MOLLUSKS 85 
limpets, for instance, there are numbers of additional eyes 
carried on the mantle edge just under the eaves of the 
shell, and forming a row completely encircling the body. 
(In the scallops there are two rows of brilliantly colored 
eyes, set like jewels on the edges of the mantle just within 
the halves of the shell.) In the chitons the eyes of the 
head disappear by the time the animal attains maturity, 
and in some species at least their place appears to be taken 
by great numbers of eyes, sometimes thousands, which are 
embedded in the shells. On the other hand, eyes are com- 
pletely absent in certain species of burrowing snails and in 
several living in the gloomy depths of the sea far from the 
surface; they appear to be absent also from fresh-water 
clams; but the fact that certain species close their shell 
when a shadow falls upon them, leads to the belief that 
while actual eyes are not present the skin is extremely 
sensitive to light. This is also the case with many snails. 
86. Smell.—Since the sense of sight is generally unde- 
veloped in the mollusks, they rely chiefly upon touch and 
smell for recognizing the presence of enemies and food. 
Tentacles upon the head and other parts of the body, and 
a skin abundantly supplied with nerves, show them to pos- 
sess a high degree of sensibility; but in the greater num- 
ber of species the sense of smell is of chief importance. 
Many experiments show that tainted meat and strongly 
scented vegetables concealed from sight and several feet 
distant from many of our land and sea mollusks will attract 
them at once. In these forms the sense of smell appears to 
be located on the tentacles, but additional organs, possibly 
of smell, are located on various portions of the body, usu- 
ally in the neighborhood of the gills. 
87. Taste and hearing.—Several mollusks appear to be 
almost omnivorous, but others are decidedly particular in 
their choice of food, which leads us to suspect that they 
possess to some extent the sense of taste. Nerves supply- 
ing the base of the mouth have also been detected, which 
