ON THE LOCOMOTOR SYSTEM OF ECHINODERMATA. 
849 
method when first irritated, without waiting for a repetition of the stimulus. An 
Echinus, on the other hand, will not drop off a perpendicular surface unless compelled 
to do so by serious irritation; it crawls away as quickly as possible, and sometimes 
rotates upon its axis in a manner afterwards to be described, whereby, without leaving- 
go its hold of the perpendicular surface, it is able to alter its position rapidly. But of 
all the Echinoderms the most curious to observe in this connexion are the Brittle- 
stars, for these may be made to leap about in any number of directions with much 
activity, by gently stimulating different parts of their bodies successively. When any 
part of the dorsal surface of any Star-fish is irritated, not unfrequently one of the 
arms is doubled over and touches the seat of irritation, as if to endeavour to brush 
away the offending body. 
That the external surface of a Star-fish should prove itself to be excitable is what 
we should perhaps expect a priori, although we might not expect to find so high a 
degree of co-ordination manifested by the nervous system as is implied by its responses 
to the cutaneous excitations above mentioned. But that the external surface of an 
Echinus or Spatangus should be so highly excitable as it is, we should scarcely have 
anticipated—particularly before our observation of the external nervous plexus ; for at 
first sight it would seem that the numberless long and mobile feet—to say nothing of 
the spines—would be sufficient to convey all the information that the animal requires 
concerning the external world, without its exterior requiring to be rendered sensitive 
over its whole surface. Yet we find, so far is this from being the case, that the 
external surface cannot be touched with a needle’s point at any part without the 
whole animal being affected thereby. We have already described the nervous plexus 
whereby this general sensitiveness of the external surface is secured. We must now 
enter pretty fully into the functions of this plexus as revealed by sundry experiments 
on the multitudinous and wonderful system of organs which, either directly or 
indirectly, depend upon this plexus for their innervation. 
These organs are the ambulacra! feet, the spines, and the pedicel]arhe. That all 
these organs are in nervous connexion with the external plexus is proved by the fact 
that when any part of the external surface is touched, however gently, all the feet, 
spines, and pedicellarise within reach of that point, and even far beyond, immediately 
approximate and close in upon the point, so holding fast to the needle, or whatever 
other body may be used as the instrument of stimulation. This simultaneous move¬ 
ment of such a little forest of prehensile organs is a singularly beautiful spectacle to 
witness. In executing it, the pedicellarise are much the most active, the spines some¬ 
what slower, and the ambulacral feet very much slower. If the object with which 
the external surface is touched be itself small enough, or presents edges narrow 
enough, to admit of the forceps on the pedicellarise establishing a hold upon it, it is 
seen to be immediately seized by some of these organs, and held there till the spines 
and ambulacral feet come up to assist; but if the object is too large, or does not 
present any surfaces which the pedicellarise are able to catch—such, for instance, as 
5 R 
MDCCCLXXXI. 
