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EVOLUTION OF ANIMALS 



Part V 



caulus) that forms the main stalk of the colony (Fig. 23.10). The hydrorhiza 

 extends along the seaweed with many colonies springing from it. 



The tentacles of the feeding polyps are armed with stinging cells (Fig. 23.8). 

 These and the persisting motion of its tentacles are the hydranth's equipment 

 for catching the minute animals which swarm through the surrounding water. 

 The reproductive polyps (gonangia) bear the medusae which bud off from its 

 stalk much as buds of hydra develop from its body. 



The bodies of Obelia and Hydra are essentially similar. As in hydra, the 

 body wall is composed of two cellular layers, epidermis and the thin layer of 



Hydra 



Ectoderm 



•^^ Mesoglea 

 Endoderm 



Jellyfish 

 a medusa 



Sea anemone 



Fig. 23.1 1. Ground plans of the three main forms of coelenterates: the hydroid 

 polyp, the medusa or jellyfish, and the polyp of the anemone, are constructed on 

 the same general plan. The mouths of hydra and sea anemone are held upward; 

 a jellyfish swims with mouth down. 



mesoglea. In Obelia, the body is encased in a transparent casing (perisarc), 

 complete except at the tips of the polyps. The digestive processes are essentially 

 the same as those of hydra. The gastrovascular cavity is continuous throughout 

 the stalks and branches of the colony and food is shared by the community. 

 Lively protozoans are swept into the mouths of feeding polyps, moved along 

 while still in tremors through the enteron, and gradually digested and absorbed 

 along the way. 



The Medusa — Its Form and Way of Living. Medusae are the sexual links 

 in the hydrozoan life cycle. Medusae are specialized individuals devoted to 

 reproduction in contrast to the ovaries and testes of higher animals which are 

 only organs of reproduction. Hydrozoan medusae are always small, and those 

 of Obelia are minute. They live in tide pools and shallows, swimming about 

 by vigorous contractions of their umbrellas. But they are powerless against 

 currents and are carried into harbors in enormous numbers though they are 

 so small that they go unnoticed. It is the larger scyphozoan jellyfishes that 

 everybody sees (Fig. 23.14). 



