The Asteriæ, with their stellate body and flat rays, are very different in aspect from the Goniasters. The Butt-thorn (Asterias aurantiaca) owes its name to one of those strange superstitions which originate in some inexplicable manner, and are handed down by one credulous generation to the next. "The first taken by the fishermen at Scarborough is carefully made a prisoner, and placed on a seat at the stern of the boat. When they hook a butt (halibut) they immediately give the poor star-fish its liberty and commit it to its native element; but if their fishery is unsuccessful it is left to perish, and may eventually enrich the cabinet of some industrious collector."
To the family of the Asteriæ belongs also the Ling-thorn (Luidia fragilissima), the largest, and one of the most interesting of our British species. When full grown, it measures two feet across, and would appear to exceed that size occasionally, judging from fragments. The rays are from five to seven in number, quite flat, and generally five times as long as the disk is broad. The colour is brick-red above, varying in intensity, the under surface being straw-coloured. The wonderful power which the Luidia possesses, not merely of casting away its arms entire, but of breaking them voluntarily into little pieces with great rapidity, approximates it to the brittle-stars, and renders the preservation of a perfect specimen a very difficult matter.
"The first time I ever took one of these creatures," says Edward Forbes, "I succeeded in getting it into the boat entire. Never having seen one before, and quite unconscious of its suicidal powers, I spread it out on a rowing-bench, the better to admire its form and colours. On attempting to move it for preservation, to my horror and disappointment I found only an assemblage of rejected members. My conservative endeavours were all neutralised by its destructive exertions, and it is now badly represented in my cabinet by an armless disk and a diskless arm. Next time I went to dredge on the same spot, determined not to be cheated out of a specimen in such a way a second time, I brought with me a bucket of cold fresh water, to which article star-fishes have a great antipathy. As I expected, a luidia came up in the dredge, a most gorgeous specimen. As it does not generally break up before it is raised above the surface of the sea, cautiously and anxiously I sunk my bucket to a level with the dredge's mouth, and proceeded in the most gentle manner to introduce luidia to the purer element. Whether the cold air was too much for him, or the sight of the bucket too terrific, I know not, but in a moment he proceeded to dissolve his corporation, and at every mesh of the dredge his fragments were seen escaping. In despair I grasped at the largest, and brought up the extremity of an arm with its terminating eye, the spinous eyelid of which opened and closed with something exceedingly like a wink of derision."
Goniaster.
The Sea-star might be called a flattened sea-urchin, with radiated lobes, and the Sea-urchin, a contracted or condensed sea-star, so near is their relationship. In both we find the same radiating construction, in which the number five is so conspicuous, and in both also the rows of suckers, which, starting from a centre, are set into motion by a similar mechanism, and used for the same purpose. In all the sea-urchins finally, and in many of the sea-stars, we find the surface of the body covered with numerous exceedingly minute, two- or three-forked pincers, that perpetually move from side to side, and open and shut without intermission. These active little organs, which have been named Pedicellariæ, were formerly supposed to be parasites, working on their own account, but they are now almost universally recognised as organs subservient to the nutrition of the animal, and destined to seize the food floating by, and to convey it to the mouth, one passing it to the other. Even in their outward appearance, the sea-urchins are not so very different from the sea-stars as would be imagined on seeing a Butt-thorn near a globular urchin, for both orders approach each other by gradations; thus, the Goniasters, with their cushion-shaped disks and shortened rays, approximate very much in shape to the sea-urchins; and among the latter we also find a gradual progression from the flattened to the globular form. Still there are notable differences between the two classes. Thus in the sea-urchins the digestive organs form a tube with two openings, while in the true sea-stars they have but one single orifice. Their mode of life is, however, identical.
Shell of Echinus, or Sea-Urchin.
On the right side covered with spines, on the left the spines removed.
The Echinidæ move forward by means of the joint action of their suckers and spines, using the former in the manner of the true star-fishes, and the latter as the snake-stars. They also make use of the spines, which move in sockets, to bury themselves in the fine sand, where they find security against many enemies.
Some species even entomb themselves pholas-like in stone, inhabiting cavities or depressions in rocks, corresponding to their size, and evidently formed by themselves. Bennett describes each cavity of the edible Echinus lividus as circular, agreeing in form with the urchin within it, and so deep as to embrace more than two-thirds of the bulk of the inhabitant. It is large enough to admit of the creature's rising a little, but not of its coming out easily. The echinus adheres so firmly to this cavity by its suckers, as to be forced from it with extreme difficulty when alive. On the coasts of the county of Clare thousands may be seen lodged in the rock, their purple spines and regular forms presenting a most beautiful appearance on the bottoms of the grey limestone rock-pools. How the boring is performed has, like many other secrets, not yet been settled by naturalists. The first perforation is most likely effected by means of the teeth, and then the rock softened by some secreted solvent.