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 



Fig. 2. 



Medusa, Tiaropsis DIadema, after Agasslz. 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, Medusas, and Polypes, 



