MOLU'SCA 



233 



of locomotion. A shell is present in the larva, although it may 

 be lost in the adult. 



The majority of the Gastropoda are marine, though some live 

 on land and some in fresh water. They occur for the most part 

 crawling about on the sea bottom, although some live near the 

 surface and so are pelagic. They have been taken at all depths 

 to about six thousand meters, it is said. Their food varies much ; 

 some feed entirely on plants, some on dead animal matter, some 

 on living animals. Some species have glands connected with 

 the mouth, which secrete a weak sulphuric acid, from two to four 



FlG. 235. Vermetus; larvae in the veliger stage. A, lateral aspect; B, ventral, cer.g, cere- 

 bral ganglia ; eye, eye; f, foot ; mo, mouth ; ot, otocyst; sh, shell; tent, tentacles ; vel, velum. 

 (After Lacaze-Duthiers, from Parker and Haswell's Text-book.) 



per cent. With this they soften a spot on the shell of some 

 other mollusc and then bore a smooth, circular hole with the 

 radula and through this feed on the animal within. The Gastro- 

 poda may be divided into three orders. 



Order 1. Prosobranchia 



The Prosobranchia (Gr. irpoaai, toward the front, and fipdyxia, 

 gills) include almost all the marine Mollusca which have a uni- 

 valve shell and all the fresh-water snails ; thus the order is 

 largely marine, and a shell is very rarely absent. In this group 



