Chap. 32 ECHINODERMS FORERUNNERS OF THE VERTEBRATES 



659 



Radial 

 canal 



Structures of 

 adult become 

 opporent 



Division of coelomic 

 sacs into anterior 

 and posterior ports 



Late Gastrulo showing 

 Stan of coelomic pouches 



Fig. 32.8. Diagrams to show the development of the starfish. This is one more 

 example of the similarity of the early processes of development among animals 

 that are later as different as worms and echinoderms. It is clearly shown here in 

 the blastula and gastrula stages. The diagrams of the later stages can probably 

 mean little without a special study of the embryology of starfishes. Even in these 

 stages it is clear that the starfish has a two-sided symmetry before it attains the 

 five-sided one. (Courtesy, Hunter and Hunter: College Zoology. Philadelphia, 

 W, B. Saunders Co., 1949.) 



is begun. The partly digested food is sucked into the posterior or pyloric part 

 of the stomach into which five pairs of conspicuous digestive glands open. 

 They are hollow so that food passes into them freely, and their hnings are 

 provided with cilia that keep the contents astir. Their surfaces are greatly in- 

 creased by infoldings and their cells produce powerful protein-splitting 

 enzymes which complete the digestion of the food eventually absorbed 

 through their walls (Fig. 32.6). Free-moving ameboid cells are abundant in 

 the digestive tract and they digest food just as similar ones do in hydra and 

 in the clam. Practically no indigestible food is consumed by common starfishes 



