foes and Friends_,[1] brought out last year by the publisher of this book.

Footnote 1:

Tierische Schädlinge und Nützlinge für Ackerbau, Viehzucht, Wald- und
Gartenbau. Lebensformen, Vorkommen, Einfluss und die Massregeln zu
Vertilgung und Schutz. Praktisches Handbuch v. _Dr. J. Ritzema Bos_,
Docent an der landwirtschaftl. Lehranstalt in Wageningen. Mit 477
eingedruckten Abbildungen. Preis 18 m., geb. 20 m. Verlag von Paul
Parey, 10 Hedemannstrasse, Berlin, S.W.

It is hoped that the present volume may be found serviceable, both in the teaching of agricultural institutions, and to the practical farmer.

DR. J. RITZEMA BOS.

WAGENINGEN,
_February, 1892_.

TRANSLATOR’S PREFACE.

Agricultural education is making such rapid strides in this country, that no apology is needed for translating a book which appears to fill a gap, especially as it is written by a well-known authority. Dr. Ritzema Bos has kindly allowed certain small alterations to be made which adapt the work to the requirements of British agriculture. Additions are indicated by square brackets, and small print employed in the case of some non-British animals. A few forms have been omitted for similar reasons. Constant reference has been made to the published works of Miss E. A. Ormerod, who has added to my obligation by writing an Introduction, and I also wish to acknowledge my indebtedness to Mr. J. H. Salter, B.Sc., and Mr. J. Dawson Roberts, M.R.C.V.S., for kind help given by them.

J. R. AINSWORTH DAVIS.

ABERYSTWYTH,
_May, 1894_

INTRODUCTION.

By request of Professor Ainsworth Davis, the skilled translator of this “handy-book” on “Agricultural Zoology,” I add some words of introduction; and I have especial pleasure in so doing: not that any observations of mine can add value to the work of the well-known author, but because, having myself had the advantage for many years of colleagueship, and important help in my own work from the assistance of Dr. Ritzema Bos, I am well acquainted both with his extensive knowledge and also his scrupulous care in observation, and I believe that this abstract of his larger work, now given in a form in which it is available for general use, will meet a great need.

We have long wanted a book, plain in wording, and of moderate size, dealing with the wild animals or animal infestations generally which occur in connection with farm life—a manual, in fact, which, whilst suitable for the use of agricultural students and teachers, should at the same time not be too technically scientific to be intelligible to practical farmers or to general readers.

In the pages of the present volume a very serviceable amount of information will be found to be embodied. So far as can be arranged in the limited space the chief characteristics of the main divisions of the animal kingdom are given, from the _Vertebrata_—including descriptions of some of our most notable forms of what may be popularly described as beasts, birds, and reptiles,—to the _Arthropoda_, including information on a most serviceable amount of insect infestation; also regarding Mites, Ticks, etc. These are followed by the _Vermes_, including, among other families of the _Nematoda_, the eelworms which cause so much injury to crop growth; and these are followed by the intestinal tapeworms and the fluke.

The fourth sub-kingdom, that of _Mollusca_, includes, besides snails and slugs, various kinds of shell-fish; and the lower sub-kingdoms—including _Echinodermata_, which may be typified by starfishes and sea urchins, the _Cœlenterata_, or Zoophytes, and the _Protozoa_—will be found to be just entered on sufficiently to show their place in the scale.

The clear descriptions, made still more instructive by the numerous and good figures, will speak for themselves to all readers; but I should like to add a few lines to point out the serviceableness of a handbook in which the reader may turn at pleasure to the history of any common farm animal—as a weasel or a vole, a wood-pigeon or a pheasant, a blindworm or a common frog. And, in regard to the insect infestations, to which it will be seen more than a hundred pages of the book are devoted, I can bear witness to the great amount of valuable information which I constantly derive myself from the study of the writings of Dr. Ritzema Bos on this subject; and I trust this little manual of “Agricultural Zoology” may take the place in our farm and school libraries which I believe it to be excellently fitted to fill.

ELEANOR A. ORMEROD,
_Late Consulting Entomologist of the Royal
Agricultural Society of England_.

TORRINGTON HOUSE, ST. ALBAN’S,
_May 24, 1894_.

CONTENTS.

PAGE
INTRODUCTION—
I. Subdivision of the Animal Kingdom 1
II. Review of the Structure and Vital Phenomena of Animals 3–16

=First Sub-Kingdom: VERTEBRATA (Backboned Animals)= 16–82
CLASS I: MAMMALIA (Sucklers) 21–48
ORDER: Carnivora (Beasts of Prey) 24–30
Family: Felidæ (Cat Family) 24–25
Family: Canidæ (Dog Family) 25–26
(Wolf, p. 25; Fox, p. 26.)
Family: Mustelidæ (Weasel Family) 26–30
(Martens and Polecat, p. 26; Ferret, p. 27; Stoat and
Weasel, p. 28; Mink, Otter, and Badger, p. 29.)
ORDER: Insectivora (Insect-eating Mammals) 30–33
(Shrews, pp. 30, 31; Mole, pp. 31–33; Hedgehog, p. 33.)
ORDER: Cheiroptera (Bats) 33–35
ORDER: Rodentia (Gnawing Mammals) 35–43
Family: Leporidæ (Hares and Rabbits) 36–38
(Hare and Rabbit, p. 37.)
Family: Muridæ (Mouse Family) 38–41
(Hamster, Black Rat, and Brown Rat, p. 39; Common Mouse,
Long-tailed Field Mouse, and Harvest Mouse, p. 40; Corn
Mouse, p. 41.)
Family: Arvicolidæ (Vole Family) 41–43
(Bank Vole and Water Vole, p. 42; Field Vole, pp. 42, 43;
Southern Field Vole, p. 43.)
ORDER: Ruminantia (Cud-chewing Mammals) 44–47
Family: Cervidæ (Deer Family) 45–47
(Red Deer, p. 46; Roebuck and Fallow Deer, p. 47.)
ORDER: Multungula or Pachydermata (Many-hoofed or
Thick-skinned Mammals) 48
(Wild Boar, p. 48.)
ORDER: Solidungula (Single-hoofed Mammals) 48
CLASS II: AVES (Birds) 49–74
ORDER: Raptores (Birds of Prey) 53–55
ORDER: Scansores (Climbing Birds) 55–56
(Cuckoo, pp. 55, 56.)
ORDER: Passeres (Perching Birds) 57–65
Group: Hirundinidæ (Swallows) 57–58
(Swallows and Martins, p. 57; Swift and Goatsucker, p.
58.)
Group: Magnirostres (Large-beaked Perchers) 58–61
(Jackdaw, Crows, Rook, and Raven, pp. 59–61; Magpie and
Jay, p. 61.)
Group: Conirostres (Conical-beaked Perchers) 61–64
(Titmice, Larks, and Buntings, pp. 61, 62; Finches, p.
62; Sparrows, pp. 62, 63; Linnet, p. 63; Chaffinch, pp.
63, 64.)
Group: Subulirostres (Awl-beaked Perchers) 64–65
(Wagtails, Pipits, and Hedge “Sparrow,” p. 64; Warblers,
pp. 64, 65; Thrush-like birds, p. 65.)
ORDER: Gyrantes (Doves) 65–67
(Wood Pigeon, pp. 66, 67; Turtle Dove and Rock Pigeon, p.
67.)
ORDER: Rasores (Poultry) 67–68
(Pheasant, p. 68.)
ORDER: Grallatores (Wading-Birds) 68–70
ORDER: Natatores (Swimming-Birds) 70–74
Family: Lamellirostra (Ducks) 71–73
Family: Longipennes (Gulls) 73–74
CLASS III.: REPTILIA (Reptiles) 74–79
CLASS IV.: AMPHIBIA (Amphibians) 79–81
CLASS V.: PISCES (Fishes) 81–82

