262 THE HYDRA 



endoderm cells. Since the individual cells are in the same 

 intimate contact with the surrounding water as the independent 

 cells of a protozoan, or the cells of a multicellular animal with their 

 surrounding lymph (c/. Fig. 60, p. 104), similar exchanges of 

 material can be postulated for each case. Likewise, the oxy- 

 gen used in respiration in hydras without zoochlorellse is pre- 

 sumably taken directly from the surrounding medium in much 

 the same fashion as the cells of a higher organism receive their 

 oxygen from the neighboring intercellular lymph. If the meta- 

 bolic processes of the frog and the protozoa be recalled and 

 compared with those of hydra, it will, therefore, be seen that 

 the phenomena of metabolism are essentially alike in all these 

 animals. The differences are those incidental to the state of 

 organization and not to any fundamental contrasts. 



Irritability and Behavior. — Moveinents and Locomotion. — The 

 more obvious movements of the hydra, whether the animal 

 is attached or in locomotion, may be accounted for mechanically 

 by means of the longitudinal and circular muscle processes of the 

 ectodermal and endodermal cells. The fact that these processes 

 can contract and relax locally and in a coordinated fashion is 

 evidenced by the varied positions and outlines the animal may 

 assume. Less obvious movements of sliding along on tentacles 

 have been described as due to the protoplasmic flow of ectoderm, 

 while gliding on the foot has been attributed to pseudopodia. In 

 addition to extensions and contractions of body and tentacles as a 

 whole, there are peristaltic pulsations by which the food is passed 

 from one part of the enteron to another. These may be very slow, 

 consuming from fifteen to thirty minutes in traveling over the 

 length of the body; or they may be so rapid as to shift the mass 

 rather suddenly, as when f secal matter is violently egested or food 

 (quickly shifted toward the middle of the hydra. There is also a 

 sphincter, or special contractile area, formed by the circular 

 muscle processes around the mouth. The sphincter at the base 

 of each tentacle is capable of shutting off the tentacular cavity 

 from that of the body. Detachment of the base during locomo- 

 tion seems also to be due to muscular contraction. 



Locomotion is effected in a variety of ways (Fig. 120). A 

 hydra may move by imperceptible degrees, ghding upon its "foot" 

 by amoeboid extensions of this area. Under stronger stimula- 

 tion, it may extend itself laterally until the tentacles are in con- 



