SPH^ERIID^E. 193 



velopes the body, and is shaped like the shell which 

 covers it. The mantle often has tentacles or feelers 

 at the edges ; these are short filaments. The mouth, 

 as there is properly speaking no head, is contained in 

 the fold of the mantle. Some species have rudimen- 

 tary eyes placed in the " interstices of those filaments 

 where the mantle is open." The fresh- water bivalves 

 of Great Britain comprise in all three families. 

 The first is the Sphczriida, of which the body is 

 sub-globular ; in front the mantle is open, and the 

 back or inner side forms a cylinder frequently 

 divided near the opening into two tubes. If there 

 are two tubes, one of which is longer than the 

 other, it is used for respiration and nutrition, the 

 shorter for excretion. The outer edges of the mantle 

 are not furnished with filaments. The mouth is be- 

 tween the adductor muscle and the sole of the foot, 

 and has two small lips. The foot is thin, of a wedge 

 form, and can be considerably extended. The shell 

 is thin and oval, and though the valves are of the same 

 size they differ in shape, being unequally convex on 

 the outer surface. The shell within is lined with 

 nacre or mother-of-pearl, and outside has a skin or 

 epidermis ; this latter is generally removed by the col- 

 lector as hiding the beauty of the colour underneath, 

 but to the scientific collector it is a defect to have a 

 specimen deprived of its natural skin. The hinge, 

 which is an important part of the animal's shell, has 

 upper and under teeth, which close and open at the 

 pleasure of the inhabitant. A muscular cord or band 

 fastens the valves together, generally external, but 

 sometimes within the hinge. This family produce 

 o 



