320 FOSSIL OSTREID.E OF NORTH AMERICA. 



fleshy lips or i>alps p, and closed above the latter. Posteriorly, at y, 

 wbere tbe gills terniiuate, tbe mantle leaves of opposite sides are joined 

 together by a narrow transverse membrane, which extends downwards 

 and lorward.s forming the floor of the cloaca cZ and the space between 

 the ventral i)rocess of the body-mass/ and the gills. This narrow mem- 

 brane IS perforated by four parallel rows of pores, 6j), whicli lead down 

 into tiie divided internal cavities of the gills. 



J he free margins of the mantle are fringed by two rows of short, pur- 

 plish, extensible, and liighly sensitive tentacles, which are supplied with 

 nerves from the great nervous ganglion7;(7,ou the lower side of the adductor 

 M. The tentacles are protruded slightly beyond the edges of the valves 

 when the animal is feeding, but they are quickly withdrawn upon any 

 intimation of danger by the contraction of the slender, branching, mus- 

 cular bundles which radiate outward in all directions through the man- 

 tle leaves of either side from around both of the insertions of the great 

 adductor M. The radiating muscles of the mantle cross the marginal 

 muscular fibers of the mantle border at right angles. They may col- 

 lectively be called the pallial muscles. 



The oyster is classed by naturalists amongst what are called lamelli 

 bninehiate mollusks, or those which tend to have the gills or branchiie 

 (J developed as great flat parallel i)lates, or lamellte, whence the name. 

 In the oyster there are four gills, alongside of each other, which extend 

 from behind the palps j) to the point ?/. They are not really simple flat 

 plates, however, as we learn upon examining them closely. They are 

 really much more complex organs than might at first be supposed. 

 Each gill is in fact composed of two rows of conjoined fleshy parallel 

 filaments fused together at their edges and lower ends and joined above 

 to the perforated fleshy membrane already alluded to. They are there- 

 fore in reality long and narrow hollow sacks. Their cavities are, how- 

 ever, subdivided by fleshy, transverse partitions at narrow intervals. 

 The api)eaiance of the gills, with their internal cavities, when cut across, 

 is shown at (j', in Fig. 3, which represents a cross-section through the 

 mantle, gills, and body of an oyster, enlarged about two diameters. 



If we inspect the outer surfaces of the gills we will find that fine 

 parallel ridges or ribs, with intervening furrows, extend vertically np 

 and down, which give rise to a striated appearance on the surface of 

 the branchiae to the naked eye. Under the microscope these ridges in 

 turn are found to be made np of still finer parallel ridges or ribs. A still 

 more searching examination reveals the fact that there are rows of very 

 fine pores between these ribs, which open from the outside into the cav- 

 ity in the gill. On this account we are finally forced to regard the gills 

 as sieve-like structures, a concejttion of them which is further justified 

 by the fact that the gills have an exceedingly delicate internal skeleton 

 apparently composed of a horny substance, the meshes of which are 

 square or in the form of oblong squares, and around which tbe soft parts 

 of the gills are built, and by which they are supported. It will be gath- 



