EMBRYOLOGY OF ECHIXODERMS. 



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. "5. 

 Ftg. 75. — The same, a little older, from the dorsal siirface. 

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

 Letters as in Fis;. 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 sho^\Ti 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, lea\ing 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. 



