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MANUAL OF ZOOLOGY 



rounded opening, serves as a pulmonary sac or lung (Fig. 

 1 68), its roof being richly supplied with blood-vessels: in 

 the aquatic forms its function is apparently as much hydro- 

 static as respiratory. In some of the Pulmonata there is a 

 return to a completely aquatic mode of respiration accom- 

 panied by the development of secondary gills vascular 

 processes of the wall of the mantle-cavity. 



ve 



sk 



FIG. 167. Patella VUlgata, seen from the ventral side. f. foot ; g. I. circlet of 

 gill lamellje ; in. e. edge of the mantle ; inu. attachment muscle ; si. slits in the 

 attachment muscle ; sh. shell ; v. efferent branchial vessel ; z/. aorta ; ve. smaller 

 vessels. (From the Cambridge Natural History.) 



In many Gastropods, as already mentioned, there is a 

 long proboscis capable of being everted and retracted, at 

 the extremity of which the mouth is placed. A single 

 curved horny jaw lies on the roof of the buccal cavity 

 in the Pulmonata ; in most marine Gastropoda the place 

 of this is taken by two lateral pieces. 



A characteristic feature of the alimentary canal of the 



