OBELIA. 27 



Do you notice any modifications of the perisarc below the 

 hydrotheca? Do the modifications serve any purpose? 



4. Trace the stem down to the creeping, stolon-like portion 

 of the colony, the hydrorhiza. 



Make a drawing of a colony. 



5. The fleshy continuation of the zooid down into the stalk 

 is termed the coenosarc. Is it in close contact with the perisarc? 



6. In an expanded hydranth, note the mouth, the arrange- 

 ment of the tentacles, and the number of tentacles. How is the 

 individual supported in its cup? Can you trace the ccelenteric 

 cavities down through the stalks, and prove them to be continuous 

 with each other? Motion in the fluid contents of living speci- 

 mens makes this easy to observe. 



7. Examine a hydranth with a high power and look for the 

 cell-layers characteristic of coelenterates. Determine how its 

 tentacles differ from the tentacles of Hydra, and explode nemato- 

 cysts as with Hydra. 



Make a drawing of a hydranth. 



8. Look for certain extremities which show neither tentacles 

 nor any opening in the perisarcal covering. Such a condition 

 signifies either an undeveloped hydranth or a reproductive 

 individual. If the latter, it is considerably swollen and is termed 

 a gonangium. The central core or coenosarc of a gonangium, the 

 blastostyle, should be examined for medusce buds. This may re- 

 quire a high power. 



Make a drawing of a gonangium. 



9. You may find free medusae swimming in the dish where 

 material is kept. If you do, you should examine one, but it 

 will not prove as satisfactory for study as a larger form, like 

 Gonionemus, directions for the study of which are given further 

 over. In comparing it with Gonionemus notice the small size 

 of the velum, and the usual everted position of the bell, so the 

 manubrium appears like a handle. 



It is not uncommon to find planulae escaping from gono- 

 thecae of Campanularia. Frequently the medusse that are 

 liberated have previously shed their eggs. 



