138 SEA-SHORE LIFE 



The sliell serves not only to protect the intestines, but the en- 

 tire head and foot maj' he withdrawn within it. 



In many sea-snails there is a horny or stony plate called the 

 operculum which lies on the dorsal side of the posterior end of 

 the foot, and wdien the foot is withdrawn this closes the opening 

 of the shell. 



The mantle projects as a curtain beyond the lip of the shell, 

 and protects the gills and other organs which lie in the Sf)ace be- 

 tween it and the side of the body. Often the mantle curtain is so 

 large that it is reflected upward over the outer surface of the shell, 

 which it may entirely encase. The secretions of the mantle serve to 

 keep the outer surface of the shell smooth and even highly pol- 

 ished as in the cowries, but in many cases the shell has degene- 

 rated and become permanently covered hy the mantle as in slugs. 



In most of the fresh-water snails, and in land snails and slugs, 

 the free edges of the mantle have fused with the side of the body, 

 leaving only one opening for the admission and expulsion of air on 

 the right side of the body. 



In these forms the gill has disappeared, and a sort of lung is 

 formed by the ramification of a network of blood vessels over the 

 inner surface of the mantle. These snails are obliged to take in a 

 fresh supply of air at regular intervals and if one observes a com- 

 mon pond snail it will Ije seen to come to the surface and emit a 

 bubble of air from its air-pore and then take in fresh air before 

 descending. Indeed, fresh water snails will soon drown if they be 

 not allowed to come to the surface to breathe. Curiously enough 

 practically none of these lung-breathing snails have an operculum 

 when adult. 



The gill of the operculuni-bearing snails is feathered and 

 close by the side of it one finds a smaller feathered body called the 

 osphradium which is possibly an organ for tasting the water that 

 is being breathed. In these snails one often finds opposite the gill 

 a long tubular fold of the mantle wdiicli serves to conduct water 

 into the gill chamber. This fold is often protected by a snout-like 

 projection of the shell above it. The water is usually discharged 

 through another opening which lies farther back. 



The sense organs of the Cktsteropoda are poorly developed 

 although they are very sensitive to touch. The feelers on the head 



