Figure 117. — Water exhalant system of the gills of C. virginica. Plaster of Paris cast of a large specimen. Left — 

 view from the right side. Center — view at sharp angle from the left. Right — view from the posterior side of the 

 gills, cl. — cloaca; l.d. — left demibranch; o.o. — outside opening (aperture) of the promyal chamber; pm. ch. — 

 promyal chamber; r.d. — right demibranch; s. — septum separating the right and left demibranchs. 



The relative size of the promyal chamber is 

 variable. In some specimens it extends over 

 three-fifths of the length of the gills and forms a 

 spacious pocket (fig. 118). The exhalant system 

 of C. gigas (fig. 119) is similar to that of C. vir- 

 ginica. In the specimens I examined, the promyal 

 chamber extends approximately one-half the 

 length of the giU, and the funnel-shaped cloaca 

 was much wider than in C. virginica. In 0. 

 edulis the promyal chamber is absent and the 

 entire system of water tubes, epibranchial cham- 

 bers, and cloaca is synnnetrical (fig. 120). In 

 comparison with C. virginica and C. gigas, the 

 cloaca of 0. edulis is much broader and longer, 

 which is probably the result of the almost circular 

 shape of the body. The water tubes are shorter 

 than in Crassostrea. 



The promyal chamber also has been described 

 for the Australian rock oyster, Ostrea (Crassostrea) 

 cucvllata, Born (C. comercialis I. and R.), and in 

 Ostrea (Crassostrea) Jrons L. from the mangrove 

 roots in Florida (Nelson, 1938). 



Oysters of the Crassostrea type inhabit and 

 thrive in the brackish and nmddy coastal waters, 

 which are less suitable for the mollusks of the 

 Ostrea tj^pe. It has been implied by some investi- 

 gators (Elsey, 1935; Nelson, 1938) that the toler- 

 ance to muddy waters is due to a superior cleaning 

 mechanism which is somehow associated witli the 

 presence of the promyal chamber and prevents 



Figure 118. — Cast of the water exhalant system of a very 

 large C. virginica. Note the extent of the promyal 

 chamber along the gill axis and the impression made by 

 the distended pallial arteries at the lower end of the 

 right demibranch. 



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