SHELLS. 51 



cause they have a long foot lying the whole length of the body. 

 Unlike the bivalves, they have a distinct head, in which the brain 

 is situated. Often there are tentacles or feelers, as in the snail, 

 on the ends of which the eyes are placed. Gasferopods have a 

 wonderful eating apparatus called the odontophore or tooth ribbon. 

 It is covered with hooked teeth, pointing backward, and is in the 

 lower side of the mouth, situated about the same as our tongues. 

 On the upper side of the mouth is a hard plate or jaw, and the 

 food is ground up by the toothed ribbon against this plate. The 



Violet Snail and Egg Float {lanthina fragilis). Copied from the Eiverside Natural His 

 tory by kind permission of Houghton, Mifliin & Co. 



odontophore wears out rapidly, but as the front part is used up it 

 grows from behind, and these animals are so fortunate as to have 

 a new set of teeth every little while. 



There is only one shell to take the place of two in the bivalves, 

 so most of the univalves have an operculum. This is a little lid 

 (either horny or calcareous) on the upper side of the foot which 

 exactly fits the aperture in the shell. If a Oasteropod wishes for 

 any reason to be alone and rest for a time, he only has to draw in 

 his foot, pull to the door, and he is in complete seclusion from all 

 the world. 



The shells of the Gasteropods, like those of the bivalves, are 

 often covered with a sort of horny membrane or epidermis which 

 protects them from the eroding power of the water and other ex- 

 ternal injuries. 



At the bottom of the Gasteropod group is a wonderful creature 

 which we may call a multivalve, as its shell is made up of a num- 

 ber of plates (usually eight) which look like ancient armor. It is 

 called the mail shell, or chiton, and is the only example in the 

 world of a shell composed of more than two parts. It is common 

 on the Atlantic coast, in some of the bays and inlets south of 

 Boston, on the Pacific shores, in England, and other places. Chi- 

 tons sometimes have as many as eight thousand eyes, their backs 

 being covered with them. 



The limpets range in shape from those which are almost flat 

 to a perfect cone. Some of my prettiest from Sitka are snow- 



