CHAPTER XX. 



BRANCH CCELENTERATA. 



[This branch includes the Hydroids, Jellyfishes, Sea Anemones, 

 and Coral Polyps.] 



Example. The Fresh-water Polyp, Hydra. 



Naked-eye Appearance of Hydra. In examining a jar 

 of aquatic plants one may find attached to them slender 

 cylindric bodies about half an inch long and of the thick- 

 ness of a needle. Extending from the free end are several 

 fine, threadlike tentacles, which may be as long as the 

 body itself. Hydras are often white or colorless, but occa- 

 sionally brown or green ones are found. If undisturbed, 

 the body and tentacles may occa- 

 sionally sway gently to and fro. 

 If disturbed, a hydra usually 

 shortens until it is a tiny ball. 

 The tentacles also shorten until 

 they look like a circle of tiny buds. 



Structure Of Hydra. -Micro- 

 scopic examination is required to 

 learn much of the structure of hydra. The cylindric body 

 is hollow, and the hollow extends through the tentacles, 

 which are closed at their tips. Above the circle of 

 tentacles rises a cone-shaped body called the hypostome, 

 at the apex of which is the mouth. The body wall consists 

 of two layers, the outer being the ectoderm, and the inner 

 the endoderm. Between these two is a layer called the 

 mesoglcea. 



313 



HYDRA EXTENDED. 



