52 CAUDAL GILLS OF ZYGOPTERID LARV^, 



as yet stout and unbranched; or else the length of the threads is 

 considerably greater, but the resolution ("Auflosung") of the 



tufts is not yet complete, not yet filling up the alveoli The 



individual fibrils of these tufts seem to be very regular in thick- 

 ness and of extreme fineness: I cannot discover any other 

 formation in them with my aids: the individual pale hsematoxy- 

 lin colouring for a single cell is quite uniform. It has already 

 been said that only one part of the alveoli contains these tufts: 

 the division is by no means regular; but, in reality, both in the 

 ease of the median as well as the lateral gills, there is a central 

 zone of empty alveoli surrounded by a peripheral zone of aheoli 

 filled with fibrils: towards the distal end of the organ the number 

 of empty alveoli decreases more and more, so that at the spot 

 where the two large blood-vessels unite only very few are left 



empty The alveoli containing blood lie predominantly on 



the periphery of the organ near the hypodermis. The alveoli 

 containing fibril-tufts contain these only, and are otherwise 

 empty. At first sight the appearance in a few places of sus- 

 pended fragments of fibril- tufts in the middle of tJie lumen of the 

 big blood-vessel seems to be very striking: but in at least one place 

 (the series is not quite without a break) it is possible to refer 

 this phenomenon back to a groti]) of Jibril-tufts tvhich is here 

 clinging to the wall of the big blood-vessel, in all other respects as 

 on the wall of the alveolus (" wie sonst der Alveolenwand "j. 



I have placed in italics two statements near the end of this 

 quotation, because it seems to me that they, taken in conjunc- 

 tion with Ris' excellent figures, three of which are reproduced 

 on Plate iv., figs. 3 1-33, give us the obvious solution of this ex- 

 traordinary formation. Having carefully studied numerous 

 series made from carefully fixed and prepared gills of the saccoid 

 type, and one of which [Diphlebia) is closely enough allied to 

 Pseudophcea to admit of no doubt as to the unity of structure in 

 the gills of the two genera, I have no hesitation in saying that 

 the formation so carefully described by Ris is entirely absent 

 from all sections which I have examined I am bound to con- 

 clude, therefore, — and I must call as evidence for my conclusion 

 both the two statements in italics and Ris' own figures— that 



