The third and highest class includes the Star-Fishes, Sea-Urchins, and Holothurians or Beches-de-Mer. The radiation is equally distinct in each of these; but here again the mode of execution differs from that of the two other classes. The internal cavity and the radiating tubes, instead of being connected with the outer wall of the body as in Polyps, or hollowed out of the substance of the body as in Jelly-Fishes, are here inclosed within independent walls of their own, quite distinct from the wall of the body. But notwithstanding this difference, a transverse section shows in these animals, as distinctly as in all the rest, the radiating structure typical of the whole branch. In these three classes we have no difference of plan, nor even any modification of the same plan,—for either one of them expresses it as clearly as any other,—but simply three different ways of executing one structural idea.
[Illustration: Common Sea-Urchin, Echinus, seen from above]
[Illustration: Echinarachnius, opened by a transverse or horizontal section, and showing the internal arrangement: c, mouth; eeeee, ambulacra, with their ramifications cmcmcm; wwww, interambulacra.]
I have mentioned only three classes of Radiates. Cuvier had five in his classification; for he placed among them the Intestinal Worms and the Infusoria or Animalcules. The Intestinal Worms are much better known now than they were in his day. Their anatomy and embryology have been traced, and it has been shown that the essential features of these parasites are the same as those of all Articulates, their whole body being divided into successive, movable joints or rings. Cuvier was misled by the circular arrangement of certain parts around the mouth, and by the presence of a wreath of feelers around the head of some of these Worms, resembling the tentacles of many Radiates. This is, however, no indication of radiate structure, but a superficial feature in no way related to the internal organization.
We must carefully distinguish between affinity and analogy among animals. The former is founded on identity of plan; the latter only upon external resemblance, produced by similar features, which, when they are intimately connected with the whole internal organization, as in some groups, may be considered as typical characters, but when only grafted, as it were, in a superficial manner on animals of another type, have no relation to the essential elements of structure, and become at once subordinate and unimportant. Such is the difference between the tentacles in a Radiate and the wreath of feelers in a Worm;—the external effect may be much the same; but in the former every tentacle opens into one of the chambers as in a Polyp, or connects with one of the radiating tubes as in Acalephs, or with the locomotive suckers as in Star-Fishes, and is therefore closely linked with the whole internal organization; whereas the feelers in the latter are only external appendages, in no way connected with the essential structural elements. We have a striking illustration of this superficial resemblance in the wings of Birds and Insects. In Birds, wings are a typical feature, corresponding to the front limbs in all Vertebrates, which are constructed in the same way, whether they are arms as in Man, or forelegs as in Quadrupeds, or pectoral fins as in Fishes, or wings as in Birds. The wing in an Insect, on the contrary, is a flattened, dried-up gill, having no structural relation whatever to the wing of a Bird. They are analogous only because they resemble each other in function, being in the same way subservient to flight; but as organs they are entirely different.
In adding Infusoria to the Radiates, Cuvier was false to his own principle of founding all classification on plan. He was influenced by their seeming simplicity of structure, and placed them in the lowest division of the Animal Kingdom on that account. But even this simplicity was only apparent in many of them. At certain seasons of the year myriads of these little Animalcules may be seen in every brook and road-side pool. They are like transparent little globules, without any special organization, apparently; and were it not that they are in constant rotation, exhibiting thus a motion of their own, one would hardly suspect that they were endowed with life. To the superficial observer they all look alike, and it is not strange, that, before they had been more carefully investigated, they should have been associated together as the lowest division of the Animal Kingdom, representing, as it were, a border-land between animal and vegetable life. But since the modern improvements in the microscope, Ehrenberg, the great master in microscopic investigation, has shown that many of these little globules have an extraordinary complication of structure. Subsequent investigations have proved that they include a great variety of beings: some of them belonging to the type of Mollusks; others to the type of Articulates, being in fact little Shrimps; while many others are the locomotive germs of plants, and so far from forming a class by themselves, as a distinct group in the Animal Kingdom, they seem to comprise representatives of all types except Vertebrates, and to belong in part to the Vegetable Kingdom, Siebold, Leuckart, and other modern zoölogists, have considered them as a primary type, and called them Protozoa; but this is as great a mistake as the other. The rotatory motion in them all is produced by an apparatus that exists not only in all animals, but in plants also, and is a most important agent in sustaining the freshness and vitality of their circulating fluids and of the surrounding medium in which they live. It consists of soft fringes, called Vibratile Cilia. Such fringes cover the whole surface of these little living beings, and by their unceasing play they maintain the rotating motion that carries them along in the water.
