﻿262 CONTRIBUTIONS TO THE NATURAL HISTORY 



mucositv mixed with brown matter. The cross-like figure in the bottom of this cavity, 

 as seen from below when the mouth is open, (Plate II. Fig. 21, d,) indicates the folds 

 in the roof of the digestive cavity, from which the radiating tubes arise. 



The digestion takes place very rapidly ; for the same specimen which, when brought 

 in from the sea, had this central space widely distended, as in Plate I. Fig. 4, showed it 

 soon after reduced to a more contracted star-shaped body, as in Plate I. Fig. 2 and 3, and 

 Plate II. Fig. 22, where b indicates the empty cavity, c its prominent angles folded to- 

 gether, and a, a, the four prominent external corners. The upper corners of the central 

 cavity are narrowed into the four radiating tubes, as seen in Plate I. Fig. 1, 2, 3, 4, 

 and in Plate II. Fig. 18, 19, 20. Their connection with the main cavity seems not 

 the same under all circumstances. When the central cavity is very full and distended, the 

 tubes seem to be almost direct continuations of the main cavity, as in Plate I. Fig. 4, and 

 to taper gradually into the tubes above ; but when contracted, the communication be- 

 tween the central cavity and the tubes is nearly closed, and the tubes seem to arise like 

 ligaments from the upper surface of the cavity, as seen in Plate I. Fig. 3. 



Owing to the various dilatations and contractions of this part of the main cavity, its 

 prominent corners assume various outlines. They appear, when half distended, like the 

 rays of a star, as in Plate II. Fig. 19, 20 ; and then their continuation into the tubes is 

 almost direct, though a slight contraction may be noticed at the base of the tubes, as in 

 Plate II. Fig. 20. In this state, when the main cavity is partly empty, the areolar struc- 

 ture of the inner surface may be noticed equally well from above, through the wall of the 

 body, as from below, through the opening of the mouth; as is shown in Plate II. Fig. 19, 

 20. There can be no doubt, therefore, that the whole inner surface of the central di- 

 gestive cavity has the same structure, and performs throughout similar functions. But 

 this areolar structure, and the lining epithelium, with the brown cells underneath, dis- 

 appear at the base of the radiating tubes, and through their whole length nothing similar 

 is noticed. Their walls consist of a homogeneous transparent tissue, through which 

 the digested food is seen to circulate. 



The fact that these tubes may be shut at their base, and thus temporarily cease to be 

 connected with the digestive cavity, shows that the food is not admitted into them before 

 it has undergone a certain degree of elaboration. But no sooner has it been reduced 

 into a certain state of fluidity, in which the particles of the nourishing materials appear 

 like little globules, than they open, the nutritive fluid passes into the radiating tubes, 

 circulates regularly through these tubes along the inner walls of the disk, and through 

 them passes into the circular tube around the lower margin. I have never seen in 

 this species a regurgitation of the digested materials into the main cavity, and it 



