872 
MR. G. J. ROMANES AND PROFESSOR J. C. EWART 
becomes uncertain, with a marked tendency to rotation upon the vertical axis. If a 
short interval of time be allowed to elapse between the application of two successive 
stimuli, the direction of advance will be in a straight line from the stimulus applied 
latest. If a circular band of injury be quickly made all the way round the equator 
of Echinus, the animal crawls away from the broadest part of the band—he., from the 
greatest amount of injury. 
The external nerve-plexus supplies innervation to three sets of organs—the pedicels, 
the spines, and the pedicellarke ; for when any part of the external surface of Echinus 
is touched, all the pedicels, spines, and pedicellariae within reach of the point that is 
touched immediately approximate and close in upon the point, so holding fast to 
whatever body may be used as the instrument of stimulation. In executing this 
combined movement the pedicellariae are the most active, the spines somewhat slower, 
and the pedicels very much slower. If the shape of the stimulating body admits of it, 
the forceps of the pedicellariae seize the body and hold it till the spines and pedicels 
come up to assist. 
And here we have proof of the function of the pedicellariae. In climbing perpen¬ 
dicular or inclined surfaces of rock covered with waving sea-weeds, it must be no small 
advantage to an Echinus to be provided on all sides with a multitude of forceps 
adapted, as described, to the instantaneous grasping and arresting of a passing frond. 
For in this way not only is an immediate hold obtained, but a moving piece of seaweed 
is held steady, till the pedicels have time to establish a further and more permanent 
hold upon it with their sucking discs. That this is the chief function of the pedi¬ 
cel lari se is indicated by the facts that, 1st, if a piece of sea-weed is drawn over the 
surface of an Echinus, this function may clearly be seen to be performed; 2nd, that 
the wonderfully tenacious grasp of the forceps is timed as to its duration with an 
apparent reference to the requirements of the pedicels, for after lasting about two 
minutes (which is about the time required for the suckers to bend over and fix 
themselves to the object held by the pedicellariae if such should be a suitable one) 
this wonderfully tenacious grasp is spontaneously released ; and 3rd, that the most 
excitable part of the trident pedicellariae is the inner surface of the mandibles, about 
a third of the way down their serrated edges— i.e., the part which a moving body 
cannot touch without being well within the grasp of the forceps. When the forceps 
are closed, they may generally be made immediately to expand by gently stroking the 
external surface of their bases. 
With regard to stimulation of the spines, if severe irritation be applied to any part 
of the external or internal surface of an Echinus, the spines all over the animal take 
on an active bristling movement. The tubercles at the bases of the spines are the 
most irritable points on the external surface. 
With regard to stimulation of the pedicels, if an irritant be applied to any part 
of a row, all the pedicels in that row retract in succession from the seat of stimulation, 
but the influence does not extend to other rows. A contrary effect is produced by 
