The third and highest class of Mollusks has been called Cephalopoda, in reference again to a special feature of their structure. They have long arms or feelers around the head, serving as organs of locomotion, by which they propel themselves through the water with a velocity that is quite extraordinary, when compared with the sluggishness of the other Mollusks. In these animals the head is distinctly marked,—being separated, by a contraction or depression behind it, from the rest of the body. The feelers, so prominent on the anterior extremity of the Gasteropoda, are suppressed in Cephalopoda, and the eyes are consequently brought immediately on the side of the head, and are very large in proportion to the size of the animal. A skin corresponding to the mantle envelops the body, and the gills are on either side of it;—the stomach with its winding canal, the liver, and heart occupy the centre of the body, as in the two other classes. This class includes all the Cuttle-Fishes, Squids, and Nautili, and has a vast number of fossil representatives. Many of these animals are destitute of any shell; and where they have a shell, it is not coiled from right to left or from left to right as in the spiral of the Gasteropoda, but from behind forwards as in the Nautilus. These shells are usually divided into a number of chambers,—the animal, as it grows, building a wall behind it at regular intervals, and always occupying the external chamber, retaining, however, a connection with his past home by a siphon that runs through the whole succession of chambers. The readers of the "Atlantic Monthly" cannot fail to remember the exquisite poem suggested to the Autocrat of the Breakfast-Table by this singular feature in the structure of the so-called Chambered Shells.
[Illustration: Common Squid, Loligo, cut transversely: a, foot or siphon; b, gills; c, mantle; d, shell; e, heart; f, main cavity, with intestines.]
Cuvier divided the Mollusks also into a larger number of classes than are now admitted. He placed the Barnacles with them on account of their shells; and it is only since an investigation of the germs born from these animals has shown them to be Articulates that their true position is understood. They give birth to little Shrimps that afterwards become attached to the rocks and assume the shelly covering that has misled naturalists about them. Brachiopods formed another of his classes; but these differ from the other Bivalves only in having a net-work of blood-vessels in the place of the free gills, and this is merely a complication of structure, not a difference in the general mode of execution, for their position and relation to the rest of the organization are exactly the same in both. Pteropods constituted another class in his division of the type of Mollusks; but these animals, again, form only an order in the class of Gasteropoda, as Brachiopods form an order in the class of Acephala.
In the third division of the Animal Kingdom, the Articulates, we have again three classes: Worms, Crustacea, and Insects. The lowest of these three classes, the Worms, presents the typical structure of that branch in the most uniform manner, with little individualization of parts. The body is a long cylinder divided through its whole length by movable joints, while the head is indicated only by a difference in the front-joint. There is here no concentration of vitality in special parts of the structure, as in the higher animals, but the nervous force is scattered through the whole body,—every ring having, on its lower side, either two nervous swellings, one on the right, the other on the left side, connected by nervous threads with those that precede and those that follow them, or these swellings being united in the median line. It is this equal distribution of nervous force through the whole system that gives to these animals such an extraordinary power of repairing any injured part, so that, if cut in two, the front part may even reconstruct a tail for itself, while the hind part produces a new head, and both continue to live as distinct animals. This facility of self-repair, after a separation of the parts, which is even a normal mode of multiplication in some of them, does not indicate, as may at first appear, a greater intensity of vital energy, but, on the contrary, arises from an absence of any one nervous centre such as exists in all the higher animals, and is the key to their whole organization. A serious injury to the brain of a Vertebrate destroys vitality at once, for it holds the very essence of its life; whereas in many of the lower animals any part of the body may be destroyed without injury to the rest. The digestive cavity in the Worms runs the whole length of the body; and the respiratory organs, wherever they are specialized, appear as little vesicles or gill-like appendages either along the back or below the sides, connected with the locomotive appendages.
