154 BIOLOGY 



LABORATORY WORK 



Hydra. Almost any pond will furnish Hydra, which may be found 

 clinging to the under side of pond-lily leaves. If such leaves are placed in 

 a dish of clean water, the Hydra will detach themselves from the leaves and 

 cling to the side of the dish. For study, a specimen is to be detached from 

 the dish, placed in a watch glass containing a little water, and examined 

 under the microscope with a low-power objective. The general structure 

 and motion of the animals may easily be seen. For the cellular structure 

 of the body, stained, mounted sections should be furnished the student by 

 the instructor. For a study of the nematocysts, a bit of the tentacles of a 

 brown Hydra should be cut off with delicate scissors and placed on a slide 

 in a small drop of water. A cover glass is placed upon the drop and gently 

 pressed. This will crush the tentacle and cause many of the nematocysts 

 to discharge their stinging hairs. The nematocysts may also be made to 

 discharge their stinging hairs if a little weak acetic acid is added to the 

 water. A careful examination with a ^-inch objective will show all three 

 kinds of nematocysts, both discharged and undischarged. For comparison 

 of Hydra with other Hydroids, preserved and mounted specimens should 

 be furnished by the instructor, some of which should show colonies and 

 others jelly fishes. 



BOOKS FOR REFERENCE 



COLTON, Zoology, Descriptive and Practical, D. C. Heath & Co., 

 Boston. 



PRATT, Invertebrate Zoology, Ginn & Co., Boston. 



MARSHALL and HURST, Practical Zoology, G. P. Putnam's Sons, New 

 York. 



