SCAPHOPODA. 69 
Fic. 73.—Section of dorsal or 
back eye of Onchidium ver- 
ruculatum. /, fibrous layer 
of retina; s, 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 Perzop- 
thalmus, that leaves the water 
and hops along the shore to 
obtain them. 
VALUE.—AIl slugs are scaven- 
gers. Several kinds are diicd and 
eaten by the Indians. 
Class I[I.—ScAPHOPODA. 
Tooth-Shells (Dentaliadz).—These headless mol- 
lusks have a tooth- or tusk-shaped shell (Fig. 74), open 
Fic. 74.—1, 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. 
