15 



between the siphons. At this point of concrescence of the 

 two ctenidia, a peculiar little semicircular flap of mem- 

 branous tissue projects downwards and forwards into the 

 general mantle cavity. 



This horizontal shelf is farther prolonged anteriorly by 

 the inner lamellae of the internal branchiae. Part of these 

 inner lamellae (Br. 1. 2, figs. 3 and 4) have the ordinary 

 filamentar structure and are connected to the outer lamellae 

 of the same branchiae by the inter -lamellar junctions. 

 But the remainder of the inner lamellae are simply mem- 

 branous, and fuse with each other across the middle line 

 of the body, and thus form the floor of the ventral supra- 

 branchial chamber {M.cA, fig. 4), which continues forward 

 the cavity of the dorsal siphon. The outer reflected 

 lamella? of the external branchiae, on the other hand, 

 fuse with the body-wall just beneath the origin of the 

 mantle lobes. It is evident from a consideration of fig. 4 

 that the inner and outer lamellae of each external branchia 

 must enclose a cavity, which is also continuous with the 

 cavity of the dorsal siphon ; this is the dorsal suprabran- 

 chial chamber (M.c.5, fig. 4). The ventral suprabranchial 

 chamber is single and median. There are paired, right 

 and left, dorsal suprabranchial chambers. 



Since the inner lamellae of the internal branchiae do not 

 fuse with the body-wall but with each other, the ventral 

 suprabranchial chamber is incomplete anteriorly; the dorsal 

 chambers end blindly in front. Evidently water entering 

 the general mantle cavity by the ventral siphon or through 

 the ventral opening of the shell has three courses open to 

 it. It may pass forward between the labial palps into the 

 mouth and so reach the dorsal siphon per anum ; it may 

 enter the ventral suprabranchial chamber through the 

 space included between the posterior surface of the foot 

 and the anterior margin of the fused inner lamellae of the 



