mollhscoida. 



169 



nous sac, formed by the endocyst, is pierced by two openings. 

 One of these is the mouth, and it is always surrounded by a 

 circle or crescent of hollow ciliated processes or tentacles 

 (Fig. 76, 2, a). These ciliated tentacles serve partly as respi- 

 ratory 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 com- 

 pletely pulled into the sac by means of a muscle which is fixed 

 to the gullet (3, g). The mouth leads into a gullet, and that 



FiQ. 76.— Morphology of Polyzoa. 1. Fragment of one of the Sea-mats (M-mtra truneata), 

 magnified to show tlie cells. 2. Diagram of a single polypide of a PolyzoOn (after Allman) ; 

 a Mouth surrounded by the ciliated tentacles ; b Alimentary canal ; c Anus ; d Nervous 

 ganglion; « Investing sac or " ectocyst ;" jiy Eeproductive organs ; j? Muscle. 8. Bird's- 

 head process. 



again into a stomach, sometimes with a muscular gizzard be- 

 tween. From the stomach proceeds an intestine of variable 

 length, which terminates by a distinct anus at the upper part 

 of the sac (3, c). On one side of the gullet, between it and 

 the anus, is placed a single nervous gangKon {d). Distinct 

 reproductive organs (//") are also present, and the whole cav- 

 ity of the sac is filled with fluid. From the above description 

 it will be evident that the typical polypide of a Polyzo'dn 

 differs firom the polypite of a JSydrozoon in having a distinct 

 alimentary canal suspended freely in a body-cavity, and hav- 

 ing both a mouth and vent, in having a distinct nervous sys- 

 tem, and in having the reproductive organs contained within 



