EMBEYOLOGY OP ECaaiNODERMS. 



131 



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

 of Echinoderms, 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 Searurchin, as they differ in some poinl^ 

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

 Searurchins (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 inay be seen 



Kg. 176. 



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

 are discernible, though the limestone rods are quite distinct. 

 Mgs, 173, 174, 176, 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 



