88' 



THE HYDRA 



flesh from a frog, crayfish, or water snail, raw beef, or liver may also 

 be used for food. Place a very small piece of the food carefully 

 against a tentacle with a clean needle. The hydra should be attached 

 to the bottom of the watch glass. Observe the animal throughout the 

 process, using handlens and lowest power of compound microscope. 



Fig. 43. — Nematocysts of Hydra. A, insect larva punctured by nematocysts 



of the barbed type. B, part of an appendage of a water-flea showing another 



type of nematocyst that seems to function like a grappling hook. 



(A, redrawn from H. S. Jennings, "Behavior of the Lower Organisms," copyright, 1906, 

 by the Columbia University Press, printed by permission; B, redrawn from O. Toppe, 

 1909, Zoologischer Anzeiger, vol. 33.) 



Make a series of three drawings to show how ingestion is accomplished, 

 and write a concise account of the process. 



Exercise 4. — The Cellular Structure. 



(f) Macerate a hydra with Bela Haller's fluid as follows: Place the 

 animal on a slide with very little water, and add a drop of Bela 

 Haller's fluid. After one-half minute, quickly remove the fluid with 



Fig. 44. — Diagram of the sensory-ncuro-muscular mechanism of Hydra. One 



of the cni(lol)lasts (c»), which are independent eft'ectors, is inchuled. ect, 



ectoderm; m. /., muscle fibril of large ectoderm cell; n. c, nerve cell; n-s. c, 



neuro-sensory cell; s. c, sensory cell; s. /., supporting lamella. 



filter paper, and add at once a drop of methyl violet. After about 

 two minutes, remove the stain with filter paper and add a drop of 

 water. Put on a cover glass and tap on it gently. In this way the 

 cells may be separated from one another while individual cells are left 

 intact. It may be necessary to make more than one such preparation 

 in order to find the maximum number of cell types; try adding the 



