24 



Guide to the Mollusca, 



Order 2. PECTINIBRANCHIA. 

 [Cases 22 to 94.) 



This order includes Streptoneurous Gastropods with only one 

 auricle, one kidney, and one gill. The gill has lost the filaments 

 of one side, by which it is attached to the mantle. In the first 

 sub-order, the Taenioglossa, the radula has usually several teeth 

 in each row. This sub-order includes a large number of families 

 (Viviparidae to Carinariidae). 



The ' River-Snails ' (Viviparidae) are rather sluggish, and are 



found at the bottom of ponds and rivers feeding on decaying 



animal and vegetable matter. The common British species is 



Vivipara vivipara, the female of which rears the young in a brood 



pouch at the end of the oviduct. 



Fig. 15. 



The Common British River-Snail (Vivipara vivipara). 

 a. head ; b. tentacles ; c. eyes ; rf. foot ; e. operculum. 



The Cyclophoridae and Cydostomatidac are Land-Snails with 

 the mantle-cavity converted into a lung. Some have remarkable 

 devices for admitting air to the lung when the operculum is 

 closed, e.g. the siphonal notch and groove in Cataidus (Case 24). 



The ' Apple-Snails ' (Ampullariidae) live in the rivers and 

 marshes of tropical regions, and, although represented by a large 

 number of species, exhibit comparatively slight variation in form 

 and colour. In this family a gill and a lung occur together, and 

 the latter can be used for respiration under water by means of 

 a long siphon which can be protruded upwards into the air. 



The ' Periwinkles ' (Littorinidae) are found almost on every 

 known shore ; they feed upon all kinds of marine vegetation. 

 Some species are met with at low-water mark, others on rocks 

 almost beyond the reach of the sea, and some have been dis- 

 covered inland nearly half a mile away from the shore. 



The family of Calyptraeidae includes the 'Slipper-Limpets' 



