CLASSIFICATION 157 



This sub-class is divided into Order I— Microdrili {Lumbricomorpha minora,) ; Order II -Megadrili 

 (L. majora). 



To these the terms ' -waterworms ' and ' earthworms ' may be applied. The only constant difference 

 between the two orders is the absence (Microdrili), or presence (MegadriK) of a capillary network 

 of blood-vessels on the nephridium. A number of characters usual to the two groups then follows. 



How far can the separation of the aquatic Oligochaeta, advocated by Clapaeede 

 and others, be held now in the light of greater knowledge of the organization of 

 the group? 



There are, undoubtedly, a certain number of points in which all these forms agree 

 to differ from the terrestrial Oligochaeta; that is, when the structural features to be 

 referred to are considered collectively. And there are furthermore a few points 

 which at present are peculiar to the aquatic Oligochaeta. We -will commence with 

 the latter ; among all the Oligochaeta which belong to Glapabede's ' Limicolae,' the 

 ova are of large size and full of yolk; this holds good, without a single exception, 

 from the smallest Enchytraeid up to so large a form as Fhreoryctes. Corresponding, 

 no doubt, to this, the egg-sacs are of large size and extend through several 

 segments ; both they and the sperm-sacs are very thin-walled, and their interior is 

 entirely undivided by trabeculae ; in all, or at least very nearly all, of the aquatic 

 Oligochaeta there is a head-pore, a structure which is, so far as our knowledge at 

 present goes, quite unrepresented in the terrestrial Oligochaeta. The remaining point 

 of difference concerns the structure of the body- wall; the longitudinal fibres consist 

 of a single row of deep fibres only ; this, however, does not characterize Fhreoryctes, 

 a genus which in other characters occupies an intermediate position. 



Besides these resemblances, all the aquatic Oligochaeta ^agree in a number of 

 structural features which would, if it were not for the family Moniligastridae, 

 distinguish them from all the teiTestrial Oligochaeta ; these are : — 



(i) The clitellum is only a single layer of cells thick. 



(2) It has a very anterior position, more so than in an.y terrestrial form. 



(3) The male- pores are also far forward. 



(4) The sperm-duct traverses only two segments. 



If we add the famUy Moniligastridae, it seems to me that a pertectly natural group 

 of Oligochaeta will be the result ; this group will be capable of the following definition. 



Oligochaeta with a clitdlum covimencing not later than the tenth or eleventh 

 segment, and condsting of only a single layer of cells; sperm-ducts only occupy 

 two segments, the external pore being on the segment following that into which the 

 funnel opens ; male-pore situated in front of the female-pore ■; eggs generally large, 

 always provided with abundant yolk; egg-sacs large; spermiducal glands, when 



