I] STRUCTURAL AND SYSTEMATIC 33 



former are as a rule more extensive in the number 

 of segments that they occupy than among the terres- 

 trial forms. Another difference which they show is 

 that their cavity is quite simple and not divided up 

 by trabeculae into numerous intercommunicating 

 chambers as in the earthworms. Finally the eggs 

 of the aquatic Oligochaeta are large and full of yolk 

 and thus contrast with the very small ova of the 

 earthworms which are moreover much more abundant. 

 These features are either of general or universal 

 occurrence and together form an assemblage of 

 characters which mark out the aquatic families of 

 Oligochaeta from their, at least mainly, terrestrial 

 allies. 



We may also refer to certain structures which 

 although not universal among these aquatic families 

 are nevertheless found only in them that is, are not 

 found in any family of the terrestrial worms of this 

 order. The most salient of such characters are the 

 long and hair-shaped setae tapering to a fine point and 

 often provided with a series of delicate branches like 

 a feather ; such setae are often of very great length 

 and they occur in their various modifications among 

 the Aeolosomatidae, Naididae, and Tubificidae. It 

 is clear that these delicate setae, though they may 

 not be due in any way to the aquatic life, are rendered 

 possible by it. To drag such tender processes through 

 stiff clay would surely break and tear them out. It 



B. E. 3 



