MOLLUSCA DIAGRAM 7. 161 



Snails also eat fruits and plants, but they are themselves eaten in 

 some countries, and are said to be very beneficial to consumptive 

 patients. Those which have fed ou vine-leaves are said to be the 

 best eating. 



The Limnea or Water Snail has a convoluted shell like the snail, 

 and much resembles it ; it lives in ponds, and although an aquatic 

 animal, it breathes like the slug 

 and snail by means of a lung 

 which it comes to the surface of 

 the water to open. A limnea cai^ 

 easily be seen breathing by merely 

 placing it in a glass of water. In Watersnail. 



spring, the limnese lay masses of eggs on aquatic plants, or even on 

 the surface of the vessels in which they are reared. These masses 

 are about an inch long and a quarter of an inch broad. They are 

 formed of a transparent jelly, and are perfectly transparent; the 

 nuclei are easily to be seen. These nuclei gradually enlarge, and 

 when the small limnea} in their eggs have- not yet attained the 

 size of a pin's head, they may be seen to turn round and round* 

 without this movement ever stopping. 



Many mollusca instead of having a spiral shell, like the snail 

 or the limnea, have one formed by two pieces joined by a hinge, 

 and which are called valves. These shells the oyster, cockle, 

 mussel, etc. are called bivalves; and the others are called 

 univalves. The animal sometimes opens the valves of its shell, 

 and sometimes closes them, and is thus completely protected by 

 them: When it is dead, the two valves are always half open. 

 To keep them closed, the animal has a muscle which connects the 

 two, and which must be cut to separate them. In opening an 

 oyster, the edge is broken with a knife, but this is not sufficient, 

 and the muscle which hold the valves together must be cut 

 through before the shell can be forced open. The muscle must 

 then be cut under the animal in order to detach it. The points 

 where the muscle is attached to the two valves are indicated by 

 recognizable marks, 



