ON THE LOCOMOTOR SYSTEM OF ECHINODERMATA. 
859 
the destruction of co-ordination among its rajs. Astropec-ten , under similar circum¬ 
stances, at first bends its rays about in various ways, with a preponderant disposition 
to the tulip form, and keeps its ambulacral feet in active movement. But after half an 
hour, or an hour, the feet generally become retracted and the rays nearly motionless— 
the animal, like a Brittle-star, remaining permanently on its back. In this, as in 
other species, the effect of dividing the nerve-ring on either side of a ray is that of 
destroying its physiological connexion with the rest of the animal, the feet in that 
ray, although still remaining feebly active, no longer taking part in any co-ordinated 
movement—that ray, therefore, being merely dragged along by the others. 
Under this division it only remains further to be said, that section of the nerve-ring 
in the disc, or the nerve-trunks of the rays, although as we have seen so completely 
destroying physiological continuity in the rows of ambulacral feet and muscular system 
of the animal, does not destroy 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 annual throws all the ambulacral feet and all the muscular system 
of the rays into active movement. This fact proves that the ambulacral feet and the 
muscles are all held in nervous connexion with one another by the external plexus, 
without reference to the integrity of the main nerve-trunks. 
2. Echini. —(A.) Section of external surface of shell. —If a cork-borer be applied to 
the external surface of the shell of an Echinus, and rotated there till the calcareous 
substance of the shell is reached, and therefore a continuous circular section of the 
overlying tissues effected, it is invariably found that the spines and pedicellariae within 
the circular area are physiologically separated from the contiguous spines and pedicel- 
larise, as regards local reflex excitability. That is to say, if any part of this circular 
area be stimulated, all the spines and pedicellariee within that area immediately 
respond to the stimulation in the ordinary way ; while none of the spines or pedicel- 
larise surrounding the area are affected. Similarly, if any part of the shell external to 
the circumscribed area be stimulated, the spines and pedicellariae within the area are 
not affected. These facts prove that the function which is manifested by these 
appendages, of localising and gathering round a seat of stimulation, is exclusively 
dependent upon the external nerve plexus. It is needless to add that in this experi¬ 
ment it does not signify of what size or shape or by what means the physiological 
island is made, so long as the destruction of the nervous plexus by a closed curve of 
injury is rendered complete. In order to ascertain whether, in the case of an unclosed 
curve of injury, any irradiation of a stimulus would take place round the ends of the 
curve, we made sundry kinds of section. It is, however, needless to describe these, 
for they all showed that, after injury of a part of the plexus, there is no irradiation of 
the stimulus round the ends of the injury. Thus, for instance, if a short straight line 
of injury be made, by drawing the point of a scalpel over the shell, say along the 
equator of the animal, and if a stimulus be afterwards applied on either side of that 
line, even quite close to one of its ends, no effect will be exerted on the spines or 
5 s 2 
