SNAILS AND LIMPETS. 



167 



grown. Should any portion of the shell be injured by a 

 blow or the like, the animal, if not too much hurt, will repair 

 the broken part in the same way that it formed the rest of 

 the shell. These repairs can be plainly seen, for they are 

 lacking in colour and in uniformity of execution. The colour- 

 ing of the shell is caused by the secretion of particular 

 glands in the mantle or skia. The snails obtain the chalky 

 matter for the formation of their shells from the food they 

 eat, and as aquatic plants are not rich in. lime, fresh-water 

 Mollusca have therefore thin shells. 



Certain snails are 

 very irritable, and 



when touched or 

 startled in any way 

 some will let go their 

 hold and fall to the 

 bottom of the water, 

 while others will dis- 

 charge a coloured 

 liquid. For instance, 

 under such circum- 

 stances the Planorbis 

 corneus (Fig. Ill) will 

 frequently emit red 

 liquid, and the Limncea 

 stagnalis (Fig. 112) 

 occasionally violet. 



Snails are sometimes 

 found with the apices 

 of their shells eaten 



away. The erosion is apparently caused by the action of 

 carbonic-acid gas, which is frequently present in consider- 

 able quantities in stagnant water owing to the presence of 

 decaying vegetable and animal matter. This gas is de- 

 structive to lime. The rest of the sheU is protected by the 

 epidermis; but on the apex, which was formed when the 

 animal was very young, the epidermis is so thin that it is 

 frequently hardly any protection at all. 



f— I 



Fio. 111. Enlarged Outline op a Fresh- 

 water Snail. A, Apex; B, Whorls; 

 C, Suture ; D, Spire ; B, Body- Whorl ; 

 F, Periphery ; G, Inner Lip ; H, Outer 

 Lip ; I, Operculum. 



