THE CGELOM AND MESODERM 271 



involuntary, and innervated by the autonomic nervous system, 

 except for those which are situated in the anterior region of 

 the body, in connexion with the gill-slits. The gill-slits 

 pierce through from the gut to the outside in the region of the 

 lateral plate ; between the gill-slits, in the visceral arches, the 

 lateral-plate mesoderm gives rise to the muscles which move 

 the arches, including the jaws. These muscles are striated and 

 voluntary, but they are not myotomic, and they are innervated 

 by visceral efferent fibres through the dorsal roots of the 

 cranial nerves. 



Between the myoccels and the splanchnocoels there are 

 typically little hollow stalks, through which at early stages the 

 cavities of the latter can communicate with those of the former. 

 They are segmental in arrangement. In Amphioxus, these 

 regions of the coelom represent the future gonads, and are 

 called the gonotomes with their cavities the gonocoels. In the 

 Craniates, they are called the nephrotomes (or intermediate 

 cell-masses) ; the cavities (communications between the 

 myocoels and the splanchnocoel) are the nephrocoels, and they 

 give rise to the tubules of the kidneys and associated structures, 

 eventually losing connexion both with myocoels and splanch- 

 nocoel. 



In Amphioxus the splanchnocoel is continuous from end 

 to end of the body as in the Ammocoete, for the transverse 

 septum in which the ductus Cuvieri crosses over from the 

 body- wall to the gut- wall, is not large. In Selachians, the 

 transverse septum separates an anterior pericardial cavity from 

 a posterior peritoneal or perivisceral cavity, leaving only very 

 small communications between them in the form of the 

 pericardio-peritoneal canals. In higher forms the separation 

 between pericardial and perivisceral cavities is complete. 

 Beginning in the Dipnoi, the pericardium becomes thin- walled 

 and projects backwards into the perivisceral cavity. 



All viscera are morphologically outside the coelomic cavity 

 and only suspended in it by a bag of coelomic epithelium which 

 forms a double membrane or mesentery. So the gut is 

 suspended by the dorsal mesentery from the roof of the 

 perivisceral cavity, and between the two membranes composing 



