border of the adductor muscle (fig. 7, sms.). The membrane 

 however has the appearance of being suspended from the 

 adductor muscle as connective tissue fibers extend along the 

 surface of the muscle and bend it toward the visceral mass. 

 Most of the muscles of the membrane run from the region of the 

 adductor muscle directly toward the borders of the gills attached 

 to it, but two distinct bands of muscles are present (fig. 18 1m.), 

 that occupy positions along the sides of the blood space which 

 runs along the dorsal borders of the gills and receives blood 

 from the gills. When the gills are elevated these muscles con- 

 tract and shorten the gills, at the same time throwing them into 

 a series of plaits. Each of the gills that are attached to this 

 membrane is attached by one lamella only. The outer gill is 

 attached by its inner lamella and the inner gill by its outer 

 lamella. The remaining lamellae are free along their borders 

 which are usually somewhat reflected. 



The gills of lamellibranchs are usually attached so the dorsal 

 borders of the outer lamellae of the outer gills are attached to 

 the mantle and the dorsal borders of the inner lamellae of the 

 inner gills are attached to the visceral mass, or behind the vis- 

 ceral mass, to each other. In this way the gills divide the space 

 between the lobes of the mantle into a ventral space, the 

 branchial chamber, into which the gills hang, and a dorsal 

 space, the cloacal chamber, above the gills. This dorsal space 

 is divided throughout the greater portion of its extent by the 

 visceral mass and by the membranes that support the gills on 

 each side, which in the scallop are muscular. 



Although the gills on each side do not form the attachments 

 described, the free edges of the gills are pushed out and make 

 contacts that correspond with the attachments in other forms 

 that have been described. It is important that such contacts 

 should be made as the water, that passes through the gills for 

 purposes of respiration and feeding, in a manner to be described 

 later, must be constantly renewed from the outside to be effec- 

 tive for either purpose, and if the outsides of the gills and the 

 spaces between their lamellae did not communicate with separate 

 cavities, a- current could not be formed. It is also important for 

 a scallop, which swims by throwing powerful currents of water 

 from the mantle chamber to have no permanent division between 



22 



