latter in turn lead to a common epibranchial 

 chamber which, tliruugh the cone-shaped cloaca, 

 opens to the outside. 



There are no partitions, Aalves, or any other 

 featui-es for regulating the flow of water inside the 

 chamber system. The current is maintained 

 solely by the beating of the ciliary epithelium of 

 the gills and of the lining of the chambers. Below 

 the level of the heart the fusion of the gills with 

 the median axis is lost and the two chandjers, 

 separated here by the pyloric process of the vis- 

 ceral nuiss (fig. 72, py.p.), merge together to form 

 a common! epibranchial cb.amber (figs. 72 and 116, 

 ep.br.ch.). This leads to a wide cone-shaped 

 exhalant chamber or cloaca (fig. 72, cl.) which can 

 be examined by forcing apart the posterior end of 

 the vahes and focusing a beam of light on its 

 inner sm-face. The water tubes appear as large, 

 round holes (fig. 72, w.t.). 



PROMYAL CHAMBER 



In oysters of the Ostrea type all the water col- 

 lected by the gills is discharged through a single 

 opening of the cloaca. In Cra^aostrea, however, 

 the exhalant system is modified by the presence of 

 an asymmetrical space on the riglit side of the 

 body called the promyal chamber (fig. 116, 

 pr.cii.). This irregularly shaped pocket between 

 the mantle and the visceral mass extends in a 

 dorsoposterior direction from the level of the 

 pericardium to a wide outside aperture which may 

 be seen by forcing the valves apart and examining 

 the space between them. Openings of the water 

 tubes similar to those found on the inner wall of 

 the cloaca are visible inside the chamber (fig. 75) 



The promyal chamber was first observed by 

 Kellogg (1892), who suggested that the water 

 from the gills may be discharged througli it. 

 StalTord (1913) showed the chamber in one of the 

 illustrations of his book but did not refer to it in 

 the text. The full anatomical significance of the 

 promyal chamber in C virginica was described by 

 Nelson (19.3S) and by Elsey (1935) for C. gigas. 



The position occupied by the promyal chamber 

 makes it apparent that water from the dorsoan- 

 terior part of the right demibranch is discharged 

 through this chamber and does not pass tlu'ough 

 the cloaca. After releasing carmine suspension 

 near the gills of an actively feeding oyster {C. 

 virginica), one can observe some of the red dis- 

 colored water being expelled through the promyal 

 chamber while the principal stream is passing 



THE GILLS 



through the cloaca. In spawning males the sperm 

 shed from the right gonoduct is frequently 

 discharged through the promyal chamber. 



In assessing the relative importance of the 

 promyal chamber in the movement of water 

 through the gills. Nelson (1938) states that out of 

 36 water tubes in the right demibranch of C. 

 virginica the first 14, ct)mprising more than half 

 the length of the demibranch, are in free C(3mmu- 

 nication with the promyal chamber. The remain- 

 ing 22 water tubes discharge water into the nar- 

 rowed portion of the epibranchial chamber beneath 

 the adductor muscle or into the cloaca. In the 

 absence of measurements of the amount of water 

 discharged through the promyal chamber, it is 

 impossible to state what percentage of the total 

 output is discluirged through the chamber. From 

 anatomical evidence it may be concluded that the 

 greatest part of the water used for ventilation of 

 the gills leaves through the cloaca and only a 

 minor portion through the promyal chamber. 



Relative dimensions of the different parts of the 

 exhalant system of the gills can be demonstrated 

 clearly by casts made with plaster of Paris or with 

 latex. Before being injected, the oyster should 

 be completely narcotized. The material used for 

 injection is then forced through both the cloaca 

 and the promyal chamber while the oyster is 

 gently and frequently tapped to permit good 

 penetration to the very ends of the water tubes. 

 When all the passages are fidl the valves are 

 pressed together and tied with a string. The ma- 

 terial is permitted to set for 24 hours before the 

 shells are opened and the soft parts of the oyster 

 removed. 



The various parts of the water exhalant system 

 of C. virginica are shown in figure 117, which rep- 

 resents the cast of the inner gill chambers viewed 

 from three different angles. The promyal cham- 

 ber (fig. 117, left, pm. ch.) occupies an irregular 

 area on the right side of the visceral mass and 

 ends in an aperture almost twice as large as the 

 opening of the cloaca (cl.). The relation between 

 the promyal chamber and the two demibranchs of 

 the I'ight side is shown in tlie central drawing (fig. 

 117) of the cast viewed from the left side. The 

 right and left demibranchs are separated by a 

 septum (s.) in the upper portion of the posterior 

 side of the gills (fig. 117, right). Most of the 

 iimer spaces of the gills are in direct connection 

 with the cloaca; less than one-half of the right 

 demibranch empties into the promyal chamber. 



123 



