10 OLIGOCHAETA 



suppose that after all the bundles of setae have some relation to the aquatic life; 

 it is quite possible that a number of them radiating out in a fanlike way serve 

 as an efficient swimming organ, and hence their development in Oligoehaeta which 

 occasionally at least ' swim.' If there were any other reasons for associating together 

 the families of Oligoehaeta mentioned with the Polychaeta, this question of the simi- 

 larity in the disposition of the setae would have to be reconsidered; but it cannot 

 be said that the Tubificidae and the Naids are nearer allies of the Polychaeta than 

 any other families of Oligoehaeta; hence I am inclined for the present to put down 

 the likeness in the bundles of setae in the marine and in the fresh water Annelids 

 to a similar need. 



ExsiG has dealt with this subject in his Monograph of the Capitellidae (p. 574, etc.). 

 In that group there are foiins in which the distichous arrangement, obvious elsewhere, 

 is nearly lost; a state of affairs very like that of Perichaeta thus results, or more 

 like Megascolex, since there are dorsal and ventral gaps. There can be no doubt, 

 in Eisig's opinion, that the later arrangement is secondary, it being restricted to the 

 beginning of the abdomen, and being preceded as well as succeeded by the ordinary 

 paired bundles. 



One argument in favour of deriving the fewer setae per segment of most earth- 

 worms from the perichaetous condition does not appear to me to have been considered. 

 That argument is based upon the fact that in most Perichaetidae the number of setae 

 is less in the anterior than in the posterior segments of the body. In a number of 

 species, which I formerly proposed to place in a separate genus Anisochaeta. there 

 are eight setae in a varying number of the anterior segments, while posteriorly the 

 number becomes much greater ; this is an extreme case of the point at issue ; but 

 in all Perichaetidae there seems to be a smaller number of setae on the pre-clitellian 

 segments than upon those which follow, or there is a progressive increase for a few 

 segments at any rate. In Megascolex this is, so far as we know, invariably the case ; 

 but in Perichaeta the number, after increasing up to a certain segment (for example 

 the seventh in P. taprohanae), diminishes. Now in such a foi-m as Onychochaeta the 

 absence of setae upon the first few segments of the body (seen also in other 

 Geoscolecids) must surely be regarded as an instance of what has been called 

 'cephalisation'; it is at any rate a modification paralleled by other organs of the 

 body. It would seem therefore likely that the smaller number of setae in the 

 anterior segments of the body in the Perichaetidae is due to a reduction from 

 a primitively greater number; as we know that this number is sometimes reduced 

 to eight we have the origin of the eight setae per segment of the majority of the 

 terrestrial Oligoehaeta suggested. Such general arguments as can be deduced from 



