282 J. II. ASH WORTH. 



The loop is iiearlj' 4 mm. long in tlie largest iiepliridia. In 

 young nephridia the lumina of the two limbs of the loop are 

 about equal in size. 



The terminal portion of the nephridium is generally bent 

 almost at right angles to the loop. It is seldom longer than 

 •5 ram. It is not distinguished from the adjacent portion of 

 the loop by any external structural character, except in a few 

 cases in which there is a slight dilation of the terminal tube 

 just before reaching the nephridiopore (fig. 18). 



The first nephridium is very small^ being only I'b mm. 

 long in a large worm. The sixth is usually the largest, being 

 4 to 5 mm. long. In most of the nephridia of large speci- 

 mens and in the first ten or fifteen nephridia of specimens 

 about 15 mui. long the loop is the most obvious part of the 

 nephridium, the straight tube being only one half to one 

 fourth its length ; but the young nephridia found in the pos- 

 terior segments have very short loops, considerably shorter 

 than the straight tube of the same nephridium (fig. 21). 



The nephridiopores are small oval apertures. The first is 

 situated just below and slightly anterior to the fourth neuro- 

 podium, but this is so minute that it can usually be seen only 

 in sections. The others may usually be found either in sur- 

 face view of favourable spirit specimens, or in specimens 

 cleared in oil (fig. 5). The pores are about '04 — '06 mm. 

 along their longer diameter (see also p. 244) , 



Histology, — The lips of the nephrostome are lined by a 

 single layer of elongate columnar cells with well-marked 

 nuclei. The cilia are better developed on the dorsal lip 

 (fig. 19). The straight tube behind the nephrostome is lined 

 by almost cubical ciliated cells, the nuclei of which are small 

 and spherical, and lie close to the lumen of the tube. The 

 cells of the loop are larger than those of the preceding 

 portions of the nephridium. In surface view they appear 

 pentagonal or hexagonal, and are closely fitted together at 

 their borders. Their protoplasm contains (in preserved 

 specimens) nnmerous cavities which in life were probably 

 filled with exci-etory substances (fig. 20). In some speci- 



