EMBRYOLOGY OF ECHINODERMS. 



131 



growth. In these early stages the young, or the so-called larvae 

 of Eehinoderms, have received the name of Pluteus on account 

 of their ever-changing forms. Let us look for a moment at the 

 plutean stages of the Sea-urchin, as they differ in some points 

 from those of the Star-fish. In the Pluteus of our common 

 Sea-urchins (see Fig. 176), the arms are supported by a frame- 

 work of solid limestone rods, which do not exist in that of 

 the Star-fish, and which give to the larva of the Sea-urchin a re- 

 markable rigidity. They are formed very early, as may be seen 



Fig. 175. 



in Fig. 173, representing the little Sea-urchin before any arms 

 are discernible, though the limestone rods are quite distinct. 

 Figs, 173, 174, 175, may be compared with Figs. 160, 162, 165, 

 of the young Star-fish, where it will be seen that the general out- 

 line is very similar, though, on account of the limestone rods, the 

 Pluteus of the Sea-urchin seems somewhat more complicated. In 



