PREY-PREDATOR RECOGNITION 45 



Pantin and I (Passano and Pantin, 1955) recently demonstrated the 

 great sensitivity of the disc and tentacles to a local pressure change. This 

 is detected by the mechanoreceptors (whatever element they may be). 

 Stimuli that are insufficiently "strong" or frequent enough to penetrate to 

 the through-conduction-system do cause waving movements of the tenta- 

 cles. On the other hand, tactile receptors of the column and foot regions 

 are adapted for use as danger indicators. They are gastrodermal, not epi- 

 dermal, and the cartilage-like mesoglea of Calliactis prevents them from 

 being excited by moderate stimuli. Small crustaceans can often crawl over 

 the column without causing any response. Strong stimuli, such as a vigor- 

 ous poke by a large crab, will discharge the receptors. The resultant excita- 

 tion immediately gets into the through-conduction-system (since there 

 probably is no facilitation barrier), and, if further action potentials get 

 into the system within a few seconds, a fast "protective" closure of the 

 disc spincter muscle occurs. 



Chemical stimuli have the same general effects on sea anemones ; but, 

 since the substances cannot penetrate the column epidermis and mesoglea, 

 the extra-oral portions of the animal are remarkably insensitive. Only the 

 most concentrated solutions are sufficiently strong to lead to contraction 

 of the "withdrawal" muscles. Yet in a recent series of observations Yentsch 

 and Pierce (1955) showed that Stomphia coccinea responds violently to 

 a substance from the surface of a number of starfish, including Hippastcrias 

 md Dennasterius. I recently noted (Passano, 1955) that Epmctis is also 

 very sensitive to this substance. The tentacles and disc (of both) are most 

 sensitive, as would be expected, but stimulation of the column also elicits 

 responses. This, of course, is immediately reminiscent of the sensitivity 

 shown by mollusks to starfish, but it is not known whether the active agent 

 is the same. Sfoinphia shows a striking series of motor reflexes that are 

 initiated by this stimulus — thrashing back and forth, becoming rigidly dis- 

 tended, and detaching itself from the substrate. Sund (1955) has recently 

 shown that exactly the same sequence of reflexes can be caused by a quick 

 series of six or eight electrical stimuli, perhaps one second apart. It thus 

 seems reasonable to conclude that some strong facilitation barrier must be 

 broken down by a series of spikes in the through-conduction-system, but 

 that, after that, one motor activity triggers another in a continual sequence. 



Another such specific chemical sensitivity, but this time evoking feeding 

 reflexes, has recently been demonstrated by Loomis (1955a,b). He has 

 shown that 10"^I\I solutions of the tripeptide glutathione cause Hydra to 

 show vigorous and persistent feeding responses and, furthermore, that this 

 substance leaks out of Daphnia after these Cladocera have been punctured 

 by nematocysts. There is some evidence that glutathione is also a specific 

 stimulating substance for other hydrozoans, but a few preliminary trials 



