390 OLIGOCHAETA 



found in segment xiv, which, judging from other earthworms, would seem to be its 

 natural position ; it is, in more than one species, situated in segment xiii, just above 

 the ovary. As has been remarked on another page (p. 86), this is, perhaps, to be 

 accounted for by the former existence of a pair of ovaries in the twelfth segment of 

 which traces exist in the embryos of Lumbricus and AcantJiodrilus. 



In the vast majority of the species of the genus Perichaeta there are four pairs of 

 hearts in segments x-xiii ; this is not, however, absolutely universal ; in two species, 

 P. divergens and P. queenslandica, the last pair of hearts is in segment xii. In 

 P. ferdinandi this pair is, on the contrary, a segment behind that in which it usually 

 occurs, viz. in xiv ; finally, in P. ringeana it lies in xv. 



P. vitiensis and P. queenslandica appear to be quite peculiar in the fact that they 

 have only a single pair of testes and of the corresponding funnels. I am not, however, 

 fully satisfied that my description of the former species, made upon a single individual 

 belonging to the Vienna Museum, and, therefore, to be injured as little as possible, is 

 correct. There is, of course, nothing improbable in the divergence from the normal, as 

 such is constantly met with in other families of the Oligochaeta. 



The only other peculiarity confined to a single species is the presence of the 

 gizzard in P. pentacystis in the seventh segment. 



We now come to the characters which vary greatly in the different species. 



(i) The variations in the number of the setae have been noted by many observers; 

 but they have, as a rule, contented themselves with a statement of the number of the 

 setae in a given segment. BOURNE has directed attention to the importance of 

 counting the setae in several segments. He found that the relation between the 

 numbers of setae in segments v, ix, and xxv (' the latter segment serving as a type 

 for the rest of the body ') ' varies with other important characters rather than the 

 actual numbers themselves.' The segments used by myself (ii, vi, xiii, xxvi) do not 

 materially differ from those used by BOURNE. Comparatively few species have been 

 studied from this point of view. The importance which the results arrived at from a 

 careful mapping of the setae may lead to is illustrated in the accompanying curves 

 (p. 391); these show distinctly the difference between P. taprobanae and certain other 

 species ; the differences in the setae are notably confirmed by a comparison of other 

 organs. 



(3) This character is of great use in differentiating species. On the anterior 

 segments of the body the setae are commonly larger than those upon the segments 

 which follow ; this difference has, however, been noted in but few species up to the 

 present, and thus is not so useful as it doubtless will be. It is not uncommon to 

 find one or more of the setae on either side of the ventral median line larger than 



