ANATOMY OF ASTEEIAS. 175 



tained fluid having been completely expelled from the muscular tube and 

 driven back into the vesicle, which is distended to the utmost. 



(456.) The fluid that thus fills the suckers, and performs so import- 

 ant 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 vas- 

 cular apparatus (fig. 88, 2, g, /), 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 oflice of a dental appa- 

 ratus. 



(459.) The oesophagus is very muscular, and susceptible of great 

 dilatation, its parietes being gathered into deep longitudinal folds. The 

 stomach (fig. 88, i, 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 conse- 

 quently, 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 ca3cum 

 (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 caeca! 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 / they are seen raised from 

 their natural position and carefully unravelled, so as to display more 

 distinctly their complicated structure. When thus unfolded, the casca 

 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 



