Chapter II: Front Matter (2)

The only beasts, however, which _truly_ fly are the bats, which form an order by themselves, well-named, from the structure of their wings, _Cheiroptera_. The bats which fly about in the twilight in this country, or sometimes in the afternoon of a warm day in winter, are all insect-eating forms. But in the warm regions of the Old World, and of Australia, there are large fruit-eating kinds, called "flying foxes;" while in South America there are blood-sucking bats, or vampires, some of which, as we shall hereafter see, present the most curious and interesting modifications of structure in harmony with their peculiar habits.

The creatures which are in some respects the most interesting to us, because they are the most like ourselves in form, are the apes. Moreover, not only are they so like us in form, but they are so widely marked-off from all other creatures except ourselves, that it seems impossible they can have any real affinity to one more than to another group of mammals below man. Apes and man then together form one order, which as ranking first was named by Linnæus, _Primates_. With the apes are commonly associated certain animals called Lemurs, which inhabit the vicinity of the Indian Ocean, especially Madagascar. They have not, however, any real affinity to apes; and if they are to be placed in the same order at all, they must be well distinguished from its other members. It has therefore been proposed[12] to divide the order Primates into two sub-orders (as the hoofed order is divided into the "odd-toed" and "even-toed" sub-orders), one of these to include man and apes, and to be called, from the resemblance to the human form pervading it, "_Anthropoidea_;" the other sub-order to be termed "_Lemuroidea_."

The first "sub-order" is divisible into three "families." One of these (_Hominidæ_) contains man (forming the genus _Homo_), the second (_Simiadæ_) contains all the apes of the Old World only, while a third (_Cebidæ_) contains all those of America.

Amongst the _Simiadæ_ are the orang, the chimpanzee, the gorilla, and the long-armed apes (or Gibbons), which are the most man-like of all the apes; and there can be no question but that there is very much less difference in structure between these four kinds of apes and man, than there is between them and the lowest of the apes--_i.e._, the marmosets.

Concerning this resemblance, Buffon has observed, when speaking of the ape, the most man-like (and so man-like) as to brain:[13] "Il ne pense pas: y a-t-il une preuve plus évidente que la matière seule, quoique parfaitement organisée, ne peut produire ni la pensée, ni la parole qui en est le signe, à moins qu'elle ne soit animée par un principe supérieur?"

As to the second sub-order, it contains some very curious forms. The typical lemurs (which inhabit Madagascar) have long fox-like snouts and long tails. Certain African forms (the genus _Galago_) are very active in their movements, and great leapers. A tailless group (the slender loris) is interesting, as presenting a diminutive quasi-human form, reflected, as it were, through a Lemurine prism, just as the rat-mole shows us a mole-form reflected through a rodent prism.

A little animal, the Tarsier, which is found on the islands of Celebes and Borneo, is very exceptional in its structure. Still more so is the aye-aye (_Cheiromys_). This very remarkable species was discovered by Sonnerat in Madagascar in 1770, and was never again seen till 1844, when a specimen was forwarded to Paris. It has now, however, become well known.

Inhabiting the sea are many beasts, which are, by mistake, popularly spoken of as "fishes." Such are the whales and the porpoises--animals which, in spite of their form and habit, suckle their young, and have hot blood, as all other mammals have. These creatures form an order by themselves, called _Cetacea_.

Another order of aquatic beasts is termed _Sirenia_, and the animals which compose it were long confounded with the _Cetacea_, from which, however, they are widely divergent in structure, in spite of the general similarity which exists between them in external appearance. The order _Sirenia_ contains but two existing genera. One of these is the now well-known manatee (_Manatus_), the other is the dugong (_Halicore_)--an animal very similar to the manatee, and found in the rivers of regions about the Indian Ocean. A third form, the _Rhytina_, existed in the Aleutian Isles till recent times, but was extirpated almost as soon as discovered, from its incapacity for flight or defence, and from its flesh affording a welcome change of diet to hungry sailors.

The _Cetacea_ and _Sirenia_ are examples of creatures organized for a completely aquatic life--for never coming to land.

The forest-regions of South America offer to animal life so enormous a mass of foliage that it may not unjustly be termed a sea of verdure, and creatures there exist which are specially organized for a completely arboreal life--for never coming to the ground. Such creatures are the sloths, which pass their lives hanging back-downwards, suspended to the branches by their huge claws. Thus, they sleep without effort (from the peculiar mechanism of their limbs), and they move slowly from tree to tree, having no need to hurry after food, since they live suspended in the midst of a perennial banquet.

Nearly allied to the sloths were certain huge beasts, now extinct, which formerly inhabited the same Continent--such as the _Megatherium_ and _Mylodon_, which rivalled or exceeded our largest rhinoceroses in bulk. They fed on the same food which nourishes the sloth, but obviously the branches of no tree could sustain such monsters. They obtained their leafy pasture, therefore, by a different method. Rearing themselves on their massive hind legs and powerful tail, as on a tripod, they embraced the trees with their vigorous arms, and swayed them to and fro, till the tree embraced was prostrated, and literally fell a prey to their efforts. These bulky creatures were protected against that danger which such a mode of life rendered imminent by a specially strong skull structure, which enabled them to bear a broken head with but little inconvenience.

In the same region of the earth are found the ant-eaters and armadillos, and more or less allied to them are the pangolins (_Manis_) of Africa and Asia. The horny scales which cover the bodies of the last-named animals caused them for some time to be associated with reptiles rather than with beasts, though they are true and perfect mammals. Lastly must be mentioned the aard-vark (_Orycteropus_) of South Africa.

All these creatures, from the sloths to the aard-vark, are commonly associated together in an order which is termed _Edentata_.

The whole of the orders of mammals yet mentioned agree in certain important details with respect to their reproductive processes, as well as in certain smaller anatomical peculiarities, and the whole of the creatures included within these orders are (and will be) often spoken of as _Placental Mammals_.

The only beasts which it yet remains to speak of are grouped in two other orders.

