EMBRYOLOGY OF ECHIXODERMS. 



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pre-oml arms, and the spicules whidi run into this lobe 

 bend forward atf, or run into 

 these arms to form their sup- 

 porting framework. 



At the point where the spic- 

 ule (y) , bends forward, it gives 

 rise to a very small process (Z), 

 which points towards the mid- 

 dle of the dorsal surface, and is 

 to become a transverse dorsal 

 bar. In a dorsal view at this 

 stage it is easy to see that the 

 ciliated ridge which fringes the 

 post-oi*al arms (c), bends back 

 towards the dorsal surface at 

 k, and runs forward along the 

 edge of the oral lobe («), and 

 pre-oral tentacles (a' «'). It 

 therefore forms a closed circlet 

 around the mouth. The pos- 

 terior end (6), of the body is 

 now quite transparent, and the 

 ends of the two long, lateral 

 spicules (<Z), have fused with 

 each other, thus fomiing a 

 large, irregular , perforated 

 mass (i?), which is covered 

 with small pigment spots. 



The different regions of the 

 digestive tract are much more fig 

 sharply distino^uished than they at the same stage, while swim- 

 were at earlier stages. The ^"1' J^T^ ^"""^ "^"^ ^^ 

 ® W. K. Brooks. ) 



mouth (m), is a large, circular Letters as in Fig. 65. 



Fig. c6. 

 . — Side view of a larva 