=Second Sub-Kingdom: ARTHROPODA (Jointed-limbed Animals)= 82–206
CLASS I.: INSECTA (Insects) 85–194
ORDER I.: Coleoptera (Beetles) 94–118
Family: Carabidæ (Ground Beetles) 94–96
(Corn Ground Beetle, pp. 95, 96.)
Family: Staphylinidæ (Rove Beetles) 96–97
Family: Silphidæ (Burying Beetles) 97
(Black Burying Beetle, and Beet Carrion Beetle, p. 97.)
Family: Nitidulidæ (Shine Beetles) 97–98
(Turnip-flower Beetle, pp. 97, 98.)
Family: Cryptophagidæ (Secret-eating Beetles) 98–99
(Beet Beetle, pp. 98, 99.)
Family: Lamellicornia (Chafers) 99–102
(Cockchafer, pp. 100, 101; Buckwheat Beetle, p. 101; Rye
and Garden Chafers, p. 102.)
Family: Elateridæ (Click Beetles) 102–105
(“Wireworms,” pp. 103–105.)
Family: Curculionidæ (Weevils) 105–110
(Seed Beetles, pp. 106, 107; Pea Weevil, pp. 107, 108;
Mouse-tooth Weevils, p. 108; Gall Weevils, pp.
108–110.)
Family: Chrysomelidæ (Leaf Beetles) 110–117
(Colorado Beetle, pp. 111–113; Tortoise Beetles, pp. 113,
114; Flea Beetles, 114–117.)
Family: Coccinellidæ (Lady Birds) 117–118
ORDER II.: Orthoptera (Straight-winged Insects) 118–121
(Migratory Grasshopper, pp. 119, 120; Mole Cricket, pp.
120, 121.)
ORDER III.: Neuroptera (Net-winged Insects) 121–123
(Dragon Flies, p. 122; Lace Flies, pp. 122, 123; Scorpion
Flies, p. 123.)
ORDER IV.: Hymenoptera (Membranous-winged Insects) 123–136
Family: Apidæ (Bees) 125–126
Family: Vespidæ (Wasps) 126–128
Family: Fossores (Digging Wasps) 128–129
Family: Formicidæ (Ants) 129–132
Family: Ichneumonidæ (Ichneumon Flies) 132–134
Family: Tenthredinidæ (Saw-flies) 134–136
(Turnip Saw-fly, pp. 134–136.)
ORDER V.: Lepidoptera (Butterflies and Moths) 136–159
Family: Diurna (Butterflies) 137–142
(Whites, pp. 138–142.)
Family: Noctuidæ (Owlet Moths) 142–152
(Surface Caterpillars, p. 143; Dart or Turnip Moth, pp.
143–145; Cabbage Moth, pp. 145–147; Lettuce and Pea
Moths, p. 147; Grass-root Moth, p. 148; Couch-grass
Moth, pp. 148, 149; Wheat-haulm Moth, p. 149; Grass
Moth, pp. 149, 150; Darnel Moth, p. 150; Silver Y Moth,
pp. 151, 152.)
Family: Pyralidæ (Snout Moths) 152–155
Family: Tortricidæ (Leaf-rollers) 155–157
(Fawn-coloured Pea Moth, p. 156; Crescent Pea Moth, pp.
156, 157.)
Family: Tineidæ (Leaf-miners) 157–159
(Carrot Moth, pp. 157, 158; Diamond-back Moth, pp. 158,
159.)
ORDER VI.: Hemiptera (Half-winged Insects) 159–163
Family: Aphidæ (Plant Lice) 159–163
ORDER VII.: Physopoda (Bladder-footed Insects) 163–164
(Thrips, pp. 163, 164.)
ORDER VIII.: Diptera (Flies) 164–193
Family: Culicinæ (Gnats) 164–165
Family: Gallicolæ (Gall Gnats) 165–170
(Hessian Fly, pp. 166–168; Scarlet Wheat Midge, pp. 168,
169; Wheat Midge, 169, 170.)
Family: Rostratæ (Crane Flies) 170–172
Family: Muscæformes (Gnat Flies) 173–174
(Sand Flies, pp. 173, 174.)
Family: Tabanidæ (Gad Flies) 174
Family: Muscidæ (True Flies) 175–184
(Caterpillar and Flesh Flies, p. 175; Common Flies, p.
175; Flower Flies, pp. 176, 177; Cheese-fly, p. 178;
Ribbon-footed Corn Fly, pp. 178–182; Frit Fly, pp.
182–184.)
Family: Syrphidæ (Hover Flies) 185
Family: Stomoxydæ (Stable Flies) 185–186
Family: Œstridæ (Bot Flies) 186–192
(Ox Warble-fly, pp. 186–188; Sheep Bot Fly, pp. 188–190;
Horse Bot, etc., pp. 190–192.)
Family: Pupipara (Louse Flies) 192–193
ORDER IX.: Aphaniptera (Fleas) 193
ORDER X.: Parasita (Lice) 193–194
CLASS II.: MYRIOPODA (Centipedes and Millipedes) 195
CLASS III.: ARACHNOIDEA (Scorpions, Spiders, Mites) 195–205
ORDER: Acaridea (Mites) 196–202
Family: Acaridæ (True Mites) 196–202
(Itch or Mange Mites, pp. 196–202.)
Family: Ixodidæ (Ticks) 202–204
Family: Gamasidæ (Beetle Mites) 204
(Fowl Mite, p. 204.)
Family: Trombidiidæ 205
(Plant Mite or “Red Spider,” p. 205.)
CLASS IV.: CRUSTACEA (Crustaceans) 206

