100 ORGANISMS OF TISSUES 



of Siphonophora, where no less than seven different kinds 

 of individuals may be present in the same colony, all working 

 for the common good and each dependent on the others for 

 some vital function (Fig. 39). We have in forms like Stephalia 

 and the Portuguese Man-of-War, therefore, a distinct type of 

 individual composed of many individuals, originally of a com- 

 mon type. Such forms have been termed individuals of a 

 second order, and their evolution may have been a parallel of 

 the evolution of metazoa where cells with 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 " metagene- 

 sis, " 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 OF 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 

 centralized organs which perform certain functions for the entire 

 individual, but are more or less uniformly distributed over the 

 body. Thus the nervous system is diffuse, that is, it consists of 