The Mollusks, the next great division of the Animal Kingdom, also include three classes. With them is introduced that character of bilateral symmetry, or division of parts on either side of a longitudinal axis, that prevails throughout the Animal Kingdom, with the exception of the Radiates. The lowest class of Mollusks has been named Acephala, to signify the absence of any distinct head; for though their whole organization is based upon the principle of bilateral symmetry, it is nevertheless very difficult to determine which is the right side and which the left in these animals, because there is so little prominence in the two ends of the body that the anterior and posterior extremities are hardly to be distinguished. Take the Oyster as an example. It has, like most Acephala, a shell with two valves united by a hinge on the back, one of these valves being thick and swollen, while the other is nearly flat. If we lift the shell, we find beneath a soft lining-skin covering the whole animal and called by naturalists the mantle, from the inner surface of which arise a double row of gills, forming two pendent folds on the sides of the body; but at one end of the body these folds do not meet, but leave an open space, where is the aperture we call the mouth. This is the only indication of an anterior extremity; but it is enough to establish a difference between the front and hind ends of the body, and to serve as a guide in distinguishing the right and left sides. If now we lift the mantle and gills, we find beneath the principal organs: the stomach, with a winding alimentary canal; the heart and liver; the blood-vessels, branching from either side of the heart to join the gills; and a fleshy muscle passing from one valve of the shell to the other, enabling the animal by its dilatation or contraction to open and close its shell at will. A cut across an animal of this class will show us better the bilateral arrangement of the parts. In such a section we see the edge of the two shells on either side; within these the edge of the mantle; then the double rows of gills; and in the middle the alimentary canal, the heart, and the blood-vessels branching right and left. Some of these animals have eye-specks on the edge of the mantle; but this is not a constant feature. This class of Acephala includes all the Oysters, Clams, Mussels, and the like. When named with reference to their double shells, they are called Bivalves; and with them are associated a host of less conspicuous animals, known as Ascidians, Brachiopods, and Bryozoa.
[Illustration: Common Mussel, Unio, cut transversely: a, foot; bb, gills; c, mantle; d, shell; e, heart; f, main cavity, with intestines.]
The second class in this type is that of Gasteropoda, so named from the fleshy muscular expansion on which they move, and which is therefore called a foot: a very inappropriate name; since it has no relation or resemblance to a foot, though it is used as a locomotive organ. This class includes all the Snails, Slugs, Cockles, Conchs, Periwinkles, Whelks, Limpets, and the like. Some of them have no solid covering; but the greater part are protected by a single shell, and on this account they are called Univalves, in contradistinction to the Acephala or Bivalves. These shells, though always single, differ from each other by an endless variety of form and color,—from the flat simple shell of the Limpet to the elaborate spiral and brilliant hues of the Cones and Cowries. Different as is their external covering, however, if we examine the internal structure of a Gasteropod, we find the same general arrangement of parts that prevails in the Acephala, showing that both belong to the same great division of the Animal Kingdom. The mantle envelops the animal, and lines its single shell as it lined the double shell of the Oyster; the gills are placed on either side of it; the stomach, with the winding alimentary canal, is in the centre of the body; the heart and liver are placed in the same relation to it as in the Acephala; and though the so-called foot would seem to be a new feature, it is but a muscular expansion of the ventral side of the body. There is an evident superiority in this class over the preceding one, in the greater prominence of the anterior extremity, where there are two or more feelers, with which eyes more or less developed are connected; and though there is nothing that can be properly called a head, yet there can be no hesitation as to the distinction between the front and hind ends of the body.
[Illustration: Limpet, Patella, cut transversely: a, foot; b, gills; c, mantle; d, shell; e, heart; f, main cavity, with intestines.]