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Paludina, syn. Vivipara (a river snail). 

 Viviparous. 



Buccinum (Whelk). Thick spirally-coiled 

 shell. The " foot " large and with a 

 horny operculum. A single pair of 

 tentacles at the bases of which are the 

 eyes. The mantle edge of the left side 

 forms a spout-like siphon for conducting 

 water into the mantle-cavity. One 

 ctenidium (of left side) attached to 

 mantle. Sexes separate. Marine. 



Clione (a shell-less Pteropod, or " Sea- 

 butterfly "). The sides of the " foot " 

 developed as swimming fins (" wings "). 

 Marine (pelagic). 



Doris (a sea-slug). One of the Nudi- 

 branchia or naked-gilled Molluscs. Shell 

 absent in adult. The exposed gills form 

 a retractile crown on the back. Marine 

 (littoral). 



Order PULMONATA. Land and freshwater air-breathing Gasteropods. 

 Ctenidium absent. The mantle-cavity is an air-chamber or " lung *' 

 formed by the union of the front edge of the mantle with the head. 



Examples, Limnaea truncatula. Shell spirally coiled. 

 A single pair of non-retractile tentacles 

 at the bases of which are the eyes. 

 Freshwater. The intermediate host of 

 the Liver-fluke (Fasciola, syn. Distoma 

 hepatica). 



Limax (Slug). The thin plate-like shell is 

 internal, hidden under the skin. Two 

 pairs of retractile tentacles ; the eyes 

 at the tips of the upper pair. Terres- 

 trial. 



Helix (Snail). The shell has a short spire. 

 Two pairs of retractile tentacles ; the 

 eyes at the tips of the upper pair. 

 Hermaphrodite. Terrestrial. 



Class PELECYPODA or LAMELLIBRANCHIATA (Bivalves). Bilaterally 

 symmetrical. Without a distinct head and odontophore. The 

 mantle has two flaps, and is usually more or less prolonged to form 

 two siphons (inhalent and exhalent). The shell has two valves 

 (bivalve). The " foot " is more or less wedge-shaped. The paired 

 ctenidia or gills are lamellate (numerous filaments in plate-like 

 rows). 



