!>52 THE VOYA.GE OF H.M.S. CHALLENGER. 



interieur ou amiuci des radiales, on remarque a chacun d'elles im petit trou rond, 

 d'oti sort un sillon droit et lineaire (fig. 43, s), qui longe la ligne mediane de la face 

 superieure ou ventrale de chaque article et continue son parcours en remontant aussi de 

 long du milieu de la face ventrale des radiales suivantes." This interior truncated 

 portion of the radials is really their ventral face ; while the openings at its central 

 portion are the ends of the axial radial furrows descending to the lower part of the 

 calyx, and the furrows j)roceeding outwards from them are the ventral radial furrows 

 (PI. Villa, fig. 7 ; PI. X. figs. 1, 4 — vrf) as described and figured by Sars, though 

 Ludwig took them for interbasal sutures. Sars's fig. 42 is particularly instructive in 

 this respect, as four out of the five first brachials are in situ, and their ventral furrows 

 are seen to be continued downwards on to the radials. 



The ventral interradial furrows which are so marked on the upper aspect of the calyx 

 of many Comatulse are absent or but slightly indicated in Rhizocrinus. Traces of them 

 may be seen, however, in fig. 42 on Tab. ii. of Sars's memoir. But the adjacent muscle- 

 plates of every two contiguous radials are intimately fused and also slightly everted. 

 Each is separated from its fellow on the same radial by a well marked, ventral radial furrow; 

 and the united halves of the inner faces of adjacent radials thus assume somewhat the 

 appearance of isolated interradial plates resting within and against the outer faces of 

 the radials. Ludwig was thus led to consider them as basals, and so to fall into exactly 

 the same kind of error wdth regard to their genetic relations as he attributed to Sars. 



I have nothing to add to his account of the chambered organ ; but his description of 

 the cords which proceed from its fibrillar envelope needs a little modification. He has 

 pointed out that they are interradial and not radial as descrijjed by Sars ; but he says 

 that they " verbinden sich dann in den untersten Radialien durch Commissuren, ohne dass 

 vorher eine Gabelung stattgefunden hiltte." Were this really the case, Rhizocrinus would 

 be a much more anomalous form than it actually is. For in all other Crinoids, recent or 

 fossil, in which this point has been worked out, with the exception of Batliycrinus, the 

 primary interradial cords fork within the basals, and there are two openings either on the 

 inner (Comatulse) or on the under face (Pentacrinus) of each first radial (PI. XII. 

 figs. 11, 22 ; PI. XX. fig. 9). Ludwig, however, figures these cords ' in Rhizocrinus as 

 single so long as they remain within the basals (top stem-joint, Ludwig) ; and he believes 

 them to fork in the suture between two radials, so that their branches would not enter 

 the radials through their inner or under faces, but at then- lower lateral angles. 



This is not quite the case, however, and it is probably to be explained by Ludwig's 

 having used the section-method only, without attempting to separate the pieces of the 

 calyx. This operation is one of no little difficulty, and some of the radials are sure to be 

 fractured in the process ; but others separate from the basals along the sutural lines, and 

 the arrangement of the canals can then be seen. The radials are comparatively low 



1 Zeitsclir. f. wiss. Zool, Bd. xxix. p. 72, 1877, Taf. vL fij;. 18 



