ANTEDON. 377 



on the external, internal, and lateral faces of the radial 

 plates belong to a system of canals which radiate from 

 the central funnel-like space enclosed by the five plates, 

 and also form an annulus around it. These canals lodge 

 the radial cords and commissures of the apical nervous 

 system, and their courses will be more readily understood 

 in relation to that system. 



The Rosette. —This plate (PL I., figs. 17 and 18) 

 assumes the form of a disc with a circular aperture in the 

 centre, and a margin divided by deep clefts into ten 

 radiating processes. Of the latter, five are triangular and 

 lie in nearly the same horizontal plane as the disc 

 (fig. 8) ; whilst the alternating five have nearly parallel 

 margins, which are inflected in such a way as to form a 

 shallow groove, the whole process being markedly reflected 

 in the direction of the centro-dorsal plate. The exact 

 form of the rosette can only be made out after dissociation 

 from the radial pentagon by maceration in a solution of 

 caustic potash. When in situ it almost completely shuts 

 off the funnel-shaped cavity enclosed by the five radials 

 from the cavity of the centro-dorsal plate, the circular 

 aperture in its centre being the only communication 

 (PL V., fig. 52; PL VI., fig. 59). The five triangular 

 processes are interradial, that is to say, they are directed 

 towards the sutures between the radials ; the alternating 

 processes are radial, and abut upon the axial faces of the 

 radials, from which also a number of irregular calcareous 

 outgrowths pass to the oral face of the rosette (PL VI., 

 fig. 59). 



First Primibrachial Plates. — Each of these is an 

 ellipsoid disc-like plate of moderate thickness (PL L, 

 fig. 3, pmb. 1 ; figs. 8 and 9), having two nearly parallel 

 faces, of which the inner, or axial, has much the larger 

 area (fig. 8), and articulates with the outer, or 



