60 THE BIOLOO Y OF AN ANIMAL. 



tube contains cilia which are arranged in two longitudinal bands on the 

 inner surface. At g it passes into the 



3. " Middle tube" (m.t.) (g toft), extending straight through the second 

 loop, of greater diameter, ciliated throughout, and with piginented walls. 

 At h it opens into the 



4. " Wide tube" (w.t.). This is of still greater calibre, with granular 

 glandular walls and without cilia. It extends through the second loop 

 (from h to i, II) into and through the first from i to j, and finally into the 

 third, opening at k into the 



5. Muscular part or duct (m.p.) which forms the third loop and opens to 

 the exterior at ex. This, the widest part of the entire nephridium, has 

 muscular walls and forms a kind of sac or reservoir like a bladder, in 

 which the excreted matter may accumulate and from which it may be 

 passed out to the exterior. 



The various parts of the nephridium are held together by connective 

 tissue (p. 90), and are covered with a rich network of blood-vessels, the 

 arrangement of which is shown in Fig. 27, B. The smaller vessels usually 

 show numerous pouchlike dilatations which must serve to retard the flow 

 of blood somewhat. The vessels supplying the nephridium are connected 

 (Fig. 27, B) on the one hand with the sub-intestinal vessel through the 

 ventro-lateral trunks (v.l.) ; on the other hand with the sub-neural (s.n.) and 

 dorsal vessels, through the dorso-tegumentary (d.t.). The course of the 

 blood is somewhat doubtful. According to the view here adopted (cf. p. 56) 

 the blood proceeds from the dorso-tegumentary trunk to the nephridia and 

 thence through the ventro lateral to the sub-intestinal, as shown by the 

 arrows in the figure. Benham (from whom the figures are copied) adopts 

 the reverse view. The development of the nephridium shows that its 

 ciliated and glandular portions arise from a solid cord of disk-shaped cells 

 which afterwards becomes tubular by the hollowing out of its axial portion. 

 The tube is therefore comparable to a drain-pipe in which each cylinder 

 represents a cell. Its cavity is not intercellular (between the cells, like the 

 alimentary cavity), but intracellnlar (witliin the cells, like a vacuole). 



The mode of action of the nephridia is as yet only partially 

 understood, though there is no doubt regarding their general char- 

 acter. It is certain that their principal office is to remove from 

 the body waste nitrogenous matters resulting from the decompo- 

 sition of proteids ; and there is reason to believe that these waste 

 matters are passed out either as urea ( [KH 8 ] S CO) or as a nearly 

 related substance, together with a certain quantity of water and 

 inorganic salts. 



Excretion in Lwnbricus appears, however, to involve two quite distinct 

 actions on the part of the nephridia. In the first place the glandular walls 

 of the tube, which are richly supplied with blood-vessels, elaborate certain 

 liquid waste substances from the blood and pass them into the cavity of 



