28 CCELENTERATA. 



Eggs are not formed at all seasons of the year and var}^ 

 greatly in appearance according to their stage of development. 

 Make drawings of the stages of reproduction that you find. 



Tannreuther: The Development of Hydra. Biol. Bui., 14, 1908. 

 Whitney: Artificial Removal of the Green Bodies from Hydra viridis. 

 Biol. Bui., 14, 1908. 



OBELIA. 



These small, colonial animals are common on submerged or 

 floating wood, stones, and seaweeds, where the water is rather free 

 from sediments. With the aid of a glass-bottomed pail they, 

 in company with many other forms, may usually be seen about 

 old wharfs. 



Note the appearance of large colonies of this form that are 

 growing on stones or on pieces of board or kelp. 



1. Notice the tree-like form of any single stem. Do the 

 branches have a definite size and arrangement ? 



2. At the extremities of the branches are the single individuals, 

 hydranths or zooids. Each is similar to a single Hydra in cer- 

 tain ways, but is inclosed in a vase-like formation, the hydrotheca. 



3. The latter is a continuation of a tough, membranous 

 sheath, the perisarc, which covers each part of the whole colony. 



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 coenosarc. 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? 



