42 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
are invariably radial in position; or three cirri from one node, and two 
from the adjoiming ones, so arranged that the cirri of two joints corre- 
spond in position to the five of the one joint. The size of the cirri varies 
among species, some being much stouter than others and also longer ; but as 
a rule, they increase somewhat in length downward, and taper to the end. 
They move rapidly, according to A. Agassiz,* quicker than the arms, and are 
used “as hooks to catch hold of neighboring objects, and on account of their 
sharp extremities are well adapted to retain their hold.” 
The functions of the cirri in Paleozoic Crinoids were probably more 
limited than in recent ones, for in most of them any rapid movement 
would be incompatible with the structure. This is especially the case 
with those of the Camerata, and it may be a question whether these 
appendages should not receive a different appellation. 
The cirri of the Camerata, except perhaps those of certain species of 
Dichocrinus, are mere branches of the stem, and were obviously capable of 
very little motion. They occur only in the lower part of the stem, and in 
many cases were probably restricted to the root. They rest within sockets, 
formed at the sides of the stem by a truncation or excavation of one or more 
stem joints. In Platycrinus, in which we have observed the complete stem 
more frequently than in any other genus, they generally occupy from one- 
fourth to one-third of its length, and, being given off invariably from the 
longer diameter of the stem, they follow its twist. In some of the species, 
each successive joint is cirrus-bearing, either at both sides of the stem or one 
side alternately. In others, the cirri are given off at irregular intervals, and 
vary considerably in length and width. Throughout this genus they are at- 
tached to one stem-joint only, which, when the base of the cirrus is larger 
than the usual length of the joint, is lengthened on that side to accommo- 
date it. The radicular cirri are larger than the others, and are generally 
provided with numerous small branchlets. The distal end of the stem also 
terminates in a sharp point. 
In stems with internodal joints, the cirri do not rest alone against the 
nodals, but the adjoining internodals share in the formation of the cirrus 
sockets. In some species the sockets involve only the adjacent joint above 
and below ; in others, two or three of them; but the cirrus canal invariably 
communicates with the axial canal of the stem through the nodal joint. The 
* Letter No. 5, on the Dredging Operations of the U. 8. Coast Survey steamer Blake, from December, 
1878, to March 10, 1879. Bull. Mus. Comp. Zodl., Vol. V., p. 296. 
