PHYLOGENETIC ARRANGEMENT 169 



It is undoubtedly the case that no other group than the Polychaeta comes nearer 

 to the Oligochaeta in these essential features of organization. The Hirudinea, 

 however, are quite as close, now that we know of the curious modifications of the 

 reproductive organs in the Eudrilidae, and the frequently unpaired condition of the 

 generative pores in that group and elsewhere ; a commencing division of the coelom, 

 carried much further in the Hirudinea, is to be recognized in a few earthworms. 



The Gephyrea are further away still; yet in a few points they come nearer to 

 the Oligochaeta than does any other group of worms ; for example, we have in 

 Echiurus, at the tail end of the body, circles of setae (as in Perichaeta) ; and the 

 'respiratory trees' are to some extent paralleled or foreshadowed in the anal 

 nephridia of Octochaetus. 



The chief differences between the Polychaeta and the Oligochaeta are as 

 follows : — 



(i) The testes and ovaries are limited in the Oligochaeta to a definite and 

 small number of pairs ^, whereas in the other group they are much 

 more abundant and are as a rule of one kind only in one individual. 



(2) The ducts for carrying away the genital products attain in the Oligochaeta 



to a specialization and variety quite unparalleled in the Polychaeta; 

 in the latter (Capitellidae, Saccocirridae, Alciopidae), they are but 

 slightly metamorphosed nephridia. 



(3) The nephridia of the Polychaeta are histologically very different from 



those of the Oligochaeta ; the only special resemblance between the two 

 is in the nephridia of the embryo. And these nephridia it will be 

 observed, recall those of the Platyhelminthes. 



(4) The setae of the two groups are different in form ; both, it is true, have 



long acicular setae; but when there is any peculiarity in form it is 



different in the two; and the long setae are closely paralleled by those 



of the Brachiopoda. 



These differences are not unimportant as it appears to me ; the question of the 



near relationship between the Oligochaeta and the Polychaeta has been prejudiced 



by the general term 'worm,' which has been applied to these and to so many other 



animals having even a more remote relationship. In the same way the Crustacea 



and the Insects have been confounded together owing to certain obvious similarities. 



The Insects are distinguished mainly by the possession of tracheae, which are totally 



different from any structures met with in the Crustacea; so the Oligochaeta are 



' See, however, above, where the very frequent occurrence of numerous gonads in Lumbricus is referred to. 



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