COELENTERATA AND CTENOPHORA 



mechanisms underlying feeding reactions have only recently been elucidated. 

 It has been demonstrated that the hydra opens its mouth to engulf prey only 

 after the attacked organism has been punctured by the discharge of the large, 

 barbed nematocysts. This suggests that the puncture of the prey releases 

 some substance into the water, and that the hydra reacts to the presence of 

 this substance by opening its mouth and executing engulfing movements of the 

 hypostome. Following this line of reasoning, investigators have found only 

 one specific chemical compound, reduced glutathione, which is normally 

 present in the body fluids of animals upon which hydra feeds and will elicit 

 the feeding reaction in hydras. Animals such as other hydras, which the 

 hydra never attacks or attempts to ingest, do not contain this substance; but 

 if a hydra is exposed to a minute amount of reduced glutathione in the culture 

 medium it will engulf other hydras, or in the absence of food will open its 

 mouth so widely that it turns inside-out. The operation of the mouth and 

 hypostome in engulfing food brought in by the tentacles is certainly a co- 

 ordinated response, and the reactions are probably set off by certain neuro- 

 sensory cells which are stimulated by very low concentrations of reduced 

 glutathione emanating from injured prey. The general behavior of the in- 

 dividual is influenced by its physiological state; hydras kept for a short 

 time without food are much more responsive in their feeding reactions than 

 well-fed animals. 



In addition to its specific reaction to reduced glutathione, the hydra reacts 

 to mechanical contacts — light, heat, and electrical stimulation. The eff^ective 

 stimuli in the normal behavior of the animal are probably combinations of 

 specific environmental changes, conditioned by the physiological state of the 

 individual at any given time. Responses to combinations of stimuli can be 

 more easily studied in some of the larger coelenterates. The hydrozoan jelly- 

 fish Gomonemus, for example, reacts moderately to contact with objects, to 

 objects in motion, and to chemical compounds such as acids in solution, but 

 the reaction is greater when these stimuli are presented in combination. 

 Thus, the tentacles of a resting Gomonemus are somewhat sensitive to the 

 contact of a fine pipette which merely touches them or is moved along their 

 surface, and to meat juice gently ejected against them without movement of 

 the pipette. But when the pipette is drawn rapidlv along a tentacle as the 

 juice is ejected, the entire animal goes into action. All the tentacles twist 

 and turn, bending toward the mouth; the hypostome turns toward the 

 tentacle stimulated, and nematocysts are discharged. This combination of 

 stimuli is presumably like that received from a small fish or crustacean, which 

 would normally be attacked, for the reactions are similar in the two ca.ses. 



We may now turn to an examination of the cellular organization of the 

 sensory-neuro-muscular mechanism, the physical basis for the reactions which 

 have been described. The epidermis contains manv nerve cells, or neurons, 

 and at least four diff^erent kinds of sensory receptor cells, all connected with the 

 contractile processes of the large epidermal epitheliomuscular cells. In addi- 

 tion there are neurosensory cells, so termed because they resemble nerve cells 



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