“332 MARINE INVERTEBRATES 
large orders of the Gasteropoda. The figures represent some of 
the commoner types of opercula. 
The opercula have been utilized a great deal in the classification 
Various forms of opercula. 
Turbo Turbo Livona, Ampullaria. Natica. 
(Sarmaticus). (Callopoma). 
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. Purpura, 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 
Buccinum 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. 
