Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct

II GRPST OF THE EARTH AS REBATE© TO zoology

Principles of Zoology,

TOUCHING

THE STRUCTURE, DEVELOPMENT, DISTRIBUTION, AND NATURAL ARRANGEMENT

OF THE

RACES OF ANIMALS, LIVING AND EXTINCT.

WITH NUMEROUS ILLUSTRATIONS.

I* A K T I .

COMPARATIVE PHYSIOLOGY.

FOR THE USE OF SCHOOLS AND COLLEGES.

BY

LOUIS AGASSIZ and A. A. GOULD, glcbisrb (Bbitioir.

BOSTON :

<3-0X7 1,10 AND LINCOLN.

50 WASHINGTON STREET. new YORK: SHELDON AND COMPANY.

Entered according to Act of Congress, in the year 1S7". by GOULD AND LINCOLN.

In the office of the Librarian of Congress, at Washington.

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Preface

The design of this work is to furnish an epitome of the leading principles of the science of Zoology, as deduced from the present state of knowledge, so illustrated as to he intelligible to the beginner. No similar treatise now exists in this country, and, indeed, some of the topics have not been touched upon in the English language, unless in a strictly technical form, and in scattered articles. On this account, some of the chapters, like those on Embryology and Metamorphosis, may, at first, seem too abstruse for scholars in our common schools. This may be the case, until teachers shall have made themselves somewhat familiar with subjects comparatively new to them. But so essential have these subjects now become to a correct interpretation of philosophical zoology, that the study of them will hereafter be indispensable. They furnish a key to many phenomena which have been heretofore locked in mystery.

Being intended for American students, the illustrations have been drawn, as far as possible, from American objects : some of them are presented merely as ideal outlines, which convey a more definite idea than accurate sketches from nature ; others have been left imperfect, except as to the parts especially in question ; a large proportion of them, however, are accurate portraits from original drawings. Popular names have been employed as far as pcssible, and to the scientific names an English termination has generally been given ; but the technical terms have been added, in brackets, whenever misunderstanding was apprehended. Definitions of those least likely to be understood, may be found in the Index.

The principles of Zoology developed by Professor Agassiz in his published works have been generally adopted in this, and the results rf many new researches have been added.

The authors gratefully acknowledge the aid they have received, In preparing the illustrations and working out the details, from Mr

1 •

PREFACE.

E. Desor for many years an associate of Professor Agassiz, from Count Pourtales and E. C. Cabot, Esq., and also from Professor Asa Gray, by valuable suggestions in the revision of the letter-press.

The first part is devoted to Comparative Anatomy, Physiology, and Embryology, as the basis of Classification, and also to the illustration of the geographical distribution and the geological succession of Animals ; the second to Systematic Zoology, in which the principles of Classification will be applied, and the principal groups cf animals will be briefly characterized.

Should our aim be attained, this work will produce more enlarged ideas of man’s relations to Nature, and more exalted conceptions of the Plan of Creation and its Great Author.

V

Boston, June 1, 1848.

Preface To The Revised Edition

In revising the present work, the authors have endeavored to render more precise those passages which admitted of too broad a signification or of a double interpretation; and to correct such errors as had arisen from inadvertence, or such as the rapid progress of Science has disclosed. They are indebted for many suggestions on these points to several distinguished teachers who have used the work as a text book, and more especially to Professor Wyman, of Harvard University. Several entirely new paragraphs have also been added.

A list of some of the principal authors who have made original researches, or of treatises which enter more into detail than was admissible in an elementary work, has been given at the close of the volume, for the use of those who would pursue the subject of Zoology in a more extended manner.

The work having thus been revised and enlarged, the authors subout it to the public with increased confidence in its accuracy and usefulness.

Boston, February 1, 1851.

CHAPTER FIFTH. Of Motion ... 73

SECTION I.

Apparatus of Motion ... 7S

SECTION II.

Of Locomotion 79

1. Plan of the Organs of Locomotion . . . . 82

2. Of Standing, and the Modes of Progression ... 88

Walking 90

Running ’.91

Leaping 91

Climbing 92

Flying 92

Swimming ... 93

SECTION II. Palaeozoic Age ... .... 223

Secondary Age ... 227

Tertiary Age 233

Modern Age .... ....

