xii PHYLUM MOLLUSCA 



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; 7. liver; o. ovary; sh. shell. (Prom 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 



FIG. 501. Atlanta peronii. n, cerebral ganglia; 6, pedal ganglia; c. eye; gr. ctenidia ; 

 h. heart k: nepliridium ; /. liver ; /. mouth ; o. ovary ; p. opercurnm ; 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 (oc. tent) placed behind the first. 



The mantle is usually developed into a fold the mantle flap 



VOL. I 



Y Y 



