MOLLUSC A. 199 



in his Recherches Anatomiques sur les Ascidies composees, 

 et sur les Ascidies simples, inserted in his Memoires sur les 

 Animaux sans Vertebres, 8vo, Paris, 1816. 



The covering of the animals of this group consists of an 

 external and internal sac or tunic, which are either entirely 

 united or unconnected, except at the apertures. The sur- 

 face is smooth in some, and rough in others, and in a few 

 species defended by an artificial covering of agglutinated 

 shells and sand. The sacs are furnished with muscular 

 bands, and are capable of contraction. Some of the species, 

 by means of contractile movements, float about in the water ; 

 others, receiving that element into the branchial cavity, and 

 ejecting it forcibly at the opposite one, push themselves for- 

 ward. Many, however, are fixed during life to seaweeds 

 and stones. 



The apertures of the tunic are two in number, unless in 

 the doubtful genus Mammaria. The one, frequently the 

 largest, is destined for receiving the water into the cavity 

 to supply the mouth and gills. This is termed the branch- 

 ial cavity. The other is destined for the exit t)f the water, 

 the eggs, and the faeces, and termed the anal opening. 

 These apertures are sometimes placed near each other, at 

 other times at opposite extremities of the body, and vari- 

 ously provided with tentacula or valves. 



The mouth is simple, destitute of spiral arms, and open- 

 ing in the anterior of the cavity of the body between the 

 branchiae, as in the other Acephala, It possesses neither 

 jaws nor tentacula. The alimentary canal is very simple, 

 and can scarcely be distinguished into gullet, stomach, and 

 intestine. The food is soft, and such as the bounty of the 

 waves bestows. The liver adheres to the stomach, and in 

 many species is divided into distinct lobes. 



