. Behavior of the lower organisms. Animal behavior; Invertebrates. 2l8 BEHAVIOR OF THE LOWER ORGANISMS. far as possible the nature and causes of the different factors which make it up. We may ask first, What is the cause of the discharge of the nematocysts ? Near each nematocyst there is a projecting point, the cnidocil (Fig. 132, el). This has often been compared to a trigger; touching the cnidocil is said to cause discharge of the nematocyst. That is, it is sup- posed that a mechanical stimulus is the cause of the discharge. But experiment does not bear out this supposition. Hydra may be rub
. Behavior of the lower organisms. Animal behavior; Invertebrates. 2l8 BEHAVIOR OF THE LOWER ORGANISMS. far as possible the nature and causes of the different factors which make it up. We may ask first, What is the cause of the discharge of the nematocysts ? Near each nematocyst there is a projecting point, the cnidocil (Fig. 132, el). This has often been compared to a trigger; touching the cnidocil is said to cause discharge of the nematocyst. That is, it is sup- posed that a mechanical stimulus is the cause of the discharge. But experiment does not bear out this supposition. Hydra may be rubbed roughly with a needle, without causing discharge of the nematocysts. Hard organisms, such as Os- tracods, may strike against it or run over its surface, brushing against many cnido- cils, yet no nematocysts are discharged. On the other hand, various chemicals readily cause discharge of the nematocysts; a solution of methylene blue or methyl green, for example, produces this effect in a marked de- gree. Apparently, then, some chemical stimulus must be associated with the me- chanical stimulus in order to Fig. 132 -Nematocysts and their action in Hydra. cause discharge of the nema. A, portion of a tentacle, showing the batteries of nema- ° tocysts; d., cnidocils. B, insect larva covered with tOCystS. Chemical Stimuli nematocysts as a result of capture by Hydra. of Qne SQrt Qr another wjU doubtless usually be received from the organisms which serve as prey. To what is the remainder of the behavior due? One thing which must be noticed first is that the food reaction depends upon the physio- logical condition of the animal. Not all Hydras react to suitable food, but only those which have not been recently fed. It is, of course, not surprising that only hungry Hydras should eat. Yet this brings out the important point that the behavior is not an invariable reflex, but depends on the physiological state of the organism. When the animal eats, are the determining factors of the rea
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