TYPE PROTOOHOBBATA. 633 



four parts (Fig. 289, A), whicli arrange themselves in a circle 

 around the Cyathozooid (Fig. 289, B), being enclosed with it 

 in a common test, and develop eventually into the four 

 primary individuals or Ascidiozooids, which occupy the closed 

 end of the cylindrical colony. As they develop the Cyatho- 

 zooid degenerates and finally completely disappears, the 

 cloacal cavity of the colony arising, as in Botryllus, as a de- 

 pression of the test, which grows deeper and deeper as new 

 individuals arise by budding. 



The entire process shows a marked similarity to what occurs in Botryl- 

 lus, the principal differences being that the Cyathozooid, which represents 

 the first generation of Botryllus, buds while yet in an embryonic condition, 

 and that it alone degenerates, the parents of succeeding generations per- 

 sisting and forming parts of the fully-developed colony. In each Ascidio- 

 zooid there occurs behind the branchial sac a pair of peculiar masses of 

 cells termed the elceohlast (Fig. 289, B, el), whose function is uncertain, 

 though it has been suggested that they may represent the rudimentary 

 larval tail. 



3. Order Thaliacea. 



The Thaliacea are with a single exception pelagic organ- 

 isms, and present a life-history complicated by the occur- 

 rence of an alternation of generations. In the genus Salpa 

 (Fig. 290) a well-developed test is present, and the muscula- 

 ture of the mantle is arranged in bands which do not quite 

 surround the body and furthermore show a tendency to unite 

 together on the dorsal surface of the body. At one end of 

 the body is a large branchial aperture (m) leading into a 

 branchial sac, in the floor of which is the endostyle (en), 

 while in its roof is a ridge, the dorsal lamina, which anteriorly 

 is produced into a single languet projecting into the interior 

 of the sac. On each side of the dorsal lamina is a large 

 opening which represents an enormous stigma and communi- 

 cates with the atrial cavity, whose aperture is situated at the 

 posterior end of the body. The intestine and the other viscera 

 are grouped together to form a small mass termed the nucleus 

 (nu) lying behind the branchial sac and ventrally, though in 

 some forms the intestine is more elongated and projects some- 

 what from the nucleus. The nervous system (n) has its usual 

 form and position ; it has in connection with it three pigmented 



