TUNICATES. 



63 



Class I. — TUOTCATA. 



Nowhere in the whole vertebrate alliance is there as arood an illustration of dewen- 

 eracy as in the group known as Tunicates or Ascidians. Many of the species start 

 in life with the promise of reaching a point high in the scale, but after a while they 

 turn around, and, as one might say, pursue a downward course, which results in 

 an adult which displays but few resemblances to the other vertebrates. Indeed, so 

 different do they seem, that the fact that they belong here was not suspected until 

 little over a decade ago. Before that time, ever since the days of Cuvier, they were 

 almost universally regarded as molluscs, and many facts were adduced to show that 

 they belonged near the acephals. In the later years, when the facts of develojjment 

 began to be known, this association was looked upon with suspicion, and by some they 

 were placed for a short time among the worms. Anyone who has watched the phases 

 of their development cannot help believing that they belong here, the lowest of the 

 vertebrate series. 



Before treating of the development, let us consider the structure of the adult, 

 taking as our type one of the common, simple forms, like Ascidia or Cynthia, and 

 illustrating the facts of anatomy by means of a diagrammic figure. 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 free from the body piroper, except in the 

 region of the two openings now to be mentioned. 

 One of these openings occupies a more or less cen- 

 tral position, while the other is usually at one side, 

 or 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 into 

 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 aper- 

 ture. 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-men- 

 tioned opening (which may be called the mouth) en- 

 ters a spacious chamber, the walls of which are made ujj of fine meshes, the whole ap- 

 pearing like lattice-work. Taking out a bit of this network and examining it under a 



Fig. 45. — Diagram of an ascidian ; a, atrial 

 cliamber; b, base; c, cavity of tunic; g, 

 genital organs; h, heart; i, intestine; m, 

 mouth; n, nerve; p, branchial sac; s, sto- 

 mach; tf tunic. 



