546 



NATURE 



lApril -J, 1 88 1 



When the forceps are clr sed they may generally be made imme- 

 diately to expand by gently stroking the external surface of their 

 base-. 



With regard to stimulation of the spines, if severe irritation 

 be applied to any part of the external or internal surface of an 

 echinu~, the spines all over the animal take on an active bristling 

 movement. The tubercles at the bases of the spines are the 

 mo t irritalle points en the external surface. 



Wiih regard to .stimulation of the pedicels, if an irritant be 

 applied to any part of a row, nil the pedicels in that row retract 

 ill succession from the seat of stimulation, hut the influence does 

 not extend to other rows. A contrary effect is produced by 

 aiplying an irritant to any part of the external rerve-plexus, nil 

 the pedicels being then stimulated into increased activity. Of 

 these antagonistic influences, the former, or inhibitory one, is the 

 sti on^er, f. r if they are both in operation at the same time the 

 pedicels are retracted. 



btirfish (with the exception of brittle-stars) and echini craul 

 tou aids, and remain in, the light ; but when their eye-spots are 

 rcijoved they no longer do so. When their eye-spots are left 

 intact they can distinguish light of very feeble intensity. 



Experiments in section showed that single rays detached from 

 the organism crawl as fast and in as determinate a direction as 

 do entire animals. They also crawl towards light, away from 

 injuries, up perpendicular surfaces, and when inverted, right 

 tiicmselves. Dividing the ray-nerve in any part of its length has 

 he effect of destroying all physiological continuity between ti e 

 pedicels on either side of the division. Severing the nerve at 

 the origin of each ray, or severing the nerve-ring between each 

 ray, has the effect of totally destroying all co-ordination amonij 

 the rays ; therefore the animal can no longer crawl away from 

 mjuries, and when inverted it forms no definite plan for righiint; 

 iiself. Each ray acting for itself, without reference to the 

 others, there is as a result a prrmiscuous distribution of spirals 

 and doublings, which as often as not are acting in antagonism to 

 one another. This division of the nerve usually induces, for 

 some time after the operation, more or less tetanic-like rigidity 

 of the rays. This operation however, although so completely 

 destroying physiological continuity in the rows of pedicels and uius- 

 cuhir .system of the rays, does not destroy or perceptibly in pair 

 pijysiological continuity in the external nerve-plexus ; for however 

 mucb the nerve-ring and nerve-trunks may be injured, stimulation 

 on ihe dorsal surface of the animals throw-s all the pedicels and 

 muscular system of the rays into active movement. This fact 

 proves that the pedicels and the muscles are all held in nervous 

 Connection with one another by the external plexus, without 

 reierence to the integrity of the main trunks. 



If a cork-borer be rotated against the external surface of an 

 echinus till the calcareous substance of the shell is reached, and 

 therefore a continuous circular section of the overlying tissues 

 etiected, the spines and pedicellarije within the circular area are 

 pihy biologically separated from those without it, as regards their 

 local reflex irritability. That is to say, if any part of this circular 

 area is stimulated, all the spines and pedicellarije within that 

 are . immediately respond to the stimulation in the ordinary way, 

 wtiile none of the spines or pedicellariEe surrounding the area are 

 atlccted, and conversely. Therefore it is concluded that the 

 function of the spines and pedicellarise of localising and gathering 

 round a seat of stimulation is exclusively dependent upon the 

 external nervous plexus. If the line of injury is not a closed 

 curve, so as not to produce a physiological island, the stimulating; 

 Inlluence will rafiiate in straight lines from its source, but will 

 n Jt irradiate round the ends of the curve or line of injury.. 



Although the nervous connections on which the spines and 

 peuicellarire depend for their function of localising and closing 

 round a seat of stimulation are thus shown to be completely 

 de iroyed by injury of the external plexus, other nervous con- 

 nections, upon which another function of the sjiines depends, are 

 ni I in the smallest degree impaired by such injury. This other 

 function is that which brings about the general co-ordinated 

 action of all the spines for the purposes of locomotion. That 

 this function is not impaired by injury of the external plexus is 

 pr ved by severely stimulating an area within a closed line of 

 injury on the surface of the shell ; all the sjiines over the whole 

 surface of the animal then manifest their bristling movement-, 

 and by their co-ordinated action move the animal in a straight 

 line of escape from the source of irritation. 



