MORPHOLOGY 33 



Although the stem in the Flexihilia is always circular exteriorly, the lumen 

 of the axial canal is pentagonal, or quinquelobate, so far as observed; in some 

 genera it is very small and possibly round, but in many of the most prominent 

 forms it is of good size. The base being dicyclic, the angles of the lumen, if 

 they represent the orientation of the axial organ and its downward extensions, 

 should in accordance with the law of Wachsmuth and Springer x be radial in 

 position; and in some genera, such as Ichthyocrinus, Lecanocrinus, Homalo- 

 crimis, Calpiocrinus, and Pycnosaccus, they seem to be so. But in other genera 

 Ihey are either distinctly interradial, or while not quite regular are more inter- 

 radial than radial. Observations on this point, however, are usually limited to 

 the aspect of the canal where it enters the base, being exposed by the detach- 

 ment of the stem at that level. Here the orientation is complicated by the fact 

 that there are only three infrabasals, which are unequal. The canal in the stem 

 of crinoids lodges the five axial nerve cords, which are downward prolonga- 

 tions of the lobes of the chambered organ, with their nerve-fibrillar investment; 

 if the base be monocyclic, they pass directly from the basals into the stem, and 

 are therefore interradial in position; if dicyclic, they enter the stem from the 

 infrabasals, and are radial (see Bather in Lankester's Zoology, pt. 3, 

 pp. 163-104). 



This would be simple enough in crinoids with five infrabasals; and it might 

 be supposed that the cords would branch from the basals to the infrabasals 

 symmetrically even when there are only three, treating the two larger ones as 

 representing primitive pairs. But in many of the Flexibilia, perhaps all, the 

 lumen for the passage of these cords into the stem is connected with a very 

 peculiar structure at the inner floor of the calyx where the lumen begins. Here 

 the opening is surrounded by a thin raised rim which spreads into a lobate, 

 funnel-shaped vessel, having broad, shallow channels leading down toward the 

 stem ; the channels, or chambers, are separated by septa, and the funnel as thus 

 divided undoubtedly lodged the chambered organ with its nerve-fibrillar 

 envelope. In Ichthyocrinus, where the infrabasals are generally reduced in 

 size and sometimes entirely resorbed, they are too minute to lodge such a struc- 

 ture ; it is therefore formed at the edges of the five basals, and is five-lobed and 

 subpentagonal in outline (PI. XXXIII, fig. gc, and text-fig. la). The very thin 

 internal septa which bound the chambers of the funnel curve in their down- 

 ward course sufficiently, to cause a partial revolution in position of the channels 

 from interradial in the upper part to radial where they enter the stem. A more 

 detailed account of this structure is given in the generic discussion under 

 Iclithyocrinus. 



1 Revision of the Palaeocrinoidea, pt. 3, sec. 1, 18S5, p. 7 ; N. A. Crinoidea Camerata, 1897, p. 59. At the 

 time of these discussions neither Wachsmuth nor I were aware of the important paper by Beyrich " Ueber 

 die Basis der Crinoidea brachiata," Monatsber. Akad. VViss. Berlin, Feb. 1871, p. 33, in which some of the 

 facts upon which the law was based were lucidly stated. For that reason the proper credit was not given. 



