Heart-Cockle and Fresh-water Cockle 107 



the fresh- water pond or stream, in order that we may 

 consider some of its inhabitants in their proper order. 

 There on the mud or climbing the weeds we shall find 

 a number of little bivalves, to which the popular name 

 of Fresh-water Cockle has been applied. The nine 

 species found in this country belong to two genera 

 of one family — the Sphaeridse — and are not at all 

 related to the true Cockles (Cardiiini). Their thin 

 shells are more or less globular, the valves being of 

 equal size but not equal-sided. The hinge is provided 

 wdth both cardinal and lateral teeth, and their general 

 character may be gathered from this figure of the 

 hinge of ^^pltceriuin corneum. The animals are very 

 active, and by means of the mobile foot can not only 

 climb weeds, but by spreading it out on 

 the surface of the water can float with 

 their shells inverted after the manner of Hmge-tGeth 

 the pond-snails. From such a position spiucrmvi 



they often descend gradually to the bottom by spin- 

 nincT a thread of mucus, the end of which remains at 

 the surface or attached to a floating weed. By means 

 of these threads they can reascend to the surface or 

 remain suspended half-way. They hibernate in the 

 fine soft mud at the bottom, becoming active again 

 when the warmth of spring has penetrated to those 

 depths. The eggs are retained in a brood-pouch 

 within the gills until they hatch, and the young 

 remain there for a time, finally being shot out 

 through the siphon when developed sufficiently 

 to take care of themselves. The four species of 

 Sphcuriuvi have each two siphons, a character which 

 serves to separate them from the five species of 

 Pisidiiim, which have one siphon only. 



