118 ZOOLOOY 



SECT. 



taining an abundant supply of blood -vessels. The tubules are 

 lined by a single layer of glandular epithelial cells (B, C) and 

 each ends blindly in a globular dilatation, the Malpighian capsule 

 (A, f/l.), lined with squamous epithelium. In many of the lower 

 Craniata, a branch goes off from the tubule, near the Malpighian 

 capsule, and, passing to the ventral surface of the kidney, ends 

 in a ciliated funnel-like body (Fig. 791, oist.), resembling the 

 nephrostome of a worm, and, like it, opening into the coelome. 

 At their opposite ends the tubules join with one another, and 

 finall}^ discharge into the ureter. 



The renal arteries branch extensively in the kidney, and five 

 off to each Malpighian capsule a minute afferent artcvy (Fig. 790, 

 A, va.) : this pushes the wall of the capsule before it, and breaks 

 up into a bunch of looped capillaries, called the glomerulus, sus- 

 pended 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 

 coelomoducts of Annulata which would hardly be suspected from its 

 adult structure. The pronephros (Fig. 791, A, p. npili.) 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 

 {nst.). Obviously such tubes are coelomoducts : their chief pecu- 

 liai-ity is that their outer ends do not open directly on the exterior, 

 but into a longitudinal tube, the proncpliric or segmental duet 

 (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 three tubules 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 glomerulus. 



The pronephros soon degenerates, its tubules losing their 

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

 fresh tubules appear in the segments posterior to the pro- 

 nephros, and together constitute the mesonephros or Wolffian hody 

 (B, ms. nph.), from which the permanent kidney is formed in most 

 of the lower Craniata. The mesonephric tubules open at one 

 end into the pronephric duct {sg. d.), at the other, by ciliated 



