23 



3. The water system. 



The water system in the Brisinga exhibits in the main the same normal features as 

 that of the proper star-fish. We have here to distinguish 1) the ambulacral vessels, and the 

 exterior parts which stand in connexion with the same ; 2) the water-feet with their ampollae, 

 and 3) the madreporic body with the stone-canal. 



a. The ambulacral vessels. 



The main trunks of the ambulacral vessels consist of the circular vessel belonging 

 to the disc and of the radial vessels issuing from it into the arms. The circular vessel 

 (Tab. II, fig. 12 f) has its place in the circular groove or semi-canal which is situated on 

 the skeleton of the disc (the bucal ringi round the interior wall and bounded below by the 

 thickened edge whereto the bucal skin is attached. The periosteum-like tendinous membrane 

 covering the ambulacral skeleton goes uninterrupted over this groove; so that the circular 

 vessel thus becomes enveloped in a completely closed canal. On the side of this canal which 

 is turned towards the perivisceral cavity, there appear a number of peculiar small globules 

 (fig. 11, 12 d) which immediately attract the eye by their yellowish color. They are usually 

 attached in pairs (more rarely 3 together) close below all the wedge-plates, and have the 

 >form of small thin-skinned vesicles (fig. 14) containing in their interior a finely granulated 

 matter (fig. 13). Of the nature of these globules, which also occur in the same place in 

 other Asteridse, I can not state anything positively. According to their structure they might 

 appear to form a sort of secretive apparatus. Of the ampolla-iike enlargements of the an- 

 nular vessel (the so-called Polish bladders) so strongly developed in other star-fishes, there 

 is indeed no trace to be seen in the Brisinga; but I have reason to presume that they 

 nevertheless really exist, and that the before mentioned rather wide cavities in the interior 

 of the bucal ring, bounded by the parietal plate and the wedge-plate, are destined to receive 

 such reservoirs of water for the circular vessel. 



The radial ambulacral vessels, the number of which corresponds to that of the arms, 

 issue from the circular vessel at right angles; traverse the groove wherein this vessel lies, 

 and then extend along the ventral side of the disc and the arms, close in to the ambulacral 

 skeleton, occupying a corresponding narrow groove, which runs along the whole arm at the 

 bottom of the ventral furrow, and is partially covered over with a tendinous ligament. On 

 account of its concealed position this vessel is very difficult to extract. It appears however 

 very distinctly in all the sections of the arm (see Tab. Ill, fig. 3) as also on the exterior 



