82 ORGANISMS OF TISSUES 



gastrula type of structure, while from these to the most complex 

 organisms 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 blastula 

 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 enteron, and terminates in a mouth-bear- 

 ing or oral end. The mouth is surrounded by a crown of ten- 

 tacles which vary in number, usually from five to eight (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 ten- 

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



Radial Symmetry. Because of its cylindrical body it is pos- 

 sible 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 

 symmetrically; 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 



