WOODWARD : INAUGURAL ADDRESS. 257 



structures, and that as increased facilities for respiration were required, 

 which of course implied increase of gill surface, it could only be 

 obtained in one of two ways — the flattening out into a leaf -like 

 expansion of the individual gill-filaments (aspidobranch), or their 

 prolongation (pectinibranch). The former modification is the one that 

 appears in all the ai'chaic members of the different Classes, and may be 

 recognized in the Polyplacophora, the rhipidoglossate Gastropoda, the 

 protobranchiate Pelecypoda, and the Cephalopoda. This structure, 

 nevertheless, is limited by the confined space of the pallial cavity, and 

 further increase of surface can only be gained by the corrugation of 

 the gill-filament. A beginning of such plication was observed by 

 Martin "Woodward in the case of Pleurotomaria,^ and doubtless it 

 exists in other aspidobranch s, but it is carried to a much greater 

 degree in the cephalopods, in which the gill-filaments exhibit two 

 series of plications crossing one another. 



In the Gastropoda some changes, Avhich would be startling if they 

 were not so familiar, take place. In the first instance the right 

 (morphologically left) ctenidium, as one ascends from lower to higher 

 meml3ers, atrophies and disappears. Martin Woodward shows that 

 this had begun in Plenrotomaria,''- but it is far more marked in 

 Scissurella. In the pcctinibranchs not only has one ctenidium 

 disappeared, but the other, except in the case of Valvafa, has become 

 attached by its whole length to the wall of the pallial cavity, and as 

 a consequence has parted with the whole of the row of filaments 

 on that side ; so that three-quarters of the gill potentiality of the 

 primitive mollusc is sacrificed. By way of partial compensation 

 the individual gill- filaments have been somewhat lengthened till 

 the familiar pectinibranch condition arises. In lanthina these gill- 

 filaments are, furthermore, plicated. 



It is in the Pelecypoda, however, that the most extraordinary 

 development of the gills takes place. The aspidobranch type of 

 Nueula and the rest of the protobranchs is abandoned in the others 

 for the pectinibranch type, and the lengthened filaments have to be 

 folded back on themselves to keep them within the limits of the shell. 

 The all-important monographs of Menegaux ^ and Pelseneer,* crowned 

 by the able memoir by Dr. Ride wood,* have made all malacologists 

 familiar with the successive stages whereby these gill-filaments 

 become united to form reticulate lamellae, and afterwards by plication 

 and further transverse unions fi-om lamella to lamella give rise to the 



Quart. Journ. Micro. Sci., x.s., vol. xliv, p. 224. 



Op. cit., p. 222. 



" Kecherches sur la circulation des Lamellibranches marius " : Thesis, -Ito, 



Besanyon, 1890. 

 " Contribution a I'etude des Lamellibranches " : Archives de Biol., torn, xi (1891), 



pp. 147-312. 

 " On the structure of the Gills of the Lamellibranchia " : Phil. Trans., ser. B, 



vol. cxcv (1903), pp. 147-284. The members of this Society had the advantage 



of a personal exposition of his work from Dr, Ridewood in March, 1904. 



-JUNE, 1907. 18 



