MOLLUSCA. 183 
Mollusca are divided into two great divisions, known respec- 
tively as the Molluscoida and Mollusca proper. In the Mollus- 
covda the nervous system consists of a single ganglion, or prin- 
cipal pair of ganglia, and there is either no circulatory organ 
or an imperfect heart. In this division are included the three 
classes of the Sea-mosses (Polyzoa), the Sea-squirts (7 unzcata), 
and the Lamp-shells and their allies (Brachiofoda). In the 
Mollusca proper the nervous system consists of three principal 
pairs of ganglia, and there is a well-developed heart, consist- 
ing of at least two chambers. Under this head come all the 
ordinary forms of shell-fish, 