EXPLANATION OF THE FIGURES-

Frontispiece. — The diagram opposite the title page is intended to present, at one view, the distribution of the principal types of animals, and the order of their successive appearance in the layers of the earth’s erust. The four Ages of Nature, mentioned at page 221, are represented by four zones, of different shades, each of which is subdivided by circles indicating the number of formations of which they are composed. The whole disk is divided by radiating lines into four segments, to include the four great departments of the Animal Kingdom; the Vertebrates, with Man at their head, are placed in the upper compartment, the Articulates at the left, the Mollusks at the right, and the Radiates below, as being the lowest in rank. Each of these compartments is again subdivided to include the different classes belonging to it, which are named at the outer circle. At the centre is placed a figure to represent the primitive egg, with its germinative vesicle and germinative dot, (278,) indicative of the universal origin of all animals, and the epoch of life when all are apparently alike, (275, 276.) Surrounding this, at the point from which each department radiates, are placed the symbols of the several departments, as explained on page 155. The zones are traversed by rays which represent the principal types of animals, and their origin and termination indicates the age at which they first appeared or disappeared, all those which reach the circumference being still in existence. The width of the ray indicates the greater or less prevalence of the type at different geological ages. Thus, in the class of Crustaceans, the Trilobites appear to commence in the earliest strata, and to disappear with the carboniferous formation. The Ammonites also appeared in the Silurian formation, and did not become extinct before the deposition of the Cretaceous rocks. The Belemnites appear in the lower Oolitic beds ; many forms commence in the Tertiary; a great number of types make their appearance only in the Modern age ; while only a few have continued from the Silurian, through every period to the present. Thus, the Crinoids were very numerous in the Primary Age, and are but slightly developed in the Tertiary and Modern Age. It is seen, at a glance, that the Animal Kingdom is much more diversified in the later than in the earlier Ages.

Below the circle is a section, intended to show more distinctly the relative position of the ten principal formations of stratified rocks (461) composing the four great geological ages ; the numerals corresponding to ►*»ose on the ray leading to Man, in the circular figure. See also figure 154.

EXPLANATION OF THE FIGUitES.

The Chakt op Zoological Regions, page 195, is intended to show the limits of the several Faunas of the American Continent, corresponding to the climatal regions. And as the higher regions of the mountains correspond in temperature to the climate of higher latitudes, it will be seen that the northern temperate fauna extends, along the mountains of Mexico and Central America, much farther towards the Equator than it does on the lower levels. In the same manner, the southern warm fauna extends northward, along the Andes.

Fio.

1. Simple cell, magnified, as seen in the house-leek.

2. Cells when altered by pressure upon each other ; from the pith of elder.

3. Nucleated cells, ( a ,) magnified; b, nucleolated cells.

4. Cartilaginous tissue from a horse, magnified 120 diameters.

5. Osseous tissue from a horse, magnified 450 diameters.

6. Nervous fibres, showing the loops as they terminate in the skin of a

frog.

7. Gray substance of the brain, magnified.

8. Head of an embryo fish, to show its cellular structure throughout

9. Diagram, to show the nervous system of the Vertebrates, as found

in a monkey.

10. Digram of the nervous system of the Articulates, as seen in a lobster.

11. Diagram of the nervous system of the Mollusks, as found in Natica

heros.

12. Diagram of the nervous system of the Radiates, as found in Scutella,

( Echinarachnius parma. )

13. Section of the eye. a, optic nerve ; b, sclerotic coat ; c, choroid coat;

d, retina ; e, crystalline lens ; f, cornea ; g, iris ; A, vitreous body ; t, chamber, divided by the iris.

14. Diagram, showing the effect of the eye on rays of light.

15. Position of the eye of the snail.

16. Eyes ( ocelli ) of the spider.

17. Eye-spots of a star-fish, (Echinaster sanguinolentus.)

18. Compound eyes, showing the arrangement of the faoettes, and their

connection with the optic nerve, as seen in a crab’s eye.

19. Diagram of the human ear, to show the different chambers, canals,

and bones.

20 Tympanum and small bones of the ear, twice the natural sire ; a,

tympanum ; m, malleus ; n, incus ; o, orbiculare : *, stapes.

