246 ZOOLOGY 



fresh-water forms, but the. majority of marine species 

 are gill breathers. Thus the land slugs have lungs, 



Eye -bearing 

 tentacles 



Drawing by R. Weber 

 FIG. 63. A land slug (Limax). Natural size. 



situated beneath the mantle ; but the sea slugs (nudi- 

 branchs) are very different, and possess external feather- 

 like gills. The lung-breathing aquatic mollusks, in- 

 cluding the commoner pond snails, have evidently been 

 derived from land-inhabiting ancestors. With rare ex- 

 ceptions they have to come to the surface of the water 

 to take in air. 



Shell of the 3. The snail's shell, from which the principal char- 

 acters for the description of the species are derived, is 

 nearly always very distinctive. Many kinds of mol- 

 lusks are known from the shell alone, yet we have no 

 difficulty in recognizing them. The commoner form 

 of shell is more or less conical, the upper end being 

 called the apex, the lower side the base. It is composed 

 of whorls, twisting around a central axis, which may be 

 hollow and open below, the opening being called the 

 umbilicus. The whorls are attached to each other along 

 a spiral line called the suture ; the surface of a whorl 

 may be rounded or keeled, or may have raised lines. 

 The aperture of the shell, commonly called the mouth, 

 has of course no connection with the true mouth of the 

 animal. The mouth may be surrounded by a thicken- 

 ing called the lip, and on this are often denticles or 

 lamellcs. The shells of land and fresh-water mollusks 

 are usually thinner and lighter than those found in the 



