ZooL— Vol. II.] EISEN—OLIGOCH.^TA. II3 



Subgenus Ocnerodrilus Eisen. 



Structure of Sperm-sacs. — The structure of the sperm- 

 sacs in Ocnerodrilus (sens, str.) is of considerable interest. 

 In the author's description of the type-species of the genus 

 no mention is made of the sperm-sacs, nor indeed were they 

 discovered at that time, an oversight explained by the fact 

 that in O. occidentalis the sperm-sacs do not project from 

 the septa, but are mere small caps at the end of the testes. 

 They are so small, even in fully adult specimens, that it is 

 only in sections that they can be studied. The structure 

 will be described under the species O. occidentalis, but I 

 will here mention that it is identical with that of the sperm- 

 sacs of the genus Pachydrilus which I have lately had occa- 

 sion to investigate (fig. 132.) 



Although always described as being void of sperm-sacs, 

 Pachydrilus really possesses them. In this genus the sperm- 

 sacs are also mere caps of the testes, the latter being multi- 

 lobed, each lobe carrying its own sperm-sac. This charac- 

 ter in Ocnerodrilus occidentalis — the lowest species in the 

 genus — may be an ancestral character retained from its 

 Pachydrilide ancestors. It is, however, hazardous to place 

 too much importance on the structure of the sperm-sacs 

 in a phylogenetic study of a genus, as even in closely 

 related species they differ widely. Thus in O. Eisem 

 the sperm-sacs are not racemose, while in the majority of 

 other species these sacs are racemose and traversed by 

 trabeculse. In this instance, however, I think that the 

 small cap-shaped sperm-sacs of O. occidentalis and of Pachy- 

 drilus point to a close phylogenetic relationship. Another 

 Enchytr^ide character is found in the diverticles of the 

 intestine, a character also seen in Eudrilid^, which family 

 Michaelsen (21, p. 18) claims has descended from Ocnero- 

 drilide ancestors. 



Ocnerodrilus occidentalis Eisen. 



Plate XII, Fig. 123. 



Definition. — Size 15 mm. Setae ab are present in somite XVII and situated 

 ventrally to the male pore. No spermathecae. No racemose sperm-sacs, but 

 small sperm-sacs capping the testes. Two pairs of testes in X and XI. 



