EMBRYOLOGY OF ECHINODERMS. 



129 



close thus over them. Fig. 168 represents the upper surface of 

 the Star-fish just before this junction takes place. The compli 

 cated structure of the Brachiolaria, as the larva of the Star-fish 

 has been called, hitherto so essential to the life of the animal, by 

 which it has been supported, moved about in the water, and. pro 

 vided with food during its immature condition, has made a final 

 contribution to its further development by the process of resorp- 

 tion described above, and has wholly disappeared within the Star 

 fish. At this stage the rays are only just marked out, as five 

 lobes around the margin ; Fig. 169 represents the lower surface 

 at the same moment, with the open mouth (w), around which 



Fig. 168. Fig. 170. 



the tentacles (t) are just beginning to appear ; while Fig. 170 

 shows us the animal at a more advanced stage, after the two sur 

 faces have united. It has now somewhat the outline of a Maltese 

 cross, the five arms being more distinctly marked out, while the 

 tentacles have already attained a considerable length (Fig. 171), 

 and the dorsal plates have become quite distinct. Fig. 172 rep 

 resents the same animal, at the same age, in profile. This period, 

 in which we have compared the form of the Star-fish to that of a 

 Maltese cross, is one of long duration ; two or three years must 

 elapse before the arms will elongate sufficiently to give it a star- 

 shaped form, and before the pedicellariae make their appearance, 



Fig. 163. Star-fish which has just resorbed the larva, seen from the back ; b madreporic opening. 

 Fig. 169. Fig. 168, seen from the mouth side ; m mouth, t tentacles. 



Fig. 170. Young Star-fish which has become symmetrical, seen from the back ; t odd tentacle. 

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