STAR-FISH IN WATER.
If pulled to pieces, each of the five arms, or fingers, will grow into a perfect star-fish.
The Star-fish group is represented by the Common Five-fingers, or Cross-fish, as it is sometimes called, and includes a very numerous assemblage of species of varying size and shape and colour. The British seas alone yield some twenty forms. Among the more notable of these is the Sun Star-fish, which, departing from the rule of possessing five arms only, has twelve or more, its contour, from which it derives its name, somewhat resembling that of a symbolic sun. The colours of this species are particularly brilliant, consisting usually of a variably patterned admixture of crimson, pink, and white. An extreme contrast in contour to the sun-star is presented by the so-called Bird's-foot species, in which the body is pentagonal and so flattened out as to somewhat resemble the foot of a duck. In the Cushion-stars the body, while pentagonal, is comparatively thick.
Photo by E. Connold.] [St. Leonards.
STAR-FISH, OUT OF WATER, TURNING OVER.
The sucker-tipped tubes with which the star-fish effects locomotion are well shown in this photograph.
The so-called Snake-armed Sand-stars and Brittle-stars constitute a section distinguished from the preceding by the character of the arms, which branch separately from the central body, and are composed of an innumerable series of calcareous joints, which snap asunder under the slightest provocation. The great majority of the species are provided with five simple arms only. In an exceptional form, however, known as the Shetland Argus, and its allies, these five arms, while simple at their base, bifurcate repeatedly and in geometrical progression to such an extent as to form in life a complex network of writhing, snake-like tendrils, that has been appropriately likened to a Medusa's head. It has been calculated that there are no less than 80,000 terminal arm-subdivisions in adult examples of this species.
Among the Sea-urchin Tribe there are many notable departures from the typical form previously referred to. In some, while the sub-spheroidal form of the case, or test, is still retained, the external spiny armature is greatly varied. In one series these spines are exceedingly long, slender, and of needle-like contour and sharpness. In others, while long, they are abnormally thick and cylindrical, somewhat resembling slate-pencils, for which they are sometimes used as a substitute; or they may be club-shaped, branched, or reduced to flattened plates. In other forms the shell itself is conspicuously modified. With some known as Biscuit- or Cake-urchins it is flattened out to the resemblance of a cake or biscuit, the spines being minute and inconspicuous. In another group, distinguished as Heart-urchins, the shell is oval and bilaterally symmetrical, though the dominant number of five still holds good with regard to the building up of its structural details. One of the most interesting is the Leather-urchin, so called on account of the flexible and loosely jointed character of its shell, the way being paved by such a form to the normally soft- and flexible-skinned sea-cucumbers. Sea-urchins are to a great extent vegetable-feeders, and the larger species are appreciated as an article of food in many countries, the ovaries, or roe, with which at certain periods the shell is mostly filled, forming the edible portion.
