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, d). 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, Awithout 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, ^.^^gg discharge of the nema- 

 j4, portion of a tentacle, showing the batteries of nema- ° . 



tocysts; d., cnidocils. B, insect larva covered with tOCystS. Chemical StimUU 

 nematocysts as a result of capture by Hydra. ^f ^^^ ^^^^ ^j. another wiU 



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 reaction 

 mechanical stimuli or chemical stimuH? Experiment shows that me- 

 chanical stimuU alone do not induce the food reaction. If bits of filter 

 paper, or ostracods with a hard shell, are brought in contact with the 

 tentacles or the mouth of a hungry Hydra, they are not swallowed. But 

 if the filter paper is soaked in meat juice, or if the ostracod is crushed, 



