HARDWICKE'S SCIENCE-GOSSIP. 



121 



WATER SNAILS; A STUDY OF POND LIFE. 



By the author .OF "PLANT LIFE." 



[Continued from ^a^e 105.] 



E have promised to 

 explain what are 



' otoliths 



The 



reader has proba- 

 bly investigated 

 the anatomy of a 

 cooked fish, as he 

 may be acquainted 

 with that of a roast 

 or boiled fowl. If 

 at all of an ob- 

 servant nature, he 

 must have noticed 

 in the head of a fish 

 certain "stones" 

 of a more or less 

 triangular form. 

 These are the oto- 

 liths, or "ear- 

 stones "of the fish, 

 and they were 

 contained in a sac or chamber filled with a crystalline 

 fluid. In the fish these are attached to the auditory 

 nerves. The vibrations of sound are conveyed through 

 the water there to auditory chambers, where they pass 

 through the crystalline fluid, and by agitating the 

 ■"otolith," convey the impressions to the brain. 

 Now if we examine our snail very carefully we shall 

 find at the base of the tentacle a very delicate 

 membrane sac, in which are minute calcareous 

 bodies. These bodies are the otoliths, and they are 

 in constant motion. They are the organs of hearing. 

 The whole animal is seen to be covered by a loose, 

 dark integument, into which the head and foot can be 

 retracted. This integument fits closely to the inner 

 ■surface of the shell, and is designated the " mantle." 

 It is this mantle that secretes the animal's shell. 

 Here a peculiarity should be noticed. The shell 

 is enlarged at one point only, the mouth or widest 

 portion. As the animal grows too large for its small 

 shell, the difficulty is not met, as in the crabs and 

 lobsters, by casting off the tight fit and developing 

 No. 210. — June 1882. 



a more roomy apartment. Instead, the shell is 

 lengthened at its widest part, and as it increases 

 in length increases the width of the portion in course 

 of formation, to allow of the proper development 

 of the head and foot. The peculiarity we desire to 

 note is that this addition is made by the free edge 

 of the mantle, which alone has the power to give 

 colour or impart a pattern to the shell. The 

 hidden portions of the mantle, which line the shell, 

 have the power only of thickening the shell that 

 has been formed. If you will look carefully over 

 the specimens which we obtained this afternoon 

 you will find that some of the large, and consequently 

 old, specimens have had their shells broken at some 

 period. You can see this by these rough edged 

 deep depressions, though the gap has been stopped 

 by a layer of shelly matter from inside. Now the 

 general colour of the shell is a light brown, but these 

 patches are dirty white in hue. Here is a better 

 instance. This larger thicker shell was formerly the 

 residence of a land snail, the Roman snail. Now 

 you see that this shell is marked by several dark 

 bands running parallel throughout the length of the 

 spire. But here is a patch, the result of an old 

 breakage, and here you see the lines are interrupted 

 and the patch is of a uniform dirty tint. Now that 

 patch of an odd colour let in there tells its own story. 

 That fracture was made long after the surrounding 

 portions of the shell were formed and hardened ; 

 because, if the fracture had occurred to the soft, 

 newly-formed edge of the shell, it would have been 

 repaired and coloured in harmony with the rest of 

 the shell. 



The shell is composed largely of carbonate of lime 

 mixed with animal matter. In fact tlie shell consists 

 of the upper layers of the mantle which have been 

 thrown off, after the cells have been filled by a deposit 

 of carbonate of lime. Now the shells of these water 

 snails are very thin and more horny than stony in 

 texture, from the small amount of lime contained in 

 them. All pond snails agree in this respect, and the 

 reason is easily found. Molluscs, whether they are 



