138 OYSTERS, AND ALL ABOUT THEM. 



site side to the mouth and in the aerial chamber. Singu- 

 larly enough, in many species the intestine passes directly 

 through the heart a little in front of the anus. 



The foot is an organ of very various development and 

 very various functions, in the different species. In some 

 its main office seems to be the secretion of threads, by 

 which the creatures moor themselves to rocks. These 

 threads are formed in a groove in the foot, and one end of 

 the thread, while yet viscid, sticky, and unconsolidated, is 

 applied by the foot to the rock. To this it adheres ; and 

 when the foot is pulled back, the thread is pulled out of 

 its groove and a fresh one made, so that, at length, a 

 bundle of very strong threads passes from the support to 

 the base of the foot. In other cases, as in the solen, the 

 foot is large and broad, and passes out in front of the long 

 razor-like shell by a slit in the mantle, and with this foot 

 the creature burrows in the sand. 



In the cockle the foot is long, and can be thrust out 

 and applied to the earth, so as to jerk the animal along. 

 In other species it is little else than a muscular investment 

 of the viscera. 



The heart is always systemic, that is, it drives the 

 blood, not to the respiratory organs, but to the system, 

 leaving it to return unaided by mechanical force to the 

 gills, and from thence to the heart again. In all species 

 the heart shows a higher development than in the mollusca 

 hitherto treated of, inasmuch as there are always two com- 

 partments, one less muscular, to receive the blood, and the 

 other more so, to drive it through a system of vessels, 

 where its course is impeded by their elastic walls. In 

 some species there are two auricles or receivers, and in 

 some verv wide Conchifers there are two distinct hearts, 



