200 MOLLUSC A. 



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 con- 

 sidered as hermaphrodite, and independent of reciprocal im- 

 pregnation. They appear not only to be oviparous, but to be 

 gemmiparous and compound, many individuals being organi- 

 cally connected, and capable of simultaneous movements. 

 They are all inhabitants of the sea. 



1st Subdivision. Dichitonida. 



Interior tunic detached from the external one, and united 

 only at the two orifices. 



The branchiae are large, equal, and spread on the central 

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

 membranaceous denticulated ring, or a circle of tentacula. 



1st Tribe. 



Body permanently fixed to other bodies. 



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

 site each other, and do not communicate through the bran- 

 chial cavity. This cavity at its opening is furnished with ten- 

 tacular filaments. The branchiae are conjoined anteriorly. 



A. Simple. 



This division includes the genus Ascidia of Linnaeus. 

 The individuals are independent of each other, and although 

 they frequently adhere together in clusters, they are desti- 

 tute of a common covering, or organical connexion. 



1. Apertures furnished with four rays. 



The animals of this group have the external tunic cori- 

 aceous, dry, opaque, rough, folded, and frequently covered 



