TYPES AND FAMILIES, 31 
arranged radially round an axis, passing through the 
dorsal and ventral pole. The cavity, which in most 
other animals—for instance, in man—is termed the 
abdominal cavity, the space between the intestinal wall 
and the abdominal parietes, is deficient in them ; but, 
on the other hand, from the stomach proceed in general 
various kinds of tubes and branchia, which to a certain 
extent replace the abdominal cavity. Fig. 2 represents a 
Fic, 2 
Medusa, Tiaropsis Diadema, after Agassiz. The darkly- 
shaded organs form the so-called ccelenteric apparatus. 
Of the Echinoderms, the reader is probably ac- 
quainted, at least with the star-fish (Asterias) and the 
sea-urchin (Echinus), of which the general form is like- 
wise usually radiate. Besides a peculiar chalky deposit, 
or greater or less calcification of the skin covering, a 
system of water-canals forms a characteristic of this 
family. With these are connected the rows of suckers, 
which, by protrusion and retraction, serve as organs of 
locomotion. On account of the radiate structure pre- 
vailing among the Echinoderms, Medusz, and Polypes, 
