"By far the greater part of the Zoanthoid polypes, as they grow, deposit in the cellular substance of the flesh of their back an immense quantity of calcareous matter which enlarges as the animal increases in size, and, in fact, fills up those portions of the substance of the animal, which by the growth of new parts are no longer wanted for its nourishment, and in this manner they form a hard and strong case, amongst the folds of which they contract themselves so as to be protected from external injury, and by the same means they form for themselves a permanent attachment which prevents their being tossed about by every wave of the element in which they live. The stony substances so formed are called corals, and their mode of formation causes them exactly to represent the animal which secretes them. The upper surface is always furnished with radiating plates, the remains of the calcareous particles which are deposited in the longitudinal folds of the stomach. These, in all probability were the constructors of all the strata of limestone rocks, which form some of our largest mountains--they were once the beds of seas, and were up-heaved by internal forces. The Madrepores (fig. 9) are also of this order, and form the stony matter in the same way. These are of various patterns, and the beautiful brain-stones in the British Museum are of their construction." [Illustration: FIG. 8.--TUBIPORA MUSICA.] The fourth order of Zoophytes are the Ascidioida, including formerly the Flustræ (fig. 10), but of late these have been shown to be closely allied, if not identical with, the Tunicated Mollusca, and have received the name of Polyzoa, they will therefore be noticed under that head. [Illustration: FIG. 9.--CORAL (Madrepora brachiata).] [Illustration: FIG. 10.--SEA MAT (Flustra foliacea), (A, magnified, B, natural size).] The fourth class of the Radiata, are the Radiaria, including two orders. The first of these are the Acalephæ; the Medusæ or Jelly-fishes belong to this order; these creatures float about sometimes in great numbers, they are transparent and gelatinous, of a hemispherical form, the flat surface being fringed round with fibres, some of which sting like nettles if brought into contact with the skin. The second order of the Radiaria include the Echinodermata or Sea-hedgehogs, Star-fishes, &c., these are many of them furnished with a profusion of spines, and hence their name; they are also called Sea-eggs or Sea-urchins (fig. 11), and are mostly of a conical or spherical shape, divided into five segments covered with minute holes, from which project tentaculæ serving as organs of locomotion, touch, &c., and the surface of the body is covered with shelly spines of various forms and thicknesses; the mouth is in the centre, and they feed upon marine productions. The spines, if cut across and ground thin so as to be transparent, show a most complicated and beautiful structure when under the microscope (fig. 12).