The first of these is called the order _Marsupialia_, and comprises all opossums (_Didelphys_), kangaroos (_Macropus_), phalangers (_Phalangista_), the Tasmanian wolf (_Thylacinus_), the dasyures (_Dasyurus_), the bandicoots (_Perameles_), and their allies. With the exception of the true opossums (_Didelphys_), all the members of the order are found in Australia or its vicinity, and nowhere else in the present day; although, as we shall better see hereafter, Europe once possessed animals closely allied to Australian forms of to-day--notably to a pretty little quadruped which bears the generic name _Myrmecobius_.

As last of the class of beasts, we have two extremely exceptional mammals (both found only in the Australian region), the duck-billed platypus (_Ornithorhynchus_), and the _Echidna_. The first of these, as its name implies, has a muzzle quite like the bill of a duck, with a squat, hairy body, and short limbs. The echidna is covered with strong, dense spines, and has a long and slender snout. These creatures together form the order _Monotremata_--an order which differs very much more from any other Mammalian order than any of the other orders of mammals differ one from another.

Thus, that great group which embraces man and beasts, and which group ranks as a "class"--the _class_ Mammalia--comprises (as we have now seen) a number of subordinate groups termed "orders," the orders being made up of families, and these again of genera.

It would be impossible as yet (when hardly any anatomical facts have been even referred to) to give the characters of the class _Mammalia_. It must at present suffice to point out that, in addition to mammary glands, the creatures have hot blood, and the body bears more or less hair--at least at some time of life.

We may now pass to the next class, that of birds--the class _Aves_. In spite of the great multitude of kinds which ornithologists enumerate--upwards of ten thousand species--there is very much less diversity of form amongst birds than there is amongst beasts.

Starting in the present class as in the preceding one from the most familiar kinds, we may begin with the domestic fowl. This is one of an "order" to which belong the peacock, all pheasants and tragopans (three forms which have their home in Central and Southern Asia), also the Guinea fowls (African forms), and the turkeys and curassows, which are American representatives of the order. Besides these may be mentioned partridges, grouse, black-cock, the capercalzie and quails, and, lastly, the megapodius or bush-turkey of Australia. This last is the only bird which hatches its eggs by artificial heat, depositing them in a mound of earth and decaying vegetable matter, wherein they are hatched fully-fledged, so that they can fly away immediately on leaving the egg. All the birds yet mentioned are called gallinaceous birds, or _Gallinæ_, and sometimes _Rasores_ or "Scratchers."

More or less allied to them are the doves and pigeons, which form the order _Columbæ_, in which the curious ground-pigeon _Didunculus_ is included--a form which presents an interesting resemblance to the celebrated and extinct dodo of Mauritius, long known only by certain pictures, and a foot and head preserved, one in the British Museum, and the other in the Ashmolean Museum of Oxford.

Our sparrows, robins, and all our song birds are members of an exceedingly numerous "order" "_Paseres_." In it are included the crows (with those gaily-decorated crows, the Birds of Paradise, found only in New Guinea and the Moluccas), the bower birds and the lyre bird of Australia; the flycatchers, the pittas (or ground thrushes), the water-ouzel, the weaver birds, the wrens, the tits, the creepers, the honey-eaters, those African gems, the sun birds, and also the swallows.

To another order--the order _Macrochires_--belong those most beautiful of all birds, the humming birds, found only in America, and long thought to be allied with the really very different sun birds just mentioned. With these may be associated the swifts (which have such marvellous powers of flight) and the wide-gaped goat-suckers or nightjars.

Woodpeckers are considered to form an order (_Pici_) by themselves, while the cuckoos are thought to be near relations of the beautiful and eccentric toncans, the plaintain-eaters, the touracous, the kingfishers, the hoopoes, the bee-eaters, the hornbills, and the trogons, all, from the cuckoos to the trogons, being included in the order _Coccyges_.

The parrots form an isolated group of birds--the order _Psittaci_. Their most peculiar forms are the macaws on the one hand, and the brush-tailed loris on the other. The order _Accipitres_ includes all the birds of prey--that is to say, the eagles, falcons, hawks, buzzards, vultures, and owls. In this order is included the long-legged secretary bird, which looks like a cross between a hawk and heron.

Pelicans, gannets, cormorants (or shags), and darters go together to constitute the order called _Steganopodes_. The flamingoes are isolated, and by themselves form the order _Odontoglossæ_. The same is the case with the penguins, which have the order _Impennes_ assigned exclusively to them.

The ducks and geese form alone the order _Lamellirostres_, in which is included the curious bird _Palamedea_, which is a goose adapted to live in trees in harmony with its South American forest habitat.

The rails and coots go with the bustards and cranes to constitute the order _Alectorides_. Similarly the auks, divers, puffins, terns, and grebes, noddies, and guillemots may be associated together in one order--the order _Pygopodes_. The gulls and petrels form another association--the order _Gaviæ_; while the plovers, snipes, curlews, peewits, turnstones, &c., constitute the order _Limicolæ_. The order _Heridiones_ includes the herons, the bitterns, the storks, spoonbill, ibis, &c.

All the foregoing birds have a multitude of points in common; indeed, so close is the similarity of their structure that their subdivision into orders is a matter of much difficulty and dispute. They are collectively spoken of as the _Carinatæ_, from the keeled form of their breast-bone.

Widely apart from them stands another group made up almost entirely of large birds, which agree not only in having no power of flight, but also in certain significant structural characters, amongst which may be mentioned the absence of a keel on the breast-bone.

This latter group is sometimes spoken of as the order _Struthiones_ from the ostrich (_Struthio_), which is its typical form. Sometimes these keelless birds are called _Ratitæ_. Besides the ostrich, the rhea, cassowary, and emeu are included within the group; also the small and nocturnal _Apteryx_ of New Zealand and those giants of featherdom, the huge species of dinornis, all also of New Zealand and all now extinct.

