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 serve , 



as food, since these are also attracted 

 by light. 



When subjected to heat, the trial 

 and error method 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 FIG- 70. Hydra moving like 



,. , , ,, . , a measuring worm. (From Jen- 



and toward the light, or shall sink ningS) after Wagner.) 



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." 



Reproduction. Hydra reproduces asexual! y by budding and by 

 fission, and sexually by the production of eggs and spermatozoa. 



Budding (Fig. 65, 6) is quite common, and may easily be ob- 

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

 'n 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 



