oysters are white and those full of spawn are 

 creamy-yellowish. The green color of oysters 

 from certain localities is attributed to the accumu- 

 lation of copper or to the absorption of blue-green 

 pigment from certain diatoms upon which they 

 feed. Mantle color is always a good indication of 

 the condition of the mollusk. 



Several ramifying blood vessels can be easily 

 seen on the surface of the mantle (fig. 72, bl. v.). 

 A broad blood vessel along the periphery is the 

 circumpallial artery (fig. 72, cp. a.). A narrow 

 and darker line immediately adjacent is a circum- 

 pallial nerve (fig. 72, cp.n.). A small, slightly 

 pigmented depression in the dorsal end of the 

 mantle marks the position of a nonfunctional 

 Quenstedt's muscle (fig. 72) barely attached to 

 the valve. 



The most conspicuous element of the oyster 

 anatomy visible after the removal of the valve is 

 the posterior adductor muscle. Tliis ovate organ 

 consists of a larger dorsal, translucent portion 

 (fig. 72, ad.m.t.) and a consistently smaller, 

 ventral opaque part (ad.m.w.). 



A semitransparent oval membrane covers the 

 pericardium, the chamber in which the heart is 

 suspended (fig. 71, per.). On the left side of the 

 oyster the pericardial wall lies directly under the 

 valve, but on the right side the large and asym- 

 metrical promyal chamber (fig. 73, pr.ch.) 

 separates tlie pericardium from the mantle. 



ORGANS UNDERLYING THE MANTLE 



Directly under the free edge of the mantle 

 along the entire anteroventral side of the oyster 

 lie the gills (fig. 72, g.). They can be exposed by 

 cutting off the mantle along the line of its attach- 

 ment to the base of the gills, or by lifting the 

 mantle and pulling it up. If a piece of shell is 

 sawed ofl^ at the anterior edge of the valve the 

 corresponding portion of the mantle curls up and 

 exposes the gills underneath. For several days 

 the opposite fold of the mantle retains its normal 

 position with the tentacles (t.) spread over the 

 edge of tlie shell, while the curled mantle edge 

 under the cut secretes a vertical plate. Later 

 on the mantle fold of the intact side of the oyster 

 also curls up and by depositing new shell material 

 at the angle to the valve closes the gap. Cutting 

 off a portion of one valve proved to be a useful 

 procedure for observing the functions of tlie gills 

 and mantle. 



The gills consist of two pairs of lamallae or 



gill plates, one pair on each side (figs. 71, 72, g.). 

 At the anterodorsal margin thei.r free and gently 

 curved edges touch the lower tips of the labial 

 palps (fig. 71, l.p.) and their bases are joined to 

 the mantle. In the ventroposterior part of tlie 

 body tiie gills and the two lobes of the mantle 

 join to form a channel (fig. 71, f.) which marks 

 the entrance to the cloaca (fig. 72, cl.). 



Tlie moutli (fig. 71, m.), a narrow horizontal 

 slot above the dorsal edges of the two posterior 

 labial palps, lies under the liood or cap formed by 

 the anterior fusion of the two mantle folds. It 

 can be seen by cutting off the mantle cap and 

 pressing down the upper (dorsal) edges of the 

 palps. 



The cloaca (fig. 72, cl.), a large funnel-shaped 

 space between the ventral side of the adductor 

 muscle and the gills, is a continuation of tlie 

 epibranchial chamber (fig. 72, ep.br. ch.) which 

 extends along the gills. The latter can be ex- 

 posed by cutting along the wall of the cloaca, 

 starting from the mantle junction (fig. 71, f.) and 

 following the edge of the muscle. The epi- 

 branchial chamber extends along the base of the 

 gills. Wlien the dissected portions of the cloacal 

 wall are pulled apart, the following structures are 

 revealed: the rectum and anus (figs. 71, 72, r., 

 an.), located on the ventroposterior border of the 

 adductor muscle; the blunt tip of the pyloric 

 process (fig. 72, py.p.) of the visceral mass, which 

 projects into the epibranchial chamber; the small 

 and almost invisible opening of the urinogenital 

 groove or vestibule (fig. 72, ur.v.), located on the 

 wall of the pyloric process; and the visceral 

 ganglion (fig. 72, g.), situated in a shallow de- 

 pression between the two divisions of the adductor 

 muscle and partially covered by the pyloric 

 process. 



The heart (fig. 71, ii.), seen after removal of the 

 pericardial wall, consists of one ventricle and two 

 pigmented auricles. Two aortae (not shown in 

 the diagram) emerge from the tip of the ventricle, 

 and large venous sinuses (also not shown) empty 

 into tlie auricles. The slightly pigmented struc- 

 ture extending dorsally from tlie auricular side of 

 the pericardium along tlie base of tlie gills is the 

 organ of excretion (kidney) frequently called the 

 organ of Bojanus (fig. 7:5, k.). Inasmuch as there 

 is no doubt regarding the function of this tubular 

 thin- walled organ it seems preferable to call it the 

 kidney. Urine is collected in a large reservoir in 

 the lower (ventral) part of the kidney before 



70 



FISH AND WILDLIFE SERVICE 



