SUMMARY 99 



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 

 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 tcTlorm 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 descended. 



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 modineoTas 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 Nectophores 

 or swimming bells are individuals specialized for locomotion. 

 Each specialized individual performs its particular function 

 for the good of the whole colony, and their colony organization 

 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, 



