98 ORGANISMS OF TISSUES 



POLYMORPHISM. COLONY FORMATION AND INDIVIDUAL 



DIFFERENTIATION 



The individual Hydra fusca in its relation to other individuals 

 may be compared with an Amoeba or Paramecium where, after 

 division, the daughter individuals separate and live independ- 

 ently. There are many species of Coelenterates, however, 

 more or less closely related to Hydra, in which the individuals, 

 after their formation by budding, remain attached to the 

 parent organism, thus forming colonies similar in origin and 

 mode of growth to many colonies of protozoa. Obelia is 

 such a colony form, the adult being a graceful, much branched 

 aggregate of individuals, resembling a small bush in its ap- 

 pearance and mode of growth, a fact which led Aristotle to 

 name such organisms Zoophyta or animal plants. Obelia 

 begins as a small hydroid (Hydra-like), one-sixty-fourth to one- 

 thirty-second of an inch in height with mouth and tentacles; 

 it reproduces by budding, while stalks or stems are formed by 

 the secretion of a nitrogenous material, chitin. New buds are 

 continually added to the colony, until the latter attains the 

 height of several inches and consists of thousands of individuals 

 with main stems and side branches enclosed within chitin, 

 the entire colony being connected by living substance. 



Such hydroid colonies differ from colonies of protozoa in that 

 some individuals may become differentiated for the performance 

 of different functions. This phenomenon, termed polymor- 

 phism, is represented by Obelia in a relatively simple form. 

 The only differentiation here is the formation of reproductive in- 

 dividuals distinct from the nutritive individuals. When the 

 colony is mature, certain individuals produce buds which differ 

 from the ordinary hydroid buds in structure. These buds, in- 

 stead of remaining attached as do the hydroids, are detached 

 and swim away as " medusae' 1 or jelly fish. The jelly fish 

 bear the gonads (testes or ovaries), and are the sexually dif- 

 ferentiated individuals of the colony. When eggs and sperma- 

 tozoa are ripe, they are discharged into the water where fer- 

 tilization takes place, the fertilized egg developing into a young 



