EMBRYOLOGY OF ECHINODERMS. 



127 



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

 quite a compHcated 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 



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

 gestive cavity, a number of lobes are formed (t, 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 



Fig. IM. Adult Larva, so-called Brachiolaria, lettering as before ; r back of young Star-fish, / ten- 

 Ucles of young Star-flsh, ff brachiolar appendages. 



