18 STUDIES OF ANIMAL LIFE. 



C. Tentacles and Mouth. 



Tentacles are the "arms" at the distal end of the body. 



1. How many are there ? 



2. What is their arrangement ? 



3. Are they as flexible as the body, and have they the 



same power to shorten and lengthen themselves ? 



4. Do they all shorten at once ? 



The Hydra's mouth is situated in the center of the 

 circle of tentacles at the summit of a small eleva- 

 tion. 



D. Structure. (Use prepared cross-sections.) 



1. Identify, — 



(a) the central cavity (digestive cavity) ; 



(5) the body wall surrounding the digestive cavity. 



2. The Body Wall. 



The body wall is composed of two layers of cells, 

 an outer layer (ectoderm) and an inner layer 

 (endoderm) . 



a. Compare the thickness of these two layers. 



b. Identify a very thin partition, not composed of 

 cells, between the ectoderm and the endoderm. 



3. Nettle cells. (Dem.) 



Nettle cells are pear-shaped cells in the ectoderm 

 which have the power to discharge a thread 

 together with a liquid and thus to paralyze small 

 animals. 



a. Identify the nettle cells. 



h. Where are they most numerous ? 



c. What advantage is there in this distribution of 

 the nettle cells ? 



4. Which of the structures mentioned in 1, 2 and 8 can 



you identify in a living Hydra ? 



