I] STRUCTURAL AND SYSTEMATIC 23 



while in Pheretima houlleti the clitellar setae are 

 different from those upon the other segments. 



We can in fact only define the family Geoscole- 

 cidae by an assemblage of characters which are 

 mainly these: dorsal pores absent, only a few in 

 the neck region being occasionally present ; setae 

 generally ornamented, those of the clitellum being 

 larger and more marked than the others ; sperma- 

 thecae without diverticula ; often instead of a pair of 

 those pouches in the segment a large number of very 

 small sacs, as in Mierochaeta, Kynotus. Sperm 

 ducts without terminal glandular or muscular sac, 

 except in a few cases; setae always eight in a 

 segment except in the genus Periscolex which has 

 acquired the ' perichaetous ' condition. The range of 

 variation shown in the anatomy of the Geoscolecidae 

 will be best taken in connection with the several 

 sub-families into which it has been subdivided. In the 

 first of these, the Geoscolecinae, no great differences 

 divide the genera from that selected as the type, viz. 

 Pontoscolex. The long sperm sacs attain to an 

 extraordinary length in Trichochaeta (or Hespero- 

 scolex) where the single pair extends through no less 

 than 109 segments. Though as a general rule the 

 sperm ducts open directly on to the exterior they do 

 so through the intermediary of a large pouch in 

 Glossoscolex (= Titanus). In Onychochaeta the setae 

 on the last segments of the body are very much 



