XII 



PHYLUM MOLLUSCA 



689 



The Gastropoda have a well-marked head, separated from 

 the body by a constriction or neck. The mouth, situated at the 

 anterior end of the head on its ventral aspect, is in many instances 



FIG. 590. Shell-bearing Pteropoda. /./.fins; I. liver; o. ovary; sh. shell. (From Cooke 



after Souleyet.) 



provided with a protrusible proboscis or introvert, sometimes of 

 considerable length. On the dorsal surface of the head are a pair 

 of tentacles which vary a good deal in shape, but are usually 

 cylindrical or club-shaped. In most cases the eyes are situated on 

 tubercles at the bases of the tentacles, or elevated towards the 



Fio, 501. Atlanta peronii. a, cerebral ganglia; 6, pedal ganglia; e. eye; g. ctenidia ; 

 h. heart k. nephridium ; I. liver ; m. mouth ; o. ovary ; p. operculum ; t. male reproductive 

 gland. 



middle ; but in the snails and slugs (Pulmonata) (Fig. 593) the 

 eyes are elevated on the extremities of a second, longer, pair of 

 tentacles (pc. tent) placed behind the first. 



The mantle is usually developed into a fold the mantle flap 



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