496 CONCHIFERA. 



most remarkable for their breadth. In Area, for example, there are 

 not only two auricles, but two ventricles likewise, placed upon the 

 opposite sides of the body ; that is, there is a distinct heart appropriated 

 to each pair of gills, each receiving the blood from the branchia3 to 

 which it belongs, and propelling it, through vessels common to both 

 hearts, to all parts of the system. 



(1298.) We must now, before entering upon the description of other 

 families of CONCHIFEKA, examine the character of the locomotive appa- 

 ratus with which those possessed of the power of moving about are 

 furnished. The instrument employed for this purpose is a fleshy organ 

 appended to the anterior part of the body, called the foot : but of this 

 apparatus, for obvious reasons, no vestige is met with in the fixed and 

 immoveable Oyster ; and even in the Scallop we have seen only a rudi- 

 ment of such an appendage. When largely developed, as in Mactra 

 (figs. 250 & 251), the foot forms a very important part of the animal, 

 and becomes useful for various and widely- different purposes. In 

 structure it almost exactly resembles the tongue of a quadruped, being 

 entirely made up of layers of muscles crossing each other at various 

 angles the external layers being circular or oblique in their disposi- 

 tion, while the internal strata are disposed longitudinally. In the 

 Cockle-tribe (Cardium) this organ attains to a very great size; and on 

 inspecting the figure given in a subsequent page, representing a dissec- 

 tion of the foot of Cardium rusticum (fig. 253), the complexity of its 

 muscular structure will be at once evident, and the disposition of the 

 several layers composing it more easily understood than from the most 

 elaborate verbal description. 



(1299.) Diverse are the uses to which the foot may be turned. It is 

 generally used for burrowing in the sand or soft mud ; and, by its con- 

 stant and worm-like action, those species in which it is largely deve- 

 loped can bury themselves with facility, and make their way beneath 

 the sand with a dexterity not a little remarkable. Perhaps the most 

 efficient burrowers met with upon our own shores are the Razor-shells 

 (Solenidce), in which family the fleshy foot attains to enormous propor- 

 tions ; and the rapidity of their movements beneath the soil will be 

 best appreciated by those who may" have watched the manner in which 

 the fishermen effect their capture. 



(1300.) The Solen excavates for itself a very deep hole in the sand, 

 boring its way by means of its foot to a depth of some feet, and remains 

 concealed in this retreat, usually occupying a position within a few 

 inches from the surface. The fisherman, armed with a slender iron 

 rod furnished with a barbed head, resembling a harpoon, treads care- 

 fully backwards over the beach left bare by the retreating tide, and 

 finds the holes in which Solen lodges by watching the little jet of 

 water thrown out by the animal when, being alarmed by the shaking 

 of the sand, it contracts its body. Guided by the orifice through which 



