848 
MR, G. J. ROMANES AND PROFESSOR J. C. EWART 
elaborate and efficient form—in Echinus the pedicels, spines, and peclicellarise being- 
more highly developed and useful than in any of the other groups, except Spcitangus, 
where the spines are even more so. In the common Star-fish, Sun-stars, and Sea- 
cucumbers, the ambulacra! feet are still the most important organs of locomotion, 
although even here we begin to see that the development of the general muscular 
system has begun to tell upon that of these specially locomotor organs. Again, in 
Astropecten the still greater development of the general muscular system has told 
still further upon that of the ambulacra! feet, the terminal suckers having become 
aborted. Lastly, the Brittle-stars have altogether discarded the use of them ambu- 
lacral feet in favour of the much more efficient organs of locomotion supplied by their 
muscular rays ; and not only the terminal suckers of these feet, but even the whole of 
the feet themselves, have dwindled into useless rudiments. 
§ II. Stimulation. 
1. General facts of stimulation.— All the Echinoderms we have observed 
respond to all kinds of stimulation. The period of latency varies considerably in 
different species, and in different parts of the same animal. In the Holothurians it is 
remarkably long, and from the seat of stimulation there very frequently starts a wave 
of strong contraction, which passes with extreme slowness throughout the length of 
the animal in the form of a deep constriction. Similar waves frequently occur 
spontaneously. 
All the Echinoderms seek to escape from injury. Thus, for instance, if a Star-fish 
or Echinus is advancing continuously in one direction, and if it be pricked or cut in 
any part of an excitable surface facing the direction of advance, the animal imme¬ 
diately reverses that direction ; or, if it be taken out of the water and a drop of some 
irritating fluid be placed on any part of the external surface, the animal will endeavour 
to move away from the source of irritation; whether placed upon a dry table or 
returned to the water, the Echinoderm will at once strike off in a perfectly straight 
line from the source of irritation, and for a long time will travel much more rapidly 
than usual. When two points of the surface are thus irritated, the direction of 
advance is usually the diagonal between them. When a greater number of points are 
irritated, the direction of advance becomes uncertain, but if any, even short, interval 
of tune is allowed to elapse between the application of successive stimuli to different 
parts of the surface, the direction of advance will be in a straight line from the 
stimulus applied latest. When a Star-fish is fastened upon a perpendicular surface, 
and any part of its body is irritated, as, e.g., by a nip with the forceps, the animal, if 
a Sun-star, will actively run away from the irritation. If, however, the latter be 
followed up and repeated, the Star-fish seems to make up its mind to escape in a still 
more expeditious manner, for it immediately lets go its hold with all its suckers, and 
falls to the bottom of the water. A common Star-fish will generally resort to this 
