EMBRYOLOGY OF ECHINODERMS. 



129 



close thus over them. Pig. 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, hj 

 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 ; Pig. 169 represents the lower surface 

 at the same moment, with the open mouth (m), around whiQh 



Fig. 168. jlg 1,Q 



v\^(,^N 



^"^^^'^^Qf 



the tentacles (jt,") are just beginning to appear ; while Pig. 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 (Pig. 171), 

 and the dorsal plates have become quite distinct. Pig. 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, 



ng. 168. star-fish which has just resorbed the larva, seen from the bacic ; A madreporic opening. 

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



Fig. 170. Young Star-fiah which has become symmetrical, seen from the back ; c' odd tentacle^ 

 17 



