Figure 75. — Promyal chamber (at left) and cloaca (at right) viewed from the posterior side of a large oyster (C. virginica) 

 completely relaxed by narcosis. Note the fusion of the two opposing lobes of the mantle, and the adductor muscle 

 (in the middle). Drawn from life. Actual size. ad.m. — adductor muscle; cl. — cloaca; f. — fusion of mantle lobes 

 and gills; pr.ch. — promyal chamber; r. — rectum. 



muscle. The large round openings of the water 

 tubes of the gills can be seen on the inner wall of 

 the chamber. The rectum extends along the 

 edge of the chamber, ending with a round anus 

 adhering to the side of the adductor muscle; the 

 opening of the cloaca lies to the right of the muscle. 

 The water tubes emptying into the cloaca and the 

 fusion of the mantle with the gill lamellae are 

 also clearly visible. 



The most conspicuous components of the mantle 

 are the radial muscles, the blood vessels, and the 

 nerves (fig. 76). All these structures can be 

 identified in a piece of fresh tissue stretched over 

 a glass slide and examined under strong illumina- 

 tion with a low-power microscope. For more 

 detailed study, it is necessary to prepare whole 

 mounts or to section the preserved tissues. 



The radial muscles extend from the place of 

 their attachment to the visceral mass to the edge 

 of the mantle. At about two-thirds of their 

 length from their base they begin a fanlike ex- 

 pansion toward the periphery before terminating 

 in the base of the tentacles. The majority of 

 the muscles are accompanied along their length 

 by nerves, blood vessels, and blood sinuses. Much 

 more slender than the radial muscles are the 

 concentric muscular bands wliich parallel the free 

 edge of the mantle (not shown in fig. 76) and are 

 more abundant at its thickened distal edge. 



Because of its strongly developed musculature, 

 the mantle is highly contractile. It may stretch 

 a considerable distance beyond the edge of the 

 valve, or withdraw inside the shell, and even roll 

 up into a tube. Contraction of the radial muscles 

 will throw the inner surface of the mantle into 



ridges which serve as temporary channels for 

 discarding mucus and foreign particles accumu- 

 lated on it. These movements may involve 

 either the entire surface of the mantle or only a 

 small portion of it, depending on the intensity 

 of stimulation received by the tentacles. 



The wide circumpallial artery (fig. 76, cp.a.) 

 follows the entire periphery of the mantle. At 

 low magnification it is usually visible as a wide 

 tubular structure with many branching vessels 

 which communicate with the irregular spaces 

 (blood sinuses) within the connective tissue. A 

 large pulsating blood vessel, called the accessory 

 heart (ch. XI, fig. 236), is located in the ante- 

 roventral part in each lobe of the mantle. The 

 structure and the function of this vessel are dis- 

 cussed in chapter XI, p. 254. 



Just outward from the circumpallial artery runs 

 the circumpallial nerve, which also extends along 

 the entire margin of the mantle. In whole mount 

 preparations seen under low power, the circum- 

 pallial nerve appears as a compact unbranching 

 band. Examination under higli power, however, 

 reveals a fine network of small nerves connecting 

 the circumpallial nerve with nerves and with the 

 visceral and cerebral ganglia. Since nerve fibers 

 on the surface of the mantle and in the tentacles 

 lead to the circumpallial nerve, stimuli received 

 by the neuroreceptors of these areas are trans- 

 mitted through the circumpallial nerve to the 

 radial nerves and reach either the visceral or the 

 cerebral ganglia. 



The thick and muscular border of the mantle 

 is divided into three lobes (fig. 77) which have 

 been described in the literature as "folds" (Awati 



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FISH AND WILDLIFE SERVICE 



