TUNICATA 6oi 



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 differs 

 from that of the ascidian larva described 

 above in various points, of which the fol- 

 lowing are the most important. Gonads are 

 present in the hinder region of the body: 



nearly always they are her- 



maphroditeandprotandrous. — 



Fig. 450. Oikopleura albicans in its house. Magnified. From Borradaile. 

 b. the body of the animal; t. its tail. Movements of the tail cause water to 

 enter through two funnels {i) provided with gratings by which coarse 

 particles are strained out. The water is directed (curved arrow) through a 

 filtering apparatus (/) which removes food particles : these are sucked out 

 of the filter by the animal. When the pressure rises sufficiently, the water 

 opens a spring door {e) at the broad end and passes out (wavy arrow), 

 driving the house in the opposite direction. The animal can escape by 

 pushing open a door {e') at the base of the beak. It does not return, but 

 secretes a new house, s, streamers on the house. 



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. 458 A). 



Oikopleura (Figs. 34 A, 450). Common in British waters. 