This class includes animals of various degrees of complication of structure, from those with highly developed organizations to the lowest Worms that float like long threads in the water and hardly seem to be animals. Yet even these creatures, so low in the scale of life, are not devoid of some instincts, however dim, of feeling and affection. I remember a case in point that excited my own wonder at the time, and may not be uninteresting to my readers. A gentleman from Detroit had had the kindness to send me one of those long thread-like Worms (Gordius) found often in brooks and called Horse-Hairs by the common people. When I first received it, it was coiled up in a close roll at the bottom of the bottle, filled with fresh water, that contained it, and looked more like a little tangle of black sewing-silk than anything else. Wishing to unwind it, that I might examine its entire length, I placed it in a large china basin filled with water, and proceeded very gently to disentangle its coils, when I perceived that the animal had twisted itself around a bundle of its eggs, holding them fast in a close embrace. In the process of unwinding, the eggs dropped away and floated to a little distance. Having finally stretched it out to its full length, perhaps half a yard, I sat watching to see if this singular being that looked like a long black thread in the water would give any signs of life. Almost immediately it moved towards the bundle of eggs, and, having reached it, began to sew itself through and through the little white mass, passing one end of its body through it, and then returning to make another stitch, as it were, till the eggs were at last completely entangled again in an intricate net-work of coils. It seemed to me almost impossible that this care of copying could be the result of any instinct of affection in a creature of so low an organization, and I again separated it from the eggs, and placed them at a greater distance, when the same action was repeated. On trying the experiment a third time, the bundle of eggs had become loosened, and a few of them dropped off singly into the water. The efforts which the animal then made to recover the missing ones, winding itself round and round them, but failing to bring them into the fold with the rest, because they were too small, and evaded all efforts to secure them, when once parted from the first little compact mass, convinced me that there was a definite purpose in its attempts, and that even a being so low in the scale of animal existence has some dim consciousness of a relation to its offspring. I afterwards unwound also the mass of eggs, which, when coiled up as I first saw it, made a roll of white substance about the size of a coffee-bean, and found that it consisted of a string of eggs, measuring more than twelve feet in length, the eggs being held together by some gelatinous substance that cemented them and prevented them from falling apart. Cutting this string across, and placing a small section under the microscope, I counted on one surface of such a cut from seventy to seventy-five eggs; and estimating the entire number of eggs according to the number contained on such a surface, I found that there were not less than eight millions of eggs in the whole string. The fertility of these lower animals is truly amazing, and is no doubt a provision of Nature against the many chances of destruction to which these germs, so delicate and often microscopically small, must be exposed. The higher we rise in the Animal Kingdom, the more limited do we find the number of progeny, and the care bestowed upon them by the parents is in proportion to this diminution.
The next class in the type of Articulates is that of Crustacea, including Lobsters, Crabs, and Shrimps. It may seem at first that nothing can be more unlike a Worm than a Lobster; but a comparison of the class-characters shows that the same general plan controls the organization in both. The body of the Lobster is divided into a succession of joints or rings, like that of the Worm; and the fact that the front rings in the Lobster are soldered together, so as to make a stiff front region of the body, inclosing the head and chest, while only the hind rings remain movable, thus forming a flexible tail, does not alter in the least the general structure, which consists in both of a body built of articulated rings. The nervous swellings, which were evenly distributed through the whole body in the Worm, are more concentrated here, in accordance with the prevalent combination of the rings in two distinct regions of the body, the larger ones corresponding to the more important organs; but their relation to the rest of the organization, and their connection by nervous threads with each other, remain the same. The respiratory organs, which in most of the Worms were mere vesicles on the lower part of the sides of the body, are here more highly organized gills; but their general character and relation to other parts of the structure are unchanged, and in this respect the connection of the gills of Crustacea with their legs is quite significant. The alimentary canal consists of a single digestive cavity passing through the whole body, as in Worms, the anterior part of which is surrounded by a large liver. What is true of the Lobsters is true also, so far as class-characters are concerned, of all the Crustacea.
Highest in this type are the Insects, and among these I include Spiders and Centipedes as well as Winged Insects. It is true that the Centipedes have a long uniform body like Worms, and the Spiders have the body divided into two regions like the Crustacea, while the body in true Insects has three distinct regions, head, chest, and hind body; but notwithstanding this difference, both the former share in the peculiar class-character that places them with the Winged Insects in a separate group, distinct from all the other Articulates. We have seen that in the Worms the respiratory organs are mere vesicles, while in the Crustacea they are more highly organized gills; but in Centipedes, Spiders, and Winged Insects, the breathing-apparatus is aerial, consisting of air-holes on the sides of the body, connected with a system of tubes and vessels extending into the body and admitting air to all parts of it. In the Winged Insects this system is very elaborate, filling the body with air to such a degree as to render it exceedingly light and adapted to easy and rapid flight. The general arrangement of parts is the same in this class as in the two others, the typical character being alike in all.
