THE ANATOMY. GENITAL SETAE 123 



therefore, it seems reasonable to regard the genital setae as being the modified 

 representatives of the ordinary setae which should occupy their place. But another 

 question requires answering : the bundles of genital setae generally contain more 

 fully developed setae than would usually be present; not, of course, in the genus 

 Megascolex, but in such a genus as Acanthodrilua. I have suggested that this points 

 to the former perichaetous condition, a number of setae having been retained for 

 the purpose of serving this new function. Though this view has not met with 

 acceptance, there are other considerations which seem to me to favour it. In 

 Perionyx, as it appears to me, we have an early stage in the development of the 

 genital setae preserved. In that genus there is (in some species) a row of the more 

 ventrally placed setae, which are modified in structure, being beset with ridges at 

 the distal end and being somewhat larger than the ordinary setae, which are smooth. 

 These setae are sometimes imbedded in a groove into which opens the male-pore 

 on each side ; if this groove were withdrawn, so as to convert it into a more strongly 

 marked recess, it would follow that the setae would be crowded together, as is the 

 case with the genital setae of the other genera, and that they would at the same time 

 tend to become longer, in order to be capable of projecting out of the invagination 

 in which they are imbedded. This suggestion is to my mind reinforced by the 

 distribution of genital setae associated with the sperm-duct apertures; they occur 

 in the Megascolicidae in bundles; the setae are numerous, though varying in 

 number. Now it is at least arguable that all the worms belonging to this family 

 are traceable to a form with a complete circle of setae. On the other hand, in the 

 Eudrilidae the penial setae are not in bundles ; there is only a single seta so 

 modified on each side of the body ; and this family is one in which perhaps 

 the evidence of a descent from some form like Perichaeta is least arguable among 

 the terrestrial Oligochaeta. It is true that the Geoscolicidae seem to be opposed to 

 this way of regarding the matter. I am on other grounds disposed to connect them 

 closely with the Eudrilidae, and this view of their affinities undoubtedly affords 

 a way out of the difficulty. As will be seen later, I believe that the aquatic 

 Oligochaeta are not near to the terrestrial ; and it will be noticed that in none of 

 these that have paired setae are there bundles of genital setae. 



A curious fact has been observed in a few earthworms, for example in Benhamia 

 annae, and in other Acanthodrilidae, and that is that the penial setae in a single 

 bundle are of two kinds ; there are setae with an extremity marked by the presence 

 of spines, and others in which the extremity is quite smooth ; that both kinds of 

 setae are fully mature, that the one kind are not the immature kind of the other, 

 seems to be proved by their being of the same size. 



K 2 



