94 THE BIOLOGY OF THE ANIMAL 



structure from the feeding zooids? Do you 

 find coenosarc, manubrium, digestive cavity, 

 etc. ? Examine the medusae in the reproductive 

 buds, and note their attachment to the central 

 stalk or blastostyle, which morphologically 

 represents a rudimentary hydranth. Of what 

 is the blastostj^le composed? How are the 

 medusae attached to it ? How many medusae 

 in each gonangium ? How do those in the same 

 gonangium differ from one another ? How do 

 you account for these differences ? 

 Draw a reproductive zooid showing medusae in va- 

 rious stages of development. 



3. The young zooids. — Note their position, shape, 

 and structure. Look for them in different 

 stages of development. Can you distinguish a 

 young feeding zooid from a young reproduc- 

 tive zooid ? Compare these young zooids as to 

 position in the colony, shape, structure, etc., 

 with the buds on a hydra. 

 Draw young zooids to show the various stages in their 

 growth. 



PHYSIOLOGY 



If living hydroids are obtainable the following topics 

 may be studied. Many of the questions, however, may 

 be satisfactorily answered by a careful comparison of 

 the shapes, positions, contents, etc., of dead specimens. 



a. Movements. — Does the colony as a whole sway back 

 and forth on the hydrocaulus? Is the hydro- 

 oaulus flexible enough to be bent by currents in 

 the water ? Can you determine whether or not 

 the colony can loosen the hydrorhiza from its 



