CHAPTER XV 



THE CCELENTERATES AND SPONGES 



The hydroids, jelly fish, sea anemones, coral polyps, 

 and their relatives constitute the phylum Ccelenterata. 

 Primarily the Ccelenterates are radiate animals with their 

 organs symmetrically disposed about a central axis, but 

 there are some of the higher members of the group which 



have become to a greater or less 

 extent two-sided or bilateral. 



One can secure in almost any 

 part of the country a living 

 representative of this group in 

 the common fresh- water Hydra 

 which is frequently found at- 

 tached to aquatic plants in 

 ponds and streams. The tubu- 

 lar body of Hydra is furnished 

 at one end with a variable 

 number of tentacles (6-8 com- 

 monly) surrounding the mouth. 

 At the opposite end is the foot 

 which may become attached to 

 objects by means of an adhe- 

 sive secretion. The attachment is not permanent, how- 

 ever, as the Hydra can break loose at any time and crawl 

 by a looping motion to another locality. 



The internal structure of Hydra is very simple. The 

 body may be regarded as a sort of sac composed of two 

 layers of cells separated by a thin membrane. The inner 



130 



FIG. 107. Fresh water Hydra. 

 Ov, ovum; T, testes. 



