GENERAL ZOOLOGY 



enterates is presumably related to the attached growth habit, since radial 

 symmetry is commonly associated with a fixed mode of life or indicates 

 descent from attached ancestors. 



The fixed growth habit, together with the radial symmetry, led early 

 naturalists to classify the coelenterates as plants. Their animal nature 

 became apparent to zoologists in the eighteenth century when it was recog- 

 nized that the organisms have mouths, tentacles, and digestive cavities. How- 

 ever, Aristotle's view that they were intermediate between plants and animals 

 persisted until well into the nineteenth century and is reflected by the inclu- 

 sion of the coelenterates, with the echinoderms, in a group called Radiata or 

 Zoophyta ("animal-plants"). Only when it became clear that the structural 

 organization of coelenterates is vastly simpler and more primitive than that of 

 echinoderms was the modern phylum Coelenterata finally established. 



Remains of coelenterates such as hydroids and corals appear in some of the 

 earliest fossil-bearing rocks. Even jellyfishes have left a fossil record as thin, 

 delicate carbon-film impressions in ancient sedimentary beds. Thus, the early 

 evolution of coelenterates long antedates the beginning of our fossil record, 

 and we have no direct information about their antecedents. Phylogenetic 

 speculation must be based on the facts of development and probable interrela- 

 tionships deduced from the life cycles of modern representatives. 



Although they are by no means "typical" hydrozoans, the fresh-water 

 hydras, long studied by zoologists, are widely distributed, easily obtainable, 

 and relatively simple representatives of the phylum. They are also repre- 

 sentative of the metazoans higher than sponges, for they have a digestive 

 cavity and cell layers comparable with these features of the more advanced 

 animals. Because of its relative simplicity, the hydra is particularly well 

 suited to illustrate the structure of a metazoan. Accordingly, we shall ex- 

 amine the hydra as a coelenterate, but more especially as a simple metazoan, 

 to be compared in structure and function with the vertebrate and with the 

 protozoan. 



THE CLASS HYDROZOA 



The Hydra: Habitat and Activities. Several species of the genus Hydra 

 occur in the United States. Related hydras are placed in different genera 

 because of special characteristics. A common brown form, with very long 

 tentacles and with its trunk region differentiated into a slender basal stalk 

 and a stouter body, has been designated Pelmalohydra oligactis (Fig. 10.1). 

 A green hydra, deriving its distinctive color from the presence of alga-like 

 zoochlorellae in cells lining its digestive cavity, is recognized as Chlorohydra 

 vindissima. Unless specific exceptions are noted, the following account is 

 applicable in general to any type of hydra. 



The individual hydra is usually found attached by its base to submerged 

 objects, with its body extended. It may float freely, however; in an aquarium 



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