310 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



Primitively the nephridial tube of the vertebrate opens at its inner 

 end by a funnel-like nephrostome from the coelomic body-cavity or 

 splanchnocoele. One of its primitive functions is to get rid of the 

 fluid passed into that cavity by the coelomic lining. In correlation 

 with this the portion of coelomic lining facing the nephrostome develops 

 greatly increased powers of secreting coelomic fluid : it also becomes 

 increased in area, bulging into the coelomic cavity as a rounded swelling 

 — the glomerulus — and receives a special blood supply, a branch from the 

 aorta breaking up into a network in the interior of the glomerulus, in 

 close contact with the secretory epithelium. 



a.n.d 



Fig. 129. 



Diagram of renal tubule of vertebrate with its Malpighian bpdy. a.n.d, Arcbinephiic duct ; 

 b.v, blood-vessels — afferent and efferent ; gl, glomerulus, network of vessels closely ensheathed by 

 epithelial covering ; m.b, Malpighian body ; ns, nephrostome ; p, peritoneal cavity ; p.c, peritoneal 

 canal ; p.f, peritoneal funnel ; t, tubule. 



In the case of the pronephros the glomeruli are very large and undergo 

 fusion together, so that the group of pronephric tubules on each side has 

 a large compound glomerulus projecting into the coelomic cavity towards 

 them. In the opisthonephros on the other hand the small individual 

 glomeruli remain separate and the portion of coelome immediately 

 surrounding each one of them becomes more or less completely separated 

 off from the main splanchnocoele. Consequently each glomerulus in the 

 opisthonephros is surrounded by a narrow space enclosed by a mem- 

 branous wall, the whole constituting what is known as a Malpighian 

 body. Where a communication remains between the cavity of the 

 Malpighian body and the main splanchnocoele this communicating channel 

 is called the peritoneal eanal (Figs. 128, B, and 129, p.c) and its splanchno- 

 coelic opening the peritonear funnel (Figs. 128, B, and 129, p.f). 



Important changes also take place in connexion with the longitudinal 



