OBELIA 63 



7. Examine hydranth and stalk with low power and look 

 for the cell-layers which you discovered in the study of Hydra. 

 Draw a hydranth and part of the stalk as seen in optical section 

 under the low power, making the hydrotheca 80 mm. long. Cell 

 layers but not cytoplasmic contents should be shown in this 

 draAving. 



8. Find reprochictive individuals and draw one. These are 

 large sac-like structures containing numerous huds formed 

 asexually. These buds when fully developed, become small, 

 free-swimming, jellyfish (see demonstration microscope) which 

 reproduce sexually. The fertilized egg develops not into a jelly- 

 fish, but into a hydroid such as you have been studying. This 

 condition where the offspring is not like the parent but like the 

 grandparent, is termed metagenesis or alternation of generations. 

 The hydroids produce jellyfish; jellyfish produce hydroids, etc. 



9. Examine demonstrations of undeveloped hydranths. 

 These must not be confused with old or injured hydranths which 

 have lost their tentacles. An undeveloped hydranth is a bud, the 

 free end of which is covered by a continuous layer of the 

 perisarc. 



