OBELIA 57 



3. Study specimen mounted on a slide and under low power 

 compare the structure of an individual zooid with the structure 

 of Hydra. Note, in addition to what was found in Hydra, a 

 tough, membranous sheath, the perisarc, covering the surface of 

 the colony. The vase-like expansion of the perisarc around each 

 zooid is called the hydrotheca. 



4. Do you notice any modifications of the perisarc below 

 the hydrotheca? Do these serve any purpose? Notes required. 



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

 is termed the cocnosarc. 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. The mouth 

 opens into the coelenteric cavity as in the Hydra. This cavity 

 continues down through the coenosarc so the coelenteric cavities 

 of all of the individuals of a colony are directly continuous with 

 each other. 



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 under the high 

 power, making the hydrotheca 80 mm. long. Cell boundaries but 

 not cytoplasmic contents should be shown in this drawing. In 

 your study of the hydranth you have noticed that the tentacles 

 are very densely grouped around the mouth. In this drawing 

 omit the tentacles from the front side of the hydranth, showing 

 only where they are attached to the body by drawing only the 

 bases of the tentacles across the front of the hydranth. 



8. Find reproductive individuals and draw. These are large 

 sack-like structures containing numerous buds formed asexually. 

 These buds when fully developed, become small, free-swimming, 

 jellyfish (see demonstration microscope) which reproduce sex- 

 ually. The fertilized egg develops not into another jellyfish, 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 hy- 

 droids produce jellyfish ; jellyfish produce hydroids, etc. 