=Third Sub-Kingdom: VERMES (Worms)= 206–245
CLASS: ANNELIDA (Segmented Worms) 207–209
(Earthworms, pp. 207–209.)
CLASS: NEMATELMINTHES (Round Worms) 209–231
ORDER: Nematoda (Thread Worms) 210–231
Family: Strongylidæ (Palisade Worms) 212–215
Family: Trichotrachelidæ (Whip Worms) 215–218
(Trichina, pp. 216–218.)
Family: Filaridæ (Slender Thread Worms) 218
Family: Ascaridæ (Round Worms) 218–219
Family: Anguillulidæ (Eelworms) 219–231
(Stem Eelworm, pp. 220–224; Wheat Eelworm, pp. 224–227;
Beet Eelworm, pp. 227–230; Root-knot Eelworm, p. 231.)
CLASS: PLATYHELMIA (Flat Worms) 231–245
ORDER: Cestoda (Tapeworms) 231–240
ORDER: Trematoda (Flukes) 240–245

=Fourth Sub-Kingdom: MOLLUSCA (Mollusca)= 245–251 CLASS: CEPHALOPODA (Cuttlefishes) 247 CLASS: GASTROPODA (Snails and Slugs) 247–251 (Grey Field Slug, pp. 249–251.) CLASS: LAMELLIBRANCHIATA (Bivalve Molluscs) 251

=Fifth Sub-Kingdom: ECHINODERMATA (Hedgehog-skinned Animals)= 252–253

=Sixth Sub-Kingdom: CŒLENTERATA (Zoophytes)= 253–255

=Seventh Sub-Kingdom: PROTOZOA (One-celled Animals)= 255–256

ILLUSTRATIONS.

FIG. PAGE
1. Schematic Longitudinal Section of the Human Body 4
2. Human Skeleton 7
3. Skeleton of an Ox 9
4. Bending of the Arm by Contraction of the Biceps Muscle 10
5. Diagram to explain the Action of the Motor and Sensory Nerves 11
6. Diagram of the Course of the Circulation 13
7. Life History of the Small-winged Gall-fly (_Andricus
terminalis_) 15
8. Diagram of a Fish’s Heart 18
9. Diagram of a Mammal’s Heart 20
10. Diagram of a Reptile’s Heart 20
11. Diagram of a Frog’s Heart 21
12. Vertical Section of a Human Grinding Tooth 22
13. Crown of an Ox’s Grinder 22
14. Skull of Domestic Cat 25
15. Pine Marten (_Mustela martes_) 27
16. Skull of Mole 30
17. Common Shrew (_Sorex vulgaris_) 31
18. Skeleton of Bat 34
19. Skull of Squirrel 35
20. Abnormal Tooth in Hare 37
21. Hamster (_Cricetus frumentarius_) 38
22. Long-tailed Field Mouse (_Mus sylvaticus_) 40
23. Upper Back Teeth of Brown Rat 41
24. Upper Back Teeth of Water Vole 41
25. Southern Field Vole (_Arvicola arvalis_) 43
26. Skull of Sheep 44
27. Development of Roebuck Antlers 45
28. Wing of Buzzard 50
29. Section through Bird’s Egg 51
30. Eagle Owl (_Otus maximus_) 52
31. Head and Foot of Falcon 53
32. Golden Eagle (_Aquila chrysaëtus_) 54
33. Barn Owl (_Strix flammea_) 55
34. Cuckoo (_Cuculus canorus_) 56
35. Goatsucker (_Caprimulgus europæus_) 58
36. Head of Rook (_Corvus frugilegus_) 60
37. Head of Bullfinch (_Pyrrhula vulgaris_) 62
38. Nightingale (_Daulias luscinia_) 65
39. Wood Pigeon (_Columba palumbus_) 66
40. Capercailzie (_Tetrao urogallus_) 68
41. Pheasant (_Phasianus colchicus_) 69
42. Woodcock (_Scolopax rusticola_) 70
43. Crested and Little Grebes (_Podiceps cristatus and minor_) 71
44. Grey Goose (_Anser cinereus_) 72
45. Herring Gull (_Larus argentatus_) 74
46. Common Lizard (_Lacerta agilis_) 75
47. Adder (_Pelias berus_) 76
48. Grass Snake (_Tropidonotus natrix_) 77
49. Blindworm (_Anguis fragilis_) 78
50. Great Crested Newt (_Triton cristatus_) 79
51. Common Frog (_Rana temporaria_) 80
52. Natterjack (_Bufo calamita_) 81
53. The Perch (_Perca fluviatilis_) 82
54. Wood-borer (_Sirex_) 83
55. Centipede (_Scolopendra morsitans_) 84
56. Ground Beetle, showing Nervous System 84
57. Disarticulated Grasshopper 85
58. Head and Mouth-parts of a Ground Beetle 87
59. Leg of Ground Beetle 88
60. Stages of Silkworm Moth (_Bombyx mori_) 88
61. Stages of Hornet (_Vespa crabro_) 89
62. Migratory Grasshopper (_Acrydium migratorium_) 90
63. Looper Caterpillar 90
64. False Caterpillar 90
65. Stages of Cockchafer (_Melolontha vulgaris_) 91
66. Larva of a Weevil 92
67. Stages of Aphis-eating Fly (_Syrphus pyrasti_) 92
68. A Ground Beetle (_Carabus auronitens_) 95
69. Corn Ground Beetle (_Zabrus gibbus_) and larva 95
70. A Rove Beetle (_Staphylinus erythropterus_) 96
71. Black Burying Beetle (_Silpha atrata_) and larva 96
72. Antennæ of Cockchafer 99
73. Abdomens of Common and Horse-chestnut Cockchafers 100
74. Skipjack (_Agriotes lineatus_) 102
75. Skipjack about to spring 103
76. Grain-plants sown deep and shallow, to show Wireworm attack 104
77. Bean Beetle (_Bruchus rufimanus_) 106
78. Pea Weevil (_Sitones lineatus_) 107
79. Mouse-tooth Weevil (_Baridius chloris_) and larva 108
80. Turnip Gall Weevil (_Ceutorhynchus sulcicollis_) 109
81. Colorado Beetle (_Chrysomela decemlineata_) 111
82. Stages of Colorado Beetle 112
83. Cloudy Tortoise Beetle (_Cassida nebulosa_) 113
84. Rape Flea Beetle (_Psylliodes chrysocephalus_) 114
85. Stages of Seven-spotted Lady-bird (_Coccinella
septempunctata_) 118
86. Stages of Common Lace Fly (_Chrysopa vulgaris_) 122
87. Head of Honey Bee (_Apis mellifica_) 124
88. Common Wasp (_Vespa vulgaris_) and nest 127
89. Common Sand Wasp (_Ammophila sabulosa_) 129
90. Stages of Yellow-legged Ichneumon Fly (_Microgaster
glomeratus_) 133
91. Turnip Saw-fly (_Athalia spinarum_) and caterpillars 135
92. Head of Butterfly 136
93. Scales from Butterfly’s Wing 136
94. Stages of Peacock Butterfly (_Vanessa io_) 138
95. Stages of Cabbage White (_Pieris brassicæ_) 139
96. Garden White (_Pieris rapæ_), male 141
97. Garden White, female and caterpillar 141
98. Green-veined White (_Pieris napi_) 142
99. Dart or Turnip Moth (_Agrotis segetum_) and caterpillar 143
100. Stages of Cabbage Moth (_Mamestra brassicæ_) 146
101. Grass Moth (_Charæas Graminis_) and caterpillar 150
102. Stages of Silver Y Moth (_Plusia gamma_) 151
103. Hop Snout Moth (_Hypena rostralis_) 153
104. Mother-of-Pearl Moth (_Botys margaritalis_) and larva 155
105. Fawn-coloured Pea Moth (_Grapholitha nebritana_) 156
106. Larch Moth (_Coleophora laricella_) 157
107. Wings of a Bug 159
108. Bean Aphis (_Aphis papaveris_) 160
109. Corn Thrips (_Thrips cerealium_) 163
110. Wheat Midge (_Cecidomyia tritici_) 165
111. Barley attacked by Hessian Fly 167
112. Larvæ of Wheat Midge (_Cecidomyia tritici_) 169
113. Stages of Daddy Longlegs (_Tipula oleracea_) 171
114. Rain Breeze Fly (_Hæmatopota pluvialis_) 174
115. Caterpillar Fly (_Tachina fera_) 175
116. Turnip infested by Cabbage Fly (_Anthomyia brassicæ_) 177
117. Ribbon-footed Corn Fly (_Chlorops tæniopus_) 179
118. Stages of ditto 181
119. Wheat Plant distorted by winter generation of ditto 181
120. Stages, etc., of Frit Fly (_Oscinis frit_) 183
121. Stages of Horse Bot Fly (_Gastrus equi_) 190
122. Horse Louse (_Hæmatopinus macrocephalus_) 194
123. Common Snake Millipede (_Julus terrestris_) 195
124. A Spider (_Salticus scenicus_) 196
125. Mange Mite of the Pig (_Sarcoptes scabiei, var. suis_) 197
126. Ditto 198
127. The Dog Tick (_Ixodes ricinus_) 203
128. Diagrammatic transverse section through a Thread Worm 210
129. Tail of male _Strongylus armatus_ 213
130. Encapsuled Muscle Trichinæ in flesh 217
131. Male Intestinal Trichina 217
132. Rye Plant in the later stage of the Eelworm Disease 222
133. Ear Cockles of Wheat 225
134. Stages of Beet Eelworm (_Heterodera Schachtii_) 226
135. _Tænia saginata_ 232
136. Common Tapeworm (_Tænia officinalis_) 234
137. Tapeworm Larva (_Tænia solium_) 234
138. Types of Bladder-worm 235
139. Measle of _Tænia solium_ 235
140. Measles in Pork 236
141. Liver Fluke (_Distoma hepaticum_) 241
142. Life History of Liver Fluke 242
143. Diagrams of Molluscs 246
144. Grey Field Slug (_Limax agrestis_) 249
145. Common Starfish (_Asterias rubens_) 252
146. Freshwater Polype (_Hydra_) 254
147. A Jellyfish (_Pelagia noctiluca_) 254
148. A Sea Anemone (_Sagartia nivea_) 254
149. Proteus Animalcule (_Amœba_) 255

