24 STUDIES OF ANIMAL TYPES 



the foot. It is an adhesive disk wjiich gives out a sticky 

 secretion for attachment to submerged objects. 



Make a drawing of the body to show its shape. 



C. STRUCTURE OF THE BODY. Mount the living hydra 

 in a drop of water beneath a cover glass, with a hair or 

 small strip of paper on either side, to prevent the cover 

 glass from crushing it. 



Examine with both objectives. Note that the body is 

 simply a tube. The large hollow running lengthwise of 

 the body is the body cavity (gastrovascular cavity). 

 This is the digestive and circulatory cavity in one, for the 

 hydra has no alimentary canal and blood vessels. The 

 food, which consists of small crustaceans and other minute 

 animals, is passed into the mouth by the tentacles. The 

 cells lining the body cavity secrete a digestive fluid which 

 acts upon the food, preparing it for assimilation. The 

 flagella, which project from the cells lining the body cavity, 

 create currents that carry the food to all parts of the body. 



With the high objective note the two layers in the body 

 walls. There is an outer layer, the ectoderm, and an inner 

 layer, the endoderm. Compare the shape of the cells of 

 the ectoderm and of the endoderm. How do they differ? 

 If the hydra is a green one, note the green bodies (Algal 

 cells) embedded among the cells of the endoderm. 



Note the prominent, rounded, oval cells in the ectoderm 

 of the tentacles and the distal portion of the body. These 

 are the stinging thread cells. If necessary, crush a tentacle, 

 in order to get a good view of one of these cells. Note the 

 fine thread coiled within each cell. 



Draw a stinging thread cell. 



Make a diagrammatic drawing of the body showing its 

 general structure. 



