142 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
organ has been observed in Rhizocrinus, Bathycrinus, and Holopus, and it 
existed probably in the other genera. Among Jurassic and later fossil 
Crinoids, axial canals, piercing the body of the calyx plates, are known to 
exist in the Apiocrinide and Eugeniacrinide (Holopid Jaekel), and they 
are readily recognized in the Triassic Encrinus, in some of the later Paleo- 
zoic Poteriocrinide, and in MWycocrinus and Catillocrinus ; but we have found 
no trace of them in the Camerata, except in Steganocrinus pentagonus (Plate 
LXI. Fig. 3), in which, so far as observed, the five or six proximal plates of 
their tubular appendages are pierced by a canal. Such canals occur upon 
the radials in some of the Cyathocrinidee, the Cupressocrinide and Gastero- 
comidxe, and we may suppose that a chambered organ existed in these and 
other groups, if not in all Crinoids. In cases where grooves or canals for 
the reception of cords are not apparent, the cords may have rested against 
the inner wall of the plates. 
B. The Convoluted Organ. 
In the abdominal cavity of Palzeozoic Crinoids, the only organic structure 
that under very favorable conditions has been observed, is a peculiar skeleton 
which occupies the greater part of the cavity. 
It is a large convoluted body, in its outlines resembling the shell of 
a Bulla, open at both ends. Its upper part rests directly beneath the origin 
of the ambulacra, the lower end within the basal ring without touching the 
plates. It is dilated above, contracted below, its lateral faces placed parallel 
to the inner walls of the calyx; the bottom truncated. In some species it is 
subeylindrical, with the vertical axis the longer, in others globose or even 
depressed globose. In coiling around its axis, the partition walls do not 
meet each other, but leave more or less wide interspaces. The convolutions 
vary in number from 2 to 4 according to species, and are, as they pass out- 
ward, directed from right to left. The walls in the usual preservation are 
thick, and perfectly solid, as they were described by Hall; but in transverse 
sections they frequently appear as if composed of two partitions closely fitted 
together, and closed along the edges. In some specimens, however, the 
walls are simple, and constructed of an extremely fine and delicate filigree 
work, composed of minute pieces or bars, with intervening meshes, which 
do not intersect at any uniform angle, but anastomose so as to impart a kind 
of irregular regularity to the form and size of the meshes. No such structure 
has ever been observed in the other specimens, in which the pores or meshes 
