284 MOLLUSCA. 



replaced by a cartilaginous substance which is sometimes so thin 

 that it is as flexible as a membrane. We divide them into two 

 families. 



FAMILY I. 

 SEGREGATA. 



This family comprises those genera in which the individuals that 

 compose them are insulated and without any mutual organic con- 

 nection, although frequently living in society. In the 



BIPHORA, Brug. THALIA, Brown, SALPA and DAGYSA, Gmelin. 

 The mantle and its cartilaginous envelope are oval or cylindrical, and open 

 at the two extremities. Muscular bands embrace the mantle and contract the 

 body. The animal moves by taking in water at the posterior aperture, and 

 forcing it out through that near the mouth, so that it is always propelled 

 backwards, a circumstance which has led some naturalists into error by 

 causing them to mistake the posterior opening for the true mouth. It usu- 

 ally swims on its back. The branchiae form a single tube or riband, furnish- 

 ed with regular vessels, placed obliquely in the middle of the tubular cavity 

 of the mantle, in such a manner that it is constantly bathed by the water as 

 it traverses that cavity. The mantle and its envelope when exposed to the 

 sun exhibit the colours of the rainbow, and are so diaphanous, that the whole 

 structure of the animal can be seen through them: in many they are furnish- 

 ed with perforated tubercles. The animal has been seen to come out from 

 its envelope without appearing to suffer pain. 



These animals are very abundant in the Mediterranean and the warmer 

 portions of the ocean, and are frequently phosphorescent. 



ASCIDIA, Lin. Theyton of the Ancients. 



The mantle and its cartilaginous envelope, which is frequently very thick, 

 resemble sacs everywhere closed, except at two orifices, which correspond 

 to the two tubes of several bivalves. The branchiae form a large sac, at the 

 bottom of which are the mouth and the visceral mass. The envelope is 

 much larger than the mouth, which is fibrous and vascular, and on which, 

 between the two tubes, is one of the ganglions. These animals attach 

 themselves to rocks and other bodies, and are deprived of all power of lo- 

 comotion; the chief sign of vitality which they exhibit, consists in the absorp- 

 tion and evacuation of water through one of their orifices; when alarmed 

 they eject it to a considerable distance. They abound in every sea, and 

 some of them are eaten. 

 Some species are remarkable for the long pedicle which supports them. 



