1902.] Structure of the Gills of the Lamellibranchia. 



499 



" On the Structure of the Gills of the Lamellibranchia." By Dr. 

 W. G-. Eidewood. Communicated by E. Eay Lankester, 

 M.A., E.E.S. Eeceivecl May 3 — Eead May 29, 1902. 



(Abstract.) 



This paper records the results of an investigation undertaken at 

 the instance of Professor E. Eay Lankester, F.E.S., and carried on 

 under his supervision. 215 species of Lamellibranchia, belonging to 

 118 genera, were examined. The results are far-reaching in that they 

 demonstrate that, except in a very broad way, the minute structure 

 of the gill, like the grosser structure, cannot be taken as an indication 

 of genetic affinity. 



Three main types of gill structure can, however, be recognised, 

 representing apparently three successive grades of complexity. The 

 first type, found in the families Nuculidae and Solenomyidae, is dis- 

 tinguished by the mutual freedom of the gill leaflets or platelets into 

 which the embryonic gill papilla expand. These two families were 

 united by Pelseneer under the heading " Protobranchia," which title 

 it is proposed to retain. 



In the other two types the embryonic papilla? elongate into fila- 

 ments, which are held in juxtaposition by interlocking cilia disposed 

 in circular patches on the anterior and posterior sides of the filaments, 

 or by regularly arranged horizontal bars of cellular tissue, which put 

 the adjacent filaments into organic connection the one with the other, 

 and convert the interfilamentar spaces into rows of f enestrse. The 

 former type it is proposed to term eleutherorhabclic, and the latter 

 synaptorhabdic. 



Although Pelseneer so far recognised the distinctness of these two 

 last conditions as to make them the prime features of his orders Fili- 

 branchia and Eulamellibranchia, he yet instituted an intermediate 

 heterogeneous order — the Pseudolamellibranchia — to include the 

 Ostreidae, Aviculida3 and Pectinidae. Evidence is produced in the 

 present communication to show that the order Pseudolamellibranchia 

 is ill-founded, and based largely on a misconception of the relative 

 value of the flatness or plication of the gill lamellae, and the presence 

 or absence of large filaments (principal filaments) at regular intervals 

 along the lamellae. 



In the family Solenidae particularly it is shown that different species 

 and sub-genera of the same genus may have their gill lamellae flat or 

 plicate, and the filaments all of one kind or with enlarged principal 

 filaments at intervals. 



Avicula argentea has proved to be a form of supreme interest, in 

 that it demonstrates how, in some cases at least, the synaptorhabdic 



