THE CCELENTERATES AND SPONGES 



135 



charged from the body and commonly develop, not into 

 jelly fish, but into hydroids. We thus have what is called 

 alternation of generations, jelly fish producing a hydroid 

 (the asexual generation) which gives rise again to a jelly 

 fish. Not all species pass through this alteration of 

 generations. Just as there are hydroids which, like the 

 fresh-water Hydra, have no 

 medusa stage but develop di- 

 rectly from eggs produced by 

 other hydroids, so there are 

 medusae which produce eggs 

 that develop directly into me- 

 dusae and have no hydroid stage. 

 In the group of Siphono- 

 phores the division of labor 

 which has been noted among 

 certain hydroids reaches an 

 extreme development. The 

 siphonophores are all free-swim- 

 ming or floating colonies made 

 up of numerous individuals 

 modified in various ways, some 

 for swimming, some for taking 

 food, others for protection, and 

 still others for reproduction. 



One of the largest species is the Portugese man-of-war 

 which is furnished with a long oblong float filled with 

 gas, by which the colony rides upon the surface of the 

 waves. From the lower side of the float hang down 

 clusters of variously modified individuals and very con- 

 tractile tentacles which may extend to a length of fifty 

 feet. These are richly furnished with nettling cells, and 

 the poison they contain is so virulent that even a slight 

 con tact with a tentacle may produce considerable irritation. 



FIG. in. Two species of 

 jelly fish from the Tortugas. 

 (After Mayer.) 



