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7. The digestive System. 



The digestive system in the Brisinga exhibits iu its general structure the typical 

 organisation of the proper star-fishes. We have to distinguish 1) The bucal aperture with its 

 muscular apparatus, 2) the digestive cavity or stomach and 3) the radial caeca. We shall 

 consider each of these principal parts separately. 



a. The bucal aperture with its muscular apparatus (the bucal membrane). 



In the middle of the under-side of the disc in the Brisinga, there appears a more 

 or less wide or gaping circular opening, which is the mouth (Tab. I, fig. 2, 5 f. Tab. II, fig. 

 11 c). It is surrounded and limited by a naked membrane attached along the inner peri- 

 phery of the skeleton of the disc (the bucal ring) to the circular projecting edge which is 

 formed by the lower parts of the parietal plates. This bucal aperture is of a very variable 

 appearance. It is sometimes quite small and narrow, only occupying the centre of the under- 

 side of the disc, and far removed from the interior border of the skeleton; sometimes it is 

 so mucb enlarged that it occupies nearly all the ventral space within the bucal ring, and 

 thereby becomes apparently bounded only by the skeleton of the disc itself. Its form is 

 also rather variable; as it is sometimes more irregularly shaped (see Tab. II, fig. 11) or 

 drawn out in one direction or another, to an oval or elliptical shape (see Tab. 1, fig. 2). 

 This variability in size and form of the bucal aperture, depends on the great contractility 

 of the membrane which limits it (the bucal membrane). On closer examination it will also 

 be found that this membrane tTab. II, fig. lib) is interwoven by a complicated system of 

 fine muscular fibres which have partly a radial and partly a circular arrangement. The radial 

 fibres diverge from the border of the mouth, which often exhibits an extremely fine crenula- 

 tion (see Tab. II, fig. 11) and attach themselves round about to the skeleton of the 

 disc. By the action of these fibres, the bucal membrane can be contracted from all sides; 

 the consequence of which will be, that the bucal aperture will be enlarged. The circular 

 fibres act antagonistically ; as they contribute together, like a sphincter, to restrict the bucal 

 aperture and thus extend the bucal membrane. Also in other star-fishes, there is a similar 

 bucal membrane; but it is so little developed that the bucal aperture seems to be limited 

 directly by the intruding angles of the skeleton of the disc with the spines on the same; 

 these spines are often of a peculiar form, and have therefore been distinguished by the 

 appellation bucal spines; they may indeed also act partly, as a sort of masticatory apparatus. 

 If we imagine in the Brisinga the bucal membrane in its highest degree of contraction, 