With this our list of birds might close, but for a bird which anciently existed in Europe so strangely different from all modern kinds, that it must certainly be here adverted to. This bird is the _Archeopteryx_, found in fossil in the Solenhofen States.

The class Aves, like the class Mammalia, consists of animals with hot blood, but all birds have feathers and a number of other peculiarities of structure, as will appear later.

The next class to be adverted to is the class which includes all reptiles properly so-called--the class _Reptilia_.

The reptiles which exist in the world to-day may be classed in four well-marked sets, each of which has the value of an "order"--(1) crocodiles, (2) lizards, (3) serpents, and (4) tortoises. The names of these creatures alone suffice to indicate the fact that the class of reptiles presents us with an extraordinary amount of diversity of form as compared with the class of birds with which, nevertheless, reptiles have, as we shall hereafter see, very close relations. Indeed, in the diversity of kinds which it contains, the class _Reptilia_ at the least fully equals the class Mammalia, especially if the extinct kinds are taken into consideration. The number of species of reptiles, both living and extinct, much exceeds also the number of living and extinct mammals.

To begin once more with forms which are the least strange and unknown, we may start with the little elegant and harmless lizards of our heaths and commons, which will serve as types of the order to which they belong--the order _Lacertilia_. That order is an extremely numerous one, containing many families, differing much in form. Our English lizards are true lizards, belonging to the typical genus _Lacerta_ and to the typical family _Lacertidæ_. The rather well-known large American lizard, _Iguana_, is the type of another and very extensive family (almost entirely confined to America), while a nearly-allied family (_Agamidæ_) is an Old World group. Amongst the curious forms found in the latter family may be mentioned the frilled and moloch lizards of Australia, and those little harmless lizards of India which go by the formidable name of "flying dragons" (_Draco_). They are the only existing aërial reptiles--not that they can truly "fly" at all, but they are enabled to take prolonged jumps, and to sustain themselves to a considerable extent in the air by means of the extremely distensible skin of their flanks which, when extended, is supported by a peculiar solid framework hereafter to be described. Some of the largest lizards are called "monitors," and are common in Egypt; they belong to the family _Monitoridæ_.

In the warmest period of the year, certain lizards are found in the South of Europe, called geckos. They have a power of running, not only up walls, but across ceilings by means of a peculiar structure of their toes. They are types of a large family (_Geckotidæ_) widely spread over the world.

Another large family (_Scincidæ_) has also its type in the South of Europe in the skink (_Scincus_), which was formerly supposed to possess much medicinal value. This large family contains a number of species which exhibit a series of gradations in structure leading to forms which have the external aspect of serpents. One such form is the perfectly harmless slow-worm, or blind-worm, of our own country, which in spite of its scientific name, _Anguis fragilis_[14], is a legless lizard, and no snake.

Other lizards of a very different kind forming the family _Amphisbæidæ_ are also legless, with the single exception of the genus _Chirotes_, which has a pair of anterior limbs, but no posterior ones. The name of this family is derived from the similarity of appearance presented by both ends of the body, so that either end looks as if ready to take the lead as "head."

A family of lizards familiar by name to us all from our childhood is the family of chameleons (_Chameleonidæ_). There are many species of chameleons, but they are found in the Old World only; they are among the most exceptional and peculiar of all lizards, but there is one form which is yet more so.

This most exceptional of lizards is one found in New Zealand, and named _Sphenodon_. Its external aspect would not lead the ordinary observer at all to suspect that it is so remarkable a creature as its anatomy shows it really to be.

The order _Crocodilia_ contains, of course, the true crocodiles which are found both in the Old and New Worlds. It contains besides the alligators (which are peculiar to America), as well as the long and slender-snouted gavials which are now found only in India and Australia. At one time the number of kinds of this order was very much greater than at present, and interesting structural modifications have taken place in it during the course of ages, as will be pointed out later.

On the whole, the order of crocodiles makes a much nearer approach to mammals and birds--especially (strange as it may seem) to birds, than is made by any other group of existing reptiles.

Reptiles, however, once existed have left their remains fossilized (in the rocks of what is termed the "secondary" or "mesozoic" period), which reptiles in the structure of their skeleton approach much more closely to birds, and especially to birds of the ostrich order, than crocodiles do. Amongst these reptiles may be mentioned the huge Iguanododon (type of the, extinct order _Dinosauria_), which once roamed over the Weald of Kent, and has left its remains in the Isle of Wight and elsewhere. Such remains were collected by its discoverer, the late Dr. Mantell, and are now preserved in our British Museum.

The crocodilia and some of the lizards of our own day are aquatic, but none live constantly in the ocean, as do the cetacea amongst beasts. This was, however, by no means always the case. In the secondary period just adverted to, huge marine reptiles (_Ichthyosauria_ and _Plesiosauria_) lorded it over the other then inhabitants of the deep, and presented some noteworthy resemblances to the whales and porpoises which have since succeeded them.

But other remains preserved in those same secondary rocks show us that in that period which has been so deservedly called "the age of reptiles," not only did many huge species of the class stalk over the land (either browsing on its foliage or preying on their fellows), and many others swarm in the then existing waters, but it shows us that the atmosphere also had its reptilian tenants. Flying reptiles which formed the now extinct order, _Pterosauria_, and which were some of small, some of very large size, as truly "flew" as do the bats of our own day fly, and by a very similar mechanism. Moreover, if the _Dinosauria_ present, as they do present, very noteworthy and interesting resemblances to birds of the ostrich order, no less noteworthy and interesting are the resemblances presented by these flying reptiles to ordinary--_i.e._, to "carinate"--birds.

The orders of extinct reptiles just referred to are not the only ones which formerly existed and have now passed away. There were reptiles with peculiarities in their teeth such as to have caused their order to be named _Amnodontia_, and it is members of this extinct order that the lizard _Sphenodon_ more or less resembles, and it is this resemblance which gives it that special interest before noted.

