DESCRIPTIONS OF GENERA AND SPECIES 389 



(5) The presence or absence of genital papillae, and their number and arrange- 



ment if present. 



(6) The position of the male and spermathecal pores. 



(7) The position of the first dorsal pore. 



(8) The number and position of the spermathecae. 



(9) The relative size of the spermiducal glands, and the presence or absence of 



a sac at the external opening of its duct. 

 (10) The position and the number of specially thickened septa. 



Besides these ten points in which species vary, there are a few other characters 

 which are of less importance as they apply to only a very small number of species, 

 perhaps only to one. Several of the Japanese species of the genus are remarkable for 

 the rudimentary condition or the absence of the spermiducal glands ; the most extreme 

 case is that oflfered by P. hilgendorfi ( = my own P. rokugo) ; in this species not a trace 

 of the gland remains, and the pore itself is removed further back than in all other 

 species. In two or three others the gland is reduced to the muscular duct only, the 

 glandular part having disappeared. 



Another character confined to a single species is seen in P. stelleri, where there are 

 a pair of calciferous glands in the thirteenth segment. In no other Perichaeta are 

 there distinct calciferous glands at all, only a particularly vascular and thickened 

 tract of oesophagus, which probably performs the same function as the calciferous 

 glands of other Oligochaeta. In a few forms, viz. P. taprobanae, P. stelleri, P. 

 everetti, P. sarawacensis, P. kinabaluensis, and P neoguinensis, there are no intestinal 

 caeca, the presence of these being elsewhere one of the most marked peculiarities of the 

 genus. 



These same intestinal caeca afibrd an excellent character for distinguishing the two 

 species P. sieboldi and P. hilgendorfi ^ from all others ; in these two Perichaetae there 

 is not, as in all others, a single pair of caeca, but a series of six or seven pairs 

 lying one above the other on either side of the intestine ; P. sangirensis appears 

 to be unique for the existence of caeca in the fifteenth segment, Michaelsen, 

 however, is not quite certain as to this matter, which is queried in his description 

 of the species. 



The number of the egg- sacs ought to have been, perhaps, included among the 

 characters which vary in a large number of species ; very little attention, however, has 

 been paid to this matter, except by myself (55). There are either one or two pairs of 

 egg-sacs ; sometimes, indeed, no egg-sacs at all seem to exist ; where there are two 

 pairs, they lie in segments xiii, xiv ; when there is but one pair, it is not always 



' Occasionally also P. nnmica. 



