BULLA. 215 



extends beyond the body behind. The shell is rather 

 globular, very thin, and nearly transparent, covered by 

 a thin yellowish or brownish skin, and is partly covered 

 by the lobes and mantle of the animal. Slug-like as 

 this mollusc appears, it is capable of moving with 

 facility, aided in swimming by the broad flaps into 

 which its sides are expanded, and in crawling, by the 

 great extent of its foot. 

 B. Cranckii shows no eyes on its front disc; it is white, 

 like the oval shell, through which the gizzard of the 

 animal can be distinctly seen. 



