MOLLUSCA. 25D 
There are six classes of Mollusks. Of these, the first 
three forin a group by themselves, called J/olluscozdea, 
distinguished by having only one or two nervous ganglia, 
an imperfect circulatory apparatus, and generally no or- 
gans for prehension and locomotion. Of the other three, 
Lamellibranchs are headless, while Gasteropods and Ceph- 
alopods only have a distinct head, and an apparatus for 
mastication. 
Crass 1.—Polyzoa. 
These minute Mollusks resemble the Polyps in appear- 
ance, living in clusters, each individual inhabiting a del- 
icate cell, or tube, and having a simple mouth surrounded 
with ciliated tentacles. The colony often takes a plant- 
like form; sometimes spreads, like fairy-chains or lace- 
work, over other bodies; or covers rocks and sea-weeds in 
Fig. 211.--Polyzoans: 1. Hornera lichenoides; natural size. 2. Branch of the same, 
magnified. 3. Discopora Skenei; greatly enlarged. 
patches with a delicate film. The majority secrete carbo- 
nate of lime. Though an extremely low form of the Mol- 
luscan type, a Polyzoan shows its superiority to the Coral, 
which it imitates, in possessing a distinct alimentary canal 
