OBELIA. 23 



individual supported in its cup? Can you trace the coelenteric 

 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 ccelenterates. 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 medusa) 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 ease with which the bell turns wrong side 

 out, so the manubrium appears like a handle. At Woods Holl, 

 Obelia apparently does not always liberate its medusaB, and it 

 is not uncommon to find planulaB escaping from gonothecse. 

 Frequently those medusa? that are liberated have previously 

 shed their eggs. 



10. Study the cellular structure of a hydranth and of a gon- 

 angium, as seen in cross and longitudinal sections. 



Make a drawing of each. 



For comparison use any thecate forms, which may be offered 

 as loose material or as demonstrations, such as Campanularia, 

 Sertularia, and Plumularia. 



