40 INVERTEBRATE PHYSIOLOGY 



logical states" may affect the responses of the organism to specific stimuli. 

 While I see no sound reason to exclude the possibilities of conditioning, or 

 learning, in the gastropods, I know of no demonstration of this. But the 

 efTect of changing states seems to be present. Forty years ago Wenrich 

 (1916), working with the fresh water mussel Anodonta, clearly demon- 

 strated that a variety of internal conditions changed their sensitivity to 

 light. Besides the unexplained but omnipresent "individual differences," 

 the presence of eggs or embryos, of foreign material in the mantle chamber, 

 and whether the animal had been stimulated previously or not — all these 

 conditions had definite effects. As will be noted later, this modifiability of 

 responsiveness is to be found throughout all the lower metazoans. For 

 instance. Gee (1913) maintained that the internal state of leeches — 

 whether they were hungry or satiated — profoundly influenced their food- 

 finding behavior. 



Gee emphasized the importance of "random movements" in the leech 

 activity pattern. He postulated that the central nervous system of these 

 annelids had two main functions: (1) the production of "spontaneity," 

 and (2) adaptation to constant external stimuli. It is clear that he assumed 

 that the central nervous system integrated all sensory input and that the 

 reaction of the organism was the resultant of these concurrent stimuli. 

 Perhaps it is even more than this ; it should be remembered that Copeland 

 (1930) and Copeland and Brown (1934) convincingly demonstrated a 

 case of conditioning in the polychaete Nereis, in which the normal positive 

 response to food extracts was duplicated by touching the anterior end of 

 the worm, usually a withdrawal stimulus. 



Copeland and Wieman (1924) earlier had demonstrated the normal 

 chemokinetic feeding behavior of these sand worms, and it is probable 

 that this is general in the carnivorous polychaetes. On the other hand, while 

 defensive (escape) responses may be initiated by strong or specific 

 chemical stimuli, it is clear that these animals normally depend on sensitive 

 vibrational or tactile receptors, synapsing with giant fibers for this use. 

 An easily observable example of these elegant responses can be seen in 

 the "quick-as-a-wink" withdrawal of the tubeworm filterers such as the 

 sabellids. 



It is an obvious truism, in general resume of the behavior patterns of 

 these animals, that chemical and vibrational or tactile cues are the im- 

 portant external stimuli enabling effective prey-predator recognition. How 

 much the disturbances engendered by prey movement, as against the dem- 

 onstrated role of weak chemical stimuli, ensures the success of the preda- 

 cious carnivores has not yet been determined. Clearly, more knowledge is 

 necessary, both at the behavioristic and physiological levels, before specific 

 analyses can be made. However, it does seem true of all of the organisms 



