98 ORGANISMS OF TISSUES 



differentiated for purposes of locomotion; others for protection 

 and defence, and others for reproduction. The highest devel- 

 opment of this coelenterate polymorphism being found in the 

 group of Siphonophora where no less than seven different kinds 

 of individuals may be found in the same colony all working 

 for the common good and each dependent on the others for 

 some vital function. We have in forms like the Portuguese 

 Man-of-War, therefore, a distinct type of individual composed of 

 many individuals originally of a common type. Such forms 

 have been termed individuals of a second order and their evolu- 

 tion may have been a parallel of the evolution of metazoa where 

 cells of different functions, working for the good of the whole 

 were gradually evolved from colonies of protozoa in which the 

 cells are of one type. 



One other phenomenon of general biological importance was 

 briefly mentioned above in describing the sexual reproduction 

 of Obelia. The jelly fish or medusa produces eggs which, 

 when fertilized, develop not into medusae but into hydroids 

 which produce other hydroids by asexual reproduction. This 

 phenomenon termed Alternation of generations or " metagenesis" 

 is not often met with in animals but it is widely spread in the 

 plant kingdom an asexual generation giving rise to a sexual 

 generation and this in turn to an asexual in regular alternation. 



B. SUMMARY or POINTS OF GENERAL BIOLOGICAL INTEREST 

 SHOWN BY HYDRA 



Hydra and its allies are important to the student of biology 

 because of their intermediate position between organisms com- 

 posed of one cell and organisms composed of organs. One 

 essential characteristic of the metazoa is represented by them 

 in the differentiation of cells for the performance of the different 

 vital functions. Nerve and sensory cells, muscular and support- 

 ing cells, nematoblasts and digestive cells are physiologically 

 unbalanced in that they exhibit some one function which pre- 

 dominates over all of the others. In Hydra at least these 

 differentiated cells are not bound together into specialized and 



