It is not only as agents in locomotion that the ambulacral suckers are used; for, helpless as these creatures appear to be, they are among the most formidable tyrants of the deep, as will be readily admitted by any one who watches them in the act of devouring prey. When seizing its food, the rays of the Asterias are bent towards the ventral aspect, so as to form a kind of cup, in the centre of which is the opening of the mouth. The cup thus formed will, to a certain extent, lay hold of a passing victim; but, without other means of securing it, the grasp would scarcely be very formidable to animals possessed of any strength: armed, however, as the rays have been found to be, with hundreds of tenacious suckers, escape is almost impossible; for prey, once seized, is secured by every part of its surface, and, in spite of its utmost efforts, is speedily dragged into the mouth and engulfed in the capacious stomach, where its soft parts are soon dissolved.

But to continue our survey of the class before us. Having arrived at the point at which, by the diminution of the rays and consequent extension of the central part, the body has assumed a pentagonal outline, we may now advance in an equally gradual manner to those globular species of which the Echinus or Sea-urchin is the type or most perfect example.

(440). Echinidae

In the Scutellae (fig. 84), we have a flat and shield-like body, in which even the angles of the margin are lost, and the whole circumference acquires a circular form; but still the five radiating ambulacra are visible upon the centre of the disk, although evidently imperfectly developed when compared with those of the Aste-ridao above-mentioned. The nature of the integument has, in fact, become so changed, that another modification of the locomotive organs is now imperatively called for; and the means of progression are therefore proportionately altered. In the Asteridae, the integuments, especially upon the dorsal aspect, are always more or less composed of a coriaceous material, or, at least, of solid pieces so articulated together as to permit of considerable flexibility: but in the Echinidae the nature of the external covering is very different; for these creatures seem completely encased in a dense calcareous shell, composed of numerous angular pieces accurately fitted together and incapable of movement. The Scutellae, moreover, bury themselves beneath the surface of the sand - a situation in which suckers would be of little use, but for which these animals are admirably adapted by a contrivance not less calculated to excite the admiration of the observer.

The exterior of the shell is entirely covered with minute appendages resembling, when seen with the naked eye, delicate hairs; but these, when examined under a microscope, are found to be spines of most elaborate structure, as is evident from the magnified view of one represented in the annexed figure (fig. 84.) Innumerable as these spines are, every one of them is articulated to the shell by a kind of ball-and-socket joint, and susceptible of being moved in all directions; so that by their combined efforts the Scutella can speedily bury itself, either for the purpose of procuring food, or of eluding observation.

Scutella.

Fig. 84. Scutella.

(441). From the flat Scutellae, the passage to the globose Echintdae is uninterrupted; and a beautiful series of connecting forms (many still existing as living species, but a still greater number found only in a fossil state) demonstrate the progressive expansion of the shell, and its conversion into the spherical figure seen in the Echinus esculentus (fig. 85.) The Echinus in shape resembles an orange, its dense calcareous crust enclosing the viscera within its cavity, while the locomotive apparatus is placed upon the external surface. The mouth is a simple orifice in the shell, placed at one extremity of its axis; and through it, as represented in the figure, the points of five singular teeth project externally; while the anal aperture is situated at the opposite pole of the sphere. The instruments of locomotion occupy the entire superficies of the shell, and consist of two distinct sets of organs adapted to different uses. The first consists of a multitude of sharp purple spines, every one of which is articulated to a distinct and prominent tubercle whereon it moves. These numerous spines, therefore, which are essentially similar in their office to those we have already described in Scutella, differing only in proportionate size, are so many inflexible legs, upon which the Echinus rolls itself from place to place, or by their assistance it can bury itself in the sand with the greatest facility.

But these wonderfully constructed animals are by no means confined to this mode of progression; for, impossible as it might appear from their outward appearance, they are able to climb rocks in search of food, and thus destroy the corallines and shell-fish upon which they principally feed. In order to effect this, we find the shell perforated with ten rows of small orifices, so disposed as to form five pairs of ambulacra extending from one pole to the other: through these apertures a system of long suckers is made to issue, which, protruding (as represented in fig. 85) beyond the points of the spines, can be firmly fixed to any smooth surface, and, like the suckers of Asterias, become locomotive agents.

(442). Holothuridae

Having traced the development of theEchino-dermata from the polypiform Encrinite to the globular Echinus, we now shall find them perceptibly approximate an annulose or worm-like form. In the Holothuria (fig. 100), the commencement of this change is perceptible: instead of being composed of hard, calcareous pieces, the integuments of the body now become soft and irritable, a few thin laminae of earthy matter around the mouth being the only vestiges of the shell, and the spines, of course, are no longer met with; the suckers, however, remain, and, when protruded through innumerable apertures distributed over the surface of the body, they still form the principal instruments of progression.