134 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



The symmetry of the Crinoids, as a rule, is bilateral, and the anal area 

 occupies the median axis. The asymmetry which occurs in many Fistulata, 

 and in certain Ichthyocrinidae, is caused by irregularities in the radials. 

 Wherever these attain a regular form, the plate x takes its median position, 

 and the plates of the ventral tube are arranged on a strictly bilateral plan. 



Whether the symmetrical calyx, as represented in the Silurian and later 

 CyathocrinidaB, was evolved from the asymmetrical form, we are unable to 

 ascertain. It may be that the two had a common symmetrical ancestor, or 

 that all these Crinoids were primitively asymmetrical, and that the lower 

 section of the posterior radial became early resorbed in some cases. Against 

 the former theory it may be said that in the Lower Silurian Fistulata, so far 

 as we know without exception, the right posterior radial is compound, and 

 that the symmetrical form occurs with the other in the same families ; against 

 the latter, that the symmetrical form is already well represented in the 

 Upper Silurian. 



We have made no reference here to the Calceocrinidge and Catillocrinidse, 

 as we have not at present the material to study the older forms ; but we feel 

 quite certain that their structure in this respect shows no material departure 

 from that of the older Fistulata. 



The anus of the Crinoids is located in the disk, and is either central, sub- 

 central, or marginal, — in the latter case sometimes coming down to the 

 arm region. In some species there is merely a simple opening passing out 

 directly through the disk ; others have a tube with an opening at the distal 

 end or along the side. The size of the tube is quite variable. In some 

 genera it rises to a height of several inches beyond the arms ; while in 

 others it is less than half their length. The tube is composed of heavy, 

 generally nodose, wedgeform pieces, admitting but little mobility in the 

 structure. When there is no tube, the anus is generally situated within the 

 centre of a wart-like inflation, composed of very minute pieces, which 

 possibly were movable, and could be drawn in by the animal, like those 

 in the " proboscis " of recent Crinoids, so as to open or close the aperture. 



There has been some difference of opinion whether or not species with 

 an anal tube should be separated generically from those with a simple 

 opening. Considering the slight differences upon which many genera have 

 been founded, it would seem that the tubular structure ought to be of 

 sufficient importance to justify a separation ; but considering that various 

 groups, after being carefully restricted with reference to all other characters, 



