194 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



interradially, but the other systems of these series are derived by a division of the 

 five original series. 



CALCAREOUS STRUCTURES. 



Skeleton as a whole. 



For convenience the crinoid skeleton is treated under three separate heads, as 

 follows : 



(1) The primary or appendicular skeleton. This is frequently referred to 

 merely as the Skeleton; under this heading are included the calyx plates (with 

 the column), the orals, and the articulated series of ossicles which form the supports 

 of the cirri, arms and pinnules. 



Among the recent crinoids these ossicles (usually, however, excepting the 

 orals) have always been considered as forming a convenient unit. P. H. Carpenter 

 was accustomed to refer to them as composing the Radial skeleton, and he denned 

 this radial skeleton as consisting of "successive joints and rods which are developed 

 in a longitudinal direction, and are united to one another by articulation or suture." 

 The uniformity of structure throughout this skeletal system was thus attested by 

 W. B. Carpenter: "The component pieces of which the skeleton of Antedon is made 

 up, alike in its adult condition and in every previous.phase of its existence, present a 

 remarkable accordance with each other in elementary structure, consisting through- 

 out of that calcareous reticulation formed by the calcification of an animal basis 

 that seems nothing else than non-differentiated sarcode which I have shown to be 

 the essential constituent of the skeleton in every type of the class Echinoderma