MONOGRAPH OF THE EXISTING CRINOIDS. 



97 



In the exocyclic species the mouth is situated at or near the middle point 

 in a horseshoe-shaped ring running about the margin of the disk and giving off 

 branches to the various arms. As the central rings of the water-vascular, blood- 

 vascular, and ambulacral nervous systems, as well as of the ambulacral grooves, 

 retain their original position about the mouth, it is evident that the posterior 

 arms are at a considerable disadvantage, for while the circumoral rings lie directly 

 at the base of the anterior arms, the bases of the posterior arms are distant from 

 them half the periphery of the disk. 



The genital cords of the different arms are at first connected by a ring about 

 the mouth, but this soon atrophies, so that in the adults the gonads on each arm 

 are entirely independent of those on the other arms. Thus the disadvantages 

 under which the brachial organs actively connected with ring systems about the 

 mouth are placed are inoperable in the case of the genital organs. 



Heat has been shown to have the effect of stimulating the growth of the 

 genital organs, and there is no reason to support that this is not the case in 

 the crinoids as well as in other animals. 



If it be so the stimulation of the gonads would naturally result in the 

 atrophy of the brachials and of the ventral systems and the more or less complete 

 suppression of the latter in the posterior arms on account of the aggressive 

 assimilation of the available nutriment by the stimulated gonads and the relative 

 difficulty of supplying additional nutriment because of the elongation of the 

 channels through which it has to pass. 



In those species of the Comasteridse in which the mouth is central and the 

 ambulacral grooves converge to it in five furrows of equal length all the struc- 

 tures on all the arms are equally balanced and no opportunity is offered for the 

 genital organs to develop abnormally at the expense of the other systems. In 

 such types, no matter what the temperature of the water in which they occur, 

 the arms are always of equal length, as in the endocyclic species belonging to 

 other families. 



Cold appears to have an inhibiting effect upon the genital organs, so that 

 exocyclic species found in cold water have all the arms, though short, of the 

 same length. 



Carpenter believed that the condition of the respiratory apparatus was the 

 determining factor in deciding the condition on the posterior arms, and that when 

 this is well developed the arm seems to have the power of indefinite growth. 



The division series and arms may stretch out horizontally (figs. 184, 185, 

 pp. 100, 102), or they may be closely appressed, making but a small angle with the 

 dorsoventral axis of the animal (figs. 206-209. pp. 143-149). These variations are 

 not correlated with the angle made by the raclials with the plane passing at right 

 angles through the dorsoventral axis, as might be expected, both extremes occurring 

 in the Oligophreata where this angle is always comparatively low. 



When the division series and arm bases are widely divergent and approach 

 the horizontal the component elements are comparatively shallow and broad and 

 their ventrolateral edges are narrow and rounded, or more or less produced into 



