ON THE LOCOMOTOR SYSTEM OP ECHINODERMATA. 
873 
applying an irritant to any part of the external nerve-plexus, all the pedicels being 
then stimulated into increased activity. Of these antagonistic influences the former, 
or inhibitory one, is the stronger; for if they are both in operation at the same time, 
the pedicels are retracted. 
Star-fish (with the exception of Brittle stars) and Echini crawl towards, and remain 
in, the light; but when their eye-spots are removed they no longer do so. When 
their eye-spots are left intact they can distinguish light of very feeble intensity. 
3. Section (A.).— Siar-jish .— Single rays detached from the organism crawl as first 
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 themselves. 
Dividing the ray-nerve in any part of its length has the effect of destroying all 
physiological continuity between the 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 among the rays ; therefore the animal 
can no longer crawl away from injuries, and when inverted it forms no definite plan 
for righting itself—each ray acting for itself without reference to the others, there is, 
as a result, a promiscuous distribution of spirals and doublings, which as often as not 
are acting in antagonism to one another. This division of the nerves usually induces, 
for some time after the operation, more or less tetanic-like rigidity of the rays. The 
operation, however, although so completely destroying physiological continuity in the 
rows of pedicels and muscular system of the rays, does not destroy, or perceptibly 
impair, physiological continuity in the external nerve-plexus ; for however much the 
nerve-ring and nerve-trunks may be injured, stimulation of the dorsal surface of the 
animal throws all the pedicels and muscular system of the rays into active movement. 
This fact proves that the pedicels and muscles are all held in nervous connexion with 
one another by the external plexus, without reference to the integrity of the main 
trunks. 
(B.) Echini .—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 effected, the spines and pedicellarise within the circular 
area are physiologically 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 pedicellarise within that area immediately respond to the stimulation in 
the ordinary way, while none of the spines or pedicellarige surrounding the area are 
affected, and conversely. Therefore we conclude 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 influence will 
radiate in straight lines from its source, but will not irradiate round the ends of the 
curve or line of injury. 
Although the nervous connexions on which the spines and pedicellarise depend for 
MUCCCLXXXI. 5 U 
