ORDER TRACHELIPODA. 45 



These shells are found generally in lunning streams 

 of fresh water; sometimes, however, they are met with 

 in brackish water, at the mouths of rivers. The oper- 

 culum of the Pakidina is ot a strong horny substance. 

 The young, immediately they are hatched, attach them- 

 selves to the outside of the shell of the mother, where 

 they remain until they are sufficienily strong to trust 

 tlicmselves in the water. 



L Y M N .E A S T A G N A L I S . 



The Lymnaoa stagnalis is found in. stagnant v»aters, 

 particularly narrow ditches, in great abundance. As it 

 breathes air, it is necessary for its exister.ce that it 

 should frequently resort to the surface of the water ; and 

 consequently, we find these shells floating about in great 

 numbers, with the body partly out of the shelL The 

 least appearance of danger causes a Lymncea to with- 

 draw the whole of its body into its shelly covering; and 

 as this increases its specitk gravity, it instantly sinks to 

 the bottom, where it remains in safety. In order to 

 reach the surface again, it is obliged to crawl to the side 

 of the ditch, and when it has reached the level of the 

 water, it again trusts its little bark to the mercy of the 

 winds and stream, for it has very little power to direct 

 its own course. 



During the Winter, these creatures remain at the 

 bottom, buried in the mud, and in a state of torpor. 

 Although apparently useless to mankind, they form part 

 of the subsistence of many water-birds, and of fishes, 

 which are extremely fond of them. The species of this 

 shell are not easily decided, since they bear so great a 

 resemblance to each other. 



