The Sense Organs
m om enta rily e le c tric a lly negativ e w ith re sp e c t to the he ad of the fi sh . T hu s the fi sh generate s in its env iron m ent a t w o -p o le d fie ld of e le c tric c u rre nt , a n d it c a n se n se w e a k d ist u rb a n c e s in th is fie ld . T h e se n se o rga n s a re lo c at e d in a n d n e a r th e h e a d a n d c o n sist o f p o re s in t h e t h ic k s k in , w h ic h it s e lf is n o n c o n d u c t in g . T h e s e p o re s le ad int o je lly-fi lle d c h a n n e ls . A t t h e b o t t o m of the se c h a n ne ls a re fou nd g rou p s of sen sor y c e lls w h ic h h av e e lab orate n er v e c on n e c tion s w ith the bra in . It is p o ssible to tra in such fi sh to d isc rim in ate b et w e en a nonc ond u c tin g o bje c t , su c h a s a pie c e of g la ss su sp end e d in the w ater, a n d a n id e nt ic a lly sh ap e d c o n d u c t in g o bje c t , su c h a s a p orou s t u b e fi lle d w ith a sa lt solution or w ith ac id . T he se o bje c ts d istort the e le c tric fie ld a rou nd the fi sh in d ifferent w ay s, a nd the fi sh c a n fe e l the se d ifferenc e s on the su rfac e of its b o d y. T he p hy sio lo g y of the se sen se orga n s is still not q u ite k no w n , bu t their e x trem e sensitiv it y is a lre ad y c le a rly p ro v e d . W ith th is apparatu s, G y m narchu s can not detec t fi sh th at a re m ore th a n fou r inche s aw ay from it. H ow ever, d ista nce is not p a rtic u la rly im p orta nt, sin c e th e se fi sh liv e in d e n se ly p o p u late d a n d t u rbid w aters w here v isibilit y is v er y p o or a lso the y a re no c t u rn a l. T h is brief re v ie w c a n not p o ssibly prov id e m ore th a n a g lim p se into the fa sc in atin g w orld of the sen se s. T here a re c ert a in ly m ore sen se orga n s th a n I h av e m entione d . S om e m a rine a n im a ls, fo r in st a n c e , re sp o n d t o slig h t d iffe re n c e s in
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sa lin it y. O ther a n im a ls c a n d e te c t d ifferenc e s in the hu m id it y of the a ir; b e e s u se th is abilit y to fi nd their w ay to the ne c t a r in a flo w er, a nd liz a rd s to fi nd their w ay to w ater. M a ny a n im a ls p erform fe ats of orientation th at still b a ffle the sc ientist . N o one h a s y e t b e en able fu lly to e xpla in hom in g in bird s or the m ig ration of m a rine fi sh e s . A n d t h e re a re m a n y lo w e r a n im a ls in the sea th at ap p a rently sen se d ifferenc e s in the tid e s, sp aw n in g on ly du rin g sprin g tid e s, or fu ll m o on a nd ne w m o on tid e s. H o w th e y c a n te ll w h ic h is w h ic h w e d o not k no w — p erh ap s the y re g ister su btle flu c t u ation s in p re ssu re , w h ic h w ou ld b e g re ate st d u rin g sp rin g tid e s. A nd w hat about p erception that goes beyond the sen ses that w e k now ? T h is is c a lled ex trasen sory p erc e p tion , a nd it is a n issue blu rre d by v a riou s fac tors, of w h ic h im p re c ise term inolo g y is one . If on e d e fi n e s a sen se orga n a s a ny orga n th at prov id e s a n a n im a l w ith in form ation ab out the o u t sid e w o rld , th e re is p e r d e fi n it io n n o su c h th in g a s e x tra sen sory p erc e p tion . O n the other h a nd , if one ap p lie s the term to p erc e p tion b y p ro c e sse s not y e t k no w n to u s, then e x tra senso r y p e rc e p t io n a m o n g liv in g c re at u re s m ay w e ll o c c u r w id e ly. In fac t , the e c ho lo c ation of b ats , ‘the fu nc tion s of the latera l line in fi she s a n d th e w ay e le c t ric fi sh e s fi n d th e ir p re y a re a ll b a se d on p ro c e sse s w h ic h w e d id not k no w a b o u t-a n d w h ic h ~e re thu s “e x t ra se n so r y ” in th is sen se -on ly 2 5 y e a rs a go . T here is no p oint in qu a rrelin g about a term -w hat w e c an a ll ag ree on is th at w e m u st c ontinu e ou r st u d ie s of the a n im a l’s w ind o w s to the w orld .