TUNICATA 679 



It should be noticed that in budding the origin of the organs takes place 

 without regard to the germ layers from which they arise in the develop- 

 ment of the ovum, for the endodermal inner tube of ordinary stolonial 

 budding often forms atrium and nervous system, which should be of 

 ectodermal origin, and the ectodermal (atrial) inner vesicle of the 

 "pallial" budding of Botryllus forms the alimentary canal, which 

 should be endodermal, while in Perophora and Clavelina all these 

 organs arise from a mass of mesoderm. 



A zooid which arises by budding is known as a blastozooid {blasto- 

 zoite): one which arises from an ovum is an oozooid. The oozooid, 

 which in the Thaliacea differs considerably from the blastozooid, 

 has always lost the power of sexual reproduction. In the Salpida and 

 Doliolida the blastozooid has lost the power of budding, so that there 

 is a regular alternation of generations. 



The Tunicata fall into three classes. Of these, one, the Larvacea^ 

 only comprises a few little animals which spend the whole of their 

 lives in the larval condition, developing genital organs and repro- 

 ducing without metamorphosis. The other two classes both attain the 

 adult form, but whereas in one of them — the Ascidiacea — the animals 

 are sedentary and have both branchial and atrial openings directed 

 away from the substratum, the members of the other — the Thaliacea 

 — are pelagic and swim by driving water out of the atrial opening, 

 which is at the opposite end of the body from the mouth. 



Class LARVACEA 



Tunicata in which the sexually mature form retains the organization 

 of the larva. 



The test is not composed of tunicin . It forms a remarkable ' ' house ' ' 

 that does not adhere to the animal, which from time to time leaves it 

 and secretes a new one. The habitat is pelagic, and food is filtered 

 from the water by an apparatus which forms part of the house and 

 through which water is caused to flow by the movements of the tail. 



The organization of the animal diflFers from that of the ascidian 

 larva described above in various points, of which the following are 

 the most important. Gonads are present in the hinder region of 

 the body: nearly always they are hermaphrodite and protandrous. 

 The tail is attached to the ventral side, near the hinder end of the 

 body. The two simple gill clefts open ventrolaterally directly to the 

 exterior. The intestine also opens directly to the exterior, ventrally or 

 on the right-hand side. The brain. is a compact fusiform ganglion, 

 and the existence of a cavity in it or in the nerve cord is doubtful. 

 There is no eye and a statocyst lies beside the brain on the left. In 

 certain of these respects the animal resembles the larva of Doliolum 

 (Fig. 483 A). 



