PHYLUM CCELENTERATA 



about until they reach a region where the light is most favorable; 

 this may be called their optimum. They find this optimum 

 by the method of "trial and error," i.e. their movements are in- 

 definite, all directions being tried until the proper conditions are 

 encountered. In a well-lighted area they are most likely to 

 secure the small animals that sen* , 



as food, since these are also attracted \ ) 



by light. ~x7X\ 



When subjected to h$at. the jrial ' J \Nv 



and error 



is likewise em- 



ployed; the animals escape if they 

 chance to move into a cooler area, 

 but perish if they remain in a heated 

 region too long. 



The reactions of a hungry Hydra 



(to food indicate that the physio- 



logical condition of the animal de- 



termines to a large extent the kind 



[of reactions produced, not only 



spontaneously, but also by external 



stimuli. " It decides whether Hydra 



shall creep upward to the surface 



and toward the light, or shall sink 



to the bottom; how it shall react 



to chemicals and to solid objects; whether it shall remain quiet 

 in a certain position, or shall reverse this position and undertake 

 a laborious tour of exploration." 



G Reproduction. Hydra reproduces asexually by budding and by 

 >sion, and sexually by the production of eggs and spermatozoa 

 Budding (Fig. 65, b) is quite common, and may easily be ob- 

 served in the laboratory. The bud appears first as a slight bulge 

 in the body-wall. This pushes out rapidly into a stalk, which 

 soon develops a circlet of blunt tentacles about its distal end. 

 The cavities of both stalk and tentacles are at all times directly 

 connected with that of the parent. When full grown, the bud 



FIG. 70. Hydra moving like 

 a measuring worm. (From Jen- 

 nings, after Wagner.) 



tin 



;oa. _J 



