MOLLUSCOIDA. 185 
or crescent of hollow ciliated processes or tentacles (fig. 95, 
2a). These ciliated tentacles serve partly as respiratory organs, 
and partly to set up a current of water by which floating 
particles of food are brought to the mouth. The mouth and 
tentacular crown can be partially or completely pulled into 
the sac by means of a muscle which is fixed to the gullet 
(2 ¢). The mouth leads into a gullet, and that again into a 
stomach, sometimes with a muscular gizzard between. From 
the stomach proceeds an intestine of variable length, which 
Fig. 95.—Morphology of Polyzoa. x. Fragment of one of the Sea-mats (//ustra 
truncata), magnified to show the cells. 2. Diagram of a single polypide of a 
Polyzodn (after‘Allman): @ Mouth surrounded by the ciliated tentacles; 6 
Alimentary canal; c Anus; d Nervous ganglion ; e Investing sac or ‘‘ectocyst ;”” 
SS Reproductive organs ; ¢ Muscle. 3. Bird’s-head process. 
terminates by a distinct anus at the upper part of the sac 
(2c). On one side of the gullet, between it and the anus, is 
placed a single nervous ganglion (¢@), Distinct reproductive 
organs (f/) are also present, and the whole cavity of the sac is 
filled with fluid. From the above description it will be evident 
that the typical polypide of a Polyzodn differs from the polypite 
of a Hydrozoon in having a distinct alimentary canal suspended 
freely in a body-cavity, and having both a mouth and vent, in 
having a distinct nervous system, and in having the reproduc- 
