158 OLIGOCHAETA 



present, possess a muscular layer interposed between the inner epithelium and the 

 glandular layer ; sexual maturity at a fixed period. 



In the above definition a number of characters are left out which do occur in all 

 of the families which are contained in this group, but which also occur in the higher 

 Oligochaeta ; such are, for example, the invariably paired nephridia and the absence 

 of a sub-nervian blood-vessel. The only genus among those not included in this group 

 which presents any resemblance to it in any one of the points mentioned is Tetragonurus, 

 in which the male-pores (presumably : its anatomy is not known) are only a segment 

 behind that which contains the funnel. It appears to me, therefore, that the above 

 definition is really sufficient to separate off a group of natural distinctness. And 

 I propose to retain BENHAM'S name of ' Microdrili ' for this group. 



The differences which Moniligaster shows from the other members of the group 

 are not in my mind so great as the differences which it shows from other earth- 

 worms ; it agrees with earthworms in having gizzards, in the thickness of the 

 body -wall, and in the presence of specially thickened septa. But a little consideration 

 will show that these characters are not by any means distinctive of earthworms ; 

 the genus Pelodrilus is surely more akin to Phreoryctes than to any other family ; 

 and it has thick body-walls and thickened septa. The presence of gizzards is 

 really the only point in which Moniligaster is more like earthworms than any 

 aquatic form ; but are we to decide a question of affinity on so variable a structure 

 as the gizzard, absent in so many earthworms, and possibly connected to some extent 

 with the habitat of the creature? In considering this proposed grouping, too, 

 a number of other structural features must be taken into consideration, which are 

 important, though not distinctive of the group. I have described, and so has 

 BENHAM (16) in a different species, a protrusible penis in Moniligaster, which is 

 exactly like that of the Tubificidae, and has not its exact counterpart in any genus 

 of worms which do not belong to the Microdrili, as I here define them. All the 

 Microdrili, with the exception of the Moniligastridao. have no nephridia in the 

 anterior segments of the body ; but this fact cannot form the basis of a first 

 grouping, since there are some undoubted members of the Oligochaeta which could 

 not be placed in the Microdrili, though they show the same structural character : the 

 genus Pontodrilus is a case in point. Nor can any stress be laid upon the fact that 

 the nephridia of Moniligaster have a vascular network ; this is, it is true, absent 

 in the aquatic worms, but it is also absent in Gordiodrilus and some of its allies, 

 which are not in other respects close to the aquatic families. In fact, there seems 

 no way out of associating together closely the Moniligastridae and the Limicolae of 

 CLAPAREDE, and making one family of them. Is it possible to pursue this arrange- 



