No. 3.] THE DISCODRILID NEPHRIDIUM. 329 



ready passage, the large heart with its prominent loop in the 

 first and second somites, the enlargement of the alimentary 

 canal in the fourth somite, the very narrow septa, and the 

 active muscular movements of the worms. These conditions 

 would evidently render a spreading longitudinal arrangement 

 of the tubules an advantageous one, inasmuch as it would per- 

 mit a more ready adjustment of the organs to the animal's 

 movements. Sections of embryos indicate the presence of 

 provisional nephridia in certain anterior somites, as has been 

 shown to be the case in certain Oligochaeta and Hirudinea. 



Individual nephridia are highly developed and conspicuous, 

 the principal portion of each one consisting, as was originally 

 described by Henle (21), of four distinct tubules arranged in two 

 long loops, and passing into an opaque granular mass, through 

 which previous writers have failed to trace them. In the more 

 transparent species, and especially in the colorless-blooded 

 Branchiobdella instabilia and pulcherrima, these granular masses 

 may be seen with the naked eye as conspicuous, dark, and 

 opaque spots, lying within the coelom by the side of the 

 alimentary canal, one each in the second and third somites, 

 and a pair symmetrically placed in the eighth somite.^ 



Connected with the masses are the nephridial funnels and 

 efferent ducts, as well as the tubule loops, so that an under- 

 standing of their structure is necessary to a knowledge of the 

 relations of the latter. 



As indicated above, and this is true for all known species, the 

 anterior nephridia are not symmetrical, the opaque mass of one 

 lying anterior to that of the other. This has been observed by 

 all writers on B. parasita, but it is noticeable in their descrip- 

 tions and figures that they fail to agree as to whether the right 

 or left one is the more anterior. Lemoine finally states that 

 the species is variable, some individuals having the right, some 

 the left nephridium in advance. This is the case in the three 

 American species (Bdellodrilus illuminatus, B. philadelphicus, 



1 The European writers have variously enumerated the somites, some counting 

 the "head" as one, and each two of the body rings as one; others each ring of 

 head and body as a somite. Inasmuch as the constitution of the " head " is still a 

 matter of disagreement, I enumerate only post-cephalic somites, each consisting 

 of two annuli. 



