HALF AN HOUR WITH SHELL-FISH. 191 



aperture by means of this contrivance. It is on 

 account of this class of molluscs creeping about on 

 their broad foot that they go by the name of 

 Gasteropoda, or " belly-footed." In all the species, 

 the head is distinct, and usually surmounted by 

 two feelers or tentacles, and close by are two eyes, 

 generally placed on stalks. Within the mouth is 

 the " lingual ribbon," a strap-shaped organ crowded 

 with teeth, which works to and fro, and thus enables 

 the creature to bore holes even through the solid 

 shells of bivalves. The siliceous teeth and ribbon 

 are differently shaped in different species, and all 

 have long been favourite objects with microscopists. 

 The stomach of many species is provided with a 

 peculiar set of calcareous plates, used in the tritura- 

 tion of food. The intestine is usually well-developed 

 and very long, terminating in a distinct aperture, 

 called the anus. The liver, also, is very large con- 

 sidering the size of the animal, and seems to be a 

 most important organ. The heart is provided with 

 two chambers, arterial and venous. 



As regards the arrangements by which respiration 

 is carried on in the Gasteropoda, a good deal of 

 diversity exists. It may be effected in three 

 different ways ; first, by exposing the blood simply 

 to the action of the air contained in the water; 

 second, by special breathing organs in the form of 

 outward processes of the skin, exposed to view on 

 the back or sides of the animal, as in the common 

 Doris; or third, by breathing organs, as feathery 



