172 ELEMENTARY ZOOLOGY. 



2. These are the lowest animals in which we find well 

 developed nervous and muscular cells which play an 

 important part in all the higher animals. 



3. There is only one opening into the digestive tract 

 (see m, Fig. 46) . 



4. In many of them there is a beautifully exact radial 

 arrangement of the parts of each individual around a 

 central point. This is seen to best advantage in the 

 jelly-fishes. 



191. The Fresh -water Hydra: a Laboratory Study. 

 This is the simplest of the ccelenterates, and yet it gives 

 a good idea of the essential structure. They are common 

 in ponds and lakes and brooks where the water is not too 

 foul, but they are easily overlooked. The best way to 

 secure them is to bring in leaves, submerged plants, algae, 

 and the like from a number of places. Put them in 

 separate glass dishes and label them so that you will 

 be able to locate them again .if you succeed. Do not 

 allow too much decay to take place in the material. 

 Watch carefully on the sides of the vessels and on the 

 leaves for the extended hydras. There are two common 

 types a smaller green hydra and a brown species. The 

 green are more hardy, but the brown are more satis- 

 factory for study. Study alive in a' glass jar or tumbler. 



Is it free or attached? General form and regions of 

 the body? Tentacles, number and position. Enumer- 

 ate the activities that you discover. What degree of 

 change of form is possible- to it? Under what circum- 

 stances does it change form? Is it moving at all when 

 undisturbed? Watch for some time. What is its custom- 

 ary position in the water? Is it heavier or lighter than 

 water ? Does it ever become detached ? What evidences ? 



