SCAPHOPODA. 



69 



FIG. 73. Section of dorsal or 

 back eye of Onchidium ver- 

 ruculatum. /, fibrous layer 

 of retina ; .r, layer of rods and 

 cones inclosed in retina. 



pore on the mantle, while the 

 eggs of Arion are luminous for 

 the first fifteen days. 



Marine Pulmonata. 

 The Peronia frequents the sea- 

 coast. The most remarkable 

 form is the Onchidium (Fig. 

 72), that, according to Sem- 

 per, has numerous eyes upon 

 its back (Fig. 73) in addition 

 to the usual pair upon the 

 head. They form the princi- 

 pal food of a fish, the Periop- 

 tkalmus, that leaves- the water 

 and hops along the shore to 

 obtain them. 



VALUE. All slugs are scaven- 

 gers. Several kinds are dried and 

 eaten bv the Indians. 



Class III. SCAPHOPODA. 



Tooth-Shells (Dentaliadcz) These headless mol- 

 lusks have a tooth- or tusk-shaped shell (Fig. 74), open 



FIG. 74. i, Dentalium entalis, natural size ; 2, shell magnified, and broken 

 to show animal within ; 3, animal projecting from the shell ; 4, animal 

 from below, magnified ; 6, same from above ; 5, same, showing internal 

 structure. 



