9 



was not possible to determine the age of marketable oys- 

 ters from wliat appeared to be annual arrests of growth, 

 indicated on the outside of the shell by deeply impressed 

 lines or projecting laminae placed at nearly equal dis- 

 tances apart, but I failed, after examining a great num- 

 ber of living and dead shells, to arrive at any satistactory 

 conclusion. 



THE ANIMAL. 



In Fig. I the head of the animal lies directed towards 

 H; the tail end, on the other hand, lies towards A ; the 

 back or upper side of the animal extends from h to c; the 

 ventral or lower side reaches from c to a up to the mouth 

 M. The right and left sides of the animal are- covered 

 by an organ called the mantle Mt. Mt.; it is not attached 

 to the shell anywhere except around the points where 

 the ends of the adductor muscle, Mus^ are fixed to the 

 valves of either side. This organ encloses, like a curtain, 

 the internal organs of the creature on either side, and 

 lies next to the shell, and secretes and deposits the 

 layers of carbonate of lime composing the latter. The 

 free edges of the mantle, which are purplish, are gar- 

 nished with small, highly sensitive tentacles tent, of the 

 same color, and which serve the purpose of organs of 

 touch and are perhaps also to some extent sensitive to 

 light. The borders of the right and left folds of the 

 mantle are free from h to c, where they unite, and from 

 G to a, but from a to /; they are joined together. Tn the 

 space between the right and left folds of the mantle from 

 5, a to d^ the palps P, or lips of the oyster, are included ; 

 this cavity may be called the palpal space F. sp. The 

 palpal space is continuous with the inferior gill cavity /. 

 cav., from d to c ; in this space, bounded like the latter 

 on each side by the mantle, the four leaf-like branchiae 

 or gills G hang suspended by a membrane, whi(^h unites 

 with the mantle on either side along the line e ; this 

 membrane, together with the bases of tlie gills, forms a 

 cavity limited below by the upper edges of the gills at g 