ZOOLOGY.

INTRODUCTION.

I. Subdivision of the Animal Kingdom.

There are animals so like one another that they are given the same name. Such animals are ranked in the same _species_. Animals which differ so much that they have to be referred to different species, but which notwithstanding agree in the majority of their characters, especially the most important ones, are placed in the same _genus_. Hare and rabbit, or horse and donkey, are reckoned as different species of the same genus. Genera resembling one another are united into a _family_; thus, the pine marten and the beech or stone marten both belong to the _Marten genus_ (Martes), while the weasel and stoat are different species of the _Weasel genus_ (Mustela); but these two genera are so similar that they are both placed in the same family, _i.e._ the _Weasel family_ (Mustelidæ). Nearly related families together build up an _order_. Thus, the Weasel family, Dog family, Cat family, etc., collectively constitute the _order of Carnivora_, characterized, speaking generally, by the same kind of teeth, claws, habits, and food. Several related orders are united into a _class_. Thus, for example, carnivorous animals (Carnivora), ruminating animals (Ruminantia), gnawing animals (Rodentia), etc., constitute different orders of the _class of Sucklers_ (Mammalia); while birds of prey (Raptores), pigeons (Gyrantes), and poultry (Rasores), are included in a second class, that of _Birds_ (Aves). But both Birds and Mammals have a skeleton, of which the chief support is the backbone; on this account they are placed in a larger subdivision, the _sub-kingdom_ of Backboned animals (Vertebrata); while snails are grouped under the sub-kingdom of _Molluscs_, millipedes and centipedes under that of _Jointed-limbed animals_ (Arthropods).

In this way the animal kingdom is divided into _sub-kingdoms_, the sub-kingdoms into _classes_, the classes into _orders_, the orders into _families_, the families into _genera_, and the genera into _species_. Animals of the same species which differ from one another in more or less constant characters, belong to different _races_ (domestic or geographical races).

There are many species of animals the external features of which are well known to ordinary folk, and which therefore possess a definite English name, but a much larger number, of the smaller forms especially, have no English name. It is, therefore, necessary to devise new names for these species. The English names, however, are liable to cause great confusion, since in different districts the same name is often applied to widely different animals. Besides this, distinct names have usually been given to successive stages in the life history of the same form: “wireworms,” for example, are the young state of the “click beetle.”

By using the scientific method of naming invented by Linnæus, confusion is made impossible. The Latin names of this naturalist have the great advantage that they not only give a perfectly distinct name to any particular species, but also at the same time show the genus to which it belongs. Each kind of animal possesses, in fact, two names; just in the same way as every person possesses at least two names, a Christian name and a surname. The generic name comes first, and is, of course, common to all animals of the same genus. The second name is the specific one, and belongs exclusively to animals of the same species. The hare and rabbit, for example, are both included in the genus _Lepus_. The Latin name of the first is _Lepus timidus_; that of the second, _Lepus cuniculus_. Horse = _Equus caballus_; ass = _Equus asinus_.

II. Review of the Structure and Vital Phenomena of Animals.

I select as a point of departure the human body, and the bodies of domestic animals, because my readers are best acquainted with these.

