106 



INTRODUCTION TO ZOOLOGY 



Fifi. 60.— Shell of 

 Limnaea stagnalis, 

 seen from below. 



s, Spire ; hiv, body 

 wliorl ; r, reflected 

 lip of the shell. 



tion is thick. It is very hardy and can easily be kept in a 

 tank, the only drawback to it being, that it has such an 

 omnivorous and hearty appetite that it is 

 apt to destroy the plants rather too rapidly, 

 and only common weeds should be kept 

 with it. The Canadian water - weed, 

 Anacharis (Elodea) canadensis, which can 

 ^ always be easily obtained, is excellent 

 food for it, and also the microscopic green 

 algal growths whicb so often cover the 

 sides of a tank. If trained to feed on 

 these, the snail will do valuable work in 

 an aquarium in keeping the glass free 

 from these Algae which obscure the view. 

 The snails should be watched 



also more detailed observations 

 should be made on a couple of snails kept 

 for a time in a small glass bowl or jar 

 which can be easily handled so that the snail may be seen 

 from all sides. 



General The soft body is covered by a shell which is 



Structure, conical in shape, but formed of six or seven spirally 

 The Shell, twisted coils or whorls. The iirst whorl is so 

 small as to be a mere dot, but each successive whorl is larger, 

 and is in close contact with the whorl before. The seventh 

 or last whorl is bigger than all the others put together, and 

 this is called the hody whmi, all the rest of the shell forming 

 the spire ; the free edge of the body whorl is known as the lip. 

 If the shell is viewed from a point vertically above the 

 apex of the spire, the whorls are seen to descend in a right- 

 handed spiral; hence it is termed "dextral." (When the 

 spiral turns to the left a shell is called " sinistral.") 



On the body whorl many delicate lines can be seen parallel 

 to the lip, and at intervals a few more deeply marked lines 

 occur. These are due to its discontinuous method of 

 growth. It is frequently added to at its free margin, but 

 the shell substance formed during one period of growth 

 differs slightly from that formed at another; consequently 

 fine lines of demarcation divide the different short periods 

 of growth during one season. The more conspicuous lines 