21 Section of the brain of a crow, showing the origin of the nerves of

the special senses.

22. Diagram of the larynx, in man.

23. Larynx of the merganser, (Magus merganser.)

24. Nests of Ploceus Philippinus, male and female.

25. Distribution of nerves to the muscular fibres.

26. Test, or crust-like covering of an Echinodcrra, ( Cidaru .)

EXPLANATION OF THE FIGURES.

Fin

27. Muscular ribbons of the willow-moth. (Cossus ligniperda. )

28. Vertebra of a cod-fish.

29. Disposition of the muscles of the trout, (Salmo trutta .)

30. Disposition of the muscles of an owl, ( Strix brachyotis. )

32. Leech, showing the terminal cups.

33. Portion of a Nereis, showing the gills as organs of motion.

34-43. Modifications of the fore-arm.

34. Monkey. 35. Deer. 36. Tiger. 37. Whale. 38. Bat. 39. Pigeon. 40. Turtle. 41. Sloth. 42. Mole. 43. Whale.

44. Leg of a beetle.

45. Leg of a lizard.

46 Skeleton of a tiger.

47 Cuttle-fish, (Loligo illecebrosa.)

48 Sea-anemone, ( Actinia marginata;) a, mouth ; b, stomach ; c, general

cavity of the body.

49. Planaria, showing the mouth, stomach, and its branches.

50. Jaws, stomach, and intestine of a sea-urchin, ( Echinus lividus.)

61. Plan of the digestive organs of an insect.

62. Plan of the digestive organs of a land-slug, (Tebennophorus Carolini-

ensis.)

53. Globules of chyle.

54. Portion of intestine, showing the lacteals of man, and. their entrano*

into a vein.

55. Jaws of an Echinoderm, {Echinarachnius parmc.

56. Jaws of a sea-ur.chin, (Echinus granulatus .)

57- Beak of a cuttle-fish.

58. Portion of the tongue of a mollusk, (Natica heros,) magnified.

59. J aws of an Annelide, {Nereis.)

60. Trophi (organs for taking food) of a beetle.

61. “ of a bee.

62. 63. “ of a squash-bug.

64. “ of a butterfly.

65. “ of a Rotifer, (Brachionus.)

66. Jaws of ditto, magnified.

67. Skull of a tiger, showing the muscles for mastication.

68. Head of a snapping-turtle, (Emysaurus serpentina. )

69. Head of a Whale, showing the whalebone.

70. Head of an ant-eater.

71. Head of an alligator.

72. Head of a skate-fish, {Myliobatis,) showing the palate bone.

73. Head of a monkey, showing the three different kinds of tenth.

74. Teeth of an insectivorous animal, the mole.

75. Teeth of a carnivorous animal, the tiger.

76. Teeth cf a rodent.

EXPLANATION OF THE FIGURES,

Fig.

77. A polyp, ( Tvbularia indivisa;) m, mouth; o, ovaries; p. tantue’

78. Blood disks in man, magnified.

79. “ “ in birds, “

80. “ “ in reptiles, “

81. “ “ in fishes, “

82 Porti on of a vein opened, to show the valves

83. Network of capillary vessels.

84. Dorsal vessel of an insect, with its valves.

85 Cavities of the heart of mammals and birds.

86. “ “ “ of a reptile.

87. “ “ “ of a fish.

88. Heart and bloodvessels of a gasteropod mollusk, ( Natica .)

83, Trachea:, or air tubes of an insect; s, stigmata; t, trachea.

90. Relative position of the heart and lungs in man.

91. Respiratory organs of a naked mollusk, (Polycera ilhtminata )

92. Respiratory organs (gills) of a fish.

93. Vesicles and canals of the salivary glands.

94. Section of the skin, magnified, to show the sweat glands ; a, the cutii

h. blood-layer ; c, epidermis ; g, gland imbedded in the fat-layer, (_/

95. Egg of a skate-fish, (Myliobatis.)

96. Egg of hydra.

97. Egg of snow-flea, ( Podurella .)

98. Section of an ovarian egg ; d, germinative dot ; g, gcrminative veai

cle ; s, shell membrane ; v, vitelline membrane.

