SUMMARY ; 101 



external nerve and sensory cells with their long fibers or cell 

 processes which inter-cross with one another and with muscle 

 cells. In higher animals, similar nerve cells come together in 

 groups to form ganglia, and these ganglia are aggregated into 

 more or less complicated central nervous and peripheral sensory 

 systems. 



Similarly with the muscle cells of Hydra; they are not bound 

 together in complicated muscle bundles, but, like nerve cells, are 

 distributed over the entire organism, and are connected only in a 

 primitive way with, the nerve fibers by which they are stimulated 

 to contract. Histologically, therefore, Hydra differs from 

 higher animals and perhaps indicates the generalized type of 

 structures from which higher animals have been derived. 



While Hydra itself does not show the concentration of cells 

 into definite specialized organs, we do find types of coelenter- 

 ates, especially the siphonophores, where specialized organs are 

 developed, but in quite a different way from organ formation in 

 higher animals. Polymorphism, a second feature of general 

 interest, is a form of organ development in which individuals 

 themselves are modified as organs for the performance of some 

 one chief function. Medusae are reproductive in function, and 

 are special organs for sex-cell formation and distribution, pro- 

 duced on blastostyles or reproducing individuals. Dactylo- 

 zooids are individuals specialized for purposes of offence and 

 defence. Gastrozoids are feeding individuals, and Necto- 

 phores or swimming bells are individuals specialized for locomo- 

 tion. Each specialized individual performs its particular func- 

 tion for the good of the whole colony, and their colony organiza- 

 tion has led to the descriptive phrase, "individuals of a second 

 order." 



A third feature of general biological interest is an outcome of 

 these individualized organs. The medusa becomes free-living 

 as an individual of the sexual generation; it forms eggs or 

 spermatozoa (the sexes are separate), and the fertilized eggs 

 develop into Hydra-like asexual individuals, called hydroids, 

 which reproduce by budding. This phenomenon is called 

 metagenesis or alternation of generations, since one (sexual) 



