TUNICA TES. 



55 



Fig. 46. — Diagram of tadpole-like larva of an ascidian; h, 

 brain; (7« gill cliamher; i, intestine; m, mouth; n, nuto- 

 chord; a', neural eord; u, urogenital system. 



cloacal chaiiiher, tlioiigli in some the development occurs outside the body. As a tyjie 

 of the development we will consider that of one of the solitary forms, leaving the many 

 curious moditications 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 rmli- 

 mentary urogenital organs, which 

 empty either into the rectum or just 

 behind the vent. In the tail, but not 

 extending to any distance into the 

 body, is an a.\ial 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 liad the emliryology remained 

 unknown. 



After the stage described, this reti'ogradc 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-aqu.atic object and adhere 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 foi-ms like Appendicularia, this dcgenera^ 

 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 algje, 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 tliey were 

 almost entirely filled with the curious cilio-flagellatc jirotozoan, Ceratium tripos, while 

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

 are small, only a small number at-taining 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. 



