THE HYDRA 87 



can be used more effectively now that you are familiar with the details 

 as seen with the microscope. Make a drawing ( X 50) of an expanded 

 hydra to show the external features. Make also an outline drawing 

 of a contracted hydra, looking down on the mouth and tentacles, 

 (c) Note that the body is hollow, containing a digestive cavity, the 

 enteron, into which the mouth opens. Is there an anus? Does the 

 enteron extend into the tentacles? The body wall consists of two 

 layers of cells, the outer ectoderm which is thinner and more trans- 

 parent, and the inner endoderm. In the green hydra the endoderm is 



1^.^. '^■3- '^••»- 



.Jt. 



'^■S- 





I 



Fig. 42. — Locomotion in Hydra. 



(Reproduced from the original by Abraham Trembley, "Polypes d'eau douce," Leyden, 

 1744.) 



green because of the presence of symbiotic unicellular plants {Zoochlo- 

 rellae) within the cells. 



(d) Study prepared cross sections of hydra. Identify the ecto- 

 derm and endoderm and see that they are separated by the thin non- 

 cellular supporting lamella. Make a drawing, about 3 inches in 

 outside diameter, to show the layers but no cell details (state magni- 

 fication). From your observations construct a diagram to show the 

 structure as it would appear in a longitudinal section passing from 

 end to end and including a tentacle. 



Exercise 3. — The Ingestion of Food. 



(e) Specimens of one of the larger species that have been made 

 hungry by keeping them without food for at least twenty-four hours 

 should be used. In nature, hydras often feed upon water-fleas; in the 

 laboratory they will be more likely to take this natural prey if the 

 fleas have been soaked in w^ater in which several specimens of the 

 annelid Tubifex have been crushed. Short pieces of Tubifex, bits of 



