THI 



PHYLUM MOLLUSCOIDA 



335 



tactile, and also act as organs of respiration. When retracted they 

 become enclosed by the walls of the introvert as by a sheath 



tent 



au 



FIG. 281. Bugula avicularia. Two zooids, magnified, an. anus ; avic. avicularia ; emb. 

 embryo enclosed in the ocecium ; funic. funiculus ; gast. muscular bands passing from the 

 stomach to the body-wall ; int. intestine ; mo. mouth ; ooec. : ooaciuin oes. oesophagus ; ov. 

 ovary ; ph. pharynx ; ret. parieto-vaginal muscles ; sp. spermatidia ; stom. stomach ; 

 tent, tentacles. The ganglion, which is not indicated, lies just below the middle of the 

 stroke from mo. 



the tentacle-sheath. A pair of bands of muscular fibres the parieto- 

 vaginal muscles (ret.) passing to the introvert from the body- wall, 

 serve to retract the introvert and tentacles. 



