436 MOLLUSCA. 



ORDER II. 



ACEPHALA NUDA(l). 



The naked Acephala(2) are not numerous, and are suf- 

 ficiently removed from the ordinary Acephala, to form a 

 distinct class, were such a division considered requisite. 

 Their branchiae assume various forms, but are never divided 

 into four leaflets ; the shell is replaced by a cartilaginous sub- 

 stance which is sometimes so thin that it is as flexible as a 

 membrane. We divide them into two families. 



FAMILY I. 



SEGREGATA(3). 



This family comprises those genera in which the individuals 

 that compose them are insulated and without any mutual or- 

 ganic connection, 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. Near the anus, the opening is trans- 

 verse, wide, and furnished with a valve which permits the entrance 

 of water, but not its exitj near the mouth, it is simply tubular. Mus- 

 cular 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 



(1) Since called by De Blainville Acephalopiioka hetkrobranchiata. As to 

 Lamarck, he makes a separate class of them, which he calls the Tuxicata, and 

 which he places between his Radiata and his Vermes,- but these animals having a 

 brain, nerves, a heart, vessels, liver, &c. this arrangement is inadmissible. 



(2) Or the Acephales sans coquilles of our author. Am. Ed. 



(3) As this family has received no name from our author, I have been com- 

 pelled, in conformity with the plan adopted from the commencement of the 

 work, to remedy the omission, for such 1 consider it, by the above word; in the 

 selection of which I have been governed by that which the Baron himself affixes 

 to the second family, or his AggregSs. Jim. Ed- 



