MOLLUSCA. 199 
in his Recherches Anatomiques sur les Ascidies composées, 
et sur les Ascidies simples, inserted in his Mémoires sur les 
Animaux sans Vertébres, 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 5 
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 dranch- 
ial cavity. The other is destined for the exit of the water, 
the eggs, and the feces, 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 
branchize, 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. 
