PHYLUM CHORDATA 



111 



urinary tubules : finally, the blood is returned from this network 

 to the renal vein. The watery constituents of the urine are 

 separated from the blood in traversing the glomerulus, and, 

 flowing down the tubule, take up and dissolve the remaining 

 constituents urea, uric acid, &c. which are secreted by the 

 cells of the tubules. 



The development of the kidney reveals a resemblance to the 

 nephridia of worms which would hardly be suspected from its 

 adult structure. The pronephros (Fig. 747, A, p. nph.) originates 

 as two or three coiled tubes formed from mesoderm in the body- 

 wall at the anterior end of the coelome ; they are arranged meta- 

 merically and each opens into the coelome by a ciliated funnel 





FIG. 740. -A, part of a urinary tubule with blood-vessels, ai, artery ; gl, Malpighian capsule con- 

 taining glomerulus ; r. veinlet returning blood from capillary network (to the right) to vein 

 vi ; ra afferent vessel of glomerulus ; re, efferent vessel. B, longitudinal, and C, transverse 

 sections of 'urinary tubules, a, secreting part of tubules ; b, conducting part of tubules ; 

 c. capillaries ; n. nuclei. (From Foster and Shore's Physiology.) 



(nst.). Obviously such tubes are mcsonephridia : their chief pecu- 

 liarity is that their outer ends do not open directly on the exterior, 

 but into a longitudinal tube, the arckinephric or segmented duct 

 (sg. d.\ which passes backwards and discharges into the cloaca. 

 It seems probable that this arrangement is to be explained by 

 supposing that the nephridia originally opened externally into a 

 longitudinal groove, which, by the apposition of its edges, was 

 converted into a tube. All three nephridia of the pronephros 

 open, by their ciliated funnels, into the narrow anterior end of 

 the coelome, into which projects a branch of the aorta ending in 

 a single large glomeruliH. 



The pronephros soon degenerates, its nephridia losing then- 

 connection with the segmental duct (B), but in the meantime 

 fresh nephridia appear in the segments posterior to the pro- 

 nephros, and together constitute the mesonephros or Wolf/fan body 



