422 



COMPARATIVE ANATOMY 



CHAP. 



4. Ophiuroidea. In this class, as a rule, one single madre- 

 porite with one pore aperture and a single stone canal are present. 

 The pore aperture is not found, as in Asteroids and Echinoids, on the 

 apical side of the body, but, in adult Ophiuroids, on the oral side of 

 the disc, asymmetrically in an interbrachial area, and on that edge of 

 the oral shield which is turned to the bursal aperture. This oral 

 shield thus becomes the madreporic plate. The pore aperture leads 

 first into an ampulla (Fig. 361, 3), which probably corresponds with 

 the axial sinus of the Asteroidea and Ecliinoidea. Into this ampulla 

 the stone canal which descends from the water vascular ring opens. 



FIG. 361. Stone canal and neighbouring parts of Amphiura squamata, diagrammatic 

 vertical section through the madreporic interradius of the disc. 1, Water vascular ring ; 2, 

 stone canal ; 3, ampulla ; 4, madreporic canal ; 5 and 7, axial sinus (?) ; 6, circular genital sinus ; 

 8, axial organ ; 9, genital rhachis ; 10, bursal pouch ; 11, oral wall of the intestine ; 12, peristomal 

 sinus ; 13, interradial muscle ; 14, circular nerve ; 15, teeth ; 16, mouth ; 17, oral surface of the disc. 



A large part of the ampulla lies on that side of the stone canal which 

 is turned towards the mouth. In consequence of the position of the 

 pore aperture, the stone canal, which rises out of the water vascular 

 ring interradially, runs in a downward (oral) direction. 



The diagram (Fig. 361) illustrates in detail (1) the relation of the stone canal to 

 the axial sinus ; (2) the manner in which the former enters the madreporic ampulla, 

 the cylindrical epithelium of the former being directly continued into the tessellated 

 epithelium of the latter ; (3) the opening outward of the ampulla through a madre- 

 poric canal. 



It appears that in many species of the genera Amphiura, Ophiolepis, Ophio- 

 plocus, Ophionereis, and Ophiocnida, several or many pore apertures occur at the 

 edge of the oral shield. This is certainly the case in many Astropliytidcc. In 

 Trichaster, however, only one pore aperture is present ; but this and the stone canal 

 belonging to it are repeated in each interradius. 



In Ophiactis virens also, which reproduces itself asexually by division, several 

 (as many as five) stone canals occur in the adult in different interradii. In young 

 individuals only one is found. 



