478 EVOLUTION OF ANIMALS Part V 



monograph, "Polypes d'eau douce." He described them as animals, portrayed 

 their locomotion, and gave accounts of his experiments upon them. He dis- 

 covered that if one were cut into two, three, or four pieces, each piece would 

 form a new animal; and if the oral end of one were split it would form a two- 

 headed animal (Fig. 23.9). Hydra has continued to be a subject of experi- 

 mentation and R. L. Roudabush (1933) turned hydras inside out as Trembley 

 did. The striking result of the later experiments was the migration of cells, 

 discovered by studying sections of the animals killed and fixed at periods of 

 10 minutes, 2 hours, and 24 hours after they had been turned inside out. They 

 showed: the epidermis on the inside and the gastrodermis on the outside as 

 they had been turned in the experiment; and later also the cells of the gastro- 

 dermis in migration toward the inside and those of epidermis toward the out- 

 side; and finally, cells of the two layers in position as they were before the 

 experiment. 



Grafting. Trembley's grafting experiments were the first of many others. 

 Pieces of different hydras, even those of different species, have been grafted 

 together. Pieces may be too small to regenerate but will fuse and grow, the 

 ectoderm joining with ectoderm and endoderm with endoderm. 



The Invertebrates. Protozoa through Ctenophora, by L. H. Hyman, con- 

 tains an unequaled store of knowledge about coelenterates, including the re- 

 sults of the author's own extensive work on the hydras and comprehensive lists 

 of references. 



Class Hydrozoa 



The hydrozoans most frequently studied are the solitary polyp. Hydra, the 

 colonial hydroid, Obelia, and the hydrozoan medusa, Gonionemus. In Hydra, 

 only the polyp form occurs. In many hydrozoans, however, both polyp and 

 medusa are well developed as in Obelia (Figs. 23.10, 23.11). In Gonionemus, 

 the polyp is minute and rarely recognized while the medusa is well known (Fig. 

 23.12). Hydrozoans exhibit an extraordinary degree of division of labor, in 

 which different functions are performed by different kinds of individuals of 

 the same species, as in the Portuguese man-of-war instead of by different or- 

 gans in the same individual (Fig. 23.13). 



Obelia — A Colonial Hydrozoan. In the fully developed colony, there are 

 three types of individuals: hydranths, the feeding polyps with mouths and ten- 

 tacles; gonangia, modified polyps that produce medusae and lack mouths and 

 tentacles; medusae or jellyfishes that arise as buds from the gonangia and 

 grow to sexual maturity as free swimming male or female individuals (Fig. 

 23.10). In Obelia, the sex cells are shed in the open water where the eggs are 

 fertilized. In some hydroids, the sperm cells are shed and the eggs are fertilized 

 while the medusa is still attached. In its complete cycle, the life of Obelia in- 

 cludes an alternation of generations. One generation, the colony, is asexual and 



