which they adhere, leaving a narrow opening 

 between the two lobes through which sea water 

 can enter the mantle cavity. The edge of the 

 mantle, may, however, occupy various positions: 

 it may extend parallel and beyond the edge of the 

 valves to leave a wide space between the two 

 opposing lobes, or it may bend inward almost 

 perpendicular to the shell surface (fig. 74) to 

 reduce or completely close the opening between the 

 two lobes and thereby limit the access of water 

 to the mantle cavity. The behavior of the 

 mantle edge as a regulatory mechanism controlling 

 the flow of water through the mollusk will be 

 discussed later (p. 185). In a closed oyster the 

 mantle edge is located about midway between 

 the distal margin of the gills and the edge of the 

 shell. Its position is marked by an impression 

 called the pallia] line, which is less pronounced in 

 the oystei's than in clams and some other bivalves. 

 At the ventroposterior end of the body the two 

 opposing lobes of the mantle join the gills to form 



the delicate outside wall of the cloaca (figs. 72 

 and 75, cl., f.). On the left side of the body the 

 mantle is joined to the visceral mass; on the 

 right side it is separated from the visceral mass 

 by the promyal chamber. The fusion of the 

 mantle with the visceral mass and with the bases 

 of gill plates forms the wall of the epibranchial 

 chamber, which leads to the cloaca (fig. 75, d.). 

 The relative position of the epibranchial and 

 promyal chambers can be seen in the cross 

 section of the oyster made through the dorsal 

 part of the body (fig. 73, ep.br.ch.; pr.ch.). 



An oblong slit between the two mantle lobes on 

 the dorsoposterior side of the body marks the 

 opening of the promyal chamber. The inside of 

 this chamber can be examined by completely 

 narcotizing the oyster and forcing its valves apart 

 as far as possible without tearing the adductor 

 muscle. Viewed from the posterior side the 

 promyal chamber in a relaxed oyster appears as 

 an oval cavity (fig. 75) to the left of the adductor 



illimeters 



FiouRE 74. — Cross sections of the valves, mantle, gills, and adjacent portion of the visceral mass of C. virginica. In both 

 diagrams the valves are open; the open pallial curtain (at left) permits free access of water to the mantle cavity; the 

 closed pallial curtain (at right) prevents water from entering the mantle cavity. The outer lobe adheres closely 

 to the valve and is not visible. Drawn from the photomicrographs of cross section of adult oyster. Bouin, hema- 

 to.\ylin-eosin. 



THE MANTLE 



75 



