ON THE LOCOMOTOR SYSTEM OF ECHINODERMATA. 
805 
Suppose, for instance, that it is an incipient sense of gravity that determines this 
central apparatus to work the feet-rows serially, in order to rotate the animal into its 
normal position; it does not follow that, under any circumstances, the stimulus sup¬ 
plied by this sense of gravity should induce the central apparatus to reverse the action 
of the feet-rows; for to do this would, under any circumstances, be to act in opposition 
to the stimulus supposed. Only if we were to imagine that the central apparatus, if 
present, must possess a true psychological element capable of sufficient intelligence to 
reflect that by temporarily acting in opposition to the sense of gravity the peculiar 
exigencies of the situation might be overcome—only then could we fairly argue that 
the result of these experiments shows the righting movements of the feet to be purely 
serial, or wholly independent of nervous co-ordination. As a matter of physiology, 
therefore, the only question which in the present connexion we have to consider is this 
—is the mechanism of the ambulacra! feet so constructed as to insure that their serial 
action shall always take place in the same direction? For if it can be shown that 
their serial action may take place indifferently in either direction, it would follow that 
the persistency with which the shaved Echini continued reared upon their equators is 
the expression of some stimulus (such as a sense of gravity) continuously acting upon 
some central apparatus, and so impelling the latter to a continuous, though fruitless, 
endeavour at co-ordinating the absent feet. If the righting movements were wholly 
independent of any such central apparatus, and due only to the serial action of the 
feet, we should expect that (supposing the feet to be able to act serially in either 
direction) when the equator position had been attained in shaved specimens, it would 
not be maintained. For if there were no constant stimulus emanating from any co¬ 
ordinating centre persistently trying to induce the absent feet to continue the serial 
action in the same direction, we should expect, if serial action can take place in either 
direction, that after a time it should begin to take place in the opposite direction ; 
upon the supposition that the feet may act serially in either direction, there is no more 
reason why a shaved Echinus should remain permanently reared upon its equator than 
there is that it should remain permanently inverted upon its pole, and therefore the 
fact that in the latter position the feet set about an immediate rotation of the animal, 
while in the former and quite as unnatural position they hold the animal in persistent 
stasis—this fact tends to show that the righting movements of the feet are something 
more than serial. The question, therefore, that we set ourselves to determine was, 
whether the serial action of the feet invariably takes place in the direction of ab-oral 
to oral pole, or may likewise take place in the opposite direction. We found that it 
may take place in the opposite direction, as the following observations prove. We 
have seen a shaved specimen, which after remaining for several hours on its equator 
was accidentally rolled over into its normal position, forthwith begin to rear itself 
upon its uninjured feet-rows. Executing this what we may call an inverted righting 
movement with activity, the Echinus was speedily reared into the equatorial position 
on the opposite side to that from which it had just fallen—and in order to do this, it 
5 T 
MDCCCLXXXI. 
