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 



Medusa, Tiaropsis Diadema, after Agassiz. The darkly- 

 shaded organs form the so-called coelenteric 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, Medusae, and Polypes, 



