472 Illinois State Laboratory of Natural History. 



characters should be regarded as generic and which as of 

 specific value merely. It is possible that an unusual varia- 

 bility in the reproductive organs exists in worms of this group, 

 and that in a similar length of time and under similar differ- 

 ences of conditions there might be a greater amount of 

 divergence in the character of these organs than there would 

 be in worms of other groups, as, for example, the earthworms, 

 and hence that differences which among the latter would be 

 generic ought perhaps to be considered as only specific when 

 found among the Lumbriculida . On the other hand, it may 

 be that species which now form the only members of the 

 genera to which they belong, will after a time, by the discov- 

 ery of other species, become types of genera which will each 

 contain two or more species more closely related to each 

 other than are the species now known, and thus the estab- 

 lishing of so many genera may be justified. In the case of 

 the three genera last referred to, when we consider that a 

 distance of a thousand miles intervenes between Florida and 

 Illinois and two thousand miles between Illinois and Califor- 

 nia, and that nothing whatever is known of the Lumbriculida 

 of the intermediate regions, it seems reasonable to suppose 

 that subsequent collections from the intervening territory 

 may bring to light other species more nearly related to one 

 or more of them than they are to each other. Until future 

 collections and study shall disclose the facts, it seems best 

 to the writer not to include in one genus species.which differ 

 so widely as do Eclipidrilus frigidus, Mesoporodrilus asym- 

 metricus, and Premnodrilus palustris. 



A comparison of these three species will be facilitated by 

 the use of the following table, which includes characters that 

 are of more or less importance from the systematic stand- 

 point. 



