472 



EVOLUTION OF ANIMALS 



Part V 



tjewafocysts 



Tenfvicl& 



Mesogle^^ 



Younger bud 



Older 

 bud 



Testis 



Gland cell 

 Ectoderm 



■Flagellated cell 

 Ovary 



£gg-cell 



•Basal disc 



Fig. 23.6. A long section of hydra with the bud of asexual reproduction and the 

 male and female organs. Such a composite is unusual; in the majority of species 

 the sexes are separate. (Courtesy, Wolcott: Animal Biology, ed. 3. New York, 

 McGraw-Hill Book Co., 1946.) 



Cuticle 

 Entoderm 



functions occur in exact places. Food enters through the mouth and nowhere 

 else. Stinging cells are most abundant on the tentacles which grapple the prey. 

 Nerve cells are most numerous near the mouth, the usual locality for a brain. 

 Like all coelenterates, hydra contains a single cavity called the enteron, 

 coelenteron, or gastrovascular cavity. Its one opening functions as an entrance 

 and exit for food, water, and waste (Figs. 23.4, 23.6). In all hydras, the 

 gastrovascular cavity is continuous into the tentacles, but is not so in the corals 

 and other hydroids. The body wall enclosing the cavity consists of the three 

 layers already mentioned, the epidermis (ectoderm), the lining of the enteron, 

 (endoderm), and the extremely thin gelatin-like mesoglea (Fig. 23.6). In 

 jellyfishes, mesoglea forms the bulk of the body and contains fibers and cells 