99. Egg cases of Pyrula.

100. Monoculus bearing its eggs, a a.

101. Section of a bird’s egg ; a, albumen ; c, chalaza ; e, embryo ; s, shell

y, yolk.

102. Cell-layer 'of the germ.

103. Separation of the cell-layer into three layers ; s, serous or nervom

layer; m, mucous or vegetative layer; v, vascular or blood layer.

104. Embryo of a crab, showing its incipient rings.

105. Embryo of a vertebrate, showing the dorsal furrow.

106—8. Sections of the embryo, showing the formation of the dorsal canaL 109. Section, showing the position of the embryo of a vertebrate, in relation to the yolk.

110 Section, showing the same in an articulate, ( Podurella .)

111 -22. Sections, showing the successive stages of development of the

embryo of the white-fish, magnified.

123, Yeung white-fish just escaped from the egg, with the yolk not yet

fully taken in.

124, 125. Sections of the embryo of a bird, showing the formation of the

allantois ; e, embryo ; x x, membrane rising to form the amnio*; a, allantois ; y, yolk.

126. The same more fully developed. The allantois (a) further de*

EXPLANATION OF THE FIGURES.

reloped, and bent upwards. The upper part of the yoljr ( d d ) is nearly separated from the yolk sphere, and is to become the in testine. The heart (A) is already distinct, and connected by threads with the blood-layer of the body.

127. Section of the egg of a mammal ; v, the thick vitelline membrane.

or chorion ; y, yolk ; s, germinative dot ; g, germinative vesicle.

128. The same, showing the empty space (k) between the vitelline sphere

and chorion.

129. Shows the first indications of the germ already divided in two layers,

the serous layer, (s,) and the mucous layer, (m.)

130. The mucous layer (m) expands over nearly half of the yolk, and be-

comes covered with many little fringes.

131. The embryo ( e ) is seen surrounded by the amnios, (6,) and covered

by a large allantois, (a;) p e, fringes of the chorion ; p m, fringes of the matrix.

132. Hydra, showing its reproduction by buds.

133. Vorticella, showing its reproduction by division.

134. Polyps, showing the same.

135. A chain of Salpae.

136. An individual salpa ; tn, the mouth; a, embryos

137. Cercaria, or early form of the Distoma.

138. Distoma, with its two suckers.

139. Nurse of the Cercaria.

140. The same, magnified, showing the included young.

141. Grand nurses of the Cercaria, enclosing the young nurses.

142. Stages of development of a jelly-fish, (Medusa ;) a, the embryo in

its first stage, much magnified; b, summit, showing the mouth; c.f, g, tentacles shooting forth ; e, embryo adhering, and forming a pedicle ; h, i, separation into segments ; d, a segment become free ; k, form of the adult.

143. Portion of a plant-like polyp, ( Campanularia ) a, the cup which

bears tentacles ; b, the female cup, containing eggs ; c, the cups in which the young are nursed, and from which they issue.

144. Young of the same, with its ciliated margin, magnified.

145. Eye of the perch, containing parasitic worms, (Distoma.)

146. One of the worms magnified.

147. Transformations of the canker-worm, (Geometra vernalis ;) a, the

canker worm ; b, its chrysalis ; c, female moth ; d, male moth.

148. Metamorphoses of the duck-barnacle, (Anatifa;) a , eggs, magnified;

b, the animal as it escapes from the egg ; c, the stem and eye appearing, and the shell enclosing them ; d, animal removed from the shell, and further magnified ; e,f the mature barnacle, affixed.

149. Metamorphoses of a star-fish, (Echinaster sanguinolenius ,) showing

the changes of the yolk, (e;) the formation of the pedicle, (/>;) and the gradual change into the pentagonal and rayed form.

EXPLANATION OF THE FIOfUHES.

Fm.

150. Comatula, a West India species, in its early stage, with its stem

151. The same detached, and swimming free.

152. Longitudinal section of the sturgeon, to show its cartilaginous vcr

tebral column.

153. Amphioxus, natural size, showing its imperfect organization.

154. Section of the earth’s crust, to show the relative positions of the

rocks composing it ; E, plutonic or massive rocks ; M, metamorphic rocks ; T, trap rocks ; L, lava. 1. Lower Silurian formation ; 2. Upper Silurian ; 3. Devonian ; 4. Carboniferous ; 5. Trias, or Saliferous ; 6. Oolitic ; 7. Cretaceous ; 8. Lower Tertiary or Eocene ; 9. Upper Tertiary, or Miocene, and Pleiocene ; 10. Drift.

