378 



LAKES AND STREAMS 



Pond-Snails. 



which is greenish black above and yellow below, and is common in ditches and 

 stagnant water. A third form is the eight-eyed leech (Nephdis vulgaris), coloured 

 dark brown above, often marked with transverse rows of yellow spots, and yellow 

 beneath. Of smaller size are the so-called snail-suckers {Clepsine), which live on 

 the juices of pond-snails. 



From the small pond to the broad river, the fresh waters harbour 

 multitudes of molluscs of various species, which play an important part 

 in consuming decaying vegetable substances, and thus keeping the water clean and 

 pure. Among these the pond-snails are most worthy of notice. All these are furnished 



with thin shells, and being 

 lung-breathers are obliged 

 occasionally to rise to the 

 surface for the purpose 

 of respiration. The most 

 common species is Li in mm 

 stagnaZis, the shell of which 

 is 2 inches long, rather 

 slender in form, in colour 

 like horn, and consisting 

 of seven or eight whorls, 

 with a long spire and deep 

 suture. There are several 

 other species of the genus, 

 and over sixty races, differ- 

 ing mainly in the number 

 of whorls, the size of the 

 lip, and the length of the 

 spire. In the ramshorn 

 (Plavorbis corneus) the 

 shell has no spire, but is 

 of the shape of a rope coiled 

 flat , with five or six whorls, 

 a deep umbilicus, and a 

 crescent - shaped mouth. 

 About an inch across, and horn-brown in colour, it may frequently be seen 

 floating on the surface of ponds and sluggish streams in which it lives, or crawling 

 underwater in its peculiar jerky manner. Representing another family is Physa 

 fontinalis, with a thin, translucent shell, in which the spire is left-handed: it is 

 about half an inch long, with four or five whorls, the spire short, and the colour 

 varying from yellow to reddish. This snail, which lives on aquatic plants, ami 

 may often be seen on the leaves above the water, is rather active in its movements. 

 These three genera are noteworthy for the threads used by certain of the 

 species in climbing to the surface of the water to breathe, and for other purposes. 

 This thread can hardly be said to be spun, being merely a thin line of mucus, and 

 bearing no analogy to the byssus of the bivalves. In Physa hypnorum it has 

 been known to attain a length of 14 inches, and the young of this species have 



vw • 



THE KAMSHllRN POND-SNAIL. 



