160 OLIGOCHAETA 



a terrestrial form in habit, and bears no superficial resemblances to the aquatic 

 Oligochaeta. All these facts seem to indicate a gulf between the Microdrili and the 

 Megadrili (as I would call the earthworms, following BENHAM'S nomenclature). 



Aeolosoma, as it appears to me, must undoubtedly form a group by itself. In 

 many respects it is barely an Oligochaet. I refer particularly to the absence of 

 specialized sperm-ducts, whose proper function is taken by at most slightly-modified 

 nephridia ; this state of affairs recalls the Capitellidae and the Saccocirridae, in both 

 of which families the generative ducts are apparently slightly- modified nephridia. 

 The clitellum is so far unlike that organ in other Oligochaeta (except certain 

 Enchytraeidae) that it is only developed on the ventral surface of the body ; where 

 the clitellum is deficient in other Oligochaeta, it is precisely here (on the ventral 

 surface) that the deficiency occurs. The only characteristically Oligochaetous feature 

 in the generative system seems to be the spermathecae. It is also the only 

 Oligochaet in which there are a pair of ciliated pits at the sides of the head. 

 These occur, as is well known, in some of the ' Flatworms ' and Nemertines ; they 

 are also found in a few Polychaeta, for instance in Saccocirrus. It may be that 

 a ciliated organ which I discovered (51) in the embryo of Octochaetus multiporus is 

 referable to the same category ; this organ, however, was a prominence, not a 

 depression, of the integument ; it had a bunch of particularly long cilia upon it. 



The ciliation of the prostomium is another absolutely distinctive character 

 of Aeolosoma ; this, again, while not occurring in any Oligochaet, is met with in 

 Saccocirrus and Ctenodrilus, two genera of perhaps doubtful position, but more 

 nearly related to the Polychaeta than to the Oligochaeta. 



As to the absence of the ventral nerve-cord, this may not be really so entirely 

 absent as would appear from most of the published descriptions of the anatomy of 

 the genus. To begin with, VEJDOVSKY found the rudiments of a cord in the shape 

 of more or less scattered cells; it is also possible that the ventral nerve-cord may 

 be in connexion with the epidermis, as the cerebral ganglia certainly are. If the 

 ventral nerve-chain be really absent, it might be put down to degeneration ; I should 

 certainly be disposed to credit a process of degeneration with the almost complete 

 absence of internal segmentation. A final peculiarity, very distinctive of the genus, is 

 its habit of temporarily encysting itself in a chitinous cyst. 



These characters, taken together, seem to me to necessitate the placing of 

 Aeolosoma in a group by itself, a group equivalent to either of the Microdrili and 

 Megadrili ; it may even be that this separation is not wide enough ; there would be 

 no great violence done to the systematic position of the genus if the group to which 

 I refer it were regarded as the equivalent of both the Microdrili and the Megadrili. 