155. Fossils of the Palaeozoic age ; a, Lingula prima ; b, Leptsena alter-

nata ; c, Euomphalus hemisphericus ; d, Trocholites ammonius ; e, Avicula decussata ; f Bucania expansa ; g, Orthoceras fusi forme ; i, Cyathocrinus ornatissimus, Hall ; j, Cariocrinus ornatus, Say; k, Melocrinus amphora, Goldf. ; l, Columnaria alveo lata ; m, Cyathophyllum quadrigeminum, Goldf. ; n, o, Caninia flexuosa ; p, Chffitetes lycoperdon.

166. Articulata of the Palajozoic age ; a, Harpes ; b, Arges ; c, Brontes , d, Platynotus ; e, Eurypterus remipes.

157. Fishes of the Palajozoic age ; a, Ptericlithys ; b, Coccosteus ; c,

Dipterus ; d, palatal bone of a shark ; e, spine of a shark.

158. Representations of the tracks of supposed birds and reptiles in the

sandstone rocks.

169. Supposed outlines of Ichthyosaurus, (a,) and Plesiosaurus, (6.)

160. Supposed outline of Pterodactyle.

161. Shells of the Secondary age; a, Terebratula; b, Goniomya; e,

Trigonia ; d, Ammonite.

162. Supposed outline of the cuttle-fish, ( a ,) furnishing the Belemnite.

163. Radiata from the Secondary age ; a, Lobophyllia flabellum ; b, Litho-

dendron pseudostylina ; c, Pentacrinus briareus ; d, Pterocoma pinnata ; e, Cidaris ; f, Dysaster ; g, Nucleolites.

164. Shells of the Cretaceous formation ; a, Ammonites ; 6, Crioceras ;

c, Scaphites ; d, Ancyloceras ; e, Ilamites ; f, Baculites ; g, Turrilites. ^

165. Shells of the Cretaceous formation; a, Magas; b, Inoceramus ; o

160. Radiata from the Cretaceous formation ; a, Diploctenium cordatum b, Marsupites; d, Galerites; c, Salenia ; e Micraster coi* anguinum.

167 Nummulite.

168. Supposed outline of Paleotherium.

169 Supposed outline of Anoplotherium.

170. Skeleton of the Mastodon, in the cabinet of Dr. J. C. Warren.

Every art and science has a language of technical terms peculiar to itself. With those terms every student must make himself familiarly acquainted at the outset ; and, first of all, he will desire to know the names of the objects about which he is to be engaged.

The names of objects in Natural History are double ; that is to say, they are composed of two terms. Thus, we speak of the white-bear, the black-bear, the hen-hawk, the sparrowhawk ; or, in strictly scientific terms, we have Felis leo , the lion, Felis tigris, the tiger, Felis catus, the cat, Canis lupus , the wolf, Canis vulpes, the fox, Canis familiaris, the dog, &c. They are always in the Latin form, and consequently the adjective name is placed last. The first is called the generic name ; the second is called the trivial, or specific name.

These two terms are inseparably associated in every object of which we treat. It is very important, therefore to have a clear idea of what is meant by the terms genus and species ; and although the most common of all others, they are not the easiest to be clearly understood. The Genus is 2*

Introduction

founded upon some of the minor peculiarities of anakmica. structure, such as the number, disposition, or proportions of the teeth, claws, fins, &c., and usually includes several kinds. Thus, the lion, tiger, leopard, cat, &c., agree in the structure of their feet, claws, and teeth, and they belong to the genus Felis ; while the dog, fox, jackal, wolf, &c., have another and a different peculiarity of the feet, claws, and teeth, and are arranged in the genus Canis.

The Species is founded upon less important distinctions, such as color, size, proportions, sculpture, &c‘. Thus we have different kinds, or species, of duck, different species of squirrel, different species of monkey, &c., varying from each other in some trivial circumstance, while those of each group agree in all their general structure. The specific name is the lowest term to which we descend, if we except certain peculiarities, generally induced by some modification of native habits, such as are seen in domestic animals. These are called varieties, and seldom endure beyond the causes which occasion them.

