40 



merit. In the lower part of the interior wall of the oral ring, we shall thus find the sec- 

 tion of the circular ambulacral vessel, of the circular blood-sinus and of the commissure of 

 the nerves, arranged precisely in the same manner and in the same serial order. In the 

 middle of the disc, we have a longitudinal section of the central part of the digestive appa- 

 ratus extended freely in the cavity of the disc; in the lower part, the oral aperture (m), 

 limited by the horisontal naked oral membrane and leading immediately to the lower strongly 

 corrugated section of the stomach (st), the periphery of which lies close in to the upper 

 margin of the oral ring and is attached to it by fine tendinous fibres; above this section is 

 the much narrower upper division of the stomach, which in its periphery is suspended by 

 ligaments from the dorsal cuticle, and the cavity of which goes over on the sides into the 

 radial caeca (r). Above the roof of the stomach, there appears finally in the centre the 

 apparatus of secretion (n) with its short exit-duct issuing on the dorsal side of the disc. 

 If the section is carried exactly through the interradial space in which the madreporic body 

 is located, we have at the same time a longitudinal section of the latter (m) and of the two 

 asymetrical parts standing in connexion with it, the stone-canal and the so-called heart (h). 

 The oral ring (m r) will then on this side be cut through, so that the wedge plate will be 

 bisected, and the 2 contiguous dorsal marginal plates, the 2 contiguous parietal plates and 

 2 pairs of contiguous adambulacral plates, will be separated from their connexion with each 

 other. Immediately below the cut wedge-plate, the triangular cavity which exists in the in- 

 terior of the oral ring at this part will appear. If the section is made on the opposite side 

 of the oral ring precisely in a radial space, the oral ring will here be cut, so that the 2 

 ambulacral vertebrae corresponding to one arm will be longitudinally bisected; the section 

 will thus coincide with the vertical sutures between the 2 pairs of ambulacral plates be- 

 longing to these vertebrae; and at the lower side (the bottom of the ventral furrow) we shall 

 have a longitudinal section of the radial ambulacral vessel, of the radial blood-sinus and of 

 the radial nerve. If we now consider an arm here in connexion with the disc, the skeleton 

 of the latter will also be divided in the same manner; so that the consecutive vertebrae of 

 the arm will all be bisected in the vertical sutures between the ambulacral plates. 



