NEPHRIDIA 



of the outer edge of the nerve-cord of its side. The external 

 opening (o.s) leads into a narrow tube (s.d), which gradually 

 dilates into a large sac (s). The narrow part is lined by small 

 epithelial cells, which are directly continuous with and perfectly 

 similar to those of the epidermis. The sac itself, which forms a 

 kind of bladder or collecting vesicle for the organ, is provided 

 with an extremely thin wall, lined with very large flattened cells. 

 The second section of the nephridium is formed by the coiled 

 tube, the epithelial lining of which varies slightly in the different 

 parts. The third section (s.o.t), 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 render this part extremely 



FIG. 11. Nephridinm from the 

 9th pair of legs of P. capensis. 

 o.s, External opening of seg- 

 mental organ ; p.f, internal 

 opening of nephridium into 

 the body cavity (lateral com- 

 partment) ; s, vesicle of seg- 

 mental organ ; s.c. 1, s.c. 2, 

 s.c.S, s.c. 4, successive regions 

 of coiled portion of nepliri- 

 dium ; s.o.t, third portion of 

 nephridium broken off at^j./ 

 from the internal vesicle, which 

 is not shown. 



conspicuous. The nuclei are arranged in several rows. It ends 

 by opening into a vesicle (Fig. 14, D), the wall of which is so 

 delicate that it is destroyed when the nephridium is removed 

 from the body, and consequently is not shown in Fig. 11. 



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 external 

 opening is placed, immediately outside one of the lateral nerve- 

 cords. The external opening, instead of being placed near the 

 base of the leg, is placed on the ventral side of the third ring 

 (counting from the outer end) of the thicker portion of the leg. 

 It leads into a portion which clearly corresponds with the collect- 

 ing vesicle of the hinder nephridia. This part is not, however, 

 diluted into a vesicle. The three pairs of nephridia in the three 

 foremost pairs of legs are rudimentary, consisting solely of a 

 vesicle and duct. The salivary glands are the modified nephridia 

 of the segment of the oral papillae. 



VOL. v c 



