(456). The fluid that thus fills the suckers, and performs so important a part in causing all their movements, is not secreted by the vesicles in which it is contained, but is conveyed into them by a special vascular apparatus (fig. 88, 2, g, f), from which branches are given off to each tube. The nature of the fluid, however, and the arrangement of the vessels through which it flows will be more properly discussed hereafter.

(457). The whole inner surface of the elaborately-constructed box that forms the skeleton as well as the integuments of the Star-fish is lined by a thin membrane, aptly enough called the peritoneum; for, like the serous tunic so named in higher animals, it not only spreads over the walls of the body, but is reflected therefrom upon the contained viscera, so that they are completely invested by it, each viscus having a distinct mesenteric fold whereby it is supported and retained in situ.

(458). The mouth of the Asterias occupies the centre of the lower surface of the body (fig. 88, 1, a.) It is usually described as being a simple orifice, entirely destitute of teeth, although it is not improbable that the osseous ring around it, and the articulated spines thereunto attached, may, to a certain extent, perform the office of a dental apparatus.

(459). The oesophagus is very muscular, and susceptible of great dilatation, its parietes being gathered into deep longitudinal folds. The stomach (fig. 88, 1, b) is a wide, sacculated bag, occupying the central portion of the body, and, like the oesophagus, is evidently calculated to undergo considerable distention. There is no anal orifice; and consequently, as in the Polyps, the indigestible parts of the food are again expelled through the mouth. The walls of the stomach, as well as those of the oesophagus, contain muscular fibres, and are further strengthened by fibrous bands, apparently of a ligamentous character, derived from the peritoneal covering that spreads over its outer surface. Ten narrow canals open by as many distinct orifices into the sides of the stomach, each of which, after a short course, expands into a voluminous caecum (fig. 88, 1, c).

(460). The whole of the digestive apparatus is displayed in fig. 89: every one of the five rays contains two of the caecal prolongations derived from the stomach or central bag (a); and in the rays marked c, d, e, these organs are represented in situ, but at f they are seen raised from their natural position and carefully unravelled, so as to display more distinctly their complicated structure. When thus unfolded, the caeca present an arborescent appearance, the central canal being dilated into numerous lateral sacculi, from which, in turn, secondary pouches are given off; and in this manner innumerable ramifications are formed, so that the extent of internal surface is enormously increased, as may be seen in the ray g, wherein, the upper walls of the caeca having been removed, their sacculated internal structure is rendered visible.

(461). With respect to the exact office of these capacious appendages to the stomach, there exists some diversity of opinion.

(462). It is scarcely possible that they can be at all instrumental in the digestion of food, the passages whereby they communicate with the central cavity being too narrow to admit any solid substance into their interior; the digestive process would therefore seem to be entirely accomplished by the receptacle into which the food is first introduced. But there is every evidence to prove that, although they can have little part in digestion, the caeca are intimately connected with the absorption of nutriment; and thus, although possessing no excretory orifice, they must be looked upon as strictly analogous in function to the intestinal canal of other animals: the great extent of surface which they present internally would alone lead to this supposition, even did not the nature of the material usually found in them, namely, a pultaceous creamy fluid, evidently a product of digestion, abundantly confirm this view of their nature. The matter seems, however, to be put beyond a doubt by the arrangement of the vascular system connected with these organs, as the veins that ramify so extensively through their walls are here, as in other Echinodermata, the only agents by which the absorption of chyle can be effected: this will be evident when we examine the organs subservient to the circulation of the nutritious fluids.

(463). Those physiologists who have adopted a different view of the nature of the caecal appendages to the stomach, consider them to be adapted to the secretion of some fluid, and probably representing a biliary apparatus. Their enormous extent, however, would alone lead us to dissent from such a conclusion, more especially as another organ has been pointed out to which the functions of a liver have been assigned. This is situated upon the base of the stomach (fig. 89, b), and is a yellow or greenish-yellow racemose sacculus, which opens into the bottom of the digestive sac by a free aperture; the contents of this organ, moreover, resemble bile both in taste and colour*.

Digestive apparatus of Asterias: a, stomach; b, hepatic (?) glands.

Fig. 89. Digestive apparatus of Asterias: a, stomach; b, hepatic (?) glands; c, d, e, caecal appendages in situ;f, the same unravelled; g, the same laid open, showing their sacculated interior.

(464). In the slender-rayed genera, such as Ophhira, the caecal appendages are not met with; but their deficiency appears to be supplied by the plicated walls of the stomach itself, the numerous folds of which resemble lateral leaflets attached to the central cavity. We are unacquainted with the precise organization of the alimentary canal in Comatula; but, from the orifices visible in the shell, it would appear that in this genus, as well as in some Crinoid species, the digestive tube was furnished with an anal aperture.