STRUCTURE OF A GENUS OF OLIGOCHiETA. o 



preserved specimens is a greenish-brown, and the body- wall is so thin that the nerve cord 

 can be plainly distinguished through it. 



None of the specimens showed any signs of a clitellum ; I suggest later in relation to 

 the affinities of the worm, that this may be due to the fact that that organ, as for instance 

 in the Naidomorpha, is only developed for a very short period. 



Among earthworms there is, as a general rule, a considerable inequality in length 

 among the anterior segments of the body ; it is only after the clitellum that the segments 

 become equal in size ; this is of course connected with the specialisation of the anterior 

 segments in other respects. On the other hand, the rule among the Limicolse — and to 

 this rule I can at present recall no exceptions — is that the anterior segments, with the 

 exception of the first, are equally sized, though perhaps having some advantage in size- 

 over the segments which follow the clitellum; I mention these facts — without desiring to 

 give them undue importance — because Moniligaster is so far allied to the Limicolse and 

 differs from earthworms. 



The prostomium is extremely small, and projects above the mouth as a short hemi- 

 spherical process ; the first and second segments # are both very narrow ; they are of equal 

 length to each other, and together are about equal to the third body segment. The setse 

 upon the first setigerous segment are much smaller than those upon the following seg- 

 ment ; this fact, combined with the very indistinct line which divides the first from the 

 second segment, led me to confound the two together, and thus to make an error of one 

 segment in my former enumeration. Indeed, any one examining the worm only by the 

 help of a hand lens would be almost certain to make this error, as with this slight magni- 

 fying power I found it impossible to distinguish the first and second segments. It is 

 necessary to examine the worm with a tolerably high magnifying power to detect the 

 setae upon the narrow first setigerous segment ; I have mounted two specimens in Canada 

 balsam, and with a preparation of this kind it is possible to reckon the segments 

 accurately. 



The prostomium is not at all evident, as it is capable of an unusual (?) amount of 

 retraction. 



In one specimen, of which the anterior end was mounted in Canada balsam, I could 

 not detect the prostomium at all; in another individual, mounted in a similar fashion, the 

 prostomium looked like fig. 1; the connection between the prostomium and the first 

 segment could not be detected, and it seemed to be surrounded by that segment. In 

 longitudinal sections (fig. 7) the prostomium could be easily made out and distinguished 

 from the first segment by its tall columnar epithelium; in that section it is seen to be 

 greatly retracted; a very deep groove separates it from the peristomial segment. 



The only apertures visible upon the outside of the body are the atrial pores in 

 segment X (see fig. 1, <j). 



* It is perhaps unnecessary to explain that the first segment throughout the following description is the peristomial 

 segment, and that I reckon the first setigerous segment as the second. 