We have, therefore, to distinguish between what may be 

 cilicd the local reflex function of the spines, which they show 

 n Common with the pedicellariae, and which is exclusively 



dependei t upon the external plexus, and what we may call the 

 universal reflex function of the spines, which consists in their 

 general coordinated action for the purposes of locomotion, and 

 which is wholly independent of the external plexus. Evidently, 

 therefore, this more universal functio - n.ust depend upon some 

 other set of nervous connections (which, however, the authors 

 were not able to detect histologically), and experiment shows 

 that these, if present, are distributed over all the niternal surface 

 of the shell. For if the internal surface be painted with acid, or 

 scoured out with emery paper and brick-dust, the spines and 

 pedici Uari^, after a short period of increa-ed activity or bristling, 

 become perfectly quiescent, lie flat, and lose both their spon- 

 taneity and irritability. After a few hours, how-ever, the spon- 

 taneity and irritability of the spines return, though in a feeble 

 degree, and also those cf the pedicellaris in a more marked 

 degree. These efiFects take place over the whole external .surface 

 of the shell, if the whole of the internal surface be pdnted with 

 acid or scoured »iih brick-dust ; but if any part of the external 

 surface be left unpainted or unsecured, the corresponding part 

 of the external surface remains uninjured. From these experi- 

 ments it is concluded : — (i) that the general co-ordination of the 

 spines is wholly dependent on the integrity of the hypothetical 

 internal plexus; (2) that the hypothetical internal plexus is 

 everywhere in intimate connection with the external, apparently 

 through the calcareous substance of the shell ; and (3) that 

 compilete destrucion of the former, while profoundly influencing 

 through shock the functions of the latter, nevertheless does not 

 wholly destroy them. 



Echini may le divided into pieces, and the pedicels, spines, 

 and I edicellarife upon these pieces will continue to exhibit their 

 functions of l-ical reflex irritability, however small the pieces 

 may be. If an entire double row of pedicels be divided out as 

 a segment and then placed upon its aboral end, it may rear itself 

 up on its oral end by the successive action of its pedicels, and 

 then proceed to crawl about the floor of the tank. We have 

 therefore to meet the question : Is the action of the ambulacral 

 feet in executing these righting movements of a merely serial 

 kind, a, b, and c fir-t securing their hold on the tank floor, owing 

 to the stimulus supplied by contact, and then by thrir traction 

 tilting over ihe gloie, till </, e, and / are able to ti uch th.e floor, 

 and so on ; or does the righting action depend upon nervous co- 

 ordination? Experiments showed that both principles are com- 

 bined, the action of the pedicels being serial, but also assisted 

 by nervous co ordination. 1 his conclusion is sustained by the 

 experiment of .shavin;; off the spines and pedicels over one-half 

 of one hemisphere, i.e. the half from the equator' to the oral 

 pole. When then inverted and forcrd to use their niutilated 

 pedicel-rows, the echini reared themselves upi'n their equators, 

 and then, having no more pedicels wherewith to continue the 

 manoeuvre, came to rest. This rest was permanent, the animal 

 remaining, if accidents were excluded, up -n its equator till it 

 died. The question then here seems to resolve itself simply into 

 this: Is the mechanism li the pedicels so constructed astoiinsure 

 that their serial action shall always take place m the samedirection ? 

 For if it can be shown that their serial actiim may take place indif- 

 ferei.tly in either directi> in it would folloKv that the persistency with 

 which the partly shaved echini continue reared upon their equators, 

 is the expression of some stimulus (such a- a sense of gravity) con- 

 tinuously acting upon some central apparatus, and impelling the 

 latter to a continuous, though fruitless, endeavour to co-ordinate 

 the absent pedicels. If the pedicels are able to act serially in 

 either direction, there is no more rea-on why a partly-shaved 

 echinus should remain permanently reared upon its equator, 

 than that it should remain permanently inveiieci upon its pole; 

 and therefore the fact that in the latter position the pedicels set 

 about an immediate rotation of the animal, while in the former, 

 and quite as unnatural position, they hold the animal in 

 I ersistcnt stasis — this fact tends to show that the righting 

 movements of the pedicels are something more than serial." 

 Thus the whole question as between the two hyi otheses amounts 

 to whether the pedicels are able to act serially from oral to 

 aboral pole. Observation showed that they are so, for echini 

 spontaneously rear themselves from their normal position on the 

 oral pole, to the positi..n of resting upon their equators. Further, 

 as additional evidence that the righting uiovements are at least 

 assisted by sonie centralising influence, is the fact that when the 

 evolution is nearly completed by the ledicel-rows engaged in 

 executing it, the lower pedicels in the other rows become 

 strongly ]>rotruried and curved do%vnwaids, in anticipation of 

 shortly coming into contact with the flo r of the tank. 



