130 



MARINE ANIMALS OF MASSACHUSETTS BAY. 



and it is only then that it can be at once recognized as the young 

 of our common Star-fish. Even then, after it has assumed its ulti- 



Fig. 171. 



mate outline, it lacks some features of the adult, having only two 

 rows of tentacles, whereas the full-grown Star-fish has four. 



/Sea-urchins. 



This extraordinary process of development which we have ana- 

 lyzed thus at length in the history of the Star-fish, but which is 



Fig. 174. 



Fig. 173. 



equally true of all Echinoderms, has been hitherto described (so 

 far as it was known) under the name of the pluteaii stages of 



Fig. 171. Lower side of ray of young Star-fish ; m mouth, b madreporic body, e. eye-speck. 



Fig. 172. Young Star-fish seen in profile ; t' odd tentacle at extremity of arm. 



Figs. 173, 174, 175. Young larvae of Toxopneustes in different stages of development ; c'-e IT arms, 

 v-v" vibratile chord, wiv' earlets (water-tubes), aod c digestive system, r'-r'" solid rods of arras, 

 m mouth, b madreporic opening. 



