496 



COMPARATIVE ANATOMY 



CHAP. 



it contains. Fig. 390, which represents the ring sinus in horizontal pro- 

 jection, illustrates its course in five outwardly directed radial and five 



Fig. 390.— Covirse of the ahoral circular sinus, wltli the ring-like strand contained in it in 

 the Ophiuroidea (diagram after Ludwig). 1, Gonads ; 2, axial sinius with axial organ ; 3, mouth; 

 4, circular sinus with ring-like strand, on the side of the bursal wall turned to the interradius ; 5, 

 interradial portion of the ring sinus and strand, bent downwards orally (Fig. 386, left cui.v) ; 6, bursal 

 aperture ; 7, radial (apical) region of the ring sinus (Fig. 3S6, right aav) ; 8, lateral branches of 

 the same on the bursal wall turned to the radius. 



inwardly, i.e. orally directed interradial curves. In this undulating 

 course the ring sinus descends on the inner wall of the disc alternately 



from the apical to the oral side, 

 and then again ascends to the 

 apical side, the radial curves lying 

 apically and the interradial (those 

 near the bursse) orally. 



This peculiar course is no doubt con- 

 nected (1) with the orally directed course 

 of the a.xial sinus, the axial organ, and 

 the stone canal which opens outward 

 orally through its madreporite (Fig. 

 361, 6, p. 422). For the ring sinus 

 is the continuation of the axial sinus, 

 and the ring-like strand is the continua- 

 tion of the axial strand. It is now im- 

 possible to determine whether the axial 

 sinus and the axial organ, in bending 

 orally, drew the ring sinus interradially 

 in the oral direction (in the first place this could of course only apply to the madre- 

 poritic interradius), or whether, on the contrary, the ring sinus, shifting orally, drew 



Fig. 391. — Bursa of Ophioglypha, seen from 

 the side turned towards the interradius (diagram 

 after Ludwig). 1, The tip of the bursa, lying 

 the dorsal side of the digestive sac ; 2, the gonads 

 sessile on the bursal wall ; 3, distal portion of a 

 bursa (that turned to the periphery of the disc) ; 

 5, proximal portion of the same (that turned to 

 the centre of the disc) ; 4, the rows of bursal 

 scales along the aperture. 



