ASTERIAS. 157 
middle of the upper surface is a minute aperture, the 
anus, and placed eccentrically in one inter-radius is a round, 
slightly convex, perforated plate called the madreporite. 
The line drawn through anus and madreporite and 
continued on either side gives the direction of the perpen- 
dicular plane of symmetry. The whole upper surface of the 
body is covered with a scattered mass of ossicles, calcareous 
nodules which can be seen and felt ¢hvough the extremely 
thin and delicate ectoderm. 
Fig. 90.—TRANSVERSE SECTION OF THE ARM OF 
ASTERIAS RuBENS. (Diagrammatic.) 
Adambulacral Ossicle. 
: Ambulacral Ossicle. - 
Radial Nerve. 
Radial Blood-vessel. 
Ampulla. 
The general cavity is the hypogastric ccelom. 
On the under or oral side of the starfish the mouth is 
situated in the centre. Along the centre of each arm there 
runs a deep groove, the ambulacral groove. This is filled 
with two double rows of ambulacra or tube-feet, a number of 
small processes terminating in suckers. They form the 
locomotive organs of the animal. If the double rows be 
pressed aside from the middle line, there can be seen a 
delicate vadiaZ nerve in the middle of the groove. Further, 
if the tube-feet be cut away it will be noticed that they 
