562 CLASS II. ONYCHOrHORA. 



two lateral, and a pericardial (Fig. 345 D). The former is by 

 far the largest, and contains the alimentary tract, the generative 

 organs, and the slime glands. It is lined by a delicate endo- 

 thelial layer, and is not divided into compartments nor traversed 

 by muscular fibres. The lateral divisions are much smaller than 

 the central, and are shut off from it by the inner transverse band 

 of muscles. They are almost entirely filled with, the nerve-cord 

 and salivary gland in front and with the nerve-cord alone behind, 

 and their lumen is broken up by muscular bands. They further 

 contain the nephridia. They are prolonged into the feet, as is 

 the embryonic body cavity of most Arthropoda. The pericardium 

 contains a peculiar cellular tissue, probably, as suggested by 

 Moseley, equivalent to the fat-bodies of insects. 



Nephridia are present in all the legs, excepting those of the 

 segment bearing the genital opening. Most of them are con- 

 structed in a very similar manner, while the two pairs situated 

 in the fourth and fifth pairs of legs are considerably larger than 

 those behind, and are in some respects very differently con- 

 stituted, and the first three pairs show some reduction. 



A nephridium from the ninth pair of legs of Peripatus capensis 

 is represented in Fig. 342. The external opening is placed at 

 the inner end of a transverse groove (coxal organ) at the base 

 of one of the legs, while the main portion of the organ lies in the 

 body cavity in the base of the leg, and extends into the trunk to 

 about the level 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 conspicuous. The nuclei are arranged in several 

 rows. It ends by opening into a vesicle (Fig. 345 D), the wall 

 of which is so delicate that it is destroyed when the nephridium 



