26 COELENTERATA. 



than one kind of cell in each or either of these layers? Where 

 are they? What are they? 



Make a careful drawing of the section showing the arrangement 

 as you see it. 



Examine longitudinal sections, for differences in the char- 

 acter of the ectoderm and endoderm in different parts of the 

 body. 



9. Reproduction. Examine living specimens in a watch- 

 glass of water for bud formation and for sexual organs. Sperm- 

 aries 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 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. 



