10 GUSTAF EISEN, ON THE ARCTIC OLIGOCILETA. 



Fam. 3. ENCHYTRMD^. 

 Enchytrseus (Henle). 



Syn.: 1861. Pachydrilus Claparede, Recherches anat. sur les Annelides etc. . 

 » 1862. Enchytrceus Claparede, Recherch. anat. sur les Oligochétes. 



It is evident, as Ratzel and others have shown, that the colour of the blood is 

 hardly a character of sufficient value to permit us to found on it the distinction of 

 genera. It may also be remembered that one of Claparedes species, Pachydrilus lacteus 

 has white blood, and that not all redblooded species live in water. I therefore here 

 unite the two genera Enchytrceus and Pachydrilus under the old genus Enchytrceus 

 (Henle). 



But as this genus seems to be very rich in species a subdivision is desirable, if 

 founded on more constant and more general characteristics. The shape and differentia- 

 tion of the supra-oesophagial ganglion, and the different stages of development of the 

 spermatozoa, when they enter the efferent duct, furnish characters of importance. In 

 some species the spermatozoa, when they enter the efferent duct, are encysted, in 

 other species they are more highly developed, the cysts being broken and the sperma- 

 tozoa free when entering the efferent duct. 



In some species the posterior margin of the supra-cesophagial ganglion is doubly 

 emarginated, in others on the contrary convex, and in a few species nearly straight, 

 and I have in no instance found the form of the above ganglion varying in any higher 

 degree in the same species. I must therefore consider characteristics taken from its 

 form tolerably constant, and, as the}' are common to a greater nuinber of species, of 

 generical value. Accordingly I propose to divide the old genus Enchytrceus s. lat. into 

 the following three subgenera. 



Subgenus I. MESENCHYTR^EUS n. 



The spermatozoa, as long as they remain in the perivisceral cavity of the body 

 or in the vesicle of the efferent duct, are not free but encysted or congregated into 

 small globules surrounded by a membrane. 



The swpra-oesofhagial ganglion is deeply divided in front, but straight behind or 

 nearly so, the emargination being very inconsiderable. 



The tube of the efferent duct is unusually short and broad, never rnore than six 

 or eight times longer than the vesicle of the said organ. 



Subgenus II. ARCHIENCHYTR^US n. 



The spermatozoa, when they enter the vesicle of the efferent duct, are free. 



The posterior margin of the sapra-oesophagial ganglion is always deeply emargi- 

 nated. 



The tube of the efferent duct is long and narrow and wound up like a clew 

 of thread. . 



