324 



APPLIED BIOLOGY 



known as the Porifera (meaning pore-bearers). This is the 

 lowest group of the many-celled animals. It is a noteworthy 

 fact that the Porifera show much similarity to the colonial pro- 

 tozoa, especially in that food is digested inside of cells, and not 

 in a stomach bounded by cells, as is true in higher animals. 



CCELENTERATA 



282. Hydra. Somewhat more differentiated than the 

 sponge-animals, but still exceedingly simple as compared 

 with a backboned animal, are those of the group to which 



belong the jelly-fishes and 

 coral-animals. Most of these 

 live in the sea, but among the 

 few species which live in fresh 

 water is the little animal 

 known to naturalists as Hy- 

 dra, and in old books often 

 called " fresh-water polyps." 

 It commonly lives in ponds 

 and streams, where it clings to 

 aquatic plants, dead leaves, 

 and sticks. Such objects 

 should be collected, placed in 

 glass jars with water, and al- 

 lowed to stand for some days. 

 Many of the hydras will move 

 to the walls of the glass jar. 



FIG. 97. Two specimens of hy- 

 dra, one contracted and one 

 expanded. The latter has three 

 buds in stages of development. 

 t, tentacles; a, captured water- 

 flea; hyp, position of mouth. 

 (From Parker.) 



I. (L) Observe hydras in glass 

 jars (aquaria) near windows. In 

 what part of the aquarium with 

 reference to light and shade are 

 they most abundant? Note the 

 long thread-like arms which are 

 attached at the free end of the 

 animal. Study a hydra which has 



