THE FRESH-WATER AQUARIUM. 



25 



thin, flat shell, through which the organs of the mollusc 

 may be viewed. Lyinncea pei'egra is exceedingly common 

 in most ponds, and its eggs will afford another embryo- 

 logical study. BitJiynia tentaadata closes its shell with 

 a horny valve (ppeiruljun)^ like the "eye" of the peri- 

 winkle. It is depicted on Fig. 11, d. The female lays 

 her eggs in bands of three rows, very carefully cleaning 

 the surface on which she is about to deposit them. These 

 bands contain from thirty to seventy eggs, which hatch 

 in about three weeks or a 

 month. The young snails 

 are two years in attaining 

 their full growth. One of 

 the most interesting small 

 molluscs is a tiny bivalve, 

 that may well be called the 

 Fresh-water Cockle, shown 

 in Fig. II, a. The projec- 

 tions from within are 

 the "foot" and the "si- 

 phons." With the first of 

 these appendages it digs 

 into mud or sand, and 

 climbs up the water-weeds, 

 respiratory process is carried on. Its shell is exceedingly 

 thin, and through it the motions of heart and gills may 

 be seen distinctly. The animal is what is termed ovo- 

 viviparous, that is to say, it carries its eggs about in its 

 gills until they are hatched. The River Snail {Paludina 

 vivipard) is a large species allied to Bithynia, found 

 in profusion in the Surrey Canal. It is truly viviparous — 

 that is, it does not produce eggs, but living young. 



Fig. II. 



{(x) Cyclas cornea ; {b) Lymfusa peregra 

 and Spawn ; {c) Plattorbis vortex and 

 Spawn ; (d) Bithynia tentactdata ; {e) 

 Spawn of Lymncea stagnalis ; (y) Spawn 

 of Plajiorbis cornciis. 



Through its siphons the 



