160 ARCHICELOMATA. 
(¢.g., Arenicola) as a general feature of the whole ccelom. 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 s/one-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.) 
q 3 Madreporite. 
Stone Canal. 
Radial Canal. 
C ) Ring Canal. 
Ampulla. 
Tube-foot,—_ > 
lateral canals which lead to the round vesicles or ampulle 
seen as a double row on the inside of the ambulacral ossicles. 
These ampullze 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 ccelom 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 perihzemal cavities. The inner walls of 
these cavities form a median mesentery, in which is contained the radial 
