HabiU 



\r, 



Many Mollusca become torpid in winter in cold climates 

 (hibernation) or in summer in hot climates (aestivation). Fresh- 

 water and terrestrial species in these circumstances bury them- 

 selves in the soil or shelter ii\ crevices. The period of hibernation 

 in the British Isles varies very much according to the severity 

 of the weather, and Land-Snails may be found emerging from their 

 retreat in mild days in January. 



Distribution. 



While the Mollusca were no doubt originally marine animals, 

 the Amphineura, Scaphopoda, and Cephalopoda are the only 

 classes that are still confined to the sea. Some families of Gastro- 

 poda and Lamellibranchia are represented by an abundance of 



Fig. 7. 



Shell-bearing Pteropod (Cavolinia tridentata). Case 96. 

 a. shell and animal ; b. side view of shell ; c. dorsal view of shell. 



species in fresh water, while the Gastropoda alone have succeeded 

 in adapting themselves to a life on dry land. 



In the sea molluscs form a large part of the bottom fauna. 

 On a sandy bottom, for example, Lamellibranchs are sometimes 

 the only large animals brought up by the dredge. On rocky 

 coasts in the British Isles the most obvious animals between tide- 

 marks are the Common Limpet, the Dog- Whelk, and the Peri- 

 winkle. On tropical shores Lamellibranchs and Gastropods of 

 large size and brilliant colours are frequently conspicuous and 

 abundant. In the ocean Mollusca are known to extend to 

 abyssal depths, even to 2,900 fathoms. The pelagic floating 

 fauna (plankton) of the sea includes certain aberrant Gastropods 

 such as the Sea-Butterflies (Pteropoda) and the Heteropoda, the 

 former sometimes occurring in vast shoals. 



On land the Stylommatophora (including the true Snails, 

 Helicidae, and Slugs) are a remarkably numerous group.- Land 

 Mollusca have a very wide range, and their habitat includes 