We may now return from these very various extinct forms to enumerate other kinds of reptiles which exist to-day. But before doing so the fact may be adverted to, that though amongst beasts many forms have become extinct, yet the proportion borne by the known extinct forms to the living kinds is much less than amongst reptiles, and that while it is the most highly-organized reptiles which have ceased to exist, the highest mammals which are in any way known to us are those which at present inhabit the earth's surface.

In passing from the orders of crocodiles and lizards to that of serpents--_i.e._, to the order _Ophidia_--we might select as first to be mentioned kinds which much resemble the legless lizards; but such kinds are not familiar ones in Europe.

The only serpents met with in England are but of three species--two harmless snakes and the common viper, which latter is the only really poisonous reptile in this country.

Of the harmless snakes, the ringed or collared snake (_Tropidonotus_) is much the commoner and more widely diffused. It ought to escape destruction on account of the ease with which it may be discriminated from the viper by means of the white collar-like mark which appears so conspicuously just behind its head.

Our viper is the type of a large and poisonous family, but by no means all poisonous snakes are vipers. The deadly cobras belong to a different group, having much more affinity with our own harmless snakes than with the vipers. The rattle-snakes again form a family (_Crotalidæ_) by themselves.

There are such things as true sea-serpents, and they are poisonous. They are not, however, allies of any "sea serpent," such as every now and again figures in startling paragraphs in our journals. The true sea-serpents are snakes of small or moderate size, which have their tails flattened from side to side, and which inhabit the Indian Ocean. Of other serpents which are not poisonous, the family of boas and pythons (which kill by crushing) is tolerably familiar to all who have visited zoological collections. There are many beautiful and harmless snakes, such as the families of tree-snakes and whip-snakes, but the snakes which more or less resemble legless lizards are burrowing forms which have the habits and more or less the appearance of earth-worms, such as those which form the families of _Uropeltidæ_ and _Typhlopsidæ_.

The last existing reptilian order (_Chelonia_) includes, besides the land tortoises of very various dimensions, a variety of aquatic forms.

The best known of these in this country, is the marine family (_Chelonidæ_), to which the edible and tortoise-shell turtles belong. The best known family in the United States and in the Continent of Europe, is the _Emydæ_, to which pertain the terrapins or ordinary river tortoises. Besides these, however, there is a very small family (_Trionicidæ_) of curious and exceptional forms, called mud-tortoises (_Trionyx_).

The creatures which have next to be glanced at are those familiar forms, the frogs, toads and efts, which, together with their allies, form another class,--the class _Batrachia_. These animals were long confounded with reptiles but are really widely distinct from them. They are arranged in four orders, three of which have living representatives. The creatures of the first order (the order of tailless Batrachians or _Anoura_)--frogs and toads--exist over almost all the habitable globe; and though the number of their kinds is very great, yet they are all extremely alike in organization. Many kinds (of both frogs and toads) are found to live in trees, the ends of their fingers and toes being dilated to enable them to cling to the surfaces of leaves. The most exceptional species of the whole group are the two tongueless toads, the _Pipa_ of South America and the _Daclytethra_ of Africa, the last-named kind being the lowest of all known animals provided with finger nails.

Closely related to the frogs and toads are the efts so common in our ponds. These familiar English forms are represented in other countries of the Northern Hemisphere by creatures, some of which (as we shall hereafter see) are of very great interest indeed. The whole group constitutes the second Batrachian order--the order _Urodela_.

One of the most noteworthy forms of the order is the eft _Proteus_, which inhabits the dark, subterranean caverns of Carniola and Istria. Allied to this is the _Menobranchus_ of North America and the Axolotl of Mexico. Other forms of the order are the American eft-genera _Spelerpes_ and _Amblystoma_, the _Menopoma_, and the gigantic Salamander (_Cryptobranchus_) of Japan and China, the eel-like _Amphiuma_--with its very long body and minute legs--and the two-legged _Siren_ of the United States.

The third order of Batrachians is one which contains very few species, but these are very strange, for though allied to frogs they have the appearance of snakes, or rather perhaps of worms. With long and slender bodies (marked by many transverse wrinkles), devoid of every rudiment of limb, they remind us of the before-noticed _Anguis_, _Typhlops_, and _Uropeltis_ amongst reptiles. The Batrachians in question (which belong to the genera _Cæcilia_ and _Siphonops_) form the order _Ophiomorpha_.

The fourth order of Batrachians is one which has entirely passed away and become extinct. It is the order _Labyrinthodonta_, and the species which composed it were, some of them, of large size, with great heads like those of crocodiles. Others bore more or less resemblance to enlarged _Ophiomorpha_.

Every one knows that frogs begin their existence in the water as tadpoles, which have the habits and mode of life of fishes. Thus, the class _Batrachia_ naturally conducts us to the class _Pisces_, the class of true fishes. This class contains a prodigious variety of forms, and is far more rich in species than any other of the classes before enumerated--even that of birds.

The fishes most familiar to us--such as the perch, carp, mackerel, cod, herring, sole, turbot, salmon, pike, dory, and eel--all belong to one great order called _Teleostei_, and which is made up of what are called "bony" fishes, though there are some bony fishes which do not belong to it. To the same order also belong the Muroena, the electric eel (_Gymnotus_), the flying fishes (_Exocetus_ and _Dactyloptera_), the sucking fish (_Remora_), the pipe-fish and sea-horse (_Hippocampus_), the diodon, the ostracion, the file-fish (_Balistes_), the largest of all fresh-water fishes (_Sudis gigas_ of South America), with a multitude of other forms.

Certain more or less singular Teleosteans are classed together in a subordinate group of "Siluroids" (of which fish the _Silurus_ is a type), and which group includes, amongst others, the singular, cuirassed fish Callichthys.