The limbs consist, beginning on the outside, of skin, flesh, and bone. The same parts can also be distinguished in the head, neck, and trunk; but in these divisions of the body they enclose a cavity, the _body-cavity_, which, again, contains various parts (“organs”), which are not everywhere attached to the body-wall. Fig. 1 represents a longitudinal section through the body. The skin is represented by a line, flesh and internal lining are shaded, while the bones are black. These parts form together the _body-wall_. In front the body-wall encloses a cavity, the body-cavity (_Kh._), which in Mammals is divided into two sections (thoracic cavity, _Bz.h_, and abdominal cavity, _B.h._) by the _midriff_ (diaphragm). In the thoracic cavity are found the lungs and heart (_H_), also most of the gullet or upper part of the gut; the abdominal cavity contains the remainder of the often much-coiled gut, which in one place widens into the stomach (_M_), also the kidneys, spleen, and parts connected with the gut (_e.g._ the liver). The cavities are bounded behind by the backbone (vertebral column), which is made up of many flattened vertebræ. The uppermost vertebra supports the skull, which encloses a _cranial cavity_ (_Sch.h._) continuous with a _vertebral canal_ bounded by the vertebræ. Cranial cavity and vertebral canal form together a second body-space, in which are contained the brain and spinal cord.

FIG. 1.—Schematic Longitudinal Section of the Human Body.
]

We will now consider the individual parts of the body, beginning with the _skeleton_. The axis of the skeleton is formed by the vertebral column (spine), which is composed of flat bones, the _vertebræ_. A vertebra usually consists of (1) the body, which occupies the front; (2) the arch, which possesses several projections or processes (neural spine, transverse processes, articular processes) and encloses the vertebral canal (_W.h._). All mammals have seven neck or cervical _vertebræ_ (Fig. 2, 1); while the number of the remaining vertebræ varies according to the species. The cervical vertebræ, which support the head, are followed by the dorsal or _thoracic vertebræ_ (12 in man, Fig. 2, 2), and these by the strong loin or _lumbar vertebræ_ (5 in man, Fig. 2, 3). Cervical, thoracic, and lumbar vertebræ are movable, but, in man, the last-named are followed by five vertebræ immovably united together to make up the _sacrum_, and these again by tail- or _caudal vertebræ_. Man has four such vertebræ, all poorly developed, and fused with one another (Fig. 2, 5); but in many animals there are a large number, movably united to make up a tail.

The ribs, which in mammals bound the chest, are jointed to the thoracic vertebræ. Man has 12 pairs of ribs; each rib consists of a bony part behind and a gristly (cartilaginous) part in front. The so-called true ribs (Fig. 2, 14) [the upper pairs] are movably united with the breast-bone, but this is not the case with the false ribs (Fig. 2, 15).

In the head we distinguish the brain-case or _cranium_, and the skeleton of the _face_. The first contains the cranial cavity in which the brain is enclosed. We distinguish—2 frontal bones (fused together in man, Fig. 2, 6); 2 parietal bones (7); 2 temporal bones (8); an occipital bone (9) composed of several pieces fused together, perforated by the foramen magnum [where brain and spinal cord unite], and bearing two elevations or condyles [for effecting union with the backbone]; and the sphenoid and ethmoid bones which make up the base of the cranium. The facial skeleton consists of the framework of the jaws and palate, and, together with some of the cranial bones, bounds the cavities in which the eyes are contained (orbits), and the nasal cavities. It consists of the maxillary bones (Fig. 2, 12), the premaxillary bones (Fig. 3, 7,—in man these 4 bones are fused together into one piece), the nasal bones, the lachrymal bones, the ploughshare bone (vomer), the turbinated bones, the cheek-bones (or malars, Fig. 2, 11), the palate-bones, and the lower jaw (Fig. 2, 13). (The last originally consists of two symmetrical halves.)

The upper and lower limbs are built on the same type, and therefore consist of corresponding parts (cp. Fig. 2). The more similar the functions of the two pairs, the closer their resemblance. In the ox they are much more alike than in man; in the bird, on the contrary, the similarity is much less. A distinction can be drawn in both limbs between the bony girdles (shoulder-girdle and hip-girdle), which serve for union with the trunk-skeleton, and the different subdivisions of the limbs themselves. I place side by side the parts of the arm and leg of man.

ARM. │ LEG.
I. Shoulder-girdle, consisting │ I. Hip-girdle, consisting of:
of: │
Shoulder-blade (Scapula) (Fig.│ Hip-bone (Ilium) (24).
2, 17). │
Collar-bone (Clavicle). │ Pubis.
Coracoid process (of Scapula).│ Rump-bone (Ischium).
II. Upper arm: │ II. Thigh:
Upper arm-bone (Humerus) (18).│ Thigh-bone (Femur) (25).
III. Fore arm: │III. Leg:
Radius (19). │ Shin-bone (Tibia) (26).
Ulna (20). │ Clasp-bone (Fibula) (27).
IV. Hand: │ IV. Foot:
Two rows of wrist-bones │ Two rows of ankle-bones
(Carpal bones) (21). │ (Tarsal bones) (28).
Metacarpal bones (22). │ Metatarsal bones (29).
Finger-bones (Phalanges) (23).│ Toe-bones (Phalanges) (30).

The differences between arm and leg are explained by their different uses. The bones of the leg, used to support the human body, are firmer and thicker, but less movable than those of the arm, which is employed in grasping. Consequently the union between the hip-girdle and the trunk-skeleton is firmer than that of the shoulder-girdle. The radius can rotate upon the ulna, so as to completely turn the hand over; a similar twisting of the foot would not be of use, and cannot be effected. The leg has a knee-pan (patella) (Fig. 31), with which there is no bone in the arm to correspond. In the foot the toes are short, and the remaining parts long; for instance, one of the tarsal bones, the calcaneum (heel-bone), is strongly developed and projects behind (28*). In the hand, the digits are relatively long, and since the tip of the thumb can be made to touch the tips of all the fingers, are admirably adapted for grasping.

FIG. 2.—The Human Skeleton.
]

The number of fingers or toes is at most five, but may be less. The horse has a single digit to each limb; the ox, two well developed and two remaining as rudiments; the pig, two large and two small; while the dog has four toes in the hind foot, five in the fore foot.

Man walks on the sole of the foot. Some other animals (dog, cat) on the toes; others again (horse, ox, pig), on the tips of the toes. In the last case there is not simply a horny structure (nail or claw) on the upper side of the toe, but a hoof sheathing the whole of its tip. In many animals the thigh and upper arm are drawn close up to the body, so that the limbs appear quite different from those of man. (Compare Fig. 2 with Fig. 3.)

