THE ANATOMY. SETAE 9 



Perichaeta, a gradual reduction in the number of the setae anteriorly leads to certain 

 species of Megascolex ; a reduction upon all the segments of the body simultaneously 

 leads through Deinodrilus to Acanthudrilus, &c. ; on the other hand, a crowding 

 together of the setae into four bundles coupled with a reduction in numbers, brings 

 about the arrangement characteristic of the Tubificidae. 



It is however by no means certain that the stages have been developed in the order 

 suggested above. There appear to be three possible lines of development; either the 

 ' perichaetous ' condition is the most primitive, or the two pairs of setae in each segment, 

 or the bundles with several setae (more than two) in each bundle. We shall consider 

 these various possibilities seriatim. If we regard the Oligochaeta as to be derived from 

 the Polychaeta it is clear that the last of the three alternatives is the one which would 

 then appear to be the most probable ; the considerable specialisation in the individual 

 setae of a bundle is a further point of resemblance to the conditions characteristic of 

 many Polychaeta. But there is not after all much detailed resemblance between the 

 setae of the Tubificidae and those of the Polychaeta ; the fact that in both groups the 

 setae of the dorsal bundles are very often different in form from those of the ventral 

 bundles is perhaps more striking as an analogy than valuable for the purposes of 

 a strict comparison ; if the differences were of the same kind it would be different. 

 Still it might be urged that in the Tubificidae there is a resemblance to the Polychaeta 

 toned down by simplification. Furthermore, we have the extinct Pronaidites ; this 

 worm has been referred to the Oligochaeta (Kusta), but it is not by any means 

 convincingly an Oligochaet. As the creature is of Carboniferous age, this argument 

 would be strongly supported if it were proved beyond doubt to be referable to the 

 Oligochaeta. 



Unfortunately, this is the only piece of palaeontological evidence bearing upon the 

 matter under discussion, and at best it is not conclusive in any direction. Besides, 

 looking at the matter in another light, there are now undoubted Polychaeta known 

 which inhabit fresh water, for example ManayunJcia. The only families of Oligochaeta 

 which show this resemblance (such as it is) to the Polychaeta in the arrangement of 

 their setae are the Tubificidae, Naididae, and Aeolosomatidae ; and they are all, it 

 will be observed, aquatic in habit. Now it seems quite reasonable to suppose that 

 long and delicate setae would be out of place in a worm having to force its way 

 through dense soil ; we need not therefore be surprised at not meeting with such 

 setae among the terricolous forms ; but there does not appear to be any valid a priori 

 reason against finding bundles of short and strong setae in the land Oligochaeta ; and 

 yet there are only the Enchytraeids among the terrestrial Annelids with such an 

 arrangement of setae, and they are largely aquatic. One is accordingly inclined to 



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