THE PISHES, MOLLUSCA, AND OTHER OBJECTS. 37 



from the water, as in the gills of fishes. Valvata 

 piscinalis is almost certain to be met with in an old 

 tarn, and it is a species not difficult to distinguish. 

 It has four well denned whorls, of a brownish yellow 

 colour, and a deep hollow beneath, called the 

 umbilicus. It is about the quarter of an inch in 

 size. You may find it not only at the bottom of the 

 water, but on the aquatic plants, up whose stems it 

 frequently crawls. The teeth of this species form 

 an exceedingly beautiful 

 microscopic object, the 

 central portion, a, being 

 relatively larger than in 

 many fresh-water mol- 

 luscs. The common 

 genera Li/mnea, Planor- . 



i * Teeth of Valvata piscinalis. 



bis, Ancylus, &c., are 

 water-air breathers that is to say, although they 

 live in water, they are compelled by the peculiar 

 structure of their branchial organs to rise frequently 

 to the surface to breathe. The eggs of many of the 

 above mollusca may be found attached to the under 

 sides and stems of aquatic plants, the egg-sacs im- 

 bedded in a transparent jelly. It takes about a 

 month to hatch them, at which time the young 

 may be seen protected by a miniature shell. 



In the rich black mud at the bottom of the tarn, 

 lie hundreds of the shells of bivalve mollusca, such as 

 Anodon, Unio, &c. The former takes its name from 

 the absence of any toothed projection near the 



