[7] DEVELOPMENT OF THE OYSTER. 785 



very minute transparent poly ftonal prisms of carbonate of lime arranged 

 vertically to the plane of the shell ; each of these prisms measures 

 y^o of an inch in diameter, and gives the appearance of an irregular 

 tesselated pavement under the microscope. They are held firmly to- 

 gether by an organic material more or less nearly identical with the 

 hard outer crust of insects. In the latter this material is called chitm ; 

 in the shell of the mollusk, where it binds the prisms of lime solidly to- 

 gether, it is called eoncMolin. As development advances the shell is 

 thickened from within by the deposition of lime carbonate on the inner 

 surface of the valves ; this lime carbonate is secreted from the blood of 

 the animal and is primarily derived from the food ; the organ, which is 

 the effective agent in laying down this deposit, is the mantle, the margin 

 of which is provided with sensitive feelers or tentaculte as indicated in 

 an undeveloped condition by the small wart-like prominences at the bor- 

 der of the mantle organ at t. 



The mantle is a highly sensitive structure, and is provided with radiat- 

 ing muscular filaments, r, which run through its substance outwards to 

 its margin in every direction. These radial muscular bands are very 

 distinct in the young, as indicated in our figure. In young oysters and 

 "silver-shells" its margin is sometimes seen protruded beyond the edges 

 of the valves, when the animal has its shell open, and is quietly feeding. 

 The mantle covers the right and left sides of the soft body of the oyster 

 like a cloak ; the leaves of opposite sides are joined together at the 

 middle line near the hinge h and at a point near the ventral hinder 

 end of the body J where the gills g end. The extent of the union of 

 the right and left leaves of the mantle behind or below the hinge I 

 have not been able to make out clearly in these young specimens of 

 spat. Only the left leaf of the mantle is shown in Fig. 2. 



The gills g of the spat are well developed at this early stage, and 

 extend between the right and left leaves of the mantle from the palps 

 or lips^ to the point.;. They are much like the gills of the adult, but 

 above them the upper gill chamber is wider, and the cloaca! space 0, 

 which lies between the adductor muscle m, the hinder part of the gills 

 and the leaves of the mantle at the sides, is remarkably spacious. The 

 gills are at this stage already very evidently of the type seen in the 

 adult ; they are really four elongated pouches suspended between the 

 leaves of the mantle with vertical rows of pores arranged in the furrows 

 on their surface ; these pores convey the water into the hollow gills, from 

 which it passes through rows of large holes above into the upper gill 

 chamber and out by way of the cloaca 0. 



In the spat, I also find in the course of more recent studies that there 

 is a very delicate branchial skeleton formed of very fine quadrangular 

 meshes of a chitinoid substance, which, as in the adult, serves to give 

 support to the soft tissues of the gills. Whether there is present a 

 delicate thin membrane in the skin covering the mantle of the spat, 

 S, ATis. 46 50 



