26 CCELENTERATES. 



combined together to form an animal of more than one 

 cell (a metazoari). Thus the ciliated cells in the water 

 cavities of the sponge have become differentiated from 

 those which possess no cilia. 1 



THE HYDRA. 



(Hydra viridis ?) 



Study of Live Specimens. In an aquarium or glass 

 jar in which hydras are kept 2 study the creatures alive. 

 Observe : 



1. Their shape, elongated, cylindrical, Avith the body 

 attached at its posterior or foot end to some support, free 

 at its anterior end, and crowned with a circle of long, 

 radiating tentacles. 



2. Their color. 



3. Their position: if all are in the same position; if all 

 are attached to the same kind of a support; in what part 

 of the vessel they are most numerous. 



4. Their actions. Note the swaying-about of the long 

 tentacles in the water. These capture food. A minute 

 animal that has been captured may sometimes be seen, 

 before it is carried to the mouth, sticking to one of 

 the tentacles. Touch the tentacles, and see them con- 

 tract. 



If the hydra jar be so placed that light reaches it 

 strongly from but one side, after some time the hydras 

 which are on the opposite side of the jar will move 

 toward the light, when their slow and very peculiar loop- 

 ing locomotion may be seen. 



1 For a further study of this and of related sponges, the student is re- 

 ferred to Potts's Monograph of Fresh-Water Sponges. For a good dis- 

 cussion of the relations and differences between plants and animals, he is 

 referred to Parker's Elementary Biology. 



2 See Appendix, p. 282. 



