58 



COELENTERATA 



annelids and Crustacea that are food for fishes it may be considered 

 injurious. Gudger has called attention to early studies of Trembley 

 and the more recent ones of Beardsley which indicate the surprising 

 ability of hydra to capture young fishes. Beardsley found that in 

 one of the hatcheries of the U. S. B. P., hydras averaged 131 per 

 square inch in certain troughs used for black-spotted trout fry and 

 that they were responsible for a considerable mortality among the 

 young trout. 



—Endoderm 



Mesoglea 



— Ectoderm 



Fig. 22C. Hydra, cross section. (After Marshall and Hurst.) 



The Hydrozoa are especially interesting to us on account of their 

 two forms of zooids, the nutritive hydranths and the reproductive 

 zooids, called medusae. In many of the Hydrozoa we find alterna- 

 tion of generations, the asexual generation being a fixed, plant-like 

 colony, while the sexual generation is a free swimming medusa. 



Obelia, a Hydroid. — Since alternation of generations is especially 

 well shown in Obelia, it is frequently used in elementary zoology 

 courses. 



Obelia is a colonial animal that looks like a plant. It has a basal 

 root, the hydrorhiza, attached to rocks, wharves and to alga, which 

 gives off stems, the hydrocauli. The side branches from the hydro- 

 cauli develop hydranths, or independent zooids, like a hydra in 

 structure. The tentacles of the hydroid are not hollow but solid. 



