170 OLIGOCHAETA 



distinguished by the elitellum, and by other appendages of the reproductive system, 

 which have not their counterpart in the Polychaeta ^. 



Only one group of Oligochaeta shows any particular resemblances to the Poly- 

 chaeta, and that is the group represented by the genus Aeolosoma. There is no 

 doubt that this Annelid does come very close in certain respects to some of the 

 lower Polychaeta. It resembles them in the following points: — 



(i) In the presence of ciliated pits on the side of the head, found also in 



Saccocirrus, Ctenodrilus, and Polygordius, &c. 

 («) Ciliation of the prostomium, characteristic of Ctenodrilus, Saccocirrus, 

 Protodrilus (general ciliation of body), &c. 



(3) Nervous system retains its primitive connexion with the skin. 



(4) Coelom divided up by irregular muscle-fibres, as in Saccocirrus. 



(5) Testicular products carried oif by very slightly metamorphosed nephridia; 



as in Saccocirrus, Capitellidae. 



(6) Setae often all acicular as in some Capitellidae. 



In other respects Aeolosoma is an undoubted Oligochaet. It has the distinctive 

 elitellum (feebly developed, liowever) and spermathecae of that group. On the whole 

 it appears to me necessary to place Aeolosoma near to the base of the Oligochaet 

 series. I am not, however, at all certain whether it would not be more in accord 

 with the anatomical facts at our disposal to place Aeolosoma among the Archian- 

 nelida. If then Aeolosoma stands at the base of the series, is it necessary to derive 

 the other Oligochaeta from this genus? Before attacking this problem it is requisite 

 "to bear in mind that Aeolosoma, in spite of its primitive characters, is in some 

 respects a degenerate worm ^. The absence of tjie ventral nerve-cord is undoubtedly 

 a mark of degeneration ; so, too, is the almost complete absence of intersegmental 

 septa ; there is at most a single septum present (see p. 178). The persistent connexion 

 of the brain with the epidermis may be also rather due to degeneration than to the 

 inheritance of an archaic character. The colourless blood, and lastly the small size 

 are facts which appear to me to point in the same direction. These facts still 

 further increase the difficulty of fixing the position of Aeolosoma in the Oligochaet 

 series. The two families to which it appears to come nearest are the Naidomorpha 

 and the Enchytraeidae. The resemblances, however, are not close to either. Both 



' EisiG considers the modification of the epidermis at the generative pores of the Capitellidae to be 

 comparable to the Oligochaet elitellum. I should rather compare it to the glandular modification which 

 surrounds the external aperture of the sperm-ducts in Lumbricus. 



' In this connexion I may be perhaps allowed to quote E. Meyek (Biol. Centralbl. x. p. sg6) to the 

 effect that asexual reproduction only occurs in undoubtedly degenerate forms, or, in those where sexual 

 products abound in the posterior, aa well as the anterior part of the body. 



