68 



CCELENTERATES 



medusae are set free from the 

 hydroids, they swim about and 

 capture their own food. Each 

 medusa is provided with either 

 ovaries (o'va-riz), organs which 

 grow egg cells, or spermaries 

 (speYma-riz), organs which 

 grow sperm cells. When the 

 eggs and sperms mature, they 

 are discharged into the water. 

 A single sperm cell must fuse 

 with an egg cell before the 

 egg can begin to grow. This 

 union of these two cells is 

 called fertilization. The egg 

 grows into an embryo (em'- 

 bri-o), an immature stage dif- 

 fering in different animals, 

 and this gradually changes into 

 a small hydroid. The several 

 steps in this complicated series 

 of changes are illustrated in 

 Figure 69. The hydroids 

 and medusa3 show a form of 

 reproduction called alternation 

 of generations, that is, they 

 reproduce alternately sexually 

 and then asexually. 



53. Sea-anemone. — Sea-anemo- 

 nes are animals allied to the 

 hydra. The interior of the 



J <^k 



Figure 69. — Pennaria Tiarella. 



a. The hydroid colony ; b, one 

 of the female medusae, much 

 enlarged ; c, the egg of the 

 medusas beginning to segment 

 after it has been fertilized; 

 d, e, f, further segmentation 

 stages ; g, the blastula stage ; 

 h, the free swimming larva 

 (planula) ; i\ /, and k show the 

 gradual transformation of the 

 larva into a hydra-like colony. 

 Branches grow on the stage 



shown in k until a colony like a results. This is the form that alterna- 

 tion of generations takes in this hydroid. (Arranged from a monograph 

 on Pennaria by C. W. Hargitt.) 



