44 INVERTEBRATE PHYSIOLOGY 



chemical stimulation, the animal may ( Fig. 1 ) first of all expand the disc, 

 then sway, then show a lengthy parietal contraction, then distend, then 

 defecate, and finally "shrivel." Here, then, we find an elaborate sequence 

 of behavior "phases" which at their start, at any rate, show apparently 

 purposeful food capture behavior. Is it any wonder that earlier workers 

 often wished to endow these organisms with rudimentary intellect ? 



The actual capture and swallowing of prey is largely effected by the 

 anemone's so-called (Parker) "independent effectors," the nematocysts, 

 cilia, and mucus glands. All of these respond to chemical stimuli from the 

 external environment (Parker, 1917a; Pantin and Pantin, 1943), al- 

 though it is probable that nematocysts, at any rate, must also be tactually 

 stimulated. Moreover, weak tactile stimulation of the tentacles may also 

 be involved in moving food towards the mouth. 



Most anemones protect themselves from forms preying on them simply 

 by contracting into a compact mass, and sometimes by extending nema- 

 tocyst-armed threads, the acontia. A few anemones will release their foot- 

 hold and move slowly away if stimulated for a relatively long time by 

 "strong" stimuli. Clearly, the terribly potent nematocysts are the chief 

 protection these animals have against their enemies. Yet how is their be- 

 havior, and how is the discharge of their nematocysts, controlled so as to 

 differentiate between foe and food? Ewer (1947) demonstrated that those 

 types of Hydra nematocysts that are used for defense have their tactile 

 threshold raised by food extracts, whereas the threshold of the discharge- 

 triggering mechanism of prey-catching and prey-holding nematocysts are 

 made more sensitive to weak mechanical stimuli by food extracts (see also 

 Pantin, 1942). "Integration" of the sensory information thus occurs 

 immediately at the sense cell-effector level. 



For sensory input that goes into the anemone's nervous system there 

 must be some sort of coding device. As in the planarians, "weak" tactile 

 or vibrational stimuli elicit food capture behavior, "strong" stimuli cause 

 withdrawal. On the tentacles of Calliactis, for example, very weak stimuli 

 will cause a slow discharge of action potentials. Decremental conduction 

 in the tentacle-disc region will prevent many of these spikes from reach- 

 ing the synapses into the through-conduction-system, since these synapses 

 require facilitation — that is, two or more impulses must reach the barrier 

 within a few seconds — only an occasional impulse will be initiated in the 

 through-conduction-system. 



These occasional spikes are enough to initiate phases of "inherent ac- 

 tivity" (Batham and Pantin, 1954), to start the expansion of the disc, or 

 the swaying response, for instance. They are not enough to cause the pow- 

 erful "quick-closure" protective response, since the innervation of these 

 muscles is also protected by facilitation requirements. 



