510 OLIGOCHAETA 



The nephridia commence in the fifth segment ; in and after the tenth they are 

 invested by a layer of clear peritoneal cells. 



The ventral surface of the eighth segment, and the anterior half of the ninth, are 

 modified, the cells of the epidermis being tall and glandular ; this layer of cells is not 

 interrupted in the middle ventral line. 



Genus OCNEBODRILTJS, EISEN. 



Syn. Pygmaeodrilus, MiCHAELSEN. 



DEFINITION. Small, often aquatic, worms. Setae paired, sometimes absent upon 

 segment XVII, upon which sperm-ducts open. Spermiducal glands lined with 

 a single layer of cells. Gizzard absent ; calciferous glands paired in IX. 

 Nephridia paired. Sperm-ducts open in common with spermiducal glands. 



For a long time this genus was only known by a single species, Ocnerodrilus 

 occidentalis, described some years ago by EISEN (9); in 1891 I described a second 

 species, 0. eiseni (20) ; more recently still (1), EISEX has published a paper containing 

 an account of eight new forms, while MICHAELSEN and I have described several 

 African species, formerly relegated to the genus Pygmaeodrilus. We are, therefore, 

 in possession of a considerable amount of information about this genus. The species 

 of the genus are all of small size ; they are partly aquatic, but usually terrestrial 

 in habit. The genus is easily separable from Gordiodrilus, on account of the paired 

 diverticula which have a different structure from those of the latter; another 

 important difference is in the fact that the sperm-duct pores are always on the 

 seventeenth segment in the present genus, and usually on the eighteenth segment 

 in the genus Gordiodrilus, and that the sperm-ducts open by the same pore as the 

 spermiducal glands ; the calciferous glands are, however, paired in G. robustus ; 

 another intermediate form is 0. limicola, where there are also two pairs of 

 spermiducal glands on exactly the same segments as those on which they occur 

 in G. robustus; but in 0. limicola the sperm-ducts open into the first pair of these 

 glands, and the similarity to Gordiodrilus is thus reduced to the mere doubling of the 

 spermiducal glands, which is of no more importance than the fact of the reduction 

 of the spermiducal glands to a single pair in G. ditheca. 



The ten species of Ocnerodrilus differ in four principal characters, besides others of 

 less importance. 



There are considerable variations in the condition of the ventral pairs of setae in 



