THE CLAM AND OTHER BIVALVED SHELL-FISH. 75 



side of tte jar. The mouth continually opens aind closes. 

 Not within the mouth is a plate forming the " jaw" (Pigs. 

 It and 78); and also a slender rasp, armed with rows of 

 minute teeth, called the lingual rihbon, which grows on the 

 " tongue." By means of this rough rasp or minute saw, 

 which can be thrust rapidly out and then withdrawn, the 

 snail can cut off bits of leaves, which it swallows, while with 

 it the drill, etc., can bore holes in other sheUs (Fig. 78). 



The snail can be killed by throwing it into hot water. It 

 can then be puUed out of its shell by means of a pin or 

 bent wire, and it will be seen that the animal fills up ' the 

 whole shell to the top of the spire. 



The land-snails lay their eggs in little packets under 

 leaves, etc., while the pond-snails deposit their eggs in the 

 spring on leaves. When a number are kept in glass jars or in 

 an aquarium, the eggs will be seen in little jelly-like masses 

 on the sides of the vessel, and it is interesting to watch the 

 young develop, until they leave the egg-mass and shift for 

 themselves. 



The common Purpura, which lives on the seashore under 

 sea^weed, lays its eggs in capsules like a vase, which may 

 be found attached to rocks (Fig. 

 79). Those of the common 

 whelk (Fig. 80) are laid in coni- 

 cal masses, sometimes two inches 

 high, composed of flattened cap- 

 sules. 



There are about 15,000 differ- 

 ent species of snails known, be- Fio. 79.— Purpura and its egg-cap- 



sides 7000 species which are ex- ^^= "^ '"«*' ""^^ 



tinct or fossil; and yet, varied as are their forms, by exam- 

 ining the few, common shells we have noticed we can ob- 

 tain an idea of the general appearance of nearly all. With 

 comparatively few exceptions, all have a head, with feelers 

 or tentacles ; they all creep on a foot or dish, and nearly 

 all are protected by a one-valved, more or less spiral shell. 

 In the squid or cuttle-fish there is no external shell, and 



