THE NEPHRIDIUM OF LUMBEIOUS. 313 



in certain species of Lumbricus and Allolobophora, where 

 he states that, as in C r i o d r i 1 u s, the whole nephridium is derived 

 from mesoblastic cells which grow outwards, and ultimately- 

 pass between the epiblast-cells to reach the exterior. 



A very interesting peculiarity in the " narrow tube " was 

 figured by me in 1886 for Microchaeta rappi (8), where I 

 show the '' smaller lumina " {I", pi. xvi bis, figs. 31, 33), as I 

 called this part of the tube, branching and anastomosing 

 so as to form a network around the two other regions of the 

 tube (/. /'.). 



No notice has been taken of this network in the recent dis- 

 cussions by Beddard and Spencer, as to the derivation of the 

 nephridia of earthworms from a " plectonephric " condition, 

 such as occurs in Acanthodrilus, Megascolides, and 

 many others ; and yet it seems to me not without interest. 



Quite recently a similar condition of things has been men- 

 tioned by Rosa (29) in the nephridium of Desmogaster, 

 one of the Moniligastridse ; Vejdovsky, too (32), figures the 

 branching of the lumen in the case ofChsetogaster. 



Again, in Brachydrilus, where there are two pairs of 

 nephridia in each segment, each having the same structure as 

 that of Lumbricus, a similar network is formed by the 

 "narrow tube^' (see PI. XXIV, fig. 26), and which is almost 

 identical with that of Perichaeta, &c. 



Now what meaning is to be attached to this network in 

 relation to the '' plectonephric " condition ? (1) Are the two 

 networks homoplastic, i. e. has each been developed indepen- 

 dently, or are they homogenetic ? and if the latter, (2) is the 

 " plectonephric '' condition derived from the " meganephric " 

 condition (of Lumbricus, Urobenus, Eudrilus, &c.) by the 

 branching of the tubule ; or (3) is it a remnant of a plec- 

 tonephric condition still retained by the meganephric genera ? 



(1) The suggestion that the network in Brachydrilus and 

 Microchseta (and no doubt this network will be found else- 

 where) is homoplastic with that of Acanthodrilus might 

 be held ; but if the tubule can branch independently of any 

 ancestral network, there is no reason to regard the plecto- 



