200 MOLLUSCA. 
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. 
Ist Subdivision.—Dichitonida. 
Interior tunic detached from the external one, and united 
only at the two orifices. 
The branchiz are large, equal, ard spread on the central 
walls of the inner sac. The branchial orifice has an inner 
membranaceous denticulated ring, or a circle of tentacula. 
Ist 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 branchiz are conjoined anteriorly. 
A. Simple. 
This division includes the genus Ascidia of Linneus. 
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 frequerttly covered 
