692 OLIGOCHAETA 



The very fact that there are species which cannot, in the present state of our 

 knowledge, be included in one or other of these groups, leads me to leave it for the 

 present. As to the two genera Lumbricus and Allolobophora, ROSA distinguishes 

 them by a better character than that adopted by EISEN and by many who have 

 followed him. In the genus Lumbricus the prostomium always completely divides 

 the buccal segment, and in addition there is invariably a single median sperm-reservoir 

 which encloses the testes and funnels ; in a few species of Allolobophora there is, either 

 occasionally or always, a similar complete division of the buccal segment, but there 

 is never a single median sperm-reservoir though there may be a number of pairs of 

 such reservoirs. This definition is accepted in the present work rather as a convenient 

 way of dividing up the numerous species than in the belief that it is a character of 

 first-rate importance ; if other families of earthworms were divided by characters as 

 slender the number of genera would be immense. 



Another genus has been separated by EISEN and thus denned : ' Setae ubique 

 binae approximatae : Tub. ventr. in segm. 12 pone segm. buccale.' 



This genus Allurus has been until quite recently accepted by all writers on the 

 group, except VAILLANT (6), who only allows it as a subgenus. Recently MICHAELSEN 

 (8, P- 9) has extended EISEN'S definition so as to include a few other forms described 

 by himself and ROSA. 



MICHAELSEN was disinclined to allow the position of the male pore (practically 

 the only distinguishing mark mentioned by EISEN) to have generic value, pointing 

 out that among the Enchytraeidae it is impossible to separate for a similar reason 

 Buchholzia apjiendiculata and Marionia sphagnetorum from the other members 

 of their respective genera. In these instances, however, it is the entire genital 

 apparatus that has been shifted forwards ; not, as in the case of Allunis, the male 

 aperture only. The genus Tetragonurus of EISEN, in which the male pores are still 

 a segment in front of the normal (for Allolobophora), is also included in the genus, 

 which is chiefly characterized by MICHAELSEN as having a more anteriorly situated 

 clitellum (not extending further back than the twenty-seventh segment) arid strictly 

 paired setae. If this be allowed it will, it might be urged, become necessary to 

 separate Allolobophora molleri as a distinct genus. In this worm the clitellum is 

 as unusually posterior in position as it is anterior in Allurus; besides A. niolleri 

 has the spermathecae in segments viii, ix, a very unusual indeed unique position 

 for these organs, when there are only two pairs. In my opinion, therefore, we 

 need a better character than that afforded by the position of the clitellum, which in 

 any case varies greatly in admitted members of the genus Allolobophora (sensu 

 MICHAELSENI). The extremes are, apart from A. molleri, A. mucosa xxv-xxxii, and 



