EMBRYOLOGY OF ECHIN01IERMS. 



127 



At this stage of the growth of the embryo, we have what seems 

 quite a complicated structure, and might be taken for a complete 

 animal ; this is after all but the prelude to its true Star-fish exist- 

 ence. While these various appendages of the embryo have been 

 forming, changes of another kind have taken place ; on one of the 



Vis- 166. 



two water-tubes above mentioned (w') at the end nearest the di- 

 gestive cavity, a number of lobes are formed (, Fig. 166) ; this is 

 the first appearance of the tentacles. In the same region of the 

 opposite water-tube (w) a number of little limestone rods arise, 

 which eventually unite to form a continuous network ; this is the 



Fi? 166. Adult Larva, so-called Brachiolaria, lettering as before ; r back of young Star-fish, t ten- 

 tack-s of young Star-fish, //' bracliiolar appendages. 



