A noteworthy feature associated with the greater portion of the structural details of the sea-urchin which have been enumerated is the dominance of the number five in the constituent elements. It is found, for instance, that the perforated areas through which the tube-feet are protruded form, as with the petals and other elements of many flowers, five symmetrically corresponding segments. The dental apparatus comprises five equivalent tooth-like structures, and there are five eye-spots and five excretory apertures at the upper pole. This particular number, with multiples of the same, is furthermore characteristic of all the typical members of the class. Thus, in the Common Star-fish, there are five so-called arms, five eye-spots, one at the tip of each arm, and five equivalent elemental components of all the more important viscera. In the Sea-cucumbers, which have elongate worm-like bodies, there is a similar apportionment of the nerves and muscles of the body generally into fives, and also of the branching tentacles which surround the mouth. Tubular locomotive organs, the so-called "tube-feet," are common to all the three types enumerated. The calcareous plates and spinules, while attaining to a maximum development in the urchins, are also abundantly represented in the other groups. In the common star-fish these calcareous elements form within the skin an openly reticulated trellis-like framework, while in the ordinary sea-cucumbers they more usually take the form of innumerable microscopically minute spicules. The two less familiarly known groups of the Feather-stars and Brittle-stars fully agree with the previously enumerated types in their five-fold structural composition. The brittle-stars have almost invariably five arms only, but they are independent outgrowths from the body proper, instead of being prolongations of it, as in the common star-fish.
Photo by W. Saville-Kent, F.Z.S.] [Milford-on-Sea.
THICK-SPINED SEA-URCHIN.
In large specimens the spines are as thick as a slate-pencil, and may be used for the same purpose.
The Feather-stars, which include some of the rarest and most beautiful representatives of the group, are mostly inhabitants of deep water, and remarkable for the circumstance that either throughout life or in their early phases they are affixed to submarine objects by slender stalks. This peculiarity imparts to the animals such a flower-like aspect that, in conjunction with the indurated calcareous nature of their skeletons, they have received the title of "Stone-lilies." This appellation, however, was originally more particularly applied to their fossilised remains, which occur in remarkable abundance in the older geological strata.
The most familiar British representative of the group is the Rosy Feather-star, occasionally obtained among seaweed in rock-pools on the southern coast, but more often brought up with the dredge from deeper water. In this form the elongate feather-like arms radiate from the central, relatively small, five-rayed body. There is no supporting foot-stalk in this adult stage, the animal being freely movable, and clinging to seaweeds and other objects by means of a cluster of claw-like filaments developed upon its under-surface. Releasing its hold upon its temporarily selected position, it can crawl about with the aid of the hooked extremities of its arms and their radiating joints. It can also propel itself through the water in a somewhat clumsy fashion by the consecutive flexion and extension of these appendages. This freedom of locomotion was not, however, always possessed by the feather-star. In its early days, and when of very small size, it was affixed to a slender foot-stalk, and dependent for its food on the animalcules and other minute organisms which drifted or swam within reach of its extended arms. The rosy feather-star takes its name from the bright rose-red tints by which it is usually characterised. Individuals of the species are, however, subject to considerable colour-variation. On the Australian coast, where many forms are abundantly represented, examples tinted deep crimson, black, bright golden yellow, or sundry admixtures of these several hues are not uncommonly found associated among a dredge-haul of these elegant sea-stars.
The Permanently Stalked Stone-lilies are at the present day of rare occurrence. Up to within comparatively recent years the so-called Medusa's-head Lily was, indeed, regarded as the only living representative of the group. This species has a pentagonal jointed foot-stalk that may be 3 feet long, with five slender appendages developed in whorls at short sub-equal distances throughout its length. From the shallow cup-shaped body at the apex of the stalk a tassel-like bundle of arms is developed, all of these being produced by repeated bifurcation from one of the five equivalent basal stem-joints. Dredging expeditions have within the last quarter of a century revealed the existence of a considerable number of previously unknown species of stone-lilies in the abysses of the ocean, a depth of no less than 3,200 fathoms representing the habitat of one such type.
Photo by N. Lazarnick] [New York.