Several genera which have certain traits in common are combined to form a family. Thus, the ale wives, herrings, shad, &c., form a family called Clupeidse; the crows, blackbirds, jays, &c., form the family Corvidae. Families are combined to form orders, and orders form classes, and finally, classes are combined to form the four primary divisions or departments, of the Animal Kingdom.

For each of these groups, whether larger or smaller, we involuntarily picture in our minds an image, made up of the traits which characterize the group. This ideal image is called a type, a term which there will be frequent occasion to employ in our general remarks on the Animal Kingdom. This image may correspond to «ome one member of the group ; but it is rare that any one species embodies all our deas of the class, family, or genus to which it be'ong*

INTRODUCTION.

Thus, we have a general idea of a bird ; but this idea does not correspond to any particular bird, or any particulai character of a bird. It is not precisely an ostrich, an owl, a hen, or a sparrow ; it is not because it has wings, or feathers, or two legs ; or because it has the power of flight, or builds nests. Any, or all, of these characters would not fully represent our idea of a bird ; and yet every one has a distinct ideal notion of a bird, a fish, a quadruped, &c. It is common, however, to speak of the animal which embodies most fully the characters of a group, as the type of that group. Thus we might, perhaps, regard an eagle as the type of a bird, the duck as the type of a swimming-bird, and the mallard as the type of a duck, and so on.

As we must necessarily make frequent allusions to animals, with reference to their systematic arrangement, it seems requisite to give a sketch of their classification in as popular terms as may be, before entering fully upon that subject, and with particular reference to the diagram fronting the titlepage.

The Animal Kingdom consists of four great divisions, which we call Departments, namely :

I. The department of Vertebrates.

II. The department of Articulates.

III. The department of Mollusks.

IV. The department of Radiates.

1. The department of Vertebrates includes all' animals which have an internal skeleton, with a back-bone for its axis. It is divided into four classes :

1. Mammals, (animals which nurse their young.)

2. Birds.

"u INTRODUCTION.

3. Reptiles.

4. Fishes.

The class of Mammals is subdivided into three orders s a Beasts of prey, ( Carnivora .) b Those which feed on vegetables, ( Herbivora .)

c. Animals of the whale kind, ( Cetaceans .)

The class of Birds is divided into four orders, namely,

a. Perching Birds, ( lnsessores .)

b. Climbers, ( Scansores .)

c. Waders, ( Grallatores.)

d. Swimmers, ( Natatores .)

The class of Reptiles is divided into five orders :

a. Large reptiles with hollow teeth, most of which are

now extinct, ( Rhizodonts .)

b. Lizards, ( Lacertians .)

c. Snakes, (Ophidians.)

d. Turtles, (Chelonians.)

e. Frogs and Salamanders, ( Batrachians .)

The class of Fishes is divided into four orders :

a. Those with enamelled scales, like the gar-pike,

(Gajioids,) fig. 157, c.

b. Those with the skin like shagreen, as the sharks and

skates, (Placoids.)

c. Those which have the edge of the scales toothed,

and usually with some bony rays to th? nns, as the perch, ( Ctenoids .)

INTRODUCTION.

d. Those whose scales are entire, end whose fin raya are soft, like the salmon, (Cycloids.)

II. Department of Articulates. Animals whose body is composed of rings or joints. It embraces three classes :

1. Insects.

2. Crustaceans, like the crab, lobster, &c.

3. Worms.

The class of Insects includes three orders :

a. Those with a trunk for sucking fluids, like the butter*

fly, ( Suctoria ,) fig. 62-64.

b. Those which have jaws for dividing their food, (Man-

ducata ,) fig. 60.

c. Those destitute of wings, like spiders, fleas, millipedes

&c., (Aptera.)

The class Crustaceans may be divided as follows .

a. Those furnished with a shield, like the crab and lob-

ster, (Malacostraca.)

b. Such as are not thus protected, ( Entomostraca .)

c. An extinct race, intermediate between these two,

( Trilobitcs ,) fig. 156.

