OBELIA 



Classification : — Phylum Coelenterata, Class Hydrozoa, Or- 

 der Leptomedusae. 



1. Obelia is a marine colonial coelenterate which becomes 

 attached to seaweed, piles, or other submerged objects. Study 

 prepared slide (or a piece of a colony on seaweed). Each in- 

 dividual of the colony is called a Jiydranth or zoo id. Is there any 

 reo^ularity in the arrangement of the zooids upon the upright 

 branches ? Each branch usually represents but a part of a colony. 

 Several such branches may be united by a continuous root-like 

 hydrorhiza which is the part by means of which attachment is 

 secured. 



2. Examine prepared slide with hand lens while holding 

 slide up to the light-. Make a simple diagram to show arrange- 

 ment of parts of the colony. In this diagram, the central stalk 

 and each of its branches should be represented by a single line. 

 Details of structure will be shown in another drawing. 



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 hydroiheca. 



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

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



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

 ment of the tentacles and the number of tentacles. The mouth 

 opens into the coeJenteric 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. 



6. Do you notice any modifications of the perisarc below 

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



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