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 Chimera. 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 wormlike 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,*"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.

*See ante, p. 14.

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 "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 subkingdoms of the world of animals.

as

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 subkingdom and contains within it at least four subordinate great groups or classes." All snails and whelks, with their allies, and also all cuttlefishes, 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 belongsthe class Gasteropoda. The snail, with the slug, are representatives of land-forms of mollusca, but the bulk of the class and of the whole subkingdom 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, surfaceswimming 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 fit 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. 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.

Of

The oyster is an animal which belongs to a much lower class of mollusca—namely, to the class called Lamellibranchiata, from the platelike (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 as " Sea-squirts," Tunicaries * See CONTEMPORARY REVIEW for July, p. 710.

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 rootlike 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 Neuropterc.