THE NEPHiUDIA OF LEECHES. 551 



coverer of the iutra-celliilar nature of any of tlie lumina of 

 the gland. That some of the cells had within their sub- 

 stance an arborescent extension of a system of ducts was 

 pointed out to me by Lankester in 1879; and that the lumen 

 of what I had called the " central duct/' and which I 

 imagined to be intercellular when I published my first paper, 

 was also intra-cellular was demonstrated to me by Lang, at 

 Naples, in 1882. 



I maintain that M. Bolsius' diagrams of the nephridium of 

 Hirudo (1, fig. 1; and 4, fig. 17) are fundamentally wrong, 

 and that my diagram, which is here reproduced, together 

 with the original description, is in the main correct.^ 



In the first place, has the duct (" canal coUecteur" — Bolsius) 

 a complicated course such as I ascribe to it, or is M. Bolsius 

 right when he says — 



" Le cours du canal collecteur, selon nous, est tout ce 

 qu'il y a de plus simple. Le canal, tortueux il est vrai, 

 s'avance toujours vers I'orifice inferieur. Durant ce trajet, il 

 est accompagne exterieurement d'une ou de plusieurs assises 

 de cellules a canaux lateraux. Jamais le canal ne revient sur 

 ses pas a travers un mem'e massif de ces cellules"? 



This question of the course of the duct is more or less 

 intimately bound up with that as to the division of the gland 

 into lobes. 



I continue to use here the terms "duct'* and " ductule," 

 as they are convenient terms which fall in with my theory of 

 the structure of the gland. I use " duct" for the lumen and 

 its walls when those walls are simple drain-pipe cells, and 

 " ductules" for lumina which branch in a cell : in this latter 

 case it is obviously inappropriate, especially when the branch- 

 ing is extensive, to include the walls of the lumen — i. e. the 

 substance of the cell — under the term. This distinction 

 between the terms is convenient and intelligible, but not, per- 

 haps, strictly accurate, as I now find lumina in some of these 



' It will be noted that in Planclie I of M. Bolsius' memoir (4), figs. 1—4 

 are Schultze's, and figs. 5—16 my own ; figs. 17—23 only are by M. Bolsius. 

 M. Bolsius invites comparison between our diagrams and his own. 



