Gastropoda. 



33 



forms gill-like structures may be developed in the mantle-cavity. 

 .Most Pulmonata arc provided with .shells, but, with the exception 

 of the . 1 mphibolidat . the adults never possess an operculum. The 

 majority are terrestrial, but some are aquatic and there are a few 

 marine forms. They are* divisible into two sub-orders, Basom- 

 matophora and Stylommatophora, characterized by tin- difference 

 in the position of the eyes. The Basommatophora, including the 

 Auriculidae, Amphibolidae, Siphonariidae, and Limnaeidae, have 

 a single pair of tentacles, at the base of which the eyes are 

 situated. The Stylommatophora (Land-Snails, Slugs, &c.) are 



Fig. 24. 



British Pond-Snail {Limnaca stagnaVs). 



1. Upper view : a. foot ; b. tentacles ; c. eye ; d. muzzle. 



2. Lower view : letters a. b. c. as above ; t. mouth ; /. respiratory orifice. 



provided, save in a few cases, with two pairs of retractile tentacles, 

 with the eyes at the summit of the hinder pair. Over 10,000 

 species of Pulmonata are known. 



Sub-order 1. Basommatophora. 



The Auriculidae chiefly inhabit salt or brackish water. The 

 largest forms are tropical and found at the mouths of rivers, 

 among the roots and stems of mangrove-trees, or in damp wood 

 near the sea. 



The * Limpet-Snails ' (Siphonariidae) look very like true 

 Limpets, but their shells are distinguished by a slight bulging on 

 one side, caused by a groove on the inside which lodges a respira- 



c 



