114 



ZOOLOGY 



SECT. 



bunch of looped capillaries, called the glomerulus, suspended in the 

 interior of the capsule. The blood is carried off from the glomerulus 

 by an efferent vessel (ve.), which joins the general capillary system 

 of the kidneys, forming a network over the 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 

 ccelomoducts of Annulata which would hardly be suspected from its 

 adult structure. The pronephros (Fig. 801, A, p. nph.) originates 

 as a small number of coiled tubes formed from mesoderm in the 

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



FIG. 800. A, part of a urinary tubule with blood-vessels, at. artery ; gL Malpighian capsule 

 containing glomerulus ; v. veinlet returning blood from capillary network (to the right) to 

 vein j)i. ; va. afferent vessel of glomerulus ; ve. 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.) 



metamerically, and each opens into the ccelome by a ciliated funnel 

 (nst.). Obviously such tubes are coelomoducts : their chief 

 peculiarity is that their outer ends do not open directly on the 

 exterior, but into a longitudinal tube, the pronephric or segment al 

 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 coelomoducts originally opened externally into 

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

 converted into a tube. All the tubules of the pronephros open, 

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

 into which projects a branch of the aorta ending in a single large 

 glomerulus. 



The pronephros soon degenerates, its tubules losing their connec- 

 tion with the pronephric duct (B), but in the meantime fresh tubules 



