CONCHIFERA. 381 



ceives the blood from the respiratory apparatus, and by its con- 

 traction transmits it through two intermediate canals (c) into the 

 more muscular ventricle (d), whence it is propelled through the 

 body by the ramifications of the arterial system (w, o, p). 



The above description of the circulatory apparatus as it exists 

 in the Oyster is applicable in all essential points to every family of 

 conchiferous Mollusca ; but there are important modifications in 

 the structure of the heart and arrangement of the blood-vessels, 

 met with in different genera, which now demand our attention. 

 Most generally, in consequence of the broad and dilated form of 

 the animals, instead of a single auricle, such as the Oyster has, 

 there are two auricular cavities, one appropriated to each pair of 

 branchial lamellae, and placed symmetrically on the two sides of 

 an elongated fusiform ventricle, into which both the auricles empty 

 themselves, still the course of the blood is similar to what we have 

 described above. 



A still greater modification is found to exist in those species 

 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 

 branchiae to which it belongs, and propelling it through vessels 

 common to both hearts, to all parts of the system. 



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

 other families of Conchifera, examine the character of the locomotive 

 apparatus 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 rudiment of such an appendage. When 

 largely developed, as in Mactra (figs. 179, 180), 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 ex- 

 ternal layers being circular or oblique in their disposition, while 

 the internal strata are disposed longitudinally. In the Cockle tribe 

 (Cardium) this organ attains to a very great size, and on inspect- 

 ing the figure given in a subsequent page, representing a dissection 

 of the foot of Cardium rusticum (Jig. 182), the complexity of 