A group of fishes, which is now very small, but which at an earlier period of the world's history was very large, includes within it all those fishes which will be hereinafter occasionally spoken of as "Ganoids," as they compose the order _Ganoidei_. Of all the forms of this order, the sturgeon is that which is least unfamiliar to us. The Ganoids are mostly fresh-water fishes and consist of the spoonbill-fish (_Polyodon_), the bony-pike (_Lepidosteus_), the African _Polypterus_, the mud fish (_Lepidosiren_), and the curious Australian fish _Ceratodus_, which last is a singular instance of piscine survival.

Another order, _Elosmobranchii_, is made up of the sharks, together with the skates (or rays) and the curious _Chimæra_. Amongst the skates may be mentioned the celebrated torpedo or electric ray.

The three groups above enumerated contain almost all known fishes, but a few other kinds, all of lowly organization, constitute two other groups of very different structure.

One of these groups is called _Marsipo-branchii_, and contains the lamprey, the _Myxine_ (or Glutinous Hag), and the _Bdellestoma_. They are fishes of parasitic habits and of relatively inferior structure.

Last of all comes a creature of such exceptional build, so widely different from, and so greatly inferior to, any kind of animal yet noticed, that it may but doubtfully be reckoned as a fish at all. The animal referred to is the lancelet (_Amphioxus_), which is a small, almost worm-like animal, living in the sand on our own coasts, and also widely distributed over other parts of the world. The _Amphioxus_ has no distinct head or heart, and its breathing apparatus--its gill structure--differs so much from that of all other fishes as to give a name to its "order" (which contains it alone)--the order _Pharyngobranchii_.

We have now, then, hastily surveyed no less than five "classes" of animals--(1) Mammalia, (2) Aves, (3) Reptilia, (4) Batrachia, and (5) Pisces.

But, as was said in the first beginning of this Essay,[15] "classes" are the groups into which "sub-kingdoms" are divided, and which, by their union, make up such "sub-kingdoms."

The five classes above-mentioned together constitute the highest of those sub-kingdoms into which the whole animal kingdom itself is divided. This highest sub-kingdom is named VERTEBRATA, and is called the vertebrate sub-kingdom, because every creature which belongs to it possesses a "spinal column," which is generally built up of bones, each of which is called a "_Vertebra_."

We ourselves are members of the genus _Homo_, of the family _Hominidæ_, of the order _Primates_, of the class _Mammalia_, of the sub-kingdom _Vertebrata_, and it is desirable to treat this sub-kingdom at considerable length, both because it is, to us who are members of it, the most interesting and important, and because, by treating it somewhat fully, a good example can be once for all given of biological classification.

But the number of animal kinds which belong to other sub-kingdoms vastly exceeds the total number of vertebrate animals, and the structural contrasts found between different non-vertebrate species is very much greater than any such contrasts as can be found to exist between any two members of the highest, or vertebrate sub-kingdom. This is only what we might expect; for non-vertebrate animals--often spoken of collectively as "_Invertebrata_"--form several distinct sub-kingdoms, each of which has a rank approximatively co-ordinate with that sub-kingdom to which we ourselves belong. Nevertheless, since the members of the invertebrata sub-kingdoms are, speaking generally, much less known and familiar than are vertebrate animals, and as the structural differences between them cannot be pointed out till an initial acquaintance has been made with comparative anatomy, for these reasons we may treat the various animal sub-kingdoms which have yet to be noticed at much less length than we have treated the vertebrata. The details of their peculiarities and the various degrees of significance and interest which they present will begin to appear when we proceed to treat of "The Forms of Animals."

The last class of vertebrates is, as we have seen, constituted by the fishes, which are fishes properly so called. But there are many animals which are familiarly and improperly spoken of as "Fishes," but which are even more below true fishes than whales and porpoises are above them. Thus, we hear of cuttle-fishes, and a variety of creatures are spoken of as "shell-fish," which are not in the least related to true fishes. Indeed, the many so-called "shell-fish" are not even nearly related one to another. Thus, the oyster and the lobster are both commonly thus named, but they belong respectively to two altogether distinct sub-kingdoms of the world of animals.

The oyster is an animal which belongs to a vast assemblage of species, with much variety of form and structure, which, on account of their soft bodies (whether or not enclosed in shells), are called MOLLUSCA or "Mollusks." This assemblage ranks as a sub-kingdom and contains within it at least four subordinate great groups, or "classes." All snails and whelks, with their allies, and also all cuttle-fishes, belong to the sub-kingdom of "soft animals."

Amongst the most familiar of mollusks is the common snail, which may serve as a type of the "class" of mollusks to which it belongs--the class _Gasteropoda_. The snail, with the slug, are representatives of land-forms of mollusca, but the bulk of the class and of the whole sub-kingdom are aquatic animals, such as the whelk (_Buccinum_), periwinkle (_Littorina_), limpet (_Patella_), &c. The Gasteropods generally possess spirally coiled shells (like the cowry or whelk), but some kinds have their shells in the form of simple cones--like a Chinaman's cap--as, _e.g._, the limpet. There are a few Gasteropods in which the shell consists of a series of similar segments as is the case with _Chiton_, while many are altogether naked. In some kinds the soft body is drawn out into a number of tufted processes, as in Doris and Eolis, and sometimes the body is almost worm-like, as in _Phylliroë_, or provided with a pair of ring-like lateral processes and a rudimentary shell, as in the sea-hare _Aplysia_.

Next above the Gasteropods comes a group of animals forming the class _Pteropoda_. These pteropods are small, active, oceanic, surface-swimming creatures, many of which live in delicate glass-like shells, and some of which form a large part of the food of the whalebone whale. They flit through the water by the aid of lateral processes which much resemble those before-mentioned as existing in the sea-hare. Allied to these pteropods is a curious little animal, the shell of which resembles a miniature elephant's tooth and which is named _Dentalium_.

