VIII 



PHYLUM MOLLUSCOIDA 



315 



by the action of their cilia, the tentacles are prehensile as well as 

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

 >come enclosed by the walls of the introvert as by a sheath 



aotc 



FIG. 255. Bugula avicularia. Two zooids, magnified, an. aims ; avic. avicularia ; emb. 

 embryo enclosed in the ooecium ; juiiic. fuhiculue ; gast, muscular bands passing from the 

 stomach to the body wall ; int. intestine ; mo. mouth ; oa>. oxjecium ; o-s. oesophagus ; ov. ovary ; 

 ph. pharynx ; ret. parieto-vaginal muscles ; sp. spermatidia ; atom, stomach. 



the tentacle-sheath. A pair of bands of muscular fibres' iheparietc- 

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

 serve to retract the introvert and tentacles. 



