i6o ARCHICCELOMATA. 



{e.g., Arenicola) as a general feature of the whole coelom. It 

 consists mainly of a ring-canal round the mouth, five radial 

 vessels just below the heads of the ambulacral ossicles, 

 and a single inter-radial stone-canal running in the median 

 mesentery to the madreporite, through which it communi- 

 cates with the exterior. Each radial canal gives off paired 



Fig. 93.— Diagram of the Water-Vascular System 

 OF THE Common Starfish. 



(Altered from Gegenbaur.) 



\\ - ..Madreporite. 



Stone Canal. _ ~' '^^ - 



Radial Canal. 

 ,Ring Canal. 



lateral canals which lead to the round vesicles or ampullce 

 seen as a double row on the inside of the ambulacral ossicles. 

 These ampullae communicate with the tube-feet, which, as 

 noticed before, protrude into the ambulacral groove, between 

 the ambulacral ossicles. The walls of this system are mus- 

 cular, and it works as a hydraulic system by means of the 

 tube-feet. 



The rest of the coelom is formed by (2) a large and spacious cavity, 

 the hypogastric cavity, surrounding the lower part of the stomach, 

 produced into the arms and forming a median mesentery in the 

 madreporic inter-radius. (3) Aboral to this, lying on the stomach and 

 produced into each arm along the aboral surface of the pyloric glands, is 

 the epigastric cavity. Its walls form, with those of the hypogastric 

 cavity, the two mesenteries along each hepatic caecum. The hypo- 

 gastric cavity is produced along the oral surface of each arm, inside 

 the radial nerve, to form a pair of perihsemal cavities. The inner walls of 

 these cavities form a median mesentery, in which is contained the radial 



