144 



DR. J. STEPHEXSON ON THE MORPHOLOGY, CLASSIFICATION, 



To get a complete series of gonads in tlie ancestors of the 

 MoniligastvicUe, however, we must supply a third pair of testes 

 in the vacant segment between the two pairs already present, as 

 well as a pair of ova,ries between the last testicular segment and 

 the actual ovarian segment. That the insertion of the third pair 

 of testes is justified is seen from the presence of three pairs of 

 prostates in Synrienodrilus. One further addition is to be made- 

 three pairs of male pores imply tln^ee pairs of spermathecal j)ores 

 apposed to them in copulation. The diagram of the ancestor thus 

 becomes that shown in text-fisr. 19. 



Text-fio-ure 19. 



The ancestral terrestrial Oligocliffite, with complete series of gonads and 

 spermathecse. 



The above dingram probably represents an animal in which as 

 yet the moniliform series of gizzards characteristic of the family 

 had not been evolved. The gizzards are in front of the testis 

 seo'ments in Syngenodrilus, behind them in the other genera : 

 that is, the muscular wall of the oesophagus has been strengthened 

 in one place in one subfamily, in another place in the other. 



Syngenodrilus (text-fig. 16) is derived from the above form by 

 a contraction, or huddling together, of the testis segments in 

 such a way that the middle one is almost squeezed out of 

 existence a,nd the other two become testis sacs, by a corresponding 

 reduction of the number of spermathecse to two pairs opening in 

 7/8 and 8/9, and by the disappearance of the anterior pair of 

 ovaries with their funnels and ducts. 



Desmogaster (text- fig. 1 1) is derived from the common ancestor 

 by the contraction of the segments of the fii\st and third pairs 

 of testes to form testis sacs, and the disappearance of the middle 

 pairs of male organs; by a corresponding diminution in the 

 number of spermathecse, the remaining pairs opening in 6/7 and 

 7/8 ; and by the disappearance, as in Syngenodrilus, of the 

 anterior ovary with its funnel and duct. 



Eupolygaster (text-fig. 12) came into existence by the further 

 disappearance of the posterior pair of male organs and the anterior 

 pair of spermathecae of Desmogaster. 



Drciwida and Moniligaster (text-fig. 13) originated from a form 

 similar to Desmogaster by a, contraction or huddling together of 

 the testis segments like that in Syngenodrilus, but carried further. 



