OH THE LOCOMOTOR SYSTEM OF ECHINODERMATA. 
839 
movements of the common Star-fish, are equally applicable to the Sun-stars. When 
placed on the dorsal surface, however, their righting movements are conducted on a 
slightly different plan. Owing to the disc being here so large in proportion to the 
length of the rays, it would be useless in the latter to endeavour to turn over the 
former by twisting themselves into spirals. They therefore adopt a device which in 
the common Star-fish is sometimes made accessory to that of twisting the rays, and 
which is also shown in Plate 81, fig. 20;* they double under the ends of a number of 
adjacent rays, laying hold of the floor of the tank with their ambulacral feet as the 
doubling progressively advances up the length of the ray. When this doubling has 
advanced up a considerable length of a number of adjacent rays, the ambulacral feet 
upon these rays obtain a sufficient purchase to drag over the whole of the large disc in 
a manner otherwise similar to that which has just been described in the case of the 
common Star-fish. 
(C.) Astropecten aurantiacus. —The ordinary locomotor movements of this species 
are highly peculiar. The form of the animal very much resembles that of the common 
Star-fish, although its disc is,proportionally larger, and the whole animal smaller. Its 
ambulacral feet are pointed tubes, rather less than a quarter of an inch long, and, 
as before stated, unprovided with any sucker at the tip. When the animal is not 
walking, these feet are nevertheless in a constant state of movement, and their move¬ 
ments are then of a peculiar writhing, almost vermiform character—twisting about 
in various directions, and frequently coiling round each other. When fully protruded, 
however, they are perfectly straight and stiff. Their protrusion—whether complete 
or partial—takes place with great suddenness, and at all times—whether the animal 
is stationary or not—a number of feet are being protruded, while a number of others 
are as continually being retracted. The feet usually remain extended for a considerable 
though indeterminate time (quarter to half a minute), and then very suddenly again 
collapse. These movements of protrusion and retraction are so sudden that the eye is 
unable to follow them, and as they are always taking place over a large number of 
feet at the same time, the appearance presented by the whole series is that of a con¬ 
tinual flick-flacking. The erection of the feet takes place obliquely from the median 
line of the ray, and the collapse takes place laterally-—the feet therefore falling over 
upon the sides of the ray. The animals, as previously observed, can crawl up perpen¬ 
dicular surfaces in the manner previously described ; but, owing probably to the 
absence of any differentiated structures in the form of sucking discs, they soon tire— 
never succeeding in crawling more than a few inches up the side of a tank before they 
drop off. 
The ordinary locomotor movements of this species are, as we have said, highly 
* Tliis figure lias been drawn so as to sbow both these devices. Very often the common Star-fish does 
not double under the ends of the rays at all, as represented in the figure; but depends entirely on the 
spiral rotation of the rays for the execution of its righting manoeuvre—the dorsal aspect of the active 
rays being therefore not raised from the floor of the tank as represented. 
