Mnge H^ or tlie point where the two valves are firmly 

 fixed to each other by a dark brown crescent- shaped 

 body, the ligament lig.^ which, while it serves to attach, 

 also tends by reason of its elastic properties to cause the 

 valves to separate at their free borders at A, in order to 

 allow the passage of water inwards to the gills 6r, and of 

 food to the mouth M^ while it also allows the water 

 which has passed through the gills to escape by way of 

 the cloaca (7Z, carrying with it in its passage the faeces 

 from the vent Y. The tendency to separate the valves, 

 inherent in the ligament, is balanced by the adductor 

 muscle Mus^ which, upon the slightest intimation of 

 danger, forcibly contracts, closing the free edges of the 

 valves tightly. The dark purple scars near the centres 

 of both valves, and vulgarly known as indicating the 

 position of the "heart," are simply the areas covered by 

 the attachment of this adductor muscle, which is com- 

 posed of a vast number of extremely fine musculaj fibers, 

 which collectively pass straight across the space between 

 the inside of the valves, being firmly fixed at either end 

 to the latter. The muscle, when closely examined, is 

 seen to be composed of bundles of fibers or fibrill^e, 

 which may be called fibrils, each with its proper sheath, 

 and polygonal in cross-section. The muscle is also made 

 up of two kinds of fibers, the larger portion, embraced 

 in the area I, are brownish gray, while those in the 

 smaller area II are whitish ; the line of contact between 

 the two is sharply defined by the curved line III, where 

 they are also easily separated as portions of a compound 

 muscle. The foregoing fairly describes the mechanism 

 of the shell, and in part the physiological significance of 

 the same. 



The structure of the shell is laminar, or in other 

 words, it is comj)osed of numerous layers of carbonate 

 of lime deposited one on the other by the mantle (the 

 latter to be described farther on,) probably from the 

 blood or possibly in some other way. These layers de- 



