lusk and Crustacean, to which the reader unfamiliar with the language of Natural History will have attached but vague ideas; and although I wanted to explain these, and convey a distinct conception of the general facts of classification, it would have been too great an interruption. So I will here make an opportunity, and finish the chapter with an indication of the five types, or plans of structure, under one of which every animal is classed. Without being versed in science, you discern at once whether the book before you is mathematical, physical, chemical, botanical, or physiological. In like manner, without being versed in Natural History, you ought to know whether the animal before you belongs to the Vertebrata, Mollusca, Articulata, Radiata, or Protozoa. A glance at the contents of our glass vases will yield us samples of each of these five divisions of the animal kingdom. We begin with this Triton (Fig. 17). It is a representative of the VERTEBRATE division or sub-kingdom. You have merely to remember that it possesses a backbone and an internal skeleton, and you will at once recognize the cardinal character which makes this Triton range under the same general head as men, elephants, whales, birds, reptiles, or fishes. All these, in spite of their manifold differences, have this one character in common-they are all backboned; they have all an internal skeleton; they are all formed according to one general type. In all vertebrate ani Fig. 17.-MALE TRITON, OR WATER-NEWT. mals the skeleton is found to be identical in plan. Every bone in the body of a triton has its corresponding bone in the body of a man or of a mouse; and every bone preserves the same connection with other bones, no matter how unlike may be the various limbs in which we detect its presence. Thus, widely as the arm of a man differs from the fin of a whale, or the wing of a bird, or the wing of a bat, or the leg of a horse, the same number of bones, and the same connections of the bones, are found in each. A fin is one modified form of the typical limb; an arm is another; a wing another. That which is true of the limbs is also true of all the other organs; and it is on this ground that we speak of the vertebrate type. From fish to man one common plan of structure prevails, and the presence of a backbone is the index by which to recognize this plan. The Triton has been wriggling grotesquely in our grasp while we have made him our text, and, now he is restored to his vase, plunges to the bottom with great satisfaction at his escape. This water-snail, crawling slowly up the side of the vase, and cleaning it of the green growth of microscopic plants, which he devours, shall be our representative of the second great division-the MOLLUSCA. I can not suggest any obvious character so distinctive as a backbone by which the word mollusk may at once call up an idea of the type which prevails in the group. It won't do to say It won't do to say "shellfish," because many mollusks have no shells, and many animals which have shells are not mollusks. The name was originally bestowed on account of the softness of the animals. But they are not softer than worms, and much less so than jellyfish. You may know that snails and slugs, oysters and cuttlefish, are mollusks; but if you want some one character by which the type may be remembered, you must fix on the imperfect symmetry of the mollusk's organs. I say imperfect symmetry, because it is an error, though a common one, to speak of the mollusk's body not being bilateral—that is to say, of its not being composed of two symmetrical halves. A vertebrate animal may be divided lengthwise, and each half will closely resemble the other; the backbone forms, as it were, an axis, on either side of which the organs are disposed; but the mollusk is said to have no such axis, no such symmetry. I admit the absence of an axis, but I deny the total absence of symmetry. Many of its organs are as symmetrical as those of a vertebrate animal—i. e., the eyes, the feelers, the jaws-and the gills in Cuttlefish, Eolids, and Pteropods; while, on the other hand, several organs in the vertebrate animal are as unsymmetrical as any of those in the mollusk—i. e., the liver, spleen, pancreas, stomach, and intestines.* As regards bilateral structure, therefore, it is only a question of degree. The vertebrate animal is not entirely symmetrical, nor is the mollusk entirely unsymmetrical. But there is a characteristic disposition of the nervous system peculiar to mollusks: it neither forms an axis for the body, as it does in the Vertebrata and Articulata, nor a centre, as it does in the Radiata, but is altogether irregular and unsymmetrical. This will be intelligible from the following diagram of the nervous systems of a mollusk and an insect, with which that of a starfish may be compared (Fig. 18). Here you perceive how the nervous centres and the nerves which * In some cases of monstrosity these organs are transposed, the liver being on the left, and the pancreas on the right side. It was in allusion to a case of this kind, then occupying the attention of Paris, that MOLIÈRE made his Medecin malgré Lui describe the heart as on the right side, the liver on the left; on the mistake being noticed, he replies, "Oui, autrefois; mais nous avons changé tout cela." |