328 MOORE. [Vol. XIII. 



with which it is in substantial agreement. The sixth and latest 

 paper (28) mentions a few novel facts observed in Bdellodrilus 

 illuminatus, but is very incomplete. A new investigation being 

 therefore desirable, I am led to present the results of some 

 recent studies. 



Odier and Henle, writing respectively in 1823 and 1835, 

 noticed that the nephridia were reduced in number to two pairs, 

 a fact which has since been repeatedly verified for Branchiob- 

 della parasita, and in the American discodrilids Bdellodrilus 

 illuminatus, B. philadelphicus, B. manus, Branchiobdella insta- 

 bilia, B. pulcherrima, Pterodrilus distichus, and P. alcicornus, as 

 well as in several undescribed species. Consequently two pairs 

 of definitive nephridia may safely be stated as characteristic of 

 the family. This is a remarkable peculiarity in an annelid, in 

 which group the metameric repetition of the nephridia is so 

 general as to have gained for them the name of segmental 

 organs, first proposed by Williams (37). Although the neph- 

 ridia are not infrequently absent from a few of the anterior 

 segments, and in Uncinais litoralis (12), and perhaps a few 

 other naids, entirely wanting, no case exactly similar to the 

 discodrilids is known. When, however, we consider the cepha- 

 lization of the anterior somites, with the nearly complete 

 obliteration of their coelom by encroaching muscle fibres and 

 glands, the parallel concentration to form the sucker and its 

 support at the posterior end, the shortening of the discodrilid 

 body until it represents only the anterior (genital and pre- 

 genital) segments, plus the anal and preanal segment of, for 

 example, an enchytraeid; and when we consider the further 

 fact that nephridia are absent from the genital segments of the 

 latter, the important factors in an explanation of the discodrilid 

 condition are in our possession. 



This does not, however, explain why with four possible neph- 

 ridia-bearing somites anterior to the genital region, only one 

 pair of pre-genital excretory organs should persist in the adult 

 worm. In this connection the following associated facts may 

 be considered as possible, but not necessarily complete expla- 

 nations, viz., the large size of the uninary calculi and granules, 

 necessitating a tube of considerable calibre to permit their 



