SNAILS AND MUSSELS 



Among the aquatic snails, one group, the Streptoneura, 

 breathes by means of gills; another, the Pulmonata, by a 

 lung-sac. The gills of the streptoneurans lie in the space 

 between the body and the mantle through which water flows 

 freely. They have an abundant circulation of blood to which 

 oxygen is supplied by the water constantly flowing across the 

 gill. Gill-bearing snails have a homy plate or operculum 

 on the upper surface of the foot near the rear end (Fig. 243). 

 When such a snail draws into its shell, pulling its foot after 

 it, the operculum comes last and entirely fills the opening of 

 the shell, thus completely protecting the body. The small, 

 smooth opercula of marine snails, commonly knowm as "eye- 

 stones," were once much used to remove irritating substances 

 from the eyes. The "eyestone" was slipped under the eyelid 

 in order to increase the flow of the lachrymal fluid and thus 

 wash out the foreign particle. Different species of snails 

 have distinctive opercula. They are often found in the 

 stomachs of fishes and the kind of snail that was eaten can 

 be determined by them. 



Fig. 243. — A gill-bearing snail showing the 

 operculum: i, snail extended, operculum on the 

 upper surface of the foot; 2, snail drawing into its 

 shell, pulling the operculum into the opening. 

 (After Baker.) 



The lung-sac of the pulmonate snail is an air chamber en- 

 closed by a part of the mantle through which, as in the gill, 

 there is an abundant circulation of blood. A pulmonate 



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