212 



ZOOLOGY 



6. Reproduction exclusively sexual; development usually- 

 indirect, i.e., with a metamorphosis. 



243. General Survey. — Most echinoderms have a central 

 disc in which are located portions of the various sets of organs. 

 Ordinarily there radiate from this disc more or less clearly 

 defined rays or arms in which lie radial outgrowths from cer- 

 tain central organs. The spaces between the rays (inter-radii) 



Fig. 97. 



Fig. 97. starfish, from chart of Leuckart and Nitsche. 



Questions on the figure.— How would you describe the symmetry of the animal? 

 Identify and name, by comparison with the diagrams and the text, all the struc- 

 tures which show in the figure. Compare this with specimens or figures of the 

 common American species and note the chief differences. 



may be bridged by growth in such a way that the distinction 

 between rays and disc is not marked (echinoids). In crinoids 

 the arms may be much branched. The oral-aboral axis is usu- 

 ally pronounced, often short, and is vertical in position (aster- 



