44 R. HOLMAN PECKi 



tion regarded the Lamellibranch gills as a pair of vascular 

 plates, placed on either side of the foot, and capable in some, 

 if not all genera, of being broken up into a series of delicate 

 adherent filaments ; further anatomical investigation of them 

 was not attempted. It was not until the year 1854 that Dr. 

 Thomas Williams,' of Swansea, published his elaborate 

 paper on the respiratory organs of Invertebrata, in which, 

 with the same fatality which attended his work on another 

 subject — namely, on the " segmental organ," the name which 

 he gave to the structure still recognised by that designation 

 — we find the greatest inaccuracy and fantasy inextricably 

 interwoven with sound observation and far-reaching induc- 

 tion. Williams recognised the filamentar character of the 

 Lamellibranch gill, but it was the eminent French zoologist 

 Henri de Lacaze Duthiers who first showed the mode of 

 development of this organ, and assigned to its parts a nomen- 

 clature which still holds its value, in his account of the 

 development of the edible mussel published in 1856. ~ 



In the same year the structure of the gill of Anodon (pre- 

 viously treated of by Rengarten) was investigated by Langer,^ 

 who made use of injections, and came to the conclusion — an 

 erroneous one — that the large vessels present in the particular 

 modification of gill presented by this genus were con- 

 nected by a complete system of branchial capillaries. 



No contribution to our knowledge of the structure of the 

 Lamellibranch gill appeared for nearly twenty years. Whilst 

 the work of Lacaze Duthiers and of Langer were confined to 

 two isolated and very distinct types, that of Williams, in 

 which a true comparative method extending to several genera 

 was adopted, failed to command any confidence. There 

 seemed, therefore, to be an exceedingly promising field of 

 inquiry open to any one who, making use of modern methods 

 of microscopical work, should examine the gills of a series of 

 Lamellibranch genera with the object of determining the 

 general plan of their structure and the particular modifications 

 thereof, exhibited in particular cases. 



When I had already applied myself to this investigation, 

 the excellent memoir of Dr. Carl Posner made its appear- 

 ance. . Posner has the merit of having first clearly demon- 

 strated by means of transverse sections that Langer was 

 misled in the conclusion he formed from the evidence of 

 injection ; viz., that there is a capillary system in the 

 Lamellibranch gill. Posner shows — what, indeed, had been 



1 ' Annals and Magaz. Nat. History,' 1854, vol. xiv. 



- ' Anuales des Sciences Naturelles Zoologie,' ser. iv, tome v, 1856, p. 1. 



3 'Denkschr. d. Wiener Akad.,' CI. viii to xii, 1856. 



