28 CCELENTERATES. 



5. The mouth, which is at the top of this prominence, 

 and may be seen in favorable positions; i.e., when it is 

 turned upward and opened. 



Structure. Mount the hydra alive in a drop of water 

 upon a slide. Place two minute slips of paper or two 

 threads alongside the hydra, and lower a cover glass upon 

 these. Examine first with low, and afterward with higher 

 power, to make out the following points of structure : 



1. The body is a hollow tube with but one opening to 

 the exterior, the mouth. The interior of this tube is the 

 gastric cavity. Observe that this cavity extends out into 

 the buds, when present, and into the tentacles. When 

 the tentacles are fully extended, food particles may be 

 seen in them. 



2. The body Avail is double. Its outer coat (the ecto- 

 derm) is made up of small transparent cells ; its inner 

 coat (the endoderm), of larger, darker cells. The body is 

 therefore a double-walled sac, both walls of which are 

 pushed outward to form buds and tentacles. 



3. In the ectoderm, in parts of the body, and especially 

 in the swollen parts of the tentacles, are certain conspic- 

 uous round or oval cells surrounded by smaller ones. 

 These larger ones are called thread cells, lasso cells, or 

 nettle cells, because they contain a long, spirally coiled 

 thread, which can be thrown out at will, and is used in 

 capturing prey. When the long, swaying tentacles ap- 

 proach some animal suitable for food, as a water flea, these 

 threads are thrown out, and, coming in contact with the 

 flea, they seem to paralyze it. It hangs apparently stunned, 

 adhering to several of these threads, and is quickly swept 

 into the mouth. If a drop of acetic acid, magenta, or 

 methyl green be drawn under the cover glass, and the ten- 

 tacles watched at the same time, the throwing-out of these 

 threads may be seen. Afterward some of these lasso cells 



