The somewhat varied assemblage of marine animals familiarly known as Star-fishes, Feather-stars, Brittle-stars, Sea-urchins, and Sea-cucumbers all agree structurally with one another and differ from all other living organisms in several conspicuous features. Prominent among these is the circumstance that their protecting skin is more or less extensively impregnated externally and strengthened internally with calcareous elements which take the form of plates and spines and spicules.
Photo by W. Saville-Kent, F.Z.S.] [Milford-on-Sea.
COMMON OR SHORT-SPINED SEA-URCHIN.
This species is used for food in Italy and other Continental countries.
The Common Sea-urchin may be cited as an example in which these calcareous elements attain their maximum development. The subspherical box-like case or shell, wherein all the vital organs are enclosed and safely protected, is a common object of the seashore, and, empty and denuded of its external coating of prickly spines, familiarly known as a "sea-egg." Examined closely, this shell is found to consist of a series of calcareous plates, which dovetail or fit together in juxtaposition with the utmost nicety. The surface of the shell is studded throughout with rounded hemispherical knobs, those of a larger size having a very distinctly symmetrical plan of distribution. These rounded knobs are the bases of attachment of the spines, which radiate at all points from the surface of the shell when the animal is alive. It will be further recognised on a nearer examination that the walls of the shell are pierced on a definitely symmetrical pattern with minute perforations, such perforations being most distinctly visible on the inner surface of the shell. These minute punctures are the apertures through which in life the delicate tubular locomotive organs, or so-called "feet," are thrust out and retracted. The majority of these tubular organs terminate in a circular sucking-disk, wherewith, collectively, the urchin is able to adhere to and travel over the surface of the smoothest rock, or even up the glass walls of an aquarium. In the empty beach-gathered urchin-shell a circular hole may be observed at the two opposite poles, the one in the centre of the lower and flatter surface being the larger of the two. It is within this lower and larger one that the mouth, with its complex apparatus of teeth, is suspended. The membranous disk which covers the upper and smaller circular aperture in the living animal is perforated centrally by the vent, and around it are grouped the eye-spots and sundry excretory apertures.
Photo by W. Saville-Kent, F.Z.S.] [Milford-on-Sea.
LONG-SPINED SEA-URCHINS.
The needle-like spines of these sea-urchins are over a foot in length. Acres of these creatures may be sometimes seen on tidally exposed areas of the Queensland Great Barrier Reef, where this photograph was taken.