196 SCIENCE PROGRESS. 



In addition to questions of relationship to other neigh- 

 bouring groups, recent investigation has brought to light 

 facts of interest in the anatomy of the Oligochaeta which 

 bear upon the mutual affinities of the families and genera 

 into which the order is divided. In this direction the main 

 discoveries of importance relate to the excretory system. In 

 all the simple aquatic genera each segment of the body 

 contains a single pair of nephridia ; as a rule these organs 

 are wanting in the anterior segments, and Professor Bourne 

 was unable to find any nephridia at all in Uncinais littoralis. 

 The absence of nephridia in the anterior segments of the 

 body, however, also characterises certain earthworms. It 

 was originally described by Perrier in Pontodrilus, and all 

 the species of this genus (6) are in the same condition. 

 More recently Benham and Risen have shown that the 

 same state of affairs characterises the aquatic Geoscolecid 

 Sparganophilus. A distinction therefore between the 

 Limicolae and Terricolse of Claparede quite breaks down. 

 That these genera have no gizzard or calciferous glands 

 (or at most the rudiments of a gizzard) is evidence of general 

 degradation, which may have something to do with their 

 aquatic or semiaquatic existence. It suggests too that 

 the simplification in structure of the Limicolae of Claparede 

 may be rather due to degeneration than to the retention of 

 primitive characters. 



Among the earthworms, however, the single pair of 

 nephridia to each segment is far from being the rule. In 

 a large number of genera the nephridia are multiple. Two 

 pairs in each segment exist in BracJiydriliis ; three pairs in 

 Trinephrus; and Eisen has lately shown that in certain 

 North American Benhamias there may be three or four 

 distinct and separate pairs each with its own internal funnel 

 and external pore. The complexity of the excretory 

 system culminates in Perichceta where a single segment may 

 be furnished with probably at least one hundred external 

 nephridiopores. It is, however, a question whether in 

 this latter case there is really an intercommunication be- 

 tween the several nephridia of each segment, and between 

 those of adjacent segments as has been alleged by Spencer 



