218 



FOUNDATIONS OF BIOLOGY 



for instance, feeding, protective, and reproductive individuals 

 are established, and thereby the HYDROID COLONY exhibits 

 what is termed POLYMORPHISM. Our present interest is confined 



FIG. 118, Life history of Obelia. A, portion of a colony: 1, ectoderm; 2, endo- 

 dsrm; 3, mouth; 4, enteric cavity; 5, stalk of colony; 6, 7, and 10, exoskeleton; 8~, 

 reproductive hydranth (blastostyle) ; 9, medusa bud. B, free swimming medusa: 

 1, mouth; 2, tentacles; S, reproductive organs; 4< radial canals; 5, sense organ. 

 C, ciliated larva of closely related species. (From Hegner, after Parker and Haswell, 

 Shipley and MacBride, and Allman.) 



to the reproductive hydranths, which in many of the Hydroids 

 are so modified that they are dependent upon the colony 

 as a whole for all the necessities of life and are merely bodies 

 which form by budding other individuals known as MEDUSAE. 



