SNAILS AND LIMPETS. 179 



thickly covered with small black spots. The tentacles are 

 pointed and blackish in coloui". The disc is narrow, and 

 ends behind in a slightly pointed tail. The shell is about Jin. 

 long, and is spindle-shaped ; it is of a dark or yellowish horn- 

 colour, glossy, and semi-transparent. There are five or six 

 whorls, and the spire is produced : the body-whorl occupies 

 more than half of the entire shell. The aperture m.ay be de- 

 scribed as pear-shaped. P. hypnorum deposits its eggs, which 

 number from three to twenty, in a roundish capsule. The fry 

 are hatched in from sixteen to twenty days. 



Physa fontinalis is much commoner than the last species. 

 It is found in sluggish or stagnant clear water in nearly every 

 part of Britain. It is frequently taken upon watercress. This 

 mollusc is also, like the P. hypnorum, a very busy "thread- 

 spinner." The body of the animal is dark grey. The tentacles 

 are of a lighter colour than the body. The disc is light grey, 

 and ends behind in a rather pointed tail. The shell, which 

 is about ^in. long, is very thin, and of a glossy light horn- 

 colour. There are four or five whorls, the body-whorl more 

 than equalling half of the shell. The spire is very short, and 

 the apex is blunt. The sutures are rather deep; the aperture 

 is large and pear- shaped. This snail lays from three to 

 twenty eggs, and incloses them in a roundish capstde. The 

 fry are hatched in from sixteen to twenty days. 



LimnsBSB are more active, greater eaters, and fonder of 

 feeding upon healthy growing water-plants than any of the 

 molluscs already described; and consequently are not, on the 

 whole, so suitable for the aquarium. However, some of the 

 Limnxee are useful in the tank, and all of them are interest- 

 ing there. A few are carnivorous as well as herbivorous. 

 I have several times seen snails of this genus, especially 

 L. peregra, sharing a worm or a dead fish with the beetles, 

 CorixsB, and the like. Although Limnxas will eat no small 

 quantity of necessary vegetation, they are at the same time 

 exceedingly useful in clearing away confervse, and in acting 

 as scavengers generally. 



I dare say I shall be laughed at by some people when 

 I say that I believe that a taste for confervoid growth can be 



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