ON THE LOCOMOTOR SYSTEM OF ECHINODERMATA. 
871 
blood-vessel, and from this into the radial blood-vessels. But when the canula was 
introduced into the proximal end of the sinus, the solution rapidly rushed along the 
sinus and escaped through the madreporic plate—proving that the blood-vessels of 
Solaster communicate far more freely with the exterior than do the water-vessels. 
The ambulacra! system of the common Star-fish only differs from that of the Sun- 
star in having no polian vesicles. Astropecten, on the other hand, has polian vesicles ; 
but in it the pedicels have departed from the usual form in being short, conical, and 
unprovided with terminal suckers. In Opkiura the pedicels are morphologically 
similar to those of Astropecten, though shorter and more slender. They diminish in 
size as they proceed outwards, and at the ends of the arms are scarcely visible. 
II. Physiology. 
1. Natural movements. —The ordinary crawling movements of Astropecten auran- 
tiacus are peculiar, the ambulacra! feet acting the part of walking poles and cilia 
combined. Brittle-stars progress by using two opposite arms upon the floor of the 
tank, with a sort of leap, and can thus travel at the rate of 6 feet per minute. The 
ordinary progression of Echinus and Spatcmgus is assisted by the co-ordinated action 
of the spines, and when placed upon a flat surface out of the water the animal 
advances by means of its spines alone. In Echinus the lantern and pedicellarise are 
also used to assist in locomotion. 
All the Echinodermata that we have observed are able, when placed upon their 
dorsal surfaces on the floor of a tank, to recover their normal position on their oral 
surface. The common Star-fish does so by twisting the ends of two or more of its 
rays round, so as to bring its terminal suckers into action upon the floor of the tank, 
and then, by a successive and similar action of the suckers further back in the series, 
the whole ray is progressively twisted round, so that its ambulacra! surface is applied 
flat against the floor. The rays which perform this action twist their semi-spirals in 
the same direction, and by their concerted action serve to drag the disc and the 
remaining rays over themselves as a fulcrum. Other species of Star-fish, which have 
not their ambulacra! suckers sufficiently developed to act in this way, execute their 
righting movements by doubling under two or three of their adjacent rays, and 
turning a somersault over them, as in the previous case. Echinus rights itself when 
placed on its ab-oral pole, by the successive action of two or three adjacent rows of 
suckers—so gradually rising from ab-oral pole to equator, and then as gradually falling 
from equator to oral pole. Spatangus executes a similar manoeuvre entirely by the 
successive pushing and propping action of its longer spines. 
2, Stimulation.- —All the Echinoderms that we have observed seek to escape from 
injury in a direct line from the source of stimulation. If two points of the surface are 
stimulated, the direction of escape is the diagonal between them. When several 
points all round the animal are simultaneously stimulated, the direction of advance 