We come now to the highest branch of the Animal Kingdom, that to which we ourselves belong,—the Vertebrates. This type is usually divided into four classes, Fishes, Reptiles, Birds, and Mammalia; and though many naturalists believe that it includes more, and I am myself of that opinion, I shall allude here only to the four generally admitted classes, as they are sufficient for my present purpose, and will serve to show the characters upon which classes are based. In a former paper I have explained in general terms the plan of structure of this type,—a backbone, with a bony arch above and a bony arch below, forming two cavities that contain all the systems of organs, the whole being surrounded by the flesh and skin. Now whether a body so constructed lie prone in the water, like a Fish,—or be lifted on imperfect legs, like a Reptile,—or be balanced on two legs, while the front limbs become wings, as in Birds,—or be raised upon four strong limbs terminating in paws or feet, as in Quadrupeds,—or stand upright with head erect, while the limbs consist of a pair of arms and a pair of legs, as in Man,—does not in the least affect that structural conception under which they are all included. Every Vertebrate has a backbone; every Vertebrate has a bony arch above that backbone and a bony arch below it, forming two cavities,—no matter whether these arches be of hard bone, or of cartilage, or even of a softer substance; every Vertebrate has the brain, the spinal marrow or spinal cord, and the organs of the senses in the upper cavity, and the organs of digestion, respiration, circulation, and reproduction in the lower one; every Vertebrate has four locomotive appendages built of the same bones and bearing the same relation to the rest of the organization, whether they be called pectoral and ventral fins, or legs, or wings and legs, or arms and legs. Notwithstanding the rudimentary condition of these limbs in some Vertebrates and their difference of external appearance in the different groups, they are all built of the same structural elements. These are the typical characters of the whole branch, and exist in all its representatives.
What now are the different modes of expressing this structural plan that lead us to associate certain Vertebrates together in distinct classes? Beginning with the lowest class,—the Fishes are cold-blooded, they breathe through gills, and they are egg-laying; in other words, though they have the same general structure as the other Vertebrates, they have a special mode of circulation, respiration, and reproduction. The Reptiles are also cold-blooded, though their system of circulation is somewhat more complicated than that of the Fishes; they breathe through lungs, though part of them retain their gills through life; and they lay eggs, but larger and fewer ones than the Fishes, diminishing in number in proportion to their own higher or lower position in their class. They also bestow greater care upon their offspring than most of the Fishes. The Birds are warm-blooded and air-breathing, having a double circulation; they are egg-laying like the two other classes, but their eggs are comparatively few in number, and the young are hatched by the mother and fed by the parent birds till they can provide for themselves.
The Mammalia are also warm-blooded and breathe through lungs; but they differ from all other Vertebrates in their mode of reproduction, bringing forth living young which they nurse with milk. Even in the lowest members of this highest group of the Vertebrates, at the head of which stands Man himself, looking heavenward it is true, but nevertheless rooted deeply in the Animal Kingdom, we have the dawning of those family relations, those intimate ties between parents and children, on which the whole social organization of the human race is based. Man is the crowning work of God on earth; but though so nobly endowed, we must not forget that we are the lofty children of a race whose lowest forms lie prostrate within the water, having no higher aspiration than the desire for food; and we cannot understand the possible degradation and moral wretchedness of Man, without knowing that his physical nature is rooted in all the material characteristics that belong to his type and link him even with the Fish. The moral and intellectual gifts that distinguish him from them are his to use or to abuse; he may, if he will, abjure his better nature and be Vertebrate more than Man. He may sink as low as the lowest of his type, or he may rise to a spiritual height that will make that which distinguishes him from the rest far more the controlling element of his being than that which unites him with them.