332 



MARINE INVERTEBRATES 



large orders of the Gasteropoda. The figures represent some of 

 the commoner types of opercula. 



The opercula have beeii utilized a great deal in the classification 



Turbo 

 (Sarmaticus). 



Turbo 

 (CaUvpoma). 



Various forms of opercula. 



I/ivona. Ampullaria. 



of the Gasteropoda, and especially in some groups that have 

 become terrestrial in habit. Often, when shell-characters utterly 

 fail to give a clue to the proper generic position of a new or rare 

 mollusk, the operculum may be relied upon to indicate it. 



Various forms of opercula. 

 Pyrula. Purpiira. Littorina. Aulopoma. Torinia. Neritopsis. Strombus. Conus, 



TENTACLES 



Two tentacles project like fleshy horns from the sides of the 

 head. These are not retractile, as in the land-snails, and their 

 office appears to be that of sensory organs. In both Fulgur and 

 Bucc'mum the tentacles are comparatively short and stout, and have 

 expanded bases ; in many other genera they are long and slender. 

 Almost without exception the Gasteropoda possess tentacles. 



