38 



THE GILLS. 



Tlie Gills or Ctenidia (PI. II., fig. 1) are conspicuous 

 in the pallial cavity wlien the valves are opened, and 

 extend like orange tinted curtains on each side of the 

 visceral mass, with their free edges reaching from the 

 labial palps to a point opposite the end of the Rectum. 

 At first sight there appear to be two gills on each side of 

 the body, but the morphological identity of the lamelli- 

 branch gills with the ctenidia of other molkisca is now 

 fully established, and there is but one ctenidium on each 

 side. These two apparent gills are two plates (double 

 for the greater part of their area), formed by a series of 

 filaments loosely attached to one another. The two 

 opposite and innermost lamellae meet, fuse, and become 

 continuous with a supporting ridge at their proximal 

 edges, but their lower or distal edges are reflected so that 

 in section each gill has the appearance of a W, the two 

 outside limbs being the reflected portions, and only two- 

 thirds the height of the middle ones by which the 

 ctenidium is attached to the body. The transverse 

 section is diagrammatically represented in fig. 20. Each 

 of these double plates is known as a demibranch so that 

 there are two demibranchs on each side, an inner and an 

 outer, which together make up a ctenidium. 



Each Ctenidium consists of a supporting axis or 

 ridge (fig. 20, Br. «./•.) from which depend two regular 

 series of long delicate filaments. These two series form 

 the two direct or descending lamellae, this part of the 

 filaments being known as the descending filament (fig. 20, 

 Br. cl.). The lower ends of the filaments .'ire reflected os 

 previously seen (fig. 20), on the outer side of the external 

 demibranch and the inner side of the internal demi- 

 branch, and the ascending or reflected jjortions of the 



