NERVOUS SYSTEM 615 



beat ; sympathetic fibres cause contraction of the arteries of the 

 penis, parasympathetic fibres their dilatation, and so the erection 

 of the organ. 



THE EXCRETORY SYSTEM 



The excretory system of vertebrates is derived from a series 

 of paired segmental coelomic tubules, which may be known col- 

 lectively as a holonephros. It is customary and convenient to 

 divide this into three sections, by time of development and 

 position in the body, but there are no fundamental differences 

 between them. Most anterior and earliest to appear is the pro- 

 nephros, behind this and a little later is the mesonephros, and 

 further back and later still is the metanephros, which is only 

 well developed in the amniotes. The tubules of all these divisions 

 have the same basic structure, and it has been shown experi- 

 mentally that their physiology is also essentially the same. 



All the tubules are derived from a segmented portion of the 

 mesoderm, which lies between the somites (or myotomes) and 

 the lateral plate. On each side of each somite an intermediate 

 cell mass becomes organised as a hollow nephrotome, containing 

 a cavity, the nephrocoele ; this is part of the coelom (p. i88) and 

 early communicates with the myocciele, and for much longer with 

 the splanchnocoele. One part of its wall grows out dorsally or 

 laterally as the excretory tubule, and another is pushed in as 

 Bowman's capsule, the wall of which becomes thin and into which 

 the blood vessels of the glomerulus grow. The tubules of adjacent 

 segments communicate so that they have a common duct, 

 and this grows back as the segmental duct to the cloaca. The 

 opening of the tubule into the cavity of Bowman's capsule is 

 the nephrocoelostome (the alternative name nephrostome may 

 cause confusion with a structure of different nature in annelids) 

 and the opening from Bowman's capsule to the splanchnocoele 

 is called the peritoneal funnel. The resulting simple structure 



is shown in Fig. 473 A. 



This arrangement of tubules is found very nearly for the full 

 length of the body in the hagfish Bdellostoma, and in the Gymno- 

 phiona. The organ in these animals used to be described as a 

 functional pronephros, but it is better to think of it as an 

 undifferentiated holonephros ; there are differences between the 

 anterior and posterior tubules, but any change is gradual and it 



