280 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



the terminal claw in its general relationships than to the dorsal spines proper; it 

 is, however, intermediate between them. It is present and well developed in many 

 species (as in all those of the Comactiniinse) in wliich no dorsal spines are ever 

 developed (figs. 394, 395, 397, 398, 400, 401, p. 309), and it exhibits the perfected 

 acutely conical shape and erect median position in many cases where the processes 

 on the preceding segments are as yet in the primitive stage of a broad serrate 

 transverse ridge. In structure it is more dense than the dorsal spines, and it 

 possesses a thick vitreous cortical layer of condensed stereom resembling that on the 

 terminal claw, though never quite so well developed. In young and in regenerating 

 cirri it is very early in making its appearance, being well developed before the cirrus 

 segments have lost their original short cylindrical form. 



The opposing spine may make but a slight angle with the median axis of the 

 penultimate segment (fig. 314, p. 273), or it may be quite erect and at right angles 

 to that axis (fig. 317, p. 273). The proportion of declination is correlated with its 

 position; if it is termally situated it makes the minimum angle with the median axis; 

 it is not erect unless its position is at the center of the dorsal surface of the penulti- 

 mate segment (fig. 352, p. 291). The degree of declination is in direct inverse 

 ratio to its distance from the distal edge. 



The opposing spine functions as a hilt for the sharp and dagger-like terminal 

 claw, preventing the cirrus from sinking too deeply into, and thereby becoming hope- 

 lessly entangled with, the substance of the organism to wliich the crinoid is clinging. 



Typically the opposing spine reaches a height about equal to the distal trans- 

 verse diameter of the penultimate segment, though it is often less, especially in those 

 species in which the preceding segments bear no dorsal processes; in the oligo- 

 phreate species it is commonly triangular, arising from the entire dorsal surface of 

 the penultimate segment, thus being considerably broader basally, and also longer, 

 than the processes on the preceding segments (fig. 318, p. 273); in the macrophreate 

 forms, as well as in certain of the oligophreate, however, the base is usually shorter, 

 and the spine arises from the outer part only of the penultimate segment (figs. 395, 

 396, p. 309); this is always the case if dorsal processes are not developed on the 

 preceding segments. 



The origin of the dorsal spines and of the opposing spine was probably some- 

 what as follows: The central canal through the cirrals is at first central in position; 

 after the middle of the cirrus it moves slowly and gradually ventralward (fig. 587, 

 pi. 13). This results in a difference in size between the dorsal and the ventral liga- 

 ment bundles by which the cirrals are articulated (the two sets at first being similar 

 and equal), the former becoming progressively larger and stronger and the latter 

 correlatively smaller and weaker. In consequence of the normal state of balanced 

 tension of the ligament fibers the cirri assume a curved shape, the curve being very 

 gradual atfirst, but increasing toward the tip, the radiusof curvature being everywhere 

 proportionate to the difference in strength between the dorsal and the ventral liga- 

 ment bundles. The calcareous elements of which the cirrals are composed are 

 deposited as rings or cylinders within the sarcode of the growing cirri; normally 

 they increase in length by the addition of calcareous matter equally all around 

 their margins; where the ligament bundles are equally balanced this occurs, but 



