(1) A vesicular portion opening to the exterior on the ventral surface of the legs by a narrow passage.
(2) A coiled portion, which is again subdivided into several sections.
(3) A section with closely packed nuclei ending by a somewhat enlarged opening.
(4) The terminal portion, which consists of a thin-walled vesicle.
The last twelve pairs of these organs are all constructed 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 constituted.
It will be convenient to commence with one of the hinder nephridia. Such a nephridium from the ninth pair of legs is represented in Fig. 11. The external opening is placed at the outer end of a transverse groove 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. 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.
Fig. 11.—Nephridium from the 9th pair of legs of P. capensis. o.s, External opening of segmental organ; p.f, internal opening of nephridium into the body cavity (lateral compartment); s, vesicle of segmental organ; s.c.1, s.c.2, s.c.3, s.c.4, successive regions of coiled portion of nephridium; s.o.t, third portion of nephridium broken off at p.f from the internal vesicle, which is not shown.
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 collecting vesicle of the hinder nephridia. This part is not, however, dilated 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.
Generative Organs.