70 OKHAMANDAL MARINE ZOOLOGY REPORT 



two lamella, the proximal or direct (fig. 12, D.hrl), where the course of the filaments 

 is directed outwards, and the distal or reflected {li.br.l), where they are folded 

 inwards. 



The reflected lamella of the external hemi-ctenidium is rather narrower than the 

 direct, due to the fact that this lamella, close to its distal margin, is again folded 

 outwards upon itself to constitute a very small fifth lamella, a peculiarity very rare and 

 seen only in the closely-related genus Anomia. In depth this accessory lamella (figs. 4 

 and 12, Br.acc.) is equal to the diff"erence between the depth of the direct and 

 reflected lamellae. As in Anomia, tlie outer reflected lamellae are free ; there is no 

 trace of concrescence with the mantle either by organic fusion or by ciliary junction. 

 Along their whole length the distal edges of the reflected lamellae of the inner branchial 

 plates unite along the middle line in weak organic union, reinforced by a short ciliary 

 concrescence on the ventral side of the organic fusion (fig. 12, Ct.j.), thus combining 

 two distinct forms of concrescence and suggesting that the original form of union was 

 entirely by ciliary concrescence as seen to persist — save for the merest thread of organic 

 fusion — in the median branchial junction of the pearl oyster. The union along this 

 median branchial line is continuous, fusing the terminals of all the filaments from the 

 inner reflected lamella of each ctenidium into one continuous band of tissue and 

 serving as a median support to the inner branchiae. 



The general arrangement of the filaments is of the Filibranch type. Except for 

 the median branchial fusion, the filaments have no organic union between themselves ; 

 they lie perfectly free along their whole course save for a series of lateral intertila- 

 mental ciliated junctions just within the crest of each branchia where the direct 

 filament passes into the reflected. At this place each filament presents an elongated 

 swelling on each lateral face in such a way that the approximation of the two swollen 

 opposing faces of the adjoining filaments reduces the space between them very 

 considerably, a space bridged over by stout interlocking cilia borne upon each lateral 

 swelling, thus constituting a ciliated disc (fig. 33). This development of ciliated discs 

 along the free edge of each branchia is noteworthy and marks an advance in 

 specialisation beyond what we find in Anomia, where true ciliated discs are wanting. 

 As a consequence the branchial framework in Placuna has greater rigidity than in 

 Anomia, the filaments less easily dissociated. 



The vascular axis of each ctenidium, as before noted, is suspended from the mantle 

 by a mesentery (figs. 4, 12, Br.ms.), extending from the base of the palps to the 

 postero-vcntral curvature of the adductor muscle. 



The narrowed anterior apex of each gill is attached between the divergent ventral 

 edges of the palps of its respective side. The extreme apex of the gill lies in the 

 palpal gutter that marks the line of junction of the outer and inner palps, so that food 

 particles passed to the anterior extremity of a ctenidium are caught by the palpar cilia 

 and propelled upwards iilong tliis palpar gutter to the mouth. 



