TUNIC ATES. 



55 



Fig. 4(j. — Diagram of tadpole-like lai-va of an ascidian; 6, 

 brain; g, gill chamber; i, intestine; m, mouth; n, noto- 

 chord; s, neural cord; u, urogeuital system. 



(iloacal chamber, though in some tlie 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. 



The egg undergoes a total segmentation and a -regular gastrulation. Soon a tail 

 appears, and under the microscope the young embryo, which now begins its free life, 

 appears much like the tadpole of the frog. It has a large oval body and a long tail 

 which lashes about, forcing the animal forward with a wriggling motion. Nor is 

 the resemblance superficial ; it pervades every part of the structure, as may be seen 

 from the adjacent diagram. The mouth is nearly terminal and communicates with 

 a gill chamber provided with gill clefts. At the posterior end of the gill chamber 

 begins the alimentary tract, which pur- 

 sues a convoluted course to the vent. 

 Just above the intestine are the rudi- 

 mentary urogenital organs, which 

 empty eitlier into the rectum or just 

 behind the vent. In the tail, but not 

 extending to any distance into the 

 body, is an axial cylinder — the noto- 

 chord, which here, as in all other verte- 

 brates, arises from the hypoblast ; and above it is the spinal cord (epiblastic in origin), 

 which extends forward to the brain, above the gill chamber. Besides, the animal is 

 provided with organs of sight and hearing. So far the correspondence between the 

 two types is very close, and if we knew nothing about the later stages, one would 

 without doubt predict that the adult tunicate would reach a high point in the scale 

 of vertebrates. These high expectations are never fulfilled ; the animal, on the con- 

 trary, pursues a retrograde course, resulting in an adult whose relationship to the 

 other vertebrates would never have been suspected had the embryology remained 

 unknown. 



After the stage described, this retrograde development begins. From various 

 parts of the body, lobes grow out, armed on their extremities with sucking discs. 

 These soon come in contact with some sub-aquatic object and adhei-e to it. Then the 

 notochord breaks down, the spinal cord is absorbed, and then the tail follows suit, 

 the intestine twists around, and the cloaca is formed, the result being much like the 

 diagram near the head of this section. In forms like Appendicularia, this degenera- 

 tion does not proceed so far; the tail, with its notochord and neural cord, persisting 

 through life. 



The tunicates, without exception, are all marine. Some are attached, and others 

 float or swim freely through the water. They feed upon diatoms, small algae. Infu- 

 soria, and minute crustaceans. Their stomachs usually prove rich collecting grounds 

 for the student of the lower forms of life. I have examined them when they were 

 almost entirely filled with the curious cilio-flagellate protozoan, Cerathim tripos, while 

 at other times they have contained large numbers of Radiolaria. Most of the species 

 ai-e small, only a small number attaining a length of a few inches. Many of them are 

 highly phosphorescent, especially the transparent pelagic forms. Owing to the entire 

 absence of any hard tissues, fossil tunicates are unknown. 



