3 8o 



LAKES AND STREAMS 



than 8. comeum. In Sphcerium the shell is almost equilateral, in Pisidium it is 

 inequilateral, and the former has two siphons, while the latter has only one. 



By far the greater number of fresh-water bivalves have no siphon. Of 

 these the most common are the river-mussels (Unionidce), which as a rule live so 

 deeply embedded in the mud that only the anterior edges of the shell are visible. 

 River-mussels possess an extraordinary adaptability, the different situations where 

 they are found influencing the formation of the shell in a remarkable manner. 

 Indeed, if they be removed by floods from rivers or old water-courses into stagnant 

 water, a modification in form results from the change. These bivalves are repre- 

 sented in central Europe by the two genei-a Unio and Anodonta, the former with 



well-marked teeth to the hinge of the 

 shell, the other with teeth rudimentary 

 or absent. The painter's mussel ( U. 

 pictorum), so called from its shell being 

 used by the old Dutch painters for 

 holding their colours, is fairly common, 

 but does not range north of Yorkshire 

 in England, or very far north on the 

 Continent. It is about 3 inches long, 

 and the shell is nearly straight along 

 its upper and lower margins. The pearl- 

 mussel ( U. margaritifer), which inhabits 

 only such streams as do not contain 

 much lime, is common to Europe and 

 North America, and is about 5 inches 

 long, with a thick, oblong shell, some- 

 times containing pearls. Should a 

 certain parasite become embedded 

 between the mantle and the shell, 

 which is lined with nacre, it speedily 

 becomes coated with a deposit of this 

 substance, so as to form, if the grain 

 pearl. In Bavaria the proportion of 



THE GREEN HYDRA. 



does not adhere to anything, a 

 pearls found is about one in a 



loose 



hundred mussels, but only those which are 

 white and sufficiently bright are of any value. Several strings of these pearls 

 are preserved in Dresden, which include some specimens as large as peas, clear 

 and white, with a beautiful lustre, while others display less brightness, and yet 

 others are only the so-called sand-pearls which just pass muster. Anodonta is 

 represented by two species, one of which (A. cygnea) grows to over 5 inches in length, 

 and is the largest of the river-mussels. The zebra-mussel (Dreisscnsiapolymorpha) 

 belongs to the Mytilidce, a family living chiefly in the sea. It is a three-cornered 

 shell, 1£ inches long, keeled in the middle of both valves, and is the only fresh- water 

 mussel attaching itself through life to other objects by means of a mooring-rope, 

 or byssus, protruding from a slit between the valves. These mussels, which 

 often adhere in clusters, so as to stop up water-pipes, and causing other damage, 

 were originally natives of the rivers flowing into the Black Sea, but have been 



