68o THE INVERTEBRATA 



The Larvacea are now generally regarded as an instance of neoteny 

 (p. 144). 



•e 



Fig- 475- 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. 



Oikopleura (Figs. 43 A, 475). Common in British waters. 



Class ASCIDIACEA 



Tunicata in which the adult is sedentary and has no tail ; a degenerate 

 nervous system; an atrium which opens dorsally; a stolon (if any) 

 of simple structure; and several gill clefts, which are nearly always 

 divided into stigmata by external longitudinal bars. 



The colonial members of this group are known as "compound 

 ascidians" and are sometimes classed together as Ascidiae compositae. 

 But they are not of one origin ; some of them have stolonial budding 

 and dorsal languets and are related to such solitary forms as Ciona; 

 the others, with pallial budding and a continuous dorsal lamina in 

 place of the languets, are related to solitary forms, such as Ascidia, 

 which have no epicardium and possess a dorsal lamina. 



Ciona (Figs. 468-471). Described above. 



Clavelina. Resembles Ciona in general features but has a stolon 

 and forms clusters of individuals by the breaking off of buds from 

 the ends of the stolon branches ; the stolon has a mesodermal septum ; 

 the zooids and their tests are free from one another. 



Polyclinum. As Clavelina; but the stolon contains an epicardial 

 tube; and the zooids are imbedded in a common test with only the 

 branchial and atrial openings at the surface. 



