902 ANATOMY AND DEVELOPMENT 



very indistinct. The fourth region is very short (fig. 28 s c 4), 

 and is formed of small columnar cells. It gradually narrows 

 till it opens suddenly into the terminal section (s o f), which 

 ends by opening into the body-cavity, and constitutes the most 

 distinct portion of the whole organ. Its walls are formed of 

 columnar cells almost filled by oval nuclei, which absorb 

 colouring matters with very great avidity, and thus renders 

 this part extremely conspicuous. The nuclei are arranged in 

 several rows. 



The study of the internal opening of this part gave me some 

 trouble. No specimens ever shew it as rounded off in the 

 characteristic fashion of tubes ending in a cul-de-sac. It is 

 usually somewhat ragged and apparently open. In the best 

 preserved specimens it expands into a short funnel-shaped 

 mouth, the free edge of which is turned back. Sections confirm 

 the results of dissections. Those passing longitudinally through 

 the opening prove its edges are turned back, forming a kind of 

 rudimentary funnel. This is represented in fig. 29, from the last 

 leg of a female. I have observed remains of what I consider 

 to be cilia in this section of the organ. The fourth region of the 

 organ is always placed close to the thin-walled collecting vesicle 

 (figs. 28 and 29). In the whole of the coiled tube just de- 

 scribed the epithelium is supported by a membrana propria, 

 which in its turn is invested by a delicate layer of peritoneal 

 epithelium. 



The fourth and fifth pairs are very considerably larger than 

 those behind, and are in other respects peculiar. The great 

 mass of each organ is placed behind the leg, on which the ex- 

 ternal opening is placed, immediately outside one of the lateral 

 nerve-cords. Its position is shewn in fig. 8. 



The external opening, instead of being placed near the base 

 of the leg, is placed on the ventral side of the third ring (count- 

 ing from the outer end) of the thicker portion of the leg. It 

 leads (fig. 27) into a portion which clearly corresponds with the 

 collecting vesicle of the hinder nephridia. This part is not, 

 however, dilated into a vesicle in the same sort of way, and the 

 cells which form the lining epithelium have not the same charac- 

 teristic structure, but are much smaller. Close to the point 

 where the vesicle joins the coiled section of the nephridium the 