FIG. 3.—Skeleton of an Ox. I. _Skull_: 1, Frontal bone, with horn
cores, _a_; 2, temporal bone; 3, malar or cheek-bone; 4, maxillary
bone; 5, lachrymal bone; 6, nasal bone; 7, premaxillary bone; 8,
lower jaw; 9, orbit; 10, occipital bone. II. _Neck and Trunk_: _H_,
7 cervical vertebræ; _R_, 13 thoracic vertebræ; _L_, 6 lumbar
vertebræ; _K_, sacrum; _Su_, caudal vertebræ; _C_, 13 pairs of ribs;
_D_, sternum. III. _Fore Limbs_: _Sc._, scapula; _A_, humerus; _S_,
radius; _E_, ulna; _U_, carpus; _M_, metacarpals; i., ii., iii.,
phalanges. IV. _Hind Limbs_: _B_, hip-girdle, _a_, ilium, _b_,
ischium; _F_, femur; _P_, patella; _T_, tibia; _Sp_, tarsus; _M_,
metatarsals; i., ii., iii., phalanges.
]

The bones are usually surrounded by flesh. This consists of a number of different pieces united together by a delicate, elastic, fibrous mass (connective tissue). The different pieces are termed _muscles_, each of which is again made up of a large number of muscle-fibres, all taking a longitudinal direction. Each fibre can contract, and a muscle becomes shorter and thicker by simultaneous contraction of all its fibres. The contraction and subsequent relaxation of muscles move other parts. There are some muscles, the _hollow muscles_, which surround a cavity, and by their contraction propel the liquid or solid substances found in their cavity. The heart, for example, is a large muscle of this sort, serving to propel the blood, while the hollow muscular coat of the gut moves on the contained food. Other muscles are fixed by their ends to other parts of the body, which they move by their contraction. We distinguish between _dermal muscles_ and _skeletal muscles_, attached respectively to the skin or by one end to an integumentary structure (hair, feather, scale), and to parts of the skeleton. The animals which are devoid of any internal skeleton, the _invertebrates_ (_i.e._ all animals except vertebrates), naturally possess no skeletal muscles. Examples of _dermal muscles_ are those by means of which a bird erects its feathers (tail-coverts of peacock!), and those which enable a hedgehog to roll itself into a ball and stick out its spines. Each end of a _skeletal muscle_ is connected with a bone. If such a muscle contracts the more easily movable bone is drawn towards the less easily movable one (Fig. 4). In order that the bones may be movable upon one another they are united together by joints.

According as muscular movements are, or are not, under the influence of the will, they are distinguished as _voluntary_ and _involuntary_. To the latter kind belong the movement of the heart, and the movements of the muscles in the wall of the gut by which the food is made to progress.

FIG. 4.—Bending of the Arm by Contraction of the Biceps Muscle. _a_,
humerus; _b_, ulna; _c_, elbow-joint; _d_, biceps muscle; _e_,
origin; _f_, insertion of the same. In the right-hand figure of the
muscle _d_ is contracted; in the left-hand figure it is slackened.
]

To destroy the contractile power of a muscle it is not necessary to injure the muscle itself. Every muscle is related to a nerve, which sends its fine branches to the fibres making up the muscle. If we cut the nerve, the corresponding muscle loses its power of contraction. But the nerve arises from the _central nervous system_, which in vertebrates principally consists of the brain and spinal cord. The muscle will therefore lose its contractile power if the connection with these central parts is broken. The true cause of movement resides in these parts. A sort of change, the essential nature of which is unknown to us, takes place in them, and is propagated along the nerve to the muscle, causing it to contract. The central nervous system is, therefore, the origin, the centre from which the order to contract proceeds; hence its name. The nerves which run from these central parts to the muscles are known as the _nerves of movement_ (motor nerves).

FIG. 5.—Diagram to explain the Action of the Motor and Sensory Nerves.
]

There is still, however, a second group of nerves, the _nerves of sensation_ (sensory nerves), which arise in the sense-organs (skin, mucous membrane of tongue, nose, ear, eye), and convey to the central nervous system the impressions they receive from the outer world by the aid of these sense-organs. In the appended diagram (Fig. 5), _C_ represents the central nervous system; _B.N._, a motor nerve, branching in the muscle _M_; _G.N._, a sensory nerve, which runs from the blood-bathed inner skin or dermis (_L.h._), underlying the outer skin or epidermis (_O.h._), to the central system. (The arrows indicate the direction in which impulses are conveyed along the corresponding nerve.)

Men or animals lose in weight if they take no food. The reason for this is that certain substances leave the body either as gases (through the lungs), or as liquids (by the kidneys and sweat-glands), without a corresponding compensation. An animal or human being could not live without taking in fresh substances, which, according as they are solid or liquid, are known as food or drink. The different kinds of food and drink, which, with few exceptions (salts, water), are taken from the animal and plant kingdoms, cannot, however, _as such_, replace the gradually diminishing body substance, for, to begin with, they contain useless matters, which pass out of the body in the _fæces_ (dung). And even the nutritious parts of the animal and vegetable substances taken into the stomach, are not always in a form in which they can be used _at once_. Digestion, which in all the higher animals takes place in a _food-tube_ (gut), serves to reduce them to a suitable condition, at the same time separating the useless matters. The action of several fluids (saliva, gastric juice, bile, etc.) secreted by glands, extracts the useful (nutritious) substances from the food and drink, converting them also into a suitable form. The smaller the pieces into which the food is separated, the better can this purpose be effected. In mammals the teeth serve to break down the food; in birds and many Invertebrates the same part is played by special secretions of the stomach or intestine provided with hard ridges.

So long as the nutritious food-stuffs remain in the food-canal, even though in a completely suitable form, they cannot nourish the body. And since waste of the substance of the body everywhere takes place, it is absolutely necessary that the food-stuffs should pass after digestion into a system of organs going to all parts of the body. This system is the _circulatory_, or _vascular system_. Food-stuffs enter it from the gut directly or indirectly, reaching it in the latter case through the _lymphatic_ (lacteal) system.

The _blood_ is the fluid into which the food-stuffs are taken up. It consists of an almost colourless liquid, together with an innumerable number of exceedingly minute blood-corpuscles.

The blood flows through the body in a system of tubes, or _blood-vessels_, which branch repeatedly, and at last become merged in the microscopic _capillary blood-vessels_. These capillaries are present in nearly all parts of the body except the epidermis and epidermal structures (hairs, feathers, scales, etc.). They have exceedingly thin walls, which present no resistance to the passage of the nutritious substances contained in the blood, so that these can be absorbed by those parts of the body which lie between the individual capillary vessels. The central organ of the circulation is the _heart_, an enlarged part of the vascular system, possessing thick muscular walls. By contraction of these, the blood is driven out of the heart (Fig. 6, _H_); and its exit is possible on one side only (_a_), as at the other side (_b_) there is a valve, which closes when the heart contracts. The vessel into which the blood leaving the heart enters is termed an _artery_ (_S.A._) It divides into several branches, also known as arteries, and the smallest arteries pass into capillaries, which again are connected with _veins_, which join larger and larger veins, until finally one or a few open into the heart (_A_).

