49^ THE INVERTEBRATA 



tween the Gasteropoda and the LamelHbranchiata. They are greatly 

 speciaHzed for burrowing. Thus the shell is tubular and perforated 

 at the apex. The foot emerges from the wider opening, while the apex 

 remains above the surface of the sand when the animal is burrowing, 



Fig. 366. Diagram of the structure of Dentaliiim. Altered from Naef. Head 

 and foot stippled, an. anus; hue. buccal mass, with radula; ce.g. cerebral 

 ganglion; eta. captacula; dig.gl. digestive gland; F. foot; gon. gonad, com- 

 municating with the cavity of the left kidney {k.)\ M. mouth; ma. mantle; 

 max. mantle cavity; pl.g. pleural ganglion; ped.g. pedal ganglion; sh. shell; 

 St. stomach; vis.g. visceral ganglion. The mollusc is represented buried in 

 sand, except for the perforated narrow end, through which both the inhalant 

 and exhalant currents (shown by arrows) flow. 



and serves alike for the entrance of water into and its exit from the 

 mantle cavity. The head is proboscis-like in form and has none of the 

 usual sense organs, but in Dentalium^ the one common genus, there 

 are extensible filaments, the captacula, with sucker-like ends, which 