Highest of all the mollusca stand the cuttle-fishes, forming (with the _Nautilus_ and many extinct animals, such as ammonites and their allies) the great class _Cephalopoda_. The Cephalopoda, such as the cuttle-fish (_Sepia_) and the Poulp (_Octopus_), have now become familiar objects through our aquaria, where their very eccentric forms and remarkable movements naturally attract attention. To this group also belongs _Spirula_, the coiled and chambered shell of which is found so abundantly, but its soft tenant so very rarely. To it also belongs the extinct Belemnite, which was provided with a dense, conical internal shell, specimens of which found in rocks were at one time taken for thunderbolts. Of a lower grade of organization is the _Nautilus_, sole existing representative of a great group of Cephalopoda (including the ammonites and other forms) which has, with the above exception, long become entirely extinct.

The oyster is an animal which belongs to a much lower class of mollusca--namely, to the class called _Lamellibranchiata_, from the plate-like (or lamellar) structure of the gill. To that class also belongs the scallop (_Pecten_), the mussel (_Magilus_), the fresh-water mussel (_Anodon_), the razor-shell (_Solen_), the cockle (_Cardium_), species with a long fleshy tube such as _Mya_, stone-perforating shells such as _Pholas_, and the well-known wood-boring "ship-worm" (_Teredo_)--which is no "worm" at all--with a multitude of other forms.

Certain other animals (which, like the Lamellibranchs, all have a shell divided into two valves) form another still lower class called _Brachiopoda_, a class which we may, at least provisionally, consider as belonging to the mollusca. These _Brachiopods_ are also called "Lamp-shells," from a certain resemblance which many of them show to the form of a classical lamp. They are interesting, because in very ancient times they seem to have held that place in the world's animal population which is now held by the Lamellibranchs, by which, as they died out, they have been gradually replaced till but comparatively few forms survive. Some of these, however, are of great antiquity, and one of them, _Lingula_, is, though still living, one of the most ancient of all known animals.

We may next pass to a small sub-kingdom which includes the curious and inert animals before referred to[16] as "Sea-squirts," Tunicaries or Ascidians, and which constitute the sub-kingdom TUNICATA. These are marine organisms of very simple but very peculiar structure which sometimes grow up in compound aggregations. Certain forms (_e.g._, _Pyrosoma_) are luminous at night and may be seen swimming about in the ocean like so many red-hot urn-heaters. As we shall hereafter see, the reproductive processes and the earlier stages of existence of these creatures possess much interest, and have afforded strong grounds for regarding them, in spite of their lowly organization, as very close allies of the highest animals or _Vertebrata_.

Returning now to the "lobster" (lately mentioned as one of those animals commonly called "shell-fish") we may regard it as an example of what is by far the most numerous of all the sub-kingdoms of animals. This sub-kingdom is made up of animals with jointed feet or "Arthropods," and the ARTHROPODA are subdivided into four classes--1, _Crustacea_; 2, _Myriapoda_; 3, _Arachnida_; and 4, _Insecta_; and it is to the first of these four classes that the lobster belongs.

The class _Crustacea_ contains, besides the lobster (and its near allies, hermit-crabs, prawns, shrimps, and cray-fish), all crabs, including those very quaint-looking animals (now so often seen in our living collections), the king-crabs (_Limulus_), and a variety of more or less strangely different forms such as the following:--

Certain Crustaceans, of the group called _Ostracods_, have the hard outer coat of their body so peculiarly modified that they have quite the appearance of Lamellibranch Mollusks, and this resemblance is even more than skin deep, as we shall see later.

Some of another group, called _Copepoda_, become, when adult, so degraded in structure as to have the appearance of mere worms, as _Lerneocera_ and _Tracheliastes_, and become strangely unlike the typical forms (crabs and lobsters) of their class.

Other animals of the class _Crustacea_, which animals form the order _Cirripedia_ (barnacles and acorn-shells), bear such an external resemblance to mollusks that they were actually classed by Cuvier in the class _Mollusca_. In some of them--the Barnacles which commonly attach themselves to the bottoms of ships--the head grows from above downwards to a relatively enormous degree, forming the long stalk or "peduncle," at the lower end of which the small body with its limbs hangs suspended.

In another group, _Rhizocephala_, the form of the adult becomes yet more strange. These creatures are parasitic on other crustacea. Having attached themselves to the surface of the soft abdomen of the Hermit crab, the head of the Rhizocephalon grows out into it as so many root-like processes, from which condition the group has received its name.

The numerous and long extinct group of _Trilobites_ also belongs to the class _Crustacea_.

The next class, _Myriopoda_, consists of the hundred-legs (centipedes), and thousand-legs (millipedes), which present us with some of the best examples of creatures the bodies of which are composed of a longitudinal series of similar segments. Allied to them is a very exceptional animal found in Africa and New Zealand, and called _Peripatus_, the anatomy of which presents many significant peculiarities.

The third class of Arthropods (_Arachnida_) consists of the scorpions and spiders with their poor relations, the mites and tics, together with the very peculiarly-shaped _Pycnogonida_ (which present us with a good image of "no body"--being all legs and no body), and the singular worm-like parasite _Linguatula_. Lastly, we come to the most zoologically important and numerous of all the classes of Arthropods--namely, to the "class" of insects--_Insecta_. Therein we meet with the power of flight in its most perfect form--_i.e._, in the Dragon-flies--and most of the species are aërial in their adult (or _Imago_) condition. Some, however, are burrowers as, for example, the mole-cricket--an insect which presents some curious analogies in structure to the beast referred to in its name. Amongst insects may be mentioned the most familiar of all, the House-fly (which belongs to the order _Diptera_), and Beetles of all kinds (which constitute the order _Coleoptera_), some of which latter are luminous, as is the well-known glow-worm, and the exotic beetles _Pyrophorus_. Another order (_Orthoptera_) is made up of the earwigs, cockroaches, crickets, grass-hoppers, and their allies the locusts, with Bamboo-insects and the curious walking-leaf (so-called from their resemblance to a Bamboo twig and a foliage leaf respectively), the praying mantis, and other curious kinds.

