OK THE LOCOMOTOR SYSTEM OE ECHINODERMATA. 
841 
the righting is effected with much more energy than in the case of any of the species 
previously mentioned. 
(D.) Brittle-stars. ( Opliiuridce ).—In these Star-fish the ambulacral feet have 
been reduced to rudiments, which, however, are exceedingly active—their mode of 
profusion and retraction being precisely similar to that which has just been described 
in the case of Astropecten. Indeed, their activity is even greater in the case of the 
Brittle-stars; but as they are very short, and not provided with suckers, it does not 
appear that they are of any use in assisting locomotion. The Brittle-stars, however, 
are much the most actively locomotive of all the Star-fish ; and the reason is that, 
having discarded the method of crawling by the ambulacral system, which is common 
to nearly all the other Echinoderms, they have adopted instead a completely new, and 
a much more effectual method. As the family name of the group implies, the 
muscular system of the rays is very perfectly developed, enabling these long and 
snake-like appendages to perform with energy and quickness a great variety of snake¬ 
like writhings. As the movement of all the arms is co-ordinated, the animal is able 
by these writhings to shuffle itself along flat horizontal surfaces at a considerable 
speed. But when it desires to move still more rapidly, it adopts another plan. If the ' 
animal is advancing in the direction of the arrow (Plate 82, fig. 22), one of its rays, 1, 
is pointed straight in that direction; the two adjacent rays, 2 and 3, are thrown 
forwards as far as possible, and then, by a strong contraction downwards upon the 
floor of the tank, these two rays partly elevate the disc, and, while keeping the disc 
so elevated, throw themselves violently backwards into the form of crescents, as 
represented in 2' and 3'. The result of this movement is to propel the animal forwards 
—ray 1 being pushed into the position 1', while rays 4 and 5 are dragged along into 
the positions 4' and 5'. As soon as the rays 2 and 3 have assumed the position 
2' and 3', they are again, without an instant’s delay, protruded straight, to be again as 
instantly thrown into the form of the curves 2' and 3'. Thus the animal advances by 
a series of leaps or bounds, which vary between 1 \ and 2 inches in length, and which 
follow one another with so much rapidity, that a lively Brittle-star can easily travel 
at the rate of 6 feet per minute. While thus travelling, the ray, 1, is usually kept 
straight pointed and partly uplifted—doubtless in order to act as a feeler; but 
sometimes the animal varies its method of progression, so as to use two pairs of arms 
for the propelling movements, and in this case the remaining arm is, of course, dragged 
behind, and so rendered useless as a feeler. The Star-fish is able to use any pair, or 
pairs, of its arms as propellers indifferently, and in all cases it so uses them by resting 
their outer, or distal, thirds upon the tank floor, and at each leap raising their remain¬ 
ing two-thircls, together with the anterior part of the disc, off the floor; at the end of 
each leap, however, the whole animal (except, perhaps, the elevated feeler-ray) lies flat 
upon the floor. 
Brittle-stars, when placed upon their backs, adopt the same method of righting 
themselves as has already been described in the case of Astropecten. They are, 
MDCCCLXXXI. 5 Q 
