IX MUSCLES, COELOME 307 



arrangements of their own; but these are originally simply prolonga- 

 tions from the ends of the myotomes which sprout out into the fin 

 rudiments and become eventually separated ofE from their parent 

 myotomes. 



The more ventrally situated portion of the coelome or splanchnocoele 

 has in the Dogfish, as in all typical vertebrates, completely lost all trace 

 of segmentation. It forms even in young stages a continuous cavity 

 from end to end. In the later stages of development, however, a secondary 

 division of the splanchnocoele takes place into a small anterior chamber 

 containing the heart — the pericardiac cavity — and a main portion con- 

 taining the greater part of the alimentary canal — the peritoneal cavity. 

 In the Dogfish and its allies, though not in the higher vertebrates, these 

 cavities remain connected with one another by a slender pericardio- 

 peritoneal canal which extends back from the pericardiac cavity, just 

 dorsal to the heart, and opens into the splanchnocoele by an inconspicuous 

 slit on each side on the ventral surface of the oesophagus (Fig. 134, /'/>c). 



As already mentioiied there is reason to believe that the coelomic 

 cavities were originally paired, consisting of left and right halves. It is 

 characteristic of the vertebrate that, while these two halves become 

 completely continuous with one another across the mesial plane on the 

 ventral side of the alimentary canal, they remain separated frota one 

 another on its dorsal side by a thin membranous partition. This is the 

 mesentery, which serves to sling up the alimentary canal to the roof 

 of the peritoneal cavity and incidentally constitutes a bridge by which 

 blood-vessels and nerves pass to the enteric wall. 



The Vertebrate, like the Annelid, possesses numerous pairs of nephridial 

 tubes, but they show this distinctive peculiarity that, instead of opening 

 independently on the surface (Fig. 127, A), the whole series of tubes on 

 each side of the body opens into a longitudinal duct which in turn opens 

 posteriorly into the cloaca or hinder portion of the alimentary canal 

 (Fig. 127, B). The whole complex of tubes on each side of the body is 

 termed the archinephros and the longitudinal duct is termed the archi- 

 nephric duet (Fig. 127, B, a.n.d). Comparing the available stages in the 

 evolution of vertebrates, as disclosed to us by different stages of individual 

 development and by the arrangements in adult members of the various 

 subdivisions of the phylum, it is seen that the archinephros has in the 

 course of evolutionary history undergone modifications of a characteristic 

 kind. In its individual development the vertebrate develops from the 

 head end backwards, segmentally arranged organs developing serially 

 one after the other. Consequently we should expect the tubes of the 

 archinephros to appear in regular sequence, one after the other from the 



