BIVALVE MOLLUSCS 335 



fact that the valves cannot be brought together behind owing 

 to the presence of enormous siphons that can only be drawn back 

 into the shell to a limited extent. The two siphons are united 

 into a single fleshy 

 mass with two orifices 

 at its tip, and pro- • 

 tected by a brown 



wrinkled layer con- Fig. 194.— Sand-Caper {Mya aretiaria) 



tinuous with the outer 



horny layer of the shell. The protective arrangement suggested in 

 the Mussel (see p. 331) is here carried to a much greater extent, 

 for when the animal is buried in the mud, with only the tip of the 

 siphon-tube projecting, it is singularly inconspicuous, though feed- 

 ing and breathing can go 

 on without interruption. 

 The foot of the Gaper is 

 small, and is of no use for 



, Fig. 195. — Razor-Shell {SoUn). Foot seen on left, siphons on right 



sprmgmg. 



A closely- related family is that of the Razor-shells (fig. 195), 

 of which two British species are very common, i.e. Solen siliqua 

 and Solen ensis. In both cases the shell is very long and narrow, 

 and gapes at both ends, but in the former species it is straight and 

 in the latter curved. 



Some very interesting boring molluscs come fairly close in 

 affinity to the Gapers. The Rock-borers {Saxicava) and Piddocks 

 {Pholas) (fig. 196), both of which can excavate burrows in hard 

 rock, include a number of British species belonging to two families, 

 while a third family is represented in the Atlantic and Mediter- 

 ranean by the so-called "Ship-worm" {Teredo navalis) that often 

 completely riddles timber with its burrows, which are lined by a 

 smooth shelly layer secreted by the surface of the long siphonal 

 tubes. 



All the preceding families belong to one, and that the largest, 

 of the five orders of bivalves. We may take the Sea Mussel 

 Family as representing another order, and of the forms included 

 by far the most familiar, largely on account of its economic import- 

 ance, is the Edible Mussel {Mytilus edulis) (fig. 197), of which 

 vast numbers are found together attached to stones, wooden piles, 

 or other firm objects, by means of strong blackish threads consti- 

 tuting the byssus. The dark bluish shell is somewhat wedge- 



