OBELIA. 27 



Do you notice any modifications of the perisarc below the 

 hydrotheca? Do the modifications serve any purpose? 



4. Trace the stem 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 ccenosarc. 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 the hydrotheca? Trace the coelenteric 

 cavity through branches and hydranths and determine whether 

 it is continuous. 



7. Can you determine what keeps the fluid in the cavity in 

 motion? 



8. 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 in Hydra. 



Make a drawing of a hydranth. 



9. Look for certain extremities which show neither tentacles 

 nor any opening in the outer covering. Such a condition sig- 

 nifies either an immature hydranth or a reproductive indi- 

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

 a gonosome. The central core of a gonosome, the blastostyle, 

 should be examined for gonophores, frequently called medusce 

 buds. This may require a high power. Determine how the 

 gonophores are arranged around the blastostyle. Are all in 

 equal stages of development? What relation has the end of the 

 blastostyle to the outer covering, the gonangium f 



Make a drawing of a gonosome. 



10. The free medusae are small, transparent, and easily 

 overlooked. During the breeding season they may usually 

 be found in abundance in dishes in which colonies have been 

 kept over night. Notice their movements and their positions 

 while at rest on the bottom. The number of tentacles and the 

 position of the sense organs is definite for the species. Two 



