120 Excretory Organs in rxlcllnstoma Stouti 



In the last four segments the duct is present as a continuous rod, the 

 outlines of which are not quite so distinct as farther forward. At four 

 places, however, situated at perfectly regular intervals, the myotomes 

 approach and are joined to the duct, and here the center of the duct is 

 filled with non-nucleated protaplasm, in which is a small cavity. These 

 can be none other than rudimentary nephrotomes. Whether they would 

 have developed farther cannot be said, but they are interesting as indi- 

 cating that the entire duct is formed from nephrotomes, for it is not 

 likely the duct would have extended farther back, at least no case has 

 been found where it extends beyond this point, the eighty-second segment. 

 In the previous paper tubules were foimd to within two segments of 

 the end of the duct, and as it was then thought that the duct was formed 

 from the ccelomic epithelium, and as there was no coelom in these seg- 

 ments it was suggested that here it had grown back independently. The 

 present observation renders this improbable. 



Fig. 21.^ — Section through the excretory organs in segments twenty-three, twenty-four 

 and twenty-five, twenty-three being to the right, d, segmental duct ; ce, ccelomic epi- 

 thelium ; nc, nephroccels. The nephrocoels in segments twenty-four and twenty-five are 

 growing towards each other, while the one in twenty-three has met and united with 

 the one in twenty-four. The duct does not show where the two ends of the nephrotomes 

 have united, as in Fig. 16. 



Turning now to the anterior end it remains to be shown how the 

 nephroccels become merged with the splanchnocoel. This is brought about 

 simply by the nephroccels in adjoining segments growing together, the 

 process beginning at the anterior end and proceeding posteriorly. Fig. 31 

 is a sagittal section through the excretory organs in segments twenty- 

 three, twenty-four and twenty-five. The nephrocoel in segment twenty- 

 five, the one to the left, has not yet united with the one in segment 

 twenty-four, although the two have approached each other; but the one 

 in segment twenty-four has united with the one in twenty-three. From 

 here forward sections corresponding to the one just given show no signs 

 of nephrocoels, but if the series be followed toward the median line it is 

 found that in the three or four sections before its entire disappearance 

 the coelom presents the appearance of segmentally arranged cavities, 

 showing that the fusion of the nephroccels is not entirely completed. 

 Between segments twenty-three and twenty-four the segmental duct, d, 

 Ig seen to be separated by a narrow space from the ccelomic epithelium. 



