A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

The following account of the structure and development of the Tunicate is taken, with considerable modification and condensation, from Professor Kingsley's chapter on the group in the Riverside Natural History. For the changes suggested I am indebted to the kindness of Professor William Emerson Ritter:

The Tunicates derive their name from the fact that the whole body is invested with a tough envelope or "tunic." This tunic or test may be either gelatinous, cartilaginous, or leathery. In some forms it is perfectly transparent, in others it is translucent, allowing enough light to pass to show the colors of the viscera, while in still others it is opaque and variously colored. The tunic is everywhere only loosely attached to the body proper, except in the region of the two openings now to be mentioned. One of these openings occupies a more or less central position, while the other is usually at one side, or it may even be placed at the opposite end of the body. On placing one of the Ascidians in a glass dish and sprinkling a little carmine or indigo in the water, we can study some of the functions of the animal. As soon as the disturbance is over, the animals will open the two apertures referred to, when it will be seen that each is surrounded with blunt lobes, the number of which varies with the species. As soon as they are opened a stream of water will be seen to rush into the central opening, carrying with it the carmine, and a moment later a reddish cloud will be ejected from the other aperture. From this we learn that the water passes through the body. Why it does so is to be our next inquiry. On cutting the animal open we find that the water, after passing through the first-mentioned opening (which may be called the mouth) enters a spacious chamber, the walls of which are made up of fine meshes, the whole appearing like lattice-work. Taking out a bit of this network and examining it under the microscope, we find that the edges of the meshes are armed with strong cilia, which are in constant motion, forcing the water through the holes. Of course, the supply has to be made good, and hence more water flows in through the mouth. This large cavity is known as the branchial or pharyngeal chamber. It is, according to Professor Ritter, "as we know from the embryology of the animal, the greatly enlarged anterior end of the digestive tract; and as the holes, or stigmata, as they are technically called, are perforations of the wall for the passage of water for purposes of respiration, they are both morphologically and physiologically comparable with the gill openings of fishes." There can be no doubt, therefore, that the pharyngeal sac of Ascidians is homologous with the pharynx of fishes.

Surrounding the mouth, or branchial orifice, just at its entrance into the branchial chamber is a circle of tentacles. These are simple in some genera, but elaborately branched in others.

In close connection with the cerebral ganglion, which is situated between the two siphons, there is a large gland with a short trumpet-shaped duct opening into the branchial sac a little distance behind the mouth. The orifice of the duct is just within a ring consisting of a ciliated groove that extends around the mouth outside the circle of branchial tentacles. On the opposite side of the mouth from the gland the ciliated groove joins another groove which is both ciliated and glandular, and which runs backward along the upper floor of the pharyngeal sac to its posterior extremity. This organ, called the endostyle, is concerned in the transportation of the animal's food through the pharyngeal sac to the opening of the œsophagus. Comparative embryology makes it almost certain that the subneural gland with its duct, described above, is homologous with the hypophesis cerebri of true vertebrates, and that the endostyle is homologous with the thyroid glands of vertebrates.

The water after passing through the branchial network is received into narrow passages and conducted to a larger cavity—the cloacal or atrial chamber. The general relations can he seen from our diagram, illustrating a vertical and horizontal section. From the atrial chamber the water flows out into the external world.

Now we can readily see how in the older works naturalists were misled as to the affinities of the Tunicates. They regarded the tunic as the equivalent of the mantle of the mollusks, while the incurrent and excurrent openings corresponded to the siphons. In one genus, Rhodosoma, the resemblance was even stronger, for there the tunic is in two parts, united by a hinge line, and closed by an adductor muscle. How and why these views were totally erroneous will be seen when we come to consider the development of these animals.

At the bottom of the pharnygeal sac is the narrow œsophagus surrounded with cilia, which force a current down into the digestive tract. The branchial meshes serve as a strainer for the water, and the larger particles which it contains fall down until they are within reach of the current going down the œsophagus. After passing through the throat, they come to the stomach, where digestion takes place, and then the ejectamenta are carried out through the intestine and poured into the bottom of the atrial cavity.

The heart lies on the ventral side of the stomach and is surrounded by a well-developed pericardium. The most remarkable fact connected with the circulation is that the heart, after beating a short time, forcing the blood through the vessels, will suddenly stop for a moment and then resume its beats; but, strange to say, after the stoppage the direction of the circulation is reversed, the blood taking an exactly opposite course from that formerly pursued. This most exceptional condition was first seen in the transparent Salpa, but it may be witnessed in the young of most genera. We have already referred to the branchial chamber. The walls of this chamber, besides acting as a strainer, are also respiratory organs. The meshes of which they are composed are in reality tubes through which the blood circulates and thus is brought in contact with a constantly renewed supply of fresh water.

The central nervous system in the adults of all except the Larvacea is reduced to a single ganglion placed near the mouth thus indicating the dorsal side. In forms like Cynthia it holds the same relative position with regard to the mouth, but by the doubling of the body (to be explained further on) it is also brought near the atrial aperture, where it is shown in our first diagram.

Development of Tunicates.—The sexes are combined in the same individual, though usually the products ripen at different times. As a rule, the earlier stages of the embryo are passed inside the cloacal chamber, though in some the development occurs outside the body. As a type of the development we will consider that of one of the solitary forms, leaving the many curious modifications to be noticed in connection with the species in which they occur. This will be best, since these forms show the relationship to the other vertebrates in the clearest manner.