738 DR. W. B. CARPENTER ON THE STRUCTURE, PHYSIOLOGY, AND 



when viewed from above, is somewhat pentagonal ; but the opening left by the inversion 

 of the lip is nearly circular. Looking clown into its cavity, we may distinguish what 

 was once the central canal (now blocked up), and the pores leading to the articulations 

 of the cirrhi. 



76. The circlet, or rather pentagon, of Basal plates (Plate XLI. figs. 5, 6, b, i), which 

 is for the most part concealed externally by the centro-dorsal basin, is found, when exposed 

 by the removal of the latter, to differ very little either in size or aspect from the circlet 

 first completed in the Pentacrinoid (§ 59). The form of each plate (Plate XLI. fig. 

 2, b) is an irregular trapezoid, with its lower angle truncated ; and it still retains the 

 solid pellucid margin which originally characterized it. But it has undergone a remark- 

 able thickening by an endogenous extension of its calcareous network ; and this has 

 taken place in such a manner as to leave its substance channelled out by a canal which 

 commences at its lower truncated angle, and almost immediately bifurcates, the two 

 branches diverging in such a manner as to pass towards the two First Radials wliich 

 severally abut on the sides of the upper triangle of each basal (Plate XLII. figs. 6, 7). 

 This canal gives passage to a large sarcodic cord that proceeds from the wall of a 

 remarkable quinquelocular organ contained within the centro-dorsal basin, which I 

 shall hereafter describe under the name of the " centro-dorsal vesicle," and which I 

 shall show to be an expansion of the original Crinoidal axis, hollowed out into a mul- 

 tiple ventricular cavity. Each of the five primary cords (which originally lay on the 

 internal surface of the basals forming the floor of the calyx) subdivides into two branches 

 within the basal whose canal it enters ; and thus each of the First liadials receives two 

 branches supplied to it through the two basals whereon it rests. 



77. The form of the First Badials (Plate XLI. fig. 2, r', fig. 5, r\ and XLII. figs. 5, 6, 

 r', /•') is now that of a trapezium having its upper and lower sides nearly straight and 

 parallel, whilst its lateral margins incline towards each other from above downwards. 

 Externally they still present their original cribriform structure ; and this is particularly 

 obvious near the upper angles, where the first-formed perforated plate has not been thick- 

 ened by internal addition. But whilst the external surface is convex, being arched from 

 side to side, the internal is nearly plane, the concavity of the cribriform plate being filled 

 up by an ingrowth of its calcareous reticulation, which still retains for the most part its 

 original type. This ingrowth, however, takes place in such a manner as to leave two 

 deep channels, which commence from the lower angles of the plate, and converge so as 

 to meet in its centre, so as to form one large canal, which becomes completely covered in 

 and passes to the upper margin of the plate, where it opens between the articular surfaces. 

 These converging channels, when the plates are in situ, are continuous with the diverging 

 canals of the two Basals whereon each Eadial abuts ; in such a manner that the primitive 

 canal that enters each basal communicates by its bifurcation Avith the converging canals 

 of two different radials ; while the single canal of each radial is fed (so to speak) by the 

 primitive canal of two different basals. At each of the lower angles of the radial, more- 

 over, the wide embouchure of the converging channel is in proximity with that of its 



