3386 



THE MOLLUSKS OR SHELLFISH 



ANIMAI, (a) AND CASE (b) OF 



Rocellaria. 



commonly called, live in deep burrows in the sand or mud, the shells often being a 

 foot below the surface. A recent writer observes that when the flats are covered 



with water, the clams extend their long siphons 

 up through the burrow to the surface of the sand, 

 and through one of these tubes the water and its 

 myriads of animalcules are drawn down into the 

 shell, furnishing the gills with oxygen, and the 

 mouth with food, and then the water, charged 

 with carbonic acid and faecal refuse, is forced out 

 of the other siphon. Two species of Mya consti- 

 tute the staple food of the walrus. The Solenidce, 

 or razor shells, are also great sand burrowers, and 

 indeed bore with such rapidity, and to such a 

 depth, that they often elude capture. They pos- 

 sess very elongate shells, and are remarkable for 



the great development of the foot. They not only burrow in sand, but also have 

 the power of darting through the water like scallops. They are eaten by the poorer 

 coast population. In the Saxicavidce the species of the typical genus Saxicava are 

 some of the few bivalves which have the power of boring into limestone and other 

 soft rocks, although they more often hide in crevices or it the roots of seaweed, 

 mooring themselves by 



a byssus. The shells 

 are very irregular, 

 their form varying ac- 

 cording to the hole or 

 crevice they inhabit. 

 In the Gastrochcenidce \ 

 forming'the last family 

 of this group, Gastro- 

 chana comprises bi- 

 valves which live 

 buried in the sand. 

 These form a long, 

 slender, club-shaped 

 fragile tube, covered 

 wi th adhering particles 

 of sand, and divided 

 off by a partition into 

 two portions, the an- 

 terior containing the 

 shell, the posterior or 

 narrower end the si- 

 phons. The animal of the allied Rocellaria is similar, but forms no tube, and has 

 the habit of boring into solid rock, shells, and other substances; R. dubia being 

 found in limestone, and even granite, on the British coasts. 



Pholas IN ITS BURROW. 

 (Natural size.) 



