UNIT AND DIVISION OF LABOR 



147 



2. What is their shape? Note. Study Figure 45 or a 

 chart. Note that the body of a hydra is hollow, and with but 

 one opening at the free end of the body, that serves both as 

 a mouth and as a 



means of getting rid 

 of waste. 



3. How many 

 threadlike organs 

 (tentacles) do you find 

 about the mouth ? 



Conclusions. 1. 

 Compare a hydra with 

 a gastrula, showing 

 how they resemble 

 each other. Note. 

 The gastrula stage of 

 most water animals 

 is ciliated so that it 

 may swim about until 

 it finally settles down 

 to develop further, 

 either with very little 

 added differences, as 

 in the hydra, or 

 many added changes 

 in order to form 

 higher and more complicated animals. 



2. Study a longitudinal section of a hydra, and show how it 

 is much like a gastrula. 



3. Show how certain parts of this animal have certain duties 

 to perform (division of labor). 



4. In what respect does a hydra agree with a sponge? 



5. Which have the better arrangement for getting food and 

 oxygen, Protozoa living singly, or many ciliated cells side 

 by side, as in the sponge ? Which have better opportunities 

 for development ? 



FIG. 45. Lengthwise section of hydra; ba, at- 

 tached end; b, bud ; m, mouth; ov, ovary; sp, 

 spermary holding sperm or male cells. 