The class of Worms comprises three orders :

a. Those which have thread-like gills about the head,

( Tubulibranchiates.)

b. Those whose gills are placed along the sides, (Dor-

sibranchiates.)

c. Those who have no exterior gills, like the earth-worm

( Abranchiates ,) and also the Intestinal Worms.

INTRODUCTION.

111. The department of Mollusks is di video into three classes, namely :

] . Those which have arms about the mouth, like the cuttle-fish, ( Ceplialopods ,) fig. 47.

2. Those which creep on a flattened disk or foot, like

snails, ( Gasteropods ,) fig. 88.

3. Those which have no distinct head, and are inclosed

in a bivalve shell, like the clams, ( Acephals .)

The Cephalopods may be divided into

a. The cuttle-fishes, properly so called, ( Teuthideans,)

b. Those having a shell, divided by sinuous partitions

into numerous chambers, ( Aimnonites ,) fig. 164.

c. Those having a chambered shell with simple par-

titions, (Nautilus.)

The Gasteropods contain four orders :

a. The land snails which breathe air, (Pulmonates.)

b. The aquatic snails which breathe water, (branch-

ifers ,) fig. 88.

c. Those which have wing-like appendages about the

head, for swimming, (Pteropods.)

d. A still lower form allied to the Polyps by their gen-

The class of Acephals contains three orders :

a. Those having shells of two valves, (bivalves,) like the

clam and oyster, (Lamellibranchiates.)

b. Those having two unequal valves, and furnished with

peculiar arms, (Brachiopods.)

c. Mollusks living in chains or clusters, like the Salpa, fig. 135 ; or upon plant-like stems, like Flustra, ( Bryo - zoa.)

- IV. The department of Radiates is divided into three classes :

1. Sea-urchins, bearing spines upon the surface, [Echin-

oderms,) figs. 12, 26.

2. Jelly-fishes, ( Acalephs ,) fig. 31.

3. Polyps, fixed like plants, and with a series of flexible

arms around the mouth, figs. 48, 77, 143.

The Echinoderms are divided into four orders :

a. Sea-slugs, like biche-le-mar, ( Holothurians .)

b. Sea-urchins, ( Echini ,) fig. 26.

c. Free star-fishes, (Asteridce,) fig. 17.

<i. Star-fishes mostly attached by a stem, ( Crinoids ,) figs. 150, 151.

The Acalephs include the following orders :

a. Those furnished with vibrating hairs, by which they

move, ( Ctenophorce .)

b. The Medusae, or common jelly-fishes, ( Biscophorce ,)

c. Those provided with aerial vesicles, ( Siphonophorce .)

The class of Polyps includes two orders.

a. The so-called fresh-water polyps, and similar marine

forms, with lobed tentacles, ( Hy droids ,) fig. 143.

b. Common polyps, like the sea-anemono and coral-

polyp, ( Aclinoids ,) fig. 48.

In addition to these, there are numberless kinds of micro-

INTRODUCTION.

sccpic animalcules, commonly united under the name of infusory animals, (Infusoria,) from their being found specially abundant in water infused with vegetable matter. These minute beings do not, however, constitute a natural group in the Animal Kingdom. Indeed, a great many that were formerly supposed to be animals are now found to be vegetables. Others are ascertained to be crustaceans, mollusks, worms of microscopic size, or the earliest stages of development of larger species. In general, however, they are exceedingly minute, and exhibit the simplest forms of animal life, and are now grouped together, under the title of Protozoa. But, as they are still very imperfectly understood, notwithstanding the beautiful researches already published on this subject, and as many of them are likely to be finally distributed among vegetables, and the legitimate classes in the Animal Kingdom to which they belong, we have not assigned any special place for thorn.

PHYSIOLOGICAL ZOOLOGY

Chapter First. The Sphere And Fundamental Principles Of Zoology

!• Zoology is that department of Natural History which relates to animals

2. To enumerate and name the animals which are found on the globe, to describe their forms, and investigate their habits and modes of life, are the principal, but by no means the only objects of this science. Animals are worthy of our regard, not merely when considered as to the variety and elegance of their forms, or their adaptation to the supply of our wants ; but the Animal Kingdom, as a whole, has a still higher signification. It is the exhibition of the divine thought, as carried out in one department of that grand whole which we call Nature ; and considered as such, it teaches us most .mportant lessons.