HYDRA, OBELIA 45 



9. Reproduction. Examine living specimens in a watch 

 glass of water for bud formation and for sexual organs. 

 Spermaries are just beneath the tentacles; ovaries, lower 

 down; buds may be found at different levels. What cells are 

 involved in the formation of each of these? 



Eggs are not formed at all seasons of the year and vary 

 greatly in appearance according to their stage of develop- 

 ment. 



Make drawings of the stages of reproduction that you 

 find. 



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

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

 Biol. Bull., 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 treelike form of any single stem. Do the 

 branches have a definite size and arrangement? 



2. At the extremities of the branches are the single in- 

 dividuals, hydranths or zooids. Each is similar to a single 

 Hydra in certain ways, but is inclosed in a vase-like forma- 

 tion, 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. 



