MOLLUSCA. 509 



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

 ing in the interior of the cavity of the body, between the 

 branchiae, as in the bivalvia. 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 be- 

 stow. The liver adheres to the stomach, and in many species 

 is divided into distinct lobes. 



The circulating system appears to be reduced to a single 

 systemic ventricle. The gills cover the walls of the cavity, 

 in the form of ridges, more or less complicated, and seldom 

 symmetrical. 



The reproductive organs consist of an ovarium, either 

 simple or complicated, with some additional glands, the 

 uses of which have not been ascertained. The species are 

 considered as hermaphrodite, and independent of reciprocal 

 impregnation. They appear, in some genera, not only to 

 be oviparous, but to be gemmiparous and compound, many 

 individuals being organically connected, and capable of si- 

 multaneous movements. They are all inhabitants of the 

 sea. 



1st Subdivision. 



Interior tunic detached from the external one, and united 

 only at the two orifices. 



The branchiae are large, equal, and spread on the cen- 

 tral walls of the inner sac. The branchial orifice has an 

 inner membranaceous denticulated rin.o-, or circle of tenta- 



o~ 



cula. 



1st Tribe. 



Body permanently fixed to other bodies. 



In this tribe the branchial and anal orifices are not op- 

 posite each other, and do not communicate through the 

 branchial cavity which, at its opening, is furnished with ten- 

 tacular filaments. The branchiae are conjoined anteriorly. 



