probably employed in the prehension of food.

If

the under surface of the disc, after having been well cleaned and dried, be examined as an opaque object with a power of 20 diameters, it will be seen that those five portions of the skeleton which form the aperture of the mouth are provided with strong, sharp-pointed moveable spines, so arranged as to give the idea of their serving the office of teeth.

The third order of the Echinodermata, the Cirrhigrada, in which are included the Asteriada, or Star-fishes, differs from the Ophiuride in this essential particular, that whilst in the latter the arms are solid and attached to the central disc, in the former the rays are prolongations of the central portion; moreover, the rays of the Star-fish are tubular, and contain very important organs auxiliary to digestion and locomotion. In some families, as the Urasteria, the rays are five in number, in the Solasteric twelve or more, and in the Goniasteria and the Asteria they scarcely exist at all. The skeleton in our common Asterias rubens, is composed of a series of joints somewhat like vertebræ ; these are arranged in the form of a triangle, and over them, as shown in Fig. 107, is a series of shorter and thicker pieces forming an arch; the whole is covered by a horny membrane, in which there is a distinct hexagonal network of calcareous matter. The outer surface of this horny covering is generally provided with spines, which in the great Asterias echinites, are as large as those of any British species of Echinus.

The largest segments of calcareous matter forming the skeleton are arranged around the mouth in an annular

A vertical section of the skeleton of one of the rays of Asterias rubens (after Sharpey).

form; there is also in the Asteriadæ a small tubercle of calcareous matter called the "madreporeform tubercle," Figs. 135 and 136, with which is connected a canal to be noticed hereafter. The digestive system of the Asteriadæ consists of a stomach, occupying the centre of the body, which communicates with, or is prolonged by, a series of cœca, extending the whole length of the rays or arms; but on this I shall not now enlarge, as our attention is for the present confined to the structure of the skeleton.

Thin sections of the outer crust or shell of a common Star-fish, the Asterias rubens, are seen to be composed of a series of layers of calcareous matter of the usual characteristic reticulated tissue, as shown in Fig. 108, the meshes or areolae being either round or oblong, and the walls exceedingly thick; indeed, all the parts of the skeleton, however massive, exhibit

VOL. II.

P

FIG. 108.

more or less of this same structure; a precisely similar appearance is found in sections of the madreporeform tubercle. Star-fish and Ophiuræ occur abundantly in the fossil state; the remains of Ophiuridæ are found in the lias limestone, and Asteriadæ are frequent in the chalk. Separate joints of the Pentagonaster are

A portion of the skele- common in the latter, they consist of reticulations with a square outline, and a circular, or rather globular cavity. In these fossil specimens the square outline is not so decided as in the recent animal, and the cavities are filled with a granular matter, which forms the usual contents of cells in fossilized specimens.

On the external surface of the shell of Asterias rubens, as shown in Fig. 109, are numerous club-shaped

Portion of the integument of Asterias been supposed to be para

rubens.

a a, Pedicellaria.

sites, but when carefully

examined, are found to have a calcareous skeleton

exhibiting the structure characteristic of the skeleton of the Asterias, and must be viewed as parts of that animal. They will be again alluded to when describing the Echini, in which they also exist. The spines of this Asterias are short and tuberculated, but those of the large A. echinites, as shown by a, a, b, in Fig. 110, are

A, Asterias echinites one-fourth of the natural size. a, a, b, spines o A. echinites of the natural size. c, a segment of a transverse section of a spine of a Star-fish of the genus Astropecten. d, a corresponding segment of a spine of Asterias echinites.

conical, and of sufficient size to be cut transversely. A segment of a transverse section, as shown at d in Fig. 110,

exhibits a series of areolæ, or meshes, surrounded by thick walls of calcareous material remarkable for their rounded figure. In sections of spines of a species of Astropecten, shown at c, the reticulated structure is firmer, and the areola are arranged in radii, their size increasing gradually from the centre to the circumference; but the structure of these spines is very different from that of the Echini, which will be noticed in a future Lecture.