CGELENTERATES 



199 



Draw the medusa twice natural size, showing all the above parts neatly 

 labeled. 



Development. — The egg of the medusa after fertilization under- 

 goes a number of changes. First the egg splits in two, then four, 

 eight, and ultimately a mass of cells. This process is known as 

 segmentation. These cells form a hollow ball of cells and swim 

 through the water by means of cilia. Ultimately this little animal 

 settles down on one end and becomes fixed to a rock, seaweed, or 

 pile. The free end becomes indented in the same manner as a 

 hollow rubber ball may be pushed in on one side. This indented 

 side becomes a mouth, tentacles develop around the orifice, and 

 we have an animal that looks very much like the hydra. This 

 animal, now known as a hydroid polyp, buds rapidly and soon 

 forms a colony of little polyps, each of which is connected with 

 its neighbor by a hollow food tube. The hydroid polyp differs 

 from its fresh- water cousin, the hydra, by 

 usually possessing a tough covering which 

 is not alive. 



Hydroid Colony {Pennaria)} — (Material put 

 up in formol may be handed out to the class in 

 small vials). In the portion of the colony you have, 

 where are the polyps located ? Examine a single 

 polyp and make out all you can regarding (a) its 

 general form, (6) the position of tentacles, (c) the 

 position of the mouth. The buds which form the 

 free-swimming medusae are frequently found bud- 

 ding out of the wall of the polyp. Can you de- 

 scribe them? 



Demonstration. — Stained polyps and part of 

 the branch, using the compound microscope. 

 Draw part of a colony four times natural size. 

 Label all the points mentioned above. 



Alternation of Generations in Coelen- 

 terates. — The lives of a hydroid and a 

 medusa are seen thus to be intimately con- 

 nected with each other. A hydroid colony 

 produces new polyps by budding. This 

 we know is an asexual method of repro- 

 duction. There come from this hydroid 

 colony, however, little buds which give rise to medusae. These 



1 See Hvinter and Valentine, MamuxL, page 155. 



A hydroid colony of six pobTJs; 

 /, feeding polyp; r, repro- 

 ductive polyp; m, a medusa; 

 y, young polyp. 