FIG. 6.—Diagram of the Course of the Circulation.
]

Since the blood in the course of its circulation gives up some of its nutriment to the various parts of the body, it would in the end become useless for the purposes of nutrition if it did not receive a fresh supply of food-stuffs from the gut, either directly or indirectly (through the lacteal system). But apart from this, the blood would ultimately become useless, and that very quickly, if it did not traverse the lungs, kidneys, and sweat-glands. It is well known to every one that a man or animal cannot live without air, or at any rate without a certain gas, _oxygen_, that is contained in air. This oxygen must be able to penetrate into the minutest particles of the body, and the blood, in the corpuscles of which it is contained, carries it everywhere. In the smallest particles (molecules) of the body an oxidation (combustion) of body substance takes place, which not only causes an evolution of heat, but also renders the body capable of doing work. But if now the blood passes from the capillaries into the veins, it contains too little oxygen. And besides, it has taken up from the molecules of the body several substances, developed in those molecules, which would be fatal to the animal if they were not removed from the body. Now, when the blood streams through the lungs, it gets rid of the poisonous gaseous matter, and when it traverses the kidneys and sweat-glands it parts with the injurious liquid and solid substances. But in the lungs the blood takes up at the same time fresh oxygen; and since in this way the air in the lungs becomes poor in oxygen, the _movements of breathing_ (respiration) provide for the passage of a fresh supply of oxygen into the lungs. Only the higher Vertebrates breathe by means of lungs; fishes and numerous aquatic Invertebrates breathe by gills, and insects by air-tubes (tracheæ).

FIG. 7.—The Small-winged Gall-fly, _d_ (_Andricus terminalis_), lays
its eggs separately in the rootlets of oak. Root-galls (_a_) result
from this, and inside of each of them a larva develops which, after
a metamorphosis, becomes a relatively large, wingless gall-fly (_c_)
known as _Biorhiza aptera_. This pierces the oak-buds in early
spring, and lays a large number of eggs in them; from part of the
bud is formed a large juicy gall (_b_), containing several larvæ,
from which the small-winged gall flies (_d_) develop. The species
here represented exist, therefore, in two forms, _e_ and _d_
(Heterogeny).
]

While _Nutrition_ is the life-process which shields the _individual_ from death, Reproduction serves to maintain the _species_. It is familiarly known that the offspring generally resemble their parents. But it is also a fact recognized by the stock-breeder, that a particular animal will not only transmit several of its _own_ characteristics to its offspring, but perhaps also various characteristics of the grandparents or of animals belonging to still more remote generations, although these characteristics are not visible in the animal which is actually breeding (_Reversion_, _Atavism_). Among insects and the lower animals there are species which, as adult animals, appear not in _one_ form, but two or several. In this case, as a regular thing, the offspring does not resemble the parents, but the grandparents, great-grandparents, or a still earlier generation. The older observers have placed the offspring and the parents, and sometimes the grandparents too, of the same animal species in different species, or even genera or families, until newer researches on the reproduction and development of these animals have proved them to belong to one and the same species (see Fig. 7 and explanation). The method of reproduction by which a species appears in two or several forms is distinguished as _heterogeny_ and _metagenesis_, or _alternation of generations_. In the first (Fig. 7) sexually reproducing animals alternate with other sexual animals. It may be that these are of separate sexes, or else they may possess both male and female organs (_hermaphrodite_). In metagenesis a sexual generation regularly alternates with one or several generations reproducing asexually.

The animal kingdom falls (cf. p. 2) into sub-kingdoms or main divisions. Seven of these are commonly distinguished: I. Backboned animals; II. Jointed-limbed animals; III. Worms; IV. Molluscs; V. Echinoderms; VI. Cœlenterates; VII. Protozoa.

First Sub-Kingdom: VERTEBRATA (BACKBONED ANIMALS).

The Vertebrate body possesses a bilateral or twosided symmetry; _i.e._ it can be separated into two exactly corresponding halves, by a plane of division. The bilateral symmetry is strictly carried out as regards the external parts of the body, a single exception to this being flat-fish (plaice, flounder, etc.); but, on the other hand, it is more or less obliterated in the arrangement of the internal organs. In the Vertebrate body we find, as an axis, a vertebral column (backbone) made up of vertebræ, and traversed by the vertebral canal. As soon as this canal widens out in the skull to the cranial cavity, the spinal cord, which it contains, merges into the brain. In addition to the cavity containing the central nervous system, and placed on the upper side (= dorsal side) of the animal, a cavity, the _body-cavity_, is found in the under side (= ventral side). It contains for the most part the organs of respiration, circulation, digestion, and excretion (Fig. 1), and in Mammals is divided by the diaphragm into _thoracic_ and _abdominal cavities_. In all the other subdivisions of the animal kingdom the central nervous system is situated in the same cavity as the above-mentioned organs.

Various bones are connected with the vertebral column, and they serve for the attachment of muscles. The bones collectively constitute the skeleton, which is one of the most distinctive features of a Vertebrate.

The animals of this sub-kingdom never have more than four limbs, and their blood is red, while that of most other animal groups is colourless.

The structure of the heart in the various Vertebrates must also be noticed. In no Vertebrate is this organ so simple in structure as in the scheme given in Fig. 6; such an arrangement, moreover, would involve great difficulties. One great difficulty would be that while the blood was leaving the heart at _a_ (Fig. 6), no fresh blood could enter, so that the blood in the veins would stand still. Even in the lowest Vertebrates (the Fishes) this difficulty is obviated, for where the main vein (or veins) opens into the heart an enlargement of this vein is found, where the blood can collect as long as the heart continues to contract. This expansion is also reckoned as part of the heart, and named the _auricle_ (Fig. 8, _V.K._), while the heart proper is termed the _ventricle_ (_K._). It is also easy to see that there must be a tolerably wide opening between the two chambers, so that as soon as the ventricle becomes flaccid the auricle can force blood into it. But there being such a wide aperture between auricle and ventricle, one valve is not enough to make it impossible for the blood to pass back into the auricle during the contraction of the ventricular walls. There are two or three valves there (Fig. 8, _Kl._) fixed by fibres to the wall of the ventricle. In order that the blood which is forced into the artery (_S.A._) may not pass into the ventricle during its relaxation, there is another valve (not indicated in Fig. 8), at the base of the artery.

FIG. 8.—Diagram of the Heart in a Fish.
]

An arrangement like that so far described is found in fishes. The heart consists in them of an auricle, into which is returned the blood that has traversed the body, and of a ventricle which moves it on again. But the blood that has traversed the body is on that account poor in oxygen, and consequently unfit to be circulated again when it is returned to the heart. It is necessary for it to take up fresh oxygen before being circulated again. In fishes this difficulty is met by the blood, poor in oxygen, which flows out of the ventricle, first going to the _gills_ and streaming through them. The gills consist of a very large number of small, thin-walled outgrowths arranged in regular rows on the firm gill-arches. The blood, poor in oxygen, passing out of the ventricle and through various arterial branches to the gill-filaments, takes up fresh oxygen as it streams through these from the oxygen dissolved in the water which constantly surrounds them. For this purpose a stream of pure water is regularly taken in by the mouth and expelled again, right and left, through the gill-slits. The blood, having become rich in oxygen in the gills, is now once more fit for circulation through the body, and therefore flows out of the gill-capillaries into larger vessels, which finally unite into a single large vessel that carries the purified blood to the various parts of the body. In the arrangement of the heart here described there is the disadvantageous condition that the blood is obliged to traverse two sets of capillaries (gill and body capillaries). This is not an easy matter, for there is a great deal of friction between the blood and the walls of the capillaries, constituting a hindrance to its progress. The circulation of the blood in fishes is consequently very slow, and since the blood contains the oxygen which is used by the various parts of the body, oxidation goes on slowly in the body of a fish; hence the small amount of heat developed there. Since fishes almost immediately give off to their surroundings the small amount of heat which they develop, they have no constant body temperature, varying in this respect with the temperature of the surrounding water. Such animals are termed _cold-blooded_.

