8 OLIGOCHAETA 



Another stage is seen in many Geoscolicidae and Lumbricidae, where the setae upon 

 the clitellum are longer than the others and of a slightly different shape ; it occurs 

 for example sometimes in the Geoscolicidae that the clitellar setae are ornamented, 

 while those formed elsewhere are not. In the Eudrilidae the setae are apparently 

 similar throughout the body except in the neighbourhood of the male pore. These 

 penial setae {see under description of reproductive system) occur in many Oligochaeta. 



It is however among the aquatic genera that the greatest specialization of the setae 

 occurs. Among earthworms a single segment has never setae differing in form though 

 they may differ in size, excepting only in the case of the segments which bear the 

 specially modified penial setae, which may be a little different among themselves 

 and are as a rule very different from the ordinary locomotive setae on the same 

 segment. But the rule among aquatic Oligochaeta is that the dorsal bundles of setae 

 should contain setae different in form from those of the ventral bundle. In most 

 Tubificidae the dorsal bundles are made up of capilliform setae, with which may or may 

 not be mixed uncinate setae ; the ventral bundles are entirely composed of the latter. 

 The details in the distribution of the setae, which are most useful for classificatory 

 purposes, will be found in the systematic part of this work. 



The arrangement of the setae among the Oligochaeta is varied. The most prevalent 

 plan is the existence of four groups of setae, which of course recall the parapodia of the 

 Polychaeta. The principal ways in which the setae are disposed are indicated in the 

 following table : — 



1. Setae numerous in each segment and forming a complete ring. — Ex. Perichaeta, 

 Pleionogaster, and other Perichaetidae. 



2. Setae numerous but arranged in two lateral masses leaving dorsal and ventral 

 gaps. — Ex. Megascolex. 



3. Setae twelve in number in each segment. — Ex. Deinodrilus. 



4. Setae eight in each segment placed at equidistant intervals or grouped into four 

 pairs, or intermediate, four setae being paired and four distant. — Ex. Acanthodrilus, 

 Lumhrieus. 



5. Setae eight in number in anterior segments, and thence increasing to thirty or 

 forty. — Ex. certain Perichaetidae. 



6. Setae in four bundles with more than two setae in each bundle. — Ex. Tubificidae. 



7. One pair of setae only on each side of segments. — Ex. Anachaeta. 



8. Setae, eight in number in each segment, but irregularly arranged. — Ex. Diachaeta. 

 It will be seen from the above table that there is an almost unbroken series of stages 



in the arrangement of the setae ; we can pass from one extreme to the other without any 

 difficulty ; thus assuming for the moment that the original condition is exemplified by 



