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.
FIG. 9.—CORAL (Madrepora brachiata).
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).
FIG. 11.—SEA-URCHIN (Echinus), WITH AND WITHOUT SPINES.
