174 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



constituents of the test, and are perhaps the most important plates of the coronal 

 ring. In the crinoids there has been a general tendency, though a tendency which 

 is not in any way regular or uninterrupted, toward the suppression of their equiva- 

 lents, the infrabasals, and with the suppression of the infrabasals has come the 

 similar suppression of the following series of plates which are usually, and always 

 in the later types (excepting in the very young), dispensed with altogether save 

 for the radials (representing the ambulacrals in the echinoids which immediately 

 border the peris tomal area), which now are closely united to the closed circlet 

 of basals. 



In certain crinoids, mostly post-Silurian, in which the visceral mass is very 

 large we find a significant reversion in the form of a subradial plate inserted below 



the right posterior radial, and later beneath all 

 the other radials also. These subradial plates are 

 usually separated from the infrabasals by the 

 closed circlet of basals ; but in a few genera, as in 

 Tlienarocrinus, Sagenocrinus, and Homalocrinus 

 the one beneath the right posterior radial connects 

 that radial directly with the infrabasal. These 

 subradial plates I take to represent the entire am- 

 bulacral series in the urchins which the great en- 

 largement of the visceral mass in these types and 

 the corresponding necessity for the development 

 of protective plates has permitted to form. Es- 

 pecially significant in this connection is the genus 

 Acrocrinus in which the radial circlet is widely 

 separated from the basal circlet by a very large 

 number of plates potentially the equivalent of the 

 plates between the coronal ring and the peristome 

 in the urchins. 



FIG. 108. LATERAL VIEW OF A SPECIMEN OF 

 COMPSOMETEA LOVENI FEOM PORT JACKSON, 

 NEW SOUTH WALES, SHOWING THE RELATIVE 



PROPORTIONS OF THE ARMS, PINNULES, CENTRO- 



DORSAL, AND CIRRI. 



Effect of external mechanics upon the crinoids. 



We have become so accustomed to dealing 

 bilaterally symmetrical animals which move, 

 by means of various methods of progression, head 

 first in the direction of the longitudinal axis of the body and hence, broadly speak- 

 ing, are subject to all the same mechanical influences, that we often fail to realize 

 the importance of a thorough appreciation of the effect of purely mechanical 

 forces upon an animal which has become fixed or has almost entirely lost the 

 power of locomotion. But a close study of the mechanical forces which echino- 

 derms are called upon to meet gives us a clue to the true interpretation of many 

 features of echinodermal structure which otherwise are quite inexplicable. 



For instance, the contour of the rounded body of the urchin is determined not 

 by any inheritance on the part of the animal from its crustacean prototype, but 

 by the struggle for supremacy between a constant tendency toward a spherical 

 form, allowing of the maximum of content within a minimum surface, and a constant 



