231 



till it meets the outer wall, and forms a junction with it (m, Fig. 

 97 ; o, digestive cavity). At this point, where the juncture takes 

 place, an aperture is presently formed, which is the true mouth. 

 The digestive sac, which has thus far served as the only internal 

 cavity, now contracts at certain distances, and forms three dis- 

 tinct, though connected cavities as in Fig. 96, viz., the oesoph- 

 agus leading directly from the mouth (m) to the second cavity or 

 stomach (d), which opens in its turn into the third cavity, the ali- 

 mentary canal. Meanwhile the water-tubes have been elongating 

 till they now surround the digestive cavity, extending on the other 

 side of it beyond the mouth, where they unite, thus forming a 



1 



Y-shaped tube, n 



branches toward the other end, Fig. 98. 2 



"On the surface where the mouth is formed, and very near it on 

 either side, two small ones arise, as v in Fig. 95 ; these are cords 

 consisting entirely of vibratile cilia. They are the locomotive or- 

 gans of the young embryo, and they gradually extend until they 

 respectively enclose nearly the whole of the upper and lower half 

 of the body, forming two large shields or plastrons (Figs. 98, 99). 

 The corners of these shields project, slightly at first (Fig. 98), 

 but elongating more and more until a number of arras are formed, 

 str etchi ng in various directions (Figs. 99, 100) 3 and, by their con- 



"> ail tt... Ii-.ir.-; t, t.'nta-!.- ofy-un- -tarlhh ; //. i,..a.-!i..>lar A V \-.-i,Ui^> : r, Lark ,.f/ 

 young starfish. Fig. 101, t\ odd tentacle. 



