MOLLUSC01DA. 



311 



The Composite Ascidians are still more intimately associated together ; 

 a great number of these little beings live together in a single 

 mass. Such are the Botryllus and the Pyrosoma. 



The Botryllus is a genera 

 the most interesting of all 

 the groups under considera- 

 tion. Only imagine from 

 ten to twenty individuals, 

 oval in form, more or less 

 flattened, adhering by their 

 dorsal surface to some sub- 

 marine body, and holding on 

 by their sides, so as to form 

 a sort of wheel. " When we 

 excite one of the branches," 

 says Fredol, "a single mol- 

 lusc contracts itself; when 

 we touch the centre, they all 

 seem to contract themselves 

 (Cuvier). The buccal orifice is 

 at the outer extremity of the 

 radius ; but the intestinal 

 terminations abut on the 

 common cavity, which occu- 

 pies the centre of the wheel. 

 Here we behold certain ani- 



Fig. 125. Ascidia pedunculate (Milne Edwards). 



mals which eat separately, 



but which fulfil together as a community very singular functions a 

 kind of union and communism of which the moral world presents 

 no prototype. With our molluscs, in place of two individuals united, 

 we have a score. We may consider the entire star as one single 

 animal with many mouths. But then, we have with it a luxury of 

 organs for the function of intelligence which seeks and chooses, and 

 parsimony of the organ of stupidity, which neither seeks nor chooses." 

 While the Botryllus is fixed and adherent, the Pyrosoma, on the 

 contrary, is perfectly free. The animal colony which constitutes it 

 floats and balances itself upon the waters, like the sea-pen or the 

 physalia, of which we have spoken in treating of the zoophytes. 



