80 ORGANISMS QF TISSUES 



type of structure, while from these to the most complex organ- 

 isms there is every grade of complexity. Just as the egg cell of 

 metazoa is represented by organisms the protozoa which 

 never go beyond this single cell condition, or the bias tula by 

 colony forms which do not develop beyond this stage, so the 

 gastrula is represented by one group of organisms, termed 

 Coelenterata, which do not develop beyond the gastrula or two 

 layered stage in the development of metazoa. 



Forming as they do the lowest branch of the metazoan tree, 

 the coelenterates demand particular attention because through 

 them we are introduced to many of the essential problems in the 

 general biology of higher animal forms. A good type to begin 

 with is the common fresh water Hydra. 



A. HYDRA FUSCA AND HYDRA VIRIDIS 



Like the majority of protozoa, Hydra always lives in water, 

 and usually in fresh water although some types live in salt water. 

 They are sedentary forms attached by one end termed the pedal 

 disc, to water plants or other objects. The body is cylindrical, 

 a double wall of ectoderm and endoderm enclosing one single 

 cavity, the enter on and terminates in a mouth-bearing or oral 

 end. The mouth is surrounded by a crown of tentacles which 

 vary in number from eight to fourteen (some allied forms of 

 Hydra, e.g., Microhydra and Protohydra have no tentacles). 

 The pedal extremity is somewhat dilated forming a sucking disc 

 for attachment to foreign objects. Thus attached, it sways 

 about with the currents in the water with its tentacles widely 

 spread for the capture of prey (Fig. 5, p. 14). 



Radial Symmetry. B ecause of its cylindrical body it is possible 

 to cut Hydra vertically through the mouth in an infinite number 

 of planes, each of which would result in two symmetrical halves. 

 In the majority of other metazoa only one plane, that passing 

 through the mouth vertically, will divide the body symmetrica- 

 ly ; such higher animals are bilaterally symmetrical, whereas Hydra 

 is said to be radially symmetrical. Radial symmetry in animals 

 is supposed to be due to the fact that they have lived as attached 



