266 



ZOOLOGY 



Forest snail, showing the two 

 tentacles with an eye on 

 the end of each. From 

 photograph by Davison. 



-Wfeo^^Hti -'t 



body, including the stomach, are covered by it. 



^^■^^^ ^j T In most gastropods the body is spirally twisted in 



^^^B^^jjtKt^-^ the shell. In the garden slug, the mantle does not 



^^^B^^^^^^ secrete a shell and the naked body is symmetri- 



^^^^^m cal. The twisting of the body is not seen in very 



■HgT young snails, so that this peculiar state is believed 



to be something secondar}'" which has appeared as 

 a consequence of the animal's bearing a shell. 



Variability of Snail Shells. — Snail shells 

 arc very variable in shape and color markings, as 

 may be seen if several of the same species be ex- 

 amined carefully. Varieties of snails in mountain 

 valleys in certain of the Hawaiian Islands are found to be quite distinct, 

 each in its own valley. Yet it is quite certain that all the snails of these 

 several varieties were at one time alike. Helix nemoralis, a European 

 snail, has been introduced into this country and has multiplied so rapidly 

 and varied so greatly that in an area one thousand feet in diameter three 

 hundred and eighty-five varieties have been collected, each slightly differing 

 from the other either in color or form of shell. ^ 



Feeding Habits. — The mouth of the snail is easily found on the under 

 side of the foot. Just within 

 the mouth is the lingual rib- 

 bon. The ribbon consists of a 

 flap of membrane bearing 

 many sharp, filelike teeth, 

 microscopic in size. This 

 structure, which is moved by 

 muscles, passes over a pad of 

 cartilage and rubs, filelike, 

 against the surface to which it 

 is applied. In this manner 

 some snails can bore circular 

 holes in shells of other mol- 

 lusks, in order to get the soft 

 part, which they use as food. 

 An example of a univalve 

 moUusk which thus obtains 

 food is the oyster drill, which 

 annually does thousands of 

 dollars' worth of damage to the 

 oysters. In Europe and this 

 country slugs and some snails do considerable damage to gardens by eat- 

 ing young plants. The snail Physa may be observed feeding on tiny 



^ See Howe, Amer. Nat., December, 1898. 



%N. 



• "i 



. 



Shell full of holes bored by the oyster drill and a 

 boring sponge. 



