380 



PROF. W. J. DAKIN ON THE 



for its constant presence and the ducts opening into it. Following 

 the vesicle the nephridium is divisible into two marked sections, 

 a tubvdar portion and a terminal chamber. The tube describes a 

 rough circle and then turns abruptly on itself, so that its termin- 

 ation is close to the vesicle referred to above (see PI. IV. fig. 11). 



The terminal chamber (PI. lY. fig. 1 1, Cob.) is now well known, 

 although missed by the first investigators, who believed that the 

 nephridia opened into the lateral compartment of the body. 

 The chamber lies partly above and partly posterior to the col- 

 lecting vesicle referred to above. 



The first portion of the nephridial tube internal to the 

 collecting vesicle is lined by a very delicate and characteristic 

 epithelium of large fiat cells. As a consequence of the size of 

 the cells relative to the diameter of the duct it is possible to have 

 transverse sections with only two or three nuclei showing (see 

 PI. IV. fig. 11). The greater part of the nephridium between the 

 terminal chamber and the collecting vesicle is built in this way. 

 The section, however, which actually opens into the terminal 

 chamber is very diflferent. The wall of the nephridium becomes 

 thicker and far less delicate and is formed of a compact layer of 

 columnar epithelial cells (see PI. IV. fig. 11, Cil.R.). These 

 cells are so crowded and the nuclei stain so distinctly that most 

 previous workers have noted the peculiarity. In fact this change 

 in the character of the cells has been taken as indicating the 

 passage from the ectodermal part of the nephridium to the 

 mesodermal portion (see Glen, Q. J. M. »S. 1918, vol. Ixiii.). 



Now it is the cells of tliis section of the nephridium which bear 

 the cilia (PLs. III.-IV. figs. 11 & 12). These are so long that 

 after projecting fi-om the cell they extend along the lumen of the 

 duct for a relatively considerable distance. It is extraordinary 

 that in many figures showing the structure of the nephridia of 

 Peripiitus details of the histology are given at a high magnili cation, 

 yet no indication of cilia is presented. 



Bundles of long cilia are very characteristic of renal cells, 

 although at the same time they are in the highest degree peculiai- 

 for the arthropoda. The Annelid i-esemblances of Perijmtus 

 are certainly heightened as a result of the examination of well- 

 preserved sections through the ciliated ducts of these nephridia*. 



* Since writing the above I have been ena])led to examine a copy of Gatlron's 

 famous paper (8) on the Anatomy and HistoU>gy of Peripatus, in which the lirst 

 mention of the presence of cilia in this animal — in the lieceptacula semijiis — was 

 made. Looking througli his description of the nephridia 1 found to my surprise 

 the following lines referring to the region where the duct opens into the coelomic 

 vesicle. It must be remembered that the vesicle was unknown at the time, and its 

 remains were supposed to be a funnel-like nephrostome opening into the body-cavity. 

 " Er besitzt wie der Trichter selbst, kleinzelliges, im Lehen ivahrsclieinlich toim- 

 perndes JSpithel . . . ." (latfron never indicates that he found cilia here nor are 

 any shown in his illustrations of this region. We must conclude that the remark 

 was merely a conjecture, probably suggested by the apparent resemblance to the 

 open ne])hrostome of an annelid. It is curious, however, that his successors who 

 have studied the nephtidia have not commented on this. Either tlie cilia are only 

 found in the West Australian Feripatoides or else my preparations must be par- 

 ticularly favourable ones. 



