30 



BEGINNERS' ZOOLOGY 



How many mature hydranths are seen in the hydroid 

 shown in Fig. 40? Why are the defensive hydranths 



on the outside of the 

 colony ? Which hy- 

 dranths have no tenta- 

 cles ? Why not ? 



Jellyfish. — Alterna- 

 tion of Generations. — 

 Medusa. — With some 

 species of hydroids, a 

 very curious thing hap- 

 pens. — The hydranth 

 that is to produce the 

 eggs falls off and be- 

 comes independent of 

 the colony. More sur- 

 fig. 43. -a jellyfish. prising still, its appear- 



ance changes entirely and instead of being hydra-like, it 

 becomes the large and complex creature called jellyfish 

 (Fig. 43)- But 

 the egg of the 

 jellyfish pro- 

 duces a small 

 hydra- like ani- 

 mal which gives 

 rise by budding 

 to a hydroid, 

 and the cycle is 

 complete. 



The bud (or 

 reproductive 

 hydranth) of 

 the hydroid Fig. 44. — A Jellyfish (medusa). 