In all other Vertebrates a more rapid movement of the blood is rendered possible by the insertion of a second heart, quite similar in every respect to the other heart, in the course of the blood between the respiratory organs and the body. The first heart drives the blood through the lung capillaries, and therefore corresponds to the fish-heart; from these capillaries the blood returns to the auricle of the second heart, and from the ventricle of that heart travels to the various parts of the body. When it has completed this course, it returns to the auricle of the first heart. Although these two structures work independently, they lie close together and make up a single organ. We do not therefore speak of two individual hearts, but of one heart with two halves. The first half, which receives the blood, poor in oxygen, that is returned from the body, and sends it on to the lungs, lies on the right, and is termed the _right_ half. The second half, which receives the richly oxygenated blood from the lungs, and pumps it to the various parts of the body, is termed the _left_ half (Fig. 9 and explanation).

In the arrangement just described, which is found in Mammals and Birds, the blood returning from the lungs is propelled with new force through the body, and therefore circulates very quickly, so that the various parts receive a relatively large amount of oxygen in a short time. It is therefore intelligible that Birds and Mammals develop more warmth than Fish. They possess a special, constant body temperature, somewhat different in different species, but usually lying between 98° and 104° Fhr., and they are called _warm-blooded_ animals.

FIG. 9.—Diagram of the Mammalian Heart. 1, right, 2, left ventricle;
3, right, 4, left auricle; 5, superior, 6, inferior vena cava; 7,
pulmonary artery forking into branches for right and left lungs; 8,
the four pulmonary veins; 9, the great body-artery (aorta); the
arrows indicate the direction of the blood-stream.
]

FIG. 10.—Diagram of the Heart of a Reptile. Between the right (_r.K._)
and left (_l.K._) ventricles is a perforated partition. _r.V.K._,
right auricle; _l.V.K._, left auricle; _H.v._, vena cava, carrying
back the blood which has traversed the body into the right auricle;
_L.art._, pulmonary artery; _L.v._, pulmonary vein; _Ao._, aorta.
]

In Reptiles (snakes, lizards, etc.), the two halves of the heart are not entirely distinct, since there is an opening in the partition-wall between the two ventricles. As a consequence of this, the poorly oxygenated blood of the right half of the heart mixes with the richly oxygenated blood of the left half, the extent to which this mixing takes place being proportional to the size of the aperture. In Reptiles, therefore, the blood supplied to the lungs is not so poor in oxygen as it might be, nor, on the other hand, is the blood supplied to the other parts of the body completely oxygenated. As consequences of this: (1) respiration is feebler, and (2) the development of heat less than in Mammals and Birds (_i.e._ reptiles are cold-blooded), and (3) the chemical changes taking place in the body (the metabolism) go on more slowly than in warm-blooded animals, and we can understand why reptiles execute fewer movements in a given space of time.

FIG. 11.—Diagram of a Frog’s Heart. (The ventricle _K._ is quite
undivided: compare Figs. 9 and 10.) Other letters as in Fig. 10.
]

In Amphibians (_e.g._ frog) the two ventricles are similarly connected, but the opening is still larger than in Reptiles, and the partition-wall may even be altogether absent. It follows, therefore, that the mixing of the two kinds of blood is still more complete, and that Amphibians, too, are cold-blooded.

The vertebrate sub-kingdom embraces the classes of I. Mammals; II. Birds; III. Reptiles; IV. Amphibians; V. Fishes.

CLASS I.: =MAMMALIA= (SUCKLERS).

Warm-blooded Vertebrates (p. 16), usually covered with hair, and bringing forth living young, that suck for some time after birth. The female is provided with milk-glands on the thorax or abdomen, or both those regions.

Speaking quite broadly, the skeleton is like that of man, described on pp. 4–8. There are, however, great differences in detail. The cranium is relatively much smaller, and the bones of the face (especially of the jaws) are usually much larger than in the human skull. The number of the cervical vertebræ is seven in all Mammals, as in man; but the other kinds of vertebræ vary in number in the different species. The number of caudal vertebræ, for example, is very variable. As most Mammals go on all fours, their fore and hind limbs are much more similar than is the case in man. In many the thigh and upper-arm bones are drawn closely up to the body (horse, ox, pig). Mammals never have more than five fingers or toes, but may have fewer. The thumb or great toe is the first to disappear (hind foot of dog, fore and hind foot of pig). There may be only three (rhinoceros), two (ox, sheep), or one (horse) digit developed. In addition to fully developed digits, there are in many Mammals very small stunted ones (“dew-claws” of the stag).

FIG. 12.—Vertical Section of a Human Grinding Tooth.
]

FIG. 13.—Crown of a Grinder of the Ox. _a_, cement; _b_, enamel; _c_,
dentine; _d_, enamel; _e_, cement.
]

There are also great differences in the way of resting the feet on the ground. Man and bear tread on the soles of the feet (plantigrade); dog and cat walk on the under side of the toes (digitigrade), not on the other parts of the feet. Ox, pig, horse, etc., rest while walking only on the tips of the toes, which are sheathed in hoofs (unguligrade).

The teeth of mammals are wedged into special sockets in the jaw-bones. The structure of a mammalian tooth is made clear by Fig. 12. We first distinguish a pulp-cavity (_p_), which in the living animal is filled with a substance supplied by a bloodvessel and nerve. This cavity is surrounded by the dentine (_d_), a hard substance which makes up the greater part of the tooth. Hard enamel (_s_) covers the whole of the crown in man and many animals, while in certain other forms it is found only on part of the crown. The root of the tooth is covered with cement (_z_), a bone-like substance.

All teeth in which the entire surface of the crown is covered by enamel only are known as _simple teeth_, while those into which the enamel only penetrates in more or less deep folds, leaving the rest of the crown uncovered, are known as _compound teeth_ (Fig. 13). The structure of the teeth is related to the nature of the food. We distinguish three kinds of teeth in the same animal, which, however, are not all present in every species; these are the _incisors_, _canines_, and _grinders_. The first two kinds are changed; but only the anterior grinders, known as the _premolars_, are changed, while the hinder ones, the _true molars_, do not first appear as “milk” teeth, but rather later on with the other “permanent” teeth.