Bees and Ants, which belong to the order _Hymenoptera_, are, as every one knows, celebrated for their wonderfully complex instincts and community-life (which will occupy us later), and to the same order also belong the Ichneumon insects, which are provided with long appendages at the hinder ends of their bodies wherewith to pierce the bodies of animals in order to deposit their eggs within them, or to pierce the substance of plants, so producing "galls" which are structures of much interest from several points of view.

Butterflies and Moths form another order of insects called _Lepidoptera_, amongst which may be mentioned as (having to be referred to hereafter) the true butterflies (_Papilio_), and the hawkmoths (some of which in their flight so much resemble Humming-birds), the clear-wing moths, and those moths the grubs of which are known as "silk-worms," and certain moths of the genera _Solenobia_ and _Psyche_.

The numerous group of bugs is allied to the plant-lice (_Aphides_), which so often infest our Pelargoniums when kept in dwelling-rooms. Allied to them, again, are the small creatures the nature of which was so long disputed, though familiar to commerce as "Cochineal." Really, they are small, singularly inert, plant-lice, which adhere to the surface of certain "Cacti."

The Dragon-flies, before referred to, are the types of the order _Neuroptere_.

All the insects above mentioned, save the House-fly, have four wings, or else none; but that familiar form may serve as the type of the two-winged order (_Diptera_) to which belong all flies and gnats--including, of course, the Mosquito--and the numerous "Bots," one of which (the Tsee-Tsee fly) is so fatal to cattle in Africa.

Finally, amongst insects may be mentioned the wingless, but active order of fleas (_Aphaniptera_), the wingless but sluggish lice (_Aptera_), and the jumping and wingless springtails (_Thysanura_).

In leaving the class of insects, we leave all the more highly-organized Invertebrata. But the next group to which we may direct our attention is one which is exceedingly numerous, and contains a very varied assemblage of forms. This group is the "sub-kingdom" of Worms, VERMES. First amongst its contents may be mentioned the higher or true "worms," such as the earth-worm (_Lumbricus_), the leech (_Hirudo_), the sea-mouse (_Aphrodite_), and their allies, together with the worms which live in tubes, which are called _Tubicolous_-"_Annelids_," because the whole class of these higher worms bears the name _Annelida_.

In this connexion may be mentioned certain exceptional vermiform creatures, about the affinities of which naturalists dispute.

One of these is a marine creature (called _Sagitta_, from the way in which it shoots like an arrow through the water), which has many affinities to Arthropods.

Another is a most remarkable worm, which has been found in the Bay of Naples, and is called _Balanoglossus_. It is the type of a group called _Enteropneusta_. To it reference will have again and again to be made on account of certain singularities in its structure.

A very distinct class of creatures is termed _Bryozoa_ (or _Polyzoa_), and is composed of very minute animals which live in compound aggregations, and often grow up in an arborescent manner. The common sea-mat (_Flustra_) is one example of the class, and another--a good type--is called _Plumatella_. The _Bryozoa_ have many affinities with the _Mollusca_, to which some naturalists consider them to belong.

Other worms form the class _Nematoidea_, of which many are parasitic and many not so. Amongst the better known of the former may be mentioned the worms which tease children (_Ascarides_), the guinea-worm (_Filaria_), the scourge of Germans who eat raw meat (_Trichina_), the deadly blood-parasite of the Nile (_Bilharzia_), and many others.

Another class (_Trematoda_) is made up of parasites called "Flukes," to some of which (_e.g._, _Monostomum_) reference will have hereafter to be made with respect to their processes of development.

The class _Turbellaria_ contains a variety of other worms of a lowly kind, one or two of which (_e.g._, _Borlesia_) live coiled up in complex tangles which, if unravelled, would attain a length of forty feet. Amongst the commoner kinds may be mentioned the worm _Nemertes_, and all worms called _Planariæ_ (which are mostly fresh-water, though some live on land), allied to the flukes.

The class of tape-worms (_Cestoidea_) is one most numerous in its kinds, which are all completely parasitic in habit. Some of them are so fatal in their effects that they are estimated to occasion every seventh death which occurs in Iceland, and they cause mortality amidst our own flocks, producing in sheep the disease known as the "staggers."

Certain minute organisms, familiarly known as "Wheel-Animalcules," or Rotifers, form the "class" _Rotifera_. They have gained their name through an apparently (though, of course, not really) rotary motion, of that end of their bodies at which the mouth is situated. Here also may be mentioned certain curious aquatic worms called _Gasterotricha_, which are closely allied to the wheel animalcules.

Finally may be mentioned the class _Gephyrea_, containing animals, worm-like indeed in form, but which have much apparent affinity to the group next to be spoken of--the group of star-fishes and their allies. Amongst the _Gephyrea_ may be mentioned the worms called _Sipunculus_ and _Priapulus_.

This leads us to the sub-kingdom containing the star-fishes--the sub-kingdom ECHINODERMA, which includes, besides the star-fishes (or _Asteridea_), all sea-eggs or sea-urchins (_Echinidea_), the brittle-stars _Ophiuridea_, as well as the elongated soft animals called sea-cucumbers, or _Holothuridea_, some of which latter are known as the Japanese edible, "Trepang."

Besides these groups there are still surviving a few creatures (_Comatula_ and _Pentacrinus_) belonging to the class of "sea-lilies," or _Crinoidea_, creatures which once lived in countless multitudes, but have now almost entirely passed away. All these crinoids were like star-fishes on stalks, and of the existing forms, _Pentacrinus_ still passes the whole of its life, and _Comatula_ its youth, in a stalked condition.

The next great primary division, or sub-kingdom of animals, is COELENTERA, and a good type of the coelenterates, the sea anemone (_Actinia_), has now become a familiar object to us in our aquaria. These animals are plant-animals, or zoophytes, and some of them build up coral-reefs, or islands, and it is one kind which produces the red coral of commerce. Forms essentially similar, but the solid supporting framework of which is of a softer nature, are such as _Alcyonium_ and _Pennatula_. All these belong to the "class" _Actinozoa_. There are other coelenterates of an active free-swimming habit, such as _Beröe_ and _Cydippe_, which are balls of glassy transparency displaying iridescent hues as they move rapidly through the water by means of their peculiar locomotive organs.

