490 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



two groups the leech (Hirudo) and the earthworm 

 (Xiiimbricus), we have a striking example of the way 

 in which more lowly and more specialised characters 

 are often associated in the same individual. Starting 

 with the proposition that the generative organs are at 

 first irregularly distributed through the body cavity, 

 we are led to suppose that the more these sexual cells 

 are consolidated, or, in other words, the fewer the 

 segments in which they are developed, the higher the 

 grade of organisation. From this point of view we 

 should assign the higher position to the earthworm, 

 inasmuch as the testes are to be found in two seg- 

 ments only, while the leech has testes developed in 

 nine segments ; on the other hand, the arrangements 

 for the safe disposal of the testicular products are no 

 less indicative of superior organisation ; from this 

 point of view we should assign the higher position to 

 the leech, inasmuch as it has, and the earthworm has 

 not, a special intromittent organ, or penis, by means 

 of which the male products are safely carried to the 

 female. 



Notwithstanding the absence of a penis in most 

 Oligochsetes, different individuals copulate with one 

 another, and the male products of the one are received 

 by the other into special pouches, whence they are 

 expelled when the ova are mature and expelled also, 

 while some of the setae in the region of the body 

 where the orifices are placed are specially modified to 

 aid in copulation. The spermatozoa are often collected 

 into masses or spermatopliores, which may or may 

 not be provided with a special investment. 



While the generative products of the Nemertmea es- 

 cape directly to the exterior, and those of the Gephyrea, 

 and probably also of some Annelids, by means of the 

 nephridial canals, the Hiruclinea and some Oligochseta 

 are provided with special ducts ; in the earthworm 

 the open funnel-shaped orifice of the efferent duct 



