THE ANATOMY. GENITAL SETAE 121 



Acanthodrilus both are without it; so too in Dichogaster damonis. The question, 

 therefore, of the origin of the spermiducal glands is bound up with that of the 

 copulatorjr glands, and must be deferred until we can determine which is the most 

 primitive type of copulatory gland. It seems reasonable to suppose that the copulatory 

 glands were originally modified tracts of the body- wall, which became invaginated and 

 furnished with penial setae for their greater eflaciency ; the invagination, as it appears 

 to me, might or might not involve the muscular layers of the body-wall ; so that very 

 probably both circumstances have occurred, which would account for the corre- 

 spondence, noted above, between the copulatory and spermiducal glands. In this 

 case the spermiducal glands of the Geoscolicidae might be equally primitive with 

 those of the Megascolicidae. Another point, which has to be taken into consideratioHj 

 is the relationship of the sperm-duct to the spermiducal gland. The independence of 

 the sperm-duct and the gland is, ex hypothesi, a primitive condition. This suggests 

 that the Geoscolicidae, and particularly the Acanthodrilidae, Perichaeta ceylonica, 

 and Dichogaster are primitive forms. The spermiducal gland of Moniligaster, which 

 is characterized by the transference of the gland-cells to the outer side of the 

 muscular layer, seems to be a later stage in the evolution of the organ than that 

 found in the Eudrilidae. 



§ 8. Genital setae. 



In the neighbourhood of the male-pores there are in some OHgochaeta bundles 

 of long modified setae which protrude through the orifices in question; to these 

 structures the name of penial setae was first applied by Lankestek (1). In a very 

 few species there are bundles of quite similar setae developed in the neighbourhood 

 of the spermathecae, and in a few Geoscolicids, again, similar setae are found, not 

 only at the male-pores, but on a greater or less number of segments in their 

 immediate vicinity. Hobst has proposed to limit the name penial setae to those 

 setae which occur in the neighbourhood of the male-pores, and to call copulatory 

 setae those which are found in the vicinity of the spermathecae. It is perhaps 

 more convenient to apply the term ' genitaL setae ' to all alike. 



These setae are nearly always found associated with glandular structures ; in the 

 case of those which protrude through the male-pores, there are, of course, the 

 spermiducal glands with which they are more particularly associated ; the sacs which 

 contain them are often or at any rate have the appearance of being, diverticula 

 of the spermiducal glands. The setae which occur in the neighbourhood of the 

 spermathecae have special glands developed in connexion with them, which are 

 described in the section devoted to the spermathecae. The special setae developed 



B 