Other coelenterates, of the same essential type but of simpler structure, form the class _Hydrozoa_. Amongst these may be mentioned the little _Hydra_ of our ponds, which will often come before us in our survey of animal life. Some compound forms of Hydrozoa simulate the compound Actinozoa; such are the calcareous millipores, and those with a softer structure, called "corallines," such as _Eudendrium_ and many others. The Portuguese man-of-war (_Physalia_) and the various forms of jelly-fish (_Medusæ_) all belong to the _Hydrozoa_, as also does a very curious and very elementary form, to which the name _Tetraplatia_ has been given.

Next we come to the group of sponges, SPONGIDA, some of which--as the now well-known _Euplectella_--are of marvellous beauty and delicacy of structure; while others, as the sponge of commerce, are of much greater simplicity of form. Simplest of all the sponges is the sponge called _Ascetta Primordialis_. Some sponges have a horny, some a calcareous, and some a siliceous skeleton, and (strange as it may appear) some have a habit of boring into shells, and living in the excavations they make.

An animal recently discovered, _Dicyema_, may at this initial stage of our inquiry be left with its place and affinities undetermined. It is a minute worm-like creature of most exceptionally simple structure, which lives parasitically within cuttle-fishes.

We now pass to animals (if so they are really to be considered) which are the lowest and simplest of all, and which are mostly microscopic in size, and may be grouped together under the term HYPOZOA, or under the generally employed name _Protozoa_. With very few exceptions these animals are aquatic, and if terrestrial they are found in damp localities. Some are marine, others are fresh-water organisms.

The highest of the group are the animalcules, which are named _Infusoria_, most of which are freely swimming organisms, though a certain number of them live fixed to some supporting body.

Another group of _Hypozoa_ is that termed _Gregarinida_, a group made up of very lowly parasites, such as are often found tenanting the intestines of insects as well as those of higher animals. Finally, we have the group of _Rhizopoda_, animals which have the faculty of projecting and retracting (so to say, at will) filamentary or conical processes of their semi-fluid substance, such processes being the _Pseudopodia_, which were referred to earlier.[17]

Amongst the _Rhizopoda_, the most complex and beautiful are the delicate and symmetrical creatures known as _Radiolaria_,[18] the siliceous skeletons of which are amongst the most remarkable of microscopic objects.

Allied to them are the simpler _Heliozoa_, of which the after-mentioned _Actinophrys_ may be taken as a type.

Next come the _Flagellata_, or minute creatures which swim about by means of one or two whip-like processes, whence the name of the group.

Last of all is the group of _Foraminifera_, animals which are well worthy of note, seeing that, though they are each but as it were a minute particle of structureless jelly, they manage to build most complexly-formed, generally calcareous, shells, or to pick up from the sand of the sea minute particles, which they agglutinate around them with marvellous neatness and precision. Their calcareous shells are generally pierced by a multitude of minute pores, through which the little creatures protrude their _pseudopodia_. It is from these pores (or _foramina_) that the group receives its name. All _Foraminifera_, however, are not provided with shells. Some, as the _Amoeba_, are naked, and the simplest of all animals, _Protogenes_ and _Protamoeba_, consist of but a minute particle of semi-fluid jelly, or protoplasm, naked and as devoid of every external protection as it is of internal organization.

We have thus descended to the bottom of the animal kingdom, and passing from these rudimentary forms, which are generally reckoned as animals, we may next survey in ascending order the different organisms which together compose the kingdom of Plants, a group much less rich in species than is the animal kingdom.

At the bottom of that kingdom are very simple creatures, but little different, to all appearance, from the lowest animals. As an example of such we may take the minute plant _Protococcus_, which is an humble member of the great group of _Algæ_, to which all sea-weeds belong. Not all of this important tribe, however, are marine. Many are found in fresh water--such as the protococcus itself, and many of the green vegetable threads known as _Conferræ_. Some even live on land, and draw their moisture from the atmosphere. The _Algæ_ are exceedingly varied in their structure; some, like the protococcus, being of extreme simplicity; others attaining a large size, and presenting the appearance of a stout stem with branches and leaves.

The Algæ are divisible into the green-spored[19] (_Chlorospermeæ_), the rose-spored (_Florideæ_), and the olive-spored (_Melanospermeæ_).

It is in the first division that the _Protococcus_ may be placed, as also those microscopic plants called _Diatoms_ and _Desmids_. The former, the _Diatomaceæ_, are a very numerous group of minute organisms, some of which are used as test objects for microscopes. They contain in their outer coat or case a relatively large portion of silex, and their remains here and there form deposits--vast beds many feet in thickness--known as "tripoli," and used for polishing. The minute particle of their protoplasm is contained within the siliceous case. They may be entirely free, or cohere in aggregations, or be attached to a supporting surface by a slender stalk, which may ramify and bear a little siliceous case or "frustule" at the end of each branch.

The desmids (or _Desmidiaceæ_) are green and devoid of silex, though their protoplasm is enclosed in hard or flexible cases, often marked with beautiful and characteristic patterns.

Both diatoms and desmids may cohere together, forming more complex masses; but another creature allied to _Protococcus_ is noted for its mode of cohesion. This is the microscopic plant _Volvox_, the individuals of which cohere so as to form spheroidal aggregations, which swim about by the action of filamentary prolongations of their protoplasm, such prolongations reminding us of the pseudopodia of radiolarians and other rhizopods.

Amongst these simplest plants may be also mentioned the curious thread-like organisms, which, on account of their remarkable and as yet unexplained movements, are called _Oscillatoriæ_.

Another curious vegetable organism which may here be mentioned is _Vaucheria_. It is a green, thread-like plant, which may be several inches long, and which at one stage of its existence (when it is what is called a "spore") swims about by pseudopodial prolongations of its protoplasm.