44 



inner branchia (Br. I.), which is the larger of the two, is 

 narrowest behind where it fuses with the corresponding 

 structure of the right side, and increases gradually in 

 width towards its anterior margin which is attached to 

 the body-wall, and is slightly overlapped by the labial 

 palps. The outer branchia (Br.E.) is narrower than the 

 inner, and is broadest at about midway between its two 

 extremities. The two branchiae of each side fuse together 

 behind the viscero-pedal mass at their posterior extrem- 

 ities. 



Each branchia consists of a large number of hollow 

 filaments connected together at intervals. The direction 

 of these filaments is at right angles to the ctenidial base. 

 At the free ventral margin, where each branchia is folded 

 on itself forming the two lamellae, the filaments of the 

 inner pass over with some modification in structure into 

 those of the outer lamella. The whole ctenidium is cut 

 obliquely to the base in fig. 4, and each of the two 

 branchiae is seen to be made up of a direct and a reflected 

 lamella. For the greater part, these two lamellae are 

 intimately bound together : the precise nature of the 

 connections is not, however, shown in the figure. In the 

 inner branchia the external lamella (Br.I.l) which is 

 inserted into the base of the ctenidium, is the direct one, 

 and the internal lamella (Br. 1. 2) is the reflected one. 

 Conversely in the outer branchia, the internal lamella 

 (Br.E.l) is the direct one, and the external lamella (Br. 

 E.2) is the reflected one. This difference in the disposition 

 of the two branchia will be noticed, the inner lamellae of 

 right and left internal branchiae fuse together in the middle 

 line, and here the lamellae (Br. 1. 3) are simply mem- 

 branous plates, showing no sign of filamentar structure. 

 These membranous portions of the inner lamellae are 

 continuous at the posterior extremities of the ctenidia 



