CLASS HYDROZOA. 13 



pinge upon the nervous walls of the sac, giving to the 

 animal the sensation of sound. Here, then, we meet for 

 the first time with organs of hearing and with a con- 

 nected nervous system. 



As the Medusa slowly propels itself through the water, 

 stretching out behind is a series of long streamers armed 

 with lasso-cells and of use to the animal in procuring its 

 food and in directing its course. These free Medusas 

 carry with them eggs, which, finally, on becoming at- 

 tached to some support, develop into branchial Cam- 

 panulariaii Hydroids.* 



ORDER DISCOPHORA (dis kof'6 ra). 



The life-history of the Jelly-fish well illustrates an- 

 other group of the Hydrozoa. 



FIG. 17. FIG. 18. FIG. 19. 



Hydra-like stage. Later stage of Fig. 17. Free-swimming Jelly-fish. E, Eye- spot. 



The several stages in the development of the Jelly-fish, Aure'ti a fla vi du'la. 



The egg, on becoming attached, finally develops into 

 a form much resembling the Hydra. The portion of the 

 body above the tentacles, however, continues to grow 

 until it is enormous. Constrictions soon appear and di- 

 vide it transversely into a series of "saucers," of which 

 the upper and largest breaks away from those below it, 

 turns over and swims away as did the Medusa of the 



* It is a singular fact that through a lack of food, certain Medusae have 

 been known to return finally to their attached hydroid condition. 



