274 



THE RISE OF ANIMAL LIFE 



blost^ula 



blpinnorio 



Fig. 12-25. Life cycle of the starfish. 



Sexes of the starfish are separate. The re- 

 productive system consists of five paired 

 gonads, lying close to the hepatic caeca and 

 attached in the ano;les between the arms 

 where their external openings are located 

 (Fig. 12-21). Ova and sperm cells are dis- 

 charged into the sea water, where fertiliza- 

 tion occurs ( Fig. 12-25 ) . The larva, or bipin- 

 naria, is at first bilaterally symmetrical; 

 later, as the pentagonal shape of the adult 

 form appears, radial symmetry becomes evi- 



dent. Larval forms are partially ciliated and 

 free-swimming. After a period of swimming 

 near the surface of the water, sometimes for 

 several weeks, the larva finally drops to the 

 bottom of tlie sea, where it undergoes meta- 

 morphosis into the adult starfish. 



Other echinoderms 



While the starfish is the best-known 

 echinoderm, there are other forms belong- 

 incr to different classes that show some in- 



Fig. 12-26. Various kinds of echinoderms. 



