11 



b. The ambulacral skeleton of the disc. 



(The bucal ring). 



(Tab. IV, fig. I— U). 



When the skin and the interior soft parts are removed from the disc, there remains 

 a rather narrow circular frame or ring (fig. 1 — 3), of a firm calcareous consistenc}', representing 

 the ambulacral skeleton of the disc. This frame has (see fig. 2) its lower side flattened 

 horisontally, and divided into regularly alternating elevated and depressed spaces, of which 

 the former (a) taper a little inwards ; while the latter (b) are of more uniform breadth. At 

 the bottom of each of the depressed spaces there appear 4 circular apertures, situated in 

 pairs and indicating the attachment of the water-feet of the disc (see also fig. 5). The 

 upper side of the bucal ring is not horisontal, but rising somewhat obliquely inwards. 

 From the most salient interior part of the bucal ring, the interior wall is inclined obliquely 

 downwards; while the much lower exterior wall is more perpendicular. A section of the 

 ring will thus represent a quadrangle with 2 longer and 2 shorter sides. The 2 longer sides 

 represent the lower and interior, and the 2 shorter sides, the upper and exterior sides (comp. 

 fig. 10 & 11). The interior wall of the bucal ring is (see fig. 1, 7 & 8) rather even and 

 smooth, but has nearer to the lower side a horizontal deep groove or semi-canal (x) ex- 

 tending in a circle round the whole ring, and having at the lower edge a thickened, and 

 inwardly somewhat salient border (y) which serves as attachment for the thin strongly con- 

 tractile bucal skin. This groove is destined to receive the circular ambulacral vessel situated 

 inside of the disc, and which, by means of the stone-canal, stands in connexion with the 

 madreporic body on the dorsal side of the disc. At the bottom of the groove there appears, 

 opposite to the middle of each of the depressed spaces on the under-side of the bucal ring, 

 a small circular aperture (fig. 8. o.) indicating the place whence the ambulacral vessels ra- 

 diating to the arms issue from the annular vessel. 



The exterior, (as partly also the upper) side of the bucal ring is very uneven, with 

 numerous elevations and depressions, which however soon appear to have a very regular 

 arrangement. If lines are drawn along the middle of the alternately depressed and ele- 

 vated spaces on the lower side, and continued round the whole ring, it will be found that 

 these unevennesses correspond exactly to each other on each side of the lines. If the 

 lines are continued outwards (the arms being considered as in connexion with the disc) then 

 the lines which bisect the depressed ventral spaces will also bisect the ambulacral skeleton of 

 the arms; while the lines which bisect the elevated ventral spaces will be continued out through 

 the intervals between the arms (see Tab* V, fig. 1^2). We may therefore, for the sake of facility, 

 preliminarily call the former the radial spaces of the disc, and the latter the interradial spaces. 

 On closer inspection of the skeleton of the disc, it will be perceived that the guiding lines 



