140 



THE STARFISH 



specimens collected in nature in which the number of arms is abnor- 

 mal. How might such conditions arise? 



Exercise 10. — Reproduction and Development, 

 (b) The ova and spermatozoa of the starfish and those of many- 

 other marine animals are discharged directly into the water where 



'^^^m 



Fig. 64. — Development of the starfish. Bipinnaria from (^4) ventral and (5) 

 lateral view. Very young starfish from (C) aboral and (D) oral view. 



(From W. K. Brooks, "Handbook of Invertebrate Zoology," copyright, 1882, by S. E. 

 Cassino, reprinted by permission.) 



fertilization occurs. Such ova are usually small and have but little 

 yolk; they develop rapidly into larvae, which swim and feed for a time 

 and then take up the life of the parent upon the bottom. Since their 

 normal environment of sea water is easily provided in the laboratory, 

 these eggs are particularly favorable for study; they have become 

 classic material for observations and experiments on fertilization, arti- 

 ficial parthenogenesis, and early development. 



