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At the very lowest end of the back-boned animals, occupying a somewhat intermediate place between what may be termed true vertebrates and true invertebrates, are the curious sea squirts, forming the class Tunicata, and the equally strange, but even less known animals of the class Enteropneusta; so little known, indeed, are they as to have no common name. These are the animals referred to as vertebrates in disguise, or degenerate vertebrates, because their form is believed to have changed with their mode of life, and they have not merely failed to progress, but have actually gone backward, and lost the position occupied by their remote ancestors.

Owing to their soft texture these animals have left few traces of their existence at former periods of the earth’s history, although we do find evidence of their presence in some Miocene rocks.

Such are the great, primary divisions or classes of the great and important phylum or subkingdom of back-boned animals, or vertebrates, and these once fixed in the mind it is an easy matter to refer to their proper places the unfamiliar creatures with which we may have to deal.

Representatives of all the classes of vertebrates are found fossil, and not only examples of all existing orders, but of a number that have become extinct. Fishes are perhaps the most common of fossil vertebrates, partly on account of their numbers and partly because they had a commendable habit of dying where their remains would be preserved and subsequently found. Birds, on the other hand, are extremely rare, particularly the earlier species, which are the ones we are most anxious to know. It is commonly stated that this is on account of their power of flight, as well as the lightness of their bodies, the first preserving them from many accidents to which other animals are subject, while the last caused their bodies to float and rendered them particularly liable to destruction after death. Still this explanation is not quite satisfactory, for birds sometimes perish in great numbers in spite of their power of flight, and in some favored localities many of their bones are found.

As charity covers a multitude of sins, so the term invertebrates includes a vast number of animals which agree with one another in the negative character of lacking a backbone. Formerly the invertebrates were regarded as forming a group of equal rank with the vertebrates, the two divisions including all animal life; but it is now known that this assemblage comprised several distinct classes of animals, equal in importance if not in size, just as Rhode Island and Texas, though very different in area, are both States of the same rank so far as independence and form of government are concerned. But while “invertebrates” is no longer used to denote one of the primary divisions of the animal kingdom, it is still a most useful and comprehensive term for all the creatures which have no backbone.

Owing to the vast numbers of invertebrates and their numerous divisions, it will be possible to mention only a few of our great primary groups or phyla, bearing in mind that each of these phyla corresponds in the degree of its importance to that including all vertebrates.

A common species of squid, Gonatus amœnus. Natural size of a small specimen. On the left is shown the beak of a larger individual, and on the right the pen.

The highly specialized structure of the squids and cuttlefishes comprising the class Cephalopoda is generally considered as placing the mollusks next to the vertebrates; and while we usually associate the term mollusk with animals covered with a shell, forgetting the fact that the name means soft, yet among the highest living members of the group only the nautilus and argonaut are thus protected. On the other hand, the familiar slugs are apparently without a shell, as this is so rudimentary as to be concealed within the mantle, while the marine forms known as Nudibranchs are quite naked—another of the many cautions not to judge animals by their clothes, or lack of them.

The members of the class Lamellibranchiata, or leaf-gilled, may be readily distinguished by having a hinged shell of two parts or valves, whence the common name of bivalves. To this class belong the common oyster, mussel, clam, and other commercially important species.

The Gastropoda include the so-called univalve shells, such as limpets, ear-shells, snails, and top-shells, many of which are spirally twisted; but some of the members of this class have no shell, and the curious chitons, often placed here, are protected by a covering of several overlapping parts, on the principle of a piece of scale armor.

The little wing-shells, forming the class Pteropoda, which stands well up in the group, may or may not be protected by a shell. These animals, in spite of their small size, play quite an important part in the economy of nature owing to the vast numbers in which they exist. They are so numerous in arctic seas as to color large tracts of water a pale green and to provide an important article of food for the great Greenland whale, while their shells settle in countless myriads on the sea-floor to form the deposit known as pteropod ooze.

The Arthropoda, the joint-footed animals, contain besides other less familiar forms such well-known groups as crabs, insects, spiders, and centipeds, and comprise several hundred thousand species, forming a prominent and important portion of the living world. Doubtless all these flying, creeping, and crawling things were equally abundant in the past, although this is not indicated by their fossil remains, since, as has been said before, many were so delicate in texture as to be preserved only under very favorable circumstances. To this great group belongs the extinct order of Trilobites, of which we shall learn more later, and the great and equally extinct Eurypterids.

Going down the line, the next phylum, the Echinodermata, contains the echinoderms or sea-urchins, the starfishes and brittle stars, the crinoids or sea-lilies and the sea-cucumbers, the first two familiar to all, the last two much less widely known. The crinoids, so abundant in Carboniferous seas, now nearly extinct and represented by a few species found in deep water, may be roughly compared to a starfish growing upside down on a stalk attached to the middle of its back. As the rays of the star turn gracefully upward and outward, while the stem bears little arms that may pass for leaves, the suggestion of a lily is very strong. The sea-cucumbers, or holothurians, do not at all resemble their radiate brethren, being soft-bodied animals, that look to the untrained eye far more like some strange overgrown worm than any relative of a starfish, although a study of their internal structures has led to the recognition of their proper place.

Then come the Vermes, or worms, a group containing several orders and many members, though in general little known, partly owing to their retiring habits, partly to the unattractive appearance of many, even though it has been thought that the ancestors of vertebrates are to be sought for in this group. Here, again, while every one knows the class as represented by such typical members as the useful earthworms, there are several groups whose relationship with creeping things is so obscure that it long went unsuspected. One of these contains the Rotifera, or wheel animalcules, minute creatures which comparatively few have seen, although they abound in fresh water. They show their relationship only in their early stages, when they exhibit resemblances to the very young of worms.

A rotifer, a minute relative of the worms. Very much enlarged. (From Bulletin of U.S. Fish Commission.)

Placed sometimes with the worms, sometimes accorded the importance of divisions of their own, are Polyzoa, or moss animals, and Brachiopoda, or lamp-shells. The former are of small size, and are associated in colonies, which often bear a strong superficial likeness to a piece of seaweed, although on closer examination this resolves itself into an assemblage of little animals each occupying a sort of pocket or cell.

A polyzoan, Flustra truncata.

The brachiopods bear shells, and were long and not unnaturally considered as belonging with bivalve mollusks, which they closely resemble in outside appearance. It may be said, however, that while the two halves of such a shell as that of a clam represent the two sides of the occupant, in a brachiopod the valves cover the upper and lower portions of the animal. The inside of these shells often has curious loops or spirals for the attachment of the muscles that move them, and in some species there is a sort of stalk running through the point of the shell, by which the animal is attached to the sea-bottom. This group of shell-bearing creatures is most ancient, and held a high place during the early history of the world, so that it is frequently necessary to refer to them.