Chap. 23 COELENTERATES — SIMPLE MULTICELLULAR ANIMALS 



471 



Ingestion 



Egesfion 



■, Food 



'^A It 



• « 



Excrefion 



Fig. 23.4. Hydra, its general metabolism. Excretion is carried on by all cells; 

 exchange of respiratory gases likewise. (Courtesy, Mavor: General Biology, ed. 3. 

 New York, The Macmillan Co., 1947.) 



^- 



"^■3 



'^- 



■a; 





'^■S- 



Fig. 23.5. The more rapid ways by which hydras travel. Figures 1-4, by loop- 

 ing; 5-9 by somersaults. Drawings from work of Abraham Trembley ( 1700-1784), 

 a pioneer in the study of hydra and of experimental zoology. (From Trembley: 

 Memoires pour I'Histoire des Polypes. Leyden, Jean and Herman, 1744.) 



