ECHINODE11MATA. 151 



(191.) The whole inner surface of the elaborately constructed 

 box which forms the skeleton, as well as the integuments of the 

 star-fish, is lined by a thin membrane, aptly enough called the pe- 

 ritoneum ; for, like the serous tunic so named in higher animals, it 

 not only spreads over the walls of the body, but is reflected there- 

 from upon the contained viscera, so that they are completely in- 

 vested by it, each viscus having a distinct mesenteric fold by 

 which it is supported and retained in situ. 



(192.) The mouth of the Asterias occupies the centre of the lower 

 surface of the body (j^g. 65, a). It is usually described as being 

 a simple orifice entirely destitute of teeth, although it is not impro- 

 bable that the osseous ring around it, and the articulated spines 

 thereunto attached, may, to a certain extent, perform the office of 

 a dental apparatus. 



The oesophagus is very muscular, and susceptible of great dila- 

 tation, its parietes being gathered into deep longitudinal folds. 

 The stomach (fig. 65, b) is a wide sacculated bag, occupying the 

 central portion of the body, and, like the oesophagus, is evidently 

 calculated to undergo considerable distension. 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 

 which 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 capacious caecum 

 (fig. 65, 1, c). 



The whole of the digestive apparatus is displayed in Jig. 66 : 

 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 /, 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, in 

 which, the upper walls of the caeca having been removed, their 

 sacculated internal structure is rendered visible. 



