THE ANATOMY. GENITAL PAPILLAE 143 



that in Allolobophora foetida, though not in all species of the genus, a mucous 

 skin is thrown off by the clitellum of the worms, which binds them together more 

 firmly when pairing. Nothing whatever, unfortunately, is known with regard to 

 the pairing of exotic Oligochaeta. But it is important to notice that in many 

 of them there are various organs whose function is, possibly, that of securing a 

 firm contact between the individuals during copulation. The papillae of Penchaeta, 

 which are sometimes present, not only near to the orifices of the sperm-ducts, but 

 also in the neighbourhood of the spermathecae, very likely perform this function ; 

 the penial setae of Acanthodrilus, Megascolex, &c., may play a similar role ; whereas 

 in the Geoscolicidae there is generally an absence of any structures which might 

 be supposed to act in this way. No doubt the modified clitellar setae of these 

 worms and of the Lumbricidae have some such function, but their effect must be 

 feeble when compared with that of the suckers of the Periehaetidae. Now it is in 

 precisely these forms, in which accessory copulatory structures are highly developed, 

 that the clitellum is restricted in extent, whereas in the Geoscolicidae the . clitellum is 

 long, and accessory copulatory structures are commonly absent. It is possible, 

 therefore, that the clitellum in these species performs a double ofiice, i.e. (i) that of 

 producing the cocoon, and (2) that of secreting a ' skin ' to attach the two worms 

 together during copulation. The suggestion is, however, only put forward in the 

 most tentative fashion as it is not supported by all the facts so far as can be seen at 

 present. 



§ 13. Genital papillae. 



There are apparently three kinds of papillae among the Oligochaeta, which have 

 been more or less indiscriminately called 'genital papillae.' 



Genital papillae occur, and have been described more particularly, in the Peri- 

 ehaetidae and the Lumbricidae (under the name of tubercula pubertatis) ; but they 

 are also met with in other Oligochaeta ; in the Acanthodrilidae there are papillae in 

 the neighbourhood of the male-pores and elsewhere, to which Michaelsen has applied 

 the name of ' WoUustorgane ; ' and organs apparently most nearly corresponding to 

 these have been described by myself in FelodrUus and in Fridericia. 



In the Periehaetidae the papillae occur in two situations ; they are found either in 

 the neighbourhood of the male-pores or near to the spermathecal orifices, sometimes 

 in both situations. They have proved most useful in the determination of species ; 

 the variation exhibited by them is so great that it is very often possible to define 

 a species by their number and position. To give a few examples : in Penchaeta 

 forbesi there are series of pairs of papillae upon segments xvii, xix-xxi. In 



