Fig. 106.—The Rosy Feather Star
The commonest British species of these starfishes is the Rosy Feather Star (Antedon rosaceus); and as this creature may be kept alive in an aquarium for some considerable time without much difficulty, it will repay one to secure a specimen for the observation of its habits. It is not often, however, that the Feather Star is to be found above low-water mark, its home being the rugged bottom under a considerable depth of water, where a number usually live in company; but there is no difficulty in obtaining this and many other species of interesting starfishes in fishing towns and villages where trawlers are stationed, for they are being continually found among the contents of the net.
Although the Feather Star can hardly be described as an active creature, yet it will cover a considerable amount of ground in the course of a day, creeping over rocks and weeds by means of its arms, which are raised, extended, and again depressed in succession, each one thus in turn serving the purpose of a foot. These arms are capable of being moved freely in any direction, as are also the little more or less rigid pinnules appended to them. The latter are bent backwards on an extended arm that is being used to pull the animal along, so that they form so many grappling hooks that hold on the bottom; and then the arm in question is bent into a curve by the contraction of its muscles, thus dragging the body forward. The arms on the opposite side of the body are also used to assist the movement by pushing it in the same direction, and this is accomplished by first bending the arms, and then, after curving the pinnules in a direction from the body, again extending them. Other movements of the feather star are equally interesting. Thus, the manner in which it will suddenly extend its arms and apply its pinnules to the surface on which it rests in order to obtain a good hold when alarmed, and the way in which it apparently resents interference when one of the arms is touched, are worthy of observation. The arms themselves are readily broken, and will continue to move for some time after being severed from the body, but the loss to the animal is only temporary, for a new arm grows in the place of each one that has been broken off.
This tendency to break into pieces is much greater in the Brittle Stars, as might be expected from their popular name; and is, in fact, such a marked characteristic of the group that it is not by any means an easy matter to obtain a collection of perfect specimens. They will often snap off all their arms, as if by their own power of will, when disturbed or alarmed, and even when removed from their hold without injury, they will frequently break themselves into pieces if dropped into spirit or in any way subjected to a sudden change of conditions.
The tube-feet of Brittle Stars are very small and are not provided with suckers, but are very sensitive, serving the purpose of feelers; also, having thin, permeable walls, they probably play a large part in the process of respiration. Both arms and disc are hardened by a dense scaffolding of calcareous plates; and not only are the former attached to the latter by a well-formed joint, but the arms themselves are constructed of a number of segments that are held together by a kind of ‘tongue and groove’ joint. Round the mouth are a number of tentacles that are kept in constant motion with the object of carrying the food towards it, and of holding the larger morsels while the act of swallowing is progressing.
Fig. 107.—The Common Brittle Star