CCELENTERATA 69 



This branch consists of two rather divergent forms, 

 represented on the one hand by the sedentary type 

 (hydroid), and on the other by the free-swimming type 

 (jellyfish). These two forms may occur during the 

 course of development of one individual, thus illustrat- 

 ing the phenomenon of "alternation of generations." 

 Many of the members of the group are soft-bodied, 

 while others secrete calcareous material forming coral. 



All of the coelenterates multiply by means of eggs 

 and sperm cells, and all, except the Ctenophora, by 

 budding as well, the latter method resulting in the 

 formation of colonies in which the various members 

 often differ greatly in form and in function. 



The animals in this group are carnivorous, feeding 

 mainly on small organisms, although the sea anemone 

 can ingulf masses of considerable size. 



There are four classes : 



Class i. Hydrozoa, represented by hydra and the 



hydroids. 

 Class 2. Scyphozoa, containing the large jellyfishes, 



for example, Aurelia. 

 Class 3. Actinozoa, including the sea anemone and the 



corals. 

 Class 4. Ctenophora, including the jellyfishes which 



have comblike swimming organs. 



CLASS i . Hydrozoa 



In these coelenterates the body is a simple tube, or 

 cavity, in which there is a single aperture, the mouth. 

 The nervous system is slightly developed. Such are 

 fresh-water hydras and the oceanic hydroids (Eucope). 



The body of the hydra is tubular, soft, and sensitive, 

 of a greenish or brownish color, and seldom over half 

 an inch long. It is found spontaneously attached by 



