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 



