Evolution

by James A. S. Watson

Fig. 64.—A Tunicate.

in.ap., Opening leading to mouth; ex.ap., opening of peribranchial chamber.

From Dendy's Outlines of Evolutionary Biology (Constable).

Fig. 65.—Larva of Tunicate.

n, Nerve cord; s, sense organs; m, mouth; kd, pharynx; vd, alimentary canal; ch, notochord.

Now the remarkable fact has been made out that the young tunicate (see Fig. 65) bears a most striking resemblance to the immature lancelet. It is a free-swimming, tadpole-like creature, and possesses a notochord and nerve cord and in general the same characters that we described for Amphioxus. It is only later that the creature settles down and assumes its final degenerate sedentary form. There can be no doubt that the tunicates have been derived from some lancelet-like form, but the course of their evolution has been unique. The type is the lost brother of the vertebrate family, who has chosen a distinctly downward path in life; yet who has come to no miserable end, but lives on, more or less successfully, in his lower social sphere.

The round-mouths, including the lampreys and the hag-fish, stand midway between the lancelet and the fishes, and therefore constitute for us an important group.

The lamprey is a fairly generally known eel-like creature, of which there is a smaller fresh-water, and a larger salt-water species, the latter reaching a length of about a yard. It is found attached to, and feeding on, the dead bodies of fish, and less frequently on living specimens. The hag is much more definitely parasitic in its habits, and often occurs in the body cavity of living fish. These forms were for long regarded as fishes, and are sometimes even yet included in that group, but all their characteristics point to a very much lower position in the animal world than such a classification would indicate. One of the most striking external differences is that the round-mouths have nothing to represent the two pairs of fins which occur uniformly in the fishes, and which are, in a true sense, the forebears of our own arms and legs. Even more important than this is the absence of jaws. The mouth in this group is a simple round opening, whose edges are armed with pointed teeth, the latter, however, bearing no real resemblance to the teeth of the higher animals. By means of these teeth, and a pointed, tongue-like organ, and by suction, the round-mouths are able to bore into the tissues of the animals on which they prey. The absence of jaws and of extremities is, of course, a feature which they share with the lancelet. Turning to the internal structure, we may first observe that there is still no vertebral column, but only a simple, rod-like notochord, similar in its shape, and in its relations to other parts, to that of Amphioxus. There is, however, an additional development of cartilage in the region of the head, forming, in particular, a sheath-like covering for the brain and also a kind of basket-work support for the pharynx and gills. The original tube-like form of the dorsal nerve cord is easily recognisable, but it is distinctly distended at its front end into a brain, which shows a division into a series of three distinct portions, called respectively the fore, mid, and hind brain. This division, it is interesting to observe, is the first process in the development of the brain in all the higher animals. There is a pair of well-developed hearing organs, and in the lamprey a pair of similarly well-developed eyes. In the hag fish the latter are greatly reduced, a condition which is explained by the creature's mode of life. The nostril is unpaired, a condition which is probably primitive. Respiration is carried out by means of gills, which are situated in a series of six to eight pouches, each of which opens into the gullet and again directly to the outside, the external openings being an obvious feature of the animal. There is a very distinct, simple heart, which pumps the blood to the gills, from whence it is collected and distributed throughout the body. The digestive canal is a simple tube, provided, however, with a liver and a pancreas, the two most important digestive glands in the higher animals. The sexes are separate, but traces of a previous hermaphrodite condition seem to persist. Henceforward in the vertebrate group the sexes are always separate. A character of the sex organs which is to be regarded as primitive is that they are unconnected with the excretory system, whereas in the higher vertebrates the two systems are always strangely interconnected. As in all the higher types, there is but one pair of male or female reproductive glands. Finally, the round-mouths differ markedly from the lancelet in the structure of the skin. In the latter animal the skin is composed of a single layer of cells. In the former it consists of an epidermis, some three or four layers thick, and an underlying cuticle or 'true skin'; in other words, the skin has the same general structure as that of the higher types. The lamprey and the hag are illustrated in Figs. 66 and 67, and some of the main anatomical features of the group are shown in Figs. 63, 68, and 69.