RrLKB] LIFE-HISTORY OF THE OYSTEK. 319 



deposited in succession one upon the top of the other hy the mantle in 

 an organic; horny matrix known as conchioline. When the surface of 

 these layers is cxauiined under the microscope the calcic carbonate is 

 found to be arranged in minute polygonal blocks or prisms. The horny 

 matrix of the shell, as well as the calcareous matter, is deposited by 

 the mantle, and is primarily derived from the food and earthy matters 

 swallowed by the animal. 



The layers of calcareous matter, deposited as they are internally, as 

 growth proceeds, project in succession iiasteach other at the free edges 

 of the valves and cxteriial surfaces of the shell, so that the successive 

 deposits may be distinguished. It is also evident that deposition occurs 

 to some extent periodically, which gives rise to the rough imbricated 

 appearance of the edges of the layers which terminate on the outside 

 of the shell. Moreover, the rate of deposition varies at different points 

 around the margin of the shell, so that growth of the shell may take 

 place more rapidly at one part of the margin of the valves than at 

 another. 



Cavities filled with fluid are frequently found in the calcareous valves 

 of the oyster. They are usually shallow, and of no very great exteat, 

 and aiise in consequence of the manner in which the calcareous matter 

 is deposited by the mantle, the new layers not being laid down in im- 

 mediate contact with the preceding ones, where the cavities are formed. 

 Such cavities are also sometimes formed in consequence of the encroach- 

 ment of mud between the valves, as shown at x,x,x in Fig. 1. In such 

 cases the animal has sunken too deejtly into the ooze, which then found 

 its way into the shell while the animal had its valves parted when feed- 

 ing. Thenuid wiiichinsuch instances has insinuated itself between the 

 mantle and shell is immediately covered by thin deposits of calcareous 

 matter secreted by the border of the mantle. Inclosed in this way by 

 calcareous deposits the included mud is rendered harmless to the soft 

 and delicate structures of the inhabitant. 



In Fig. 1, the back or dorsal side of the animal, it may be observed, 

 extends anterioily from n to y; the ventral or lower side reaches from z 

 to y. The right and left sides of the animal are covered, in life, bv an 

 organ called the mantle, mt. (In the figure the mantle of the right side 

 has been entirely removed, except a small triangular patch int', which is 

 closely adherent to the front part of the body-mass.) This organ is 

 thin; it is in fact a flat membrane, which is not attached to the shell 

 anywhere except around the poiuts where the ends of the adductor 

 muscle M and the i)edal muscle j/ are affixed to the internal surfaces of 

 the valves of either side. This organ, as may be seen from the figure, 

 incloses, like the covers or lids of a book, the other soft parts, viz, thegills, 

 body, and palps, which are in truth suspended between the two great right 

 and left leaves of the mantle. The margins of the mantle-lobes of eithei* 

 side are joined together for only a short distance at the head end of the 

 aaimal, or from n to z, forming a sort of hood over the mouth and great 



