142 OLIGOCHAETA 



on to the clitellum. In the Tubificidae both have moved back. In the Lumbriculidae 

 and the Enchytraeidae there is a similar relation between the male-pore and the 

 clitellum. Among the higher Oligcchaeta there is, as a rule, no such relation 

 apparent. The position of the male-pore, though it may coincide with one of the 

 clitellar segments, does not show any alteration in position corresponding to alterations 

 in position of the clitellum. The male-pore, for instance, in nearly all the Megascolicidae 

 is upon the eighteenth segment, but the clitellum is most variable in extent, though 

 it certainly does not fluctuate much as to the segment where it commences. The 

 Geoscolicidae show greater variation than any other family of Oligochaeta in the 

 position of the male-pore, and yet it is impossible to trace any clear connexion 

 between the position of the male-pore and that of the clitellum. On the other hand, 

 the Lumbricidae show the greatest amount of variation in the position of the clitellum, 

 combined with a nearly invariable position (on the fifteenth segment) of the openings 

 of the sperm-ducts. As to this family, however, it must be admitted that when 

 {AUurus) the male-pore is advanced a couple of segments the clitellum also 

 commences earlier. It may be noted, also, that in the series of variations exhibited 

 by Perionyx excavatus (Beddard 41) an alteration in the position of the male-pore 

 is accompanied by an alteration in the position of the clitellum. 



There do not seem to be any other relations between the clitellum and any other 

 organs in the higher Oligochaeta — excepting, perhaps, that in no case are the sperma- 

 thecae situated behind the clitellum. This is, of course, connected with the function of 

 the clitellum in forming the cocoon, which is passed over the head, receiving the 

 contents of the spermathecae on its way. 



It has been mentioned that the number of segments of which the clitellum is 

 composed vary from species to species and from genus to genus. The smallest 

 number of segments, viz. two, characterizes the Naidomorpha and Tubificidae, whereas 

 we meet with the greatest number among the Geoscolicidae and Acanthodrilidae 

 (twenty-seven in Trigaster lanlcesteri) ; every intermediate number is to be met with. 

 Sometimes, as in the case of the Tubificidae, the number of segments is constant 

 throughout the entire family ; in other cases — in Perichaeta at any rate, the number 

 of clitellar segments (three) distinguishes, with very few exceptions, the genua. It 

 is difiicult to detect any relation between the development of the clitellum and any 

 structure in the worms. Why should one earthworm have a clitellum of barely 

 three segments, and another have a clitellum of over thirty? The question is no 

 easier to answer than that relating to the highly variable position of the organ in 

 the genus AUolobophora. It is possible that there is some connexion between the 

 extent of the clitellum and the phenomena of pairing among earthworms We know 



