EMBRYOLOGY OF ECHINODERMS. 



115 



pre-oral arms, and the spicules which run into this lobe 

 bend forward at/", or run into 

 these arms to form their sup- 

 porting frame work. 



At the point where the spic- 

 ule (/), bends forward, it gives 

 rise to a very small process (7), 

 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-oral 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 (c?), have fused with 

 each other, thus forming a 

 large, irregular, perforated 

 mass (^>), which is covered 

 with small pigment spots. 



The different regions of the 



t. 



digestive tract are much more 



FIG. 66. 



FIG. 66. Side view of a larva 

 sharply distinguished than they at the same stage, while swim- 

 were at earlier stages. The in f; < Dra from nature by 



W. K. Brooks.) 



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



