THE MEDUSA. 



GONIONEMUS MURBACHII. 



THE medusae formed by the hydroids studied, Pennaria 

 and Obelia, are so small that their structure can be made 

 out only with some difficulty. Therefore, a larger medusa is 

 taken as a type for study. Although Gonionemus does not 

 belong to the same order as the hydroids described, and the 

 hydroid stage -is greatly reduced, it has about the same 

 structure as the medusae which are formed from these hy- 

 droids. 



Place specimens in a watch glass with water and examine 

 with a lens. The umbrella or bell shape is rather character- 

 istic of all medusae. The external convex part is called the 

 exumbrella or aboral, and the concave part is the subumbrella 

 or oral side. The under part of the medusa is partly closed 

 by a membrane called the velum, or veil. Where are the 

 tentacles? Are they alike. How many are there? Are 

 they regularly arranged? Are nematocysts present? If 

 so note their arrangement. Near the tip of the tentacle is 

 an adhesive or muscular pad used by the medusa for hold- 

 ing against some object. 



Within the subumbrellar space note the central hanging 

 sac, the manubrium or stomach. Is a mouth present? A 

 gastric cavity? How does the mouth differ from that of 

 the hydroids? Note the radial canals, delicate tubes which 

 extend outward from the center of the bell. How many 

 canals are there? Is the number the same in all specimens? 



