29 



whereby consequently the bucal aperture will occupy the whole of the ventral space within 

 the bucal ring; then the ends of the interradial spaces formed by the adambulacral plates 

 of the interior vertebrae will make the limits of the mouth; and the spines diverging inwards 

 from these plates in fan-like fascicles may also partly be able to act as bucal spines; for 

 which reason we have also thus named them in the foregoing. 



b. The digestive cavity. 



Through the more or less gaping bucal aperture in the Brisinga, there is an uninter- 

 rupted view into the digestive cavity (Tab. 1, tig. 2 & 5) which occupies the greatest part 

 of the cavity of the disc. Its walls form below the immediate continuation of the bucal 

 membrane, as the latter (see Tab. II, fig. 9 b) suddenly curves itself round outwards at the 

 border of the bucal aperture, acquiring at the same time a different quality and structure. 

 There is thus no proper section which might be compared to the aesophagus; and immedia- 

 tely within the border of the mouth the digestive cavity or stomach begins. It has the form 

 of a flat and wide sack, the walls of which are tolerably thick, and consist of 2 distinct 

 layers; an outer fibrous, and an inner cellular, which latter forms numerous irregular folds 

 and sinuosities projecting inwards (see fig. 9). The exterior convex surface of the stomach, 

 which likewise exhibits a more or less well defined longitudinal folding, is attached to the 

 bucal ring by numerous very strong fine tendinous fibres (fig. 9 a). These tendinous fibres, 

 which form an immediate continuation of the tendinous membrane that covers the skeleton 

 of the disc, issue (see fig. 11 c) close together from the upper edge of the latter, just before 

 the insertion of the arms; that is from the dorsal side of the ambulacra! plates of the in- 

 terior vertebrae; they extend themselves horisontally inwards, diverging from each other, and 

 attach themselves at the same height, round the exterior wall of the stomach. 



The stomach is moreover fastened in the middle to the dorsal skin of the disc, by 

 a central ligament which will be hereafter described. By this ligament, and by the tendi- 

 nous fibres before mentioned, the stomach is thus kept in its place and, as it were, suspended 

 in the central cavity which it fills almost entirely; only leaving below a very small space, 

 like a canal surrounding the lower part of the stomach, limited below by the upper surface 

 of the bucal membrane, and on the outside by the interior wall of the bucal ring. This 

 space, thus representing the lower part of the perivisceral cavity, stands round the peri- 

 phery in connexion with a similar narrow upper space, which above the radial spaces of the 

 bucal ring, communicates again with the cavities of the arms. 



The upper part of the stomach is separated, by a distinctly marked annular fold, 

 from the lower proper digestive cavity, with which it nevertheless communicates by a wide 

 circular aperture. From the periphery of this narrow upper section of the alimentary cavity 

 there radiate (see Tab. II, fig. 8 c and fig. 10) on all sides the strongly developed so-called 

 radial caeca, hereafter more particularly described, extending far into the cavities of the arms. 



