STARFISH 121 



entoderm cells are giving off amoeboid cells, 

 which move about in the segmentation cavity, 

 and finally form a third layer, or mesoderm, 

 between the other two. 

 Draw a gastrula. 



6. The later larval stages. — If material be obtain- 

 able, the student can trace the change of the 

 gastrula into the Bipinnaria stage, and from 

 this the development of the young starfish, 

 noting especially the extreme changes of form 

 and structure, and drawing each stage. 



Use the sand-star {Ophiopholis), sea-urchin {Echinus 

 or Ariacid), sand -cake {EchinaroGhnius), and sea -cu- 

 cumber {Pentacta). The examination of the skeletons 

 of the first three is especially instructive, and a very 

 valuable and interesting study may be made of the 

 manner in which the same fundamental plan of struct- 

 ure is modified in various ways in these different organ- 

 isms. 



