360 OLIGOCHAETA 



The setae are as a rule similar on all the segments of the body ; there are, however, 

 exceptions to this rule. In many species, for example, the setae upon some of the 

 anterior segments are stouter than those which occur elsewhere ; in P. sinensis 

 this is the case with the setae of the segments vi, vii, viii, ix. In other species the 

 ventral setae are larger than the rest ; in P. houUeti there are three or four setae 

 on each side of the nerve-cord, which are distinctly longer as well as thicker than 

 those upon the remaining part of the segment ; in this case there is a gradual increase 

 in size, commencing a little way from the median ventral line and culminating 

 at the last seta on each side. In the genus Perichaeta there are either very few 

 or no setae upon the clitellum when this is mature. P. taprohanae is one of the 

 few species of the genus which has setae forming continuous circles upon the 

 cliteUar segments ; it is common, if not universal, among the species of Megascolex 

 to have setae upon the clitellum. When setae are present upon the clitellum they 

 are never, except in P houUeti, in any way different from those of other segments. 

 In the above-mentioned species the cliteUar setae are shorter than those upon other 

 segments, and have a truncated form and a bifid extremity. The clitellum is in the 

 majority of species (in all belonging to the genus Perichaeta itself) complete, i. e. 

 it completely encircles the body ; in this genus too, with very few exceptions, there 

 are only three segments in the clitellum — only two apparently in P. upoluensis. 

 Megascolex has the most extensive clitellum (nine segments in one species), and here 

 the clitellum is only complete anteriorly ; it is deficient ventrally behind. 



The male pore is nearly invariably upon the eighteenth segment ; the only 

 exceptions are M. attenuata, where it is upon the seventeenth, and M. cTiormis, where 

 it is between the seventeenth and eighteenth. , 



The prostomiuTn in the Perichaetidae is rarely continued by grooves as far as the 

 end of the buccal segment ; it does, however, as a rule, extend for a certain distance 

 on to the buccal segment. 



Genital papillae are very general in this family ; they are found in nearly all the 

 species. When present they may occur in the neighbourhood of the spermathecae 

 as well as near to the male pores. With the papillae are always connected glands 

 of a peculiar kind whose structure has been described above (p. 144) ; the papillae 

 in fact are but the external pores of these gltads. 



The alimentary canal of the Perichaetidae is chiefly remarkable for the two caeca 

 of the intestine, which arise in the twenty-sixth segment, and are directed forwards ; 

 these were first discovered by Vaillant (3), and have since been found to be, with 

 a few exceptions, characteristic of the genus Perichaeta itself, and they serve as a means 

 of distinguishing that genus from Megascolex. It is a remarkable fact that similar 



