68 INVERTEBRATE ZOOLOGY. 



is neither silicious nor calcareous, but composed of closely interwoven threads 

 of c'hitin (same material as horn, silk, etc). Draw a small area of this under 

 a high power to show the relation of the threads to each other. This speci- 

 men may be permanently mounted by removing the cover, wiping away the 

 excess of turpentine and adding a drop of Canada Balsam. 



Type III.— Hydra. 



The Hydra is the only fresh water representative of the group. It is found 

 in quiet pools, clinging to submerged vegetation. It is tubular or vase-shaped 

 with 6-8 long thread-like tentacles hanging from the free end. There are two 

 principle forms distinguished \>y their colors — the brown one. Hydra fusca, 

 and the green one. Hydra viridis. 



27. Field ivork. Search for specimens in stagnant water abounding 

 in aquatic plants. Hydras average I4 inch in length and may be sometimes 

 seen attached to submerged leaves and stems, their long tentacles streaming 

 in the water. They are easily seen in an aquarium, btit to find them in a nat- 

 ural pool is much more difficult. The best plan is to collect a handful of acpia- 

 tic plants, place them in a jar filled with clear water and hold it up to the 

 light. The observation should not be too hasty, for hydras are extremely 

 sensitive and contract at once when so roughly h^uidled. One should there- 

 fore wait a few minutes and allow them to expand. Repeat this test several 

 times and in different localities. 



28. Study of external form. Place a hydra in a watch crystal of water 

 and observe with a dissecting microscope. Make oiitline drawings of several 

 shapes and positions. Place under the low power of the compound micro- 

 scope, and study the structure of the tentacles, body wall, etc. Make a 

 sketch, using an outline (one of those just made), and fill in what you see. It 

 is better to draw one tentacle and a little portion of the body-wall minutely 

 and leave the rest in outline. 



29. Study of life habits. Keep two or three specimens for several 

 weeks in a large beaker, placed where it can be frefpiently watched. Feed 

 with minute fresh water Crustacea (Cyclops, etc). The results of this experi- 

 ment depend largely upon luck and continual observation. If fortunate, one 

 may observe (1) the prey caught by the tentacles and conveyed to the mouth 

 lying between them ; (2) the growth of new individuals from the sides of the 

 older ones : (3) the subsequent separation of the offspring as an iiidependent 

 hydra, etc. 



