EMBRYOLOGY OF ECHINODERMS. 



129 



The mouth of the echinoderm is now formed as a new 

 opening, which penetrates to the stomach of the pluteus 

 from the right side of the body, in the centre of the circle 

 of ambulacral feet. 



FIG. 75. 



FIG. 75. The same, a little older, from the dorsal surface, 

 from nature by Mr. B. P. Colton. ) 

 Letters as in Fig. 74. 



(Drawn 



The sea-urchin grows and protrudes more and more 

 from the opening, and the arms of the pluteus are finally 

 bent upwards so as to project from the ab-oral surface of 

 the body, as shown in Fig. 76. The integument of the 

 larva still covers the sea-urchin as a delicate, transparent, 

 outer skin, and the oral lobe can still be recognized for a 

 short time. The manner in which the arms finally disap- 

 pear is somewhat peculiar. The wall of the arm flows, 

 like a retracted pseudopodium, down onto the surface of 

 the body, leaving the bare spicule projecting from the 

 ab-oral surface. The spicules soon drop off, the dentary 

 apparatus is developed, and the young sea-urchin assumes 

 the form shown, from the oral side, in Fig. 77. 



