PTEKOPODA GASTEBOPODA. 



257 



washes them ; but Scallops, Cockles (fig. 34), and 

 Mussels (fig. 33), have the power of fixing themselves to 

 any substance they wish, by means of the "byssus" or 

 beard, which is a tuft of fibres passing out from between 

 the shells ; and it is said, the Scallops have the power 

 of progression, by suddenly opening and shutting the 

 shells. 



4. The Pteropoda are Mollusca which have no shell, or a 



very thin one ; the Clio borealis (fig. 33), which forms 

 the chief food of the Whale, is found in great multitudes 

 in the Arctic seas, it swims about by means of two 

 extensions, similar to wings. The Hyaloea has a small 

 round transparent shell. 



5. The Gasteropoda are extremely numerous ; most of 



FIG. 35. SXAIL (Helix aspersa), AND SLUG (Limax cinerius). 



them have univalve shells, but many of them have 

 none, as the Garden Slug (Limax, fig. 35). The "Whelk 

 (Buccinum undatum) , Perriwinkle (Littorina littoria), 

 Garden Snail (Helix aspersa, fig. 35), and the Wentle- 

 trap (Scalaria, fig. 36), are the most familiar examples 

 of this class. These Mollusca walk by means of the 



s 



