134 



Descriptive Zoology. 



The eggs are laid in clusters, usually enveloped in a gelat- 

 inous mass ; and in an aquarium are usually deposited on 

 the side of the glass in a very favorable place for watching 

 their development. There are three common genera, 

 Limnea, a right-hand shell ; Planorbis, a discoid shell, or 

 flat spiral ; and Physa, a left-hand spiral (Fig. 80). 



The River Snails. These are not entirely distinct from 

 the pond snails ; still, they nearly all breathe by means 

 of gills, and most of them have operculums. Being gill 

 breathers, they of course do not come to the surface to 

 breathe, hence are not usually so conspicuous. Some of 

 them have a projecting tube on each side of the neck, 

 the water entering through one of the tubes to the gill, and 

 passing out through the other. The eyes are like those of 



the pond snails in being 

 borne at the bases of the one 

 pair of tentacles. In some 

 of the river snails the young 

 are brought forth alive. 



Sea Snails. These are 

 found chiefly along the 

 shore, not often in very deep 

 water. They are numerous 

 in kinds and individuals, and 

 vary greatly in both color 

 and form. In size they vary 

 from almost microscopic to 

 a foot and a half or more 

 in length. They also vary 

 greatly in shape, from globu- 

 lar (Fig. 88) to slender tapering forms resembling screws. 

 The shell is usually right-handed, and the majority have 

 operculums. Nearly all breathe by means of gills. 



FIG. 88. A LARGE SEA SNAIL 



(NATICA). 



It feeds on clams, etc., boring through 

 their shells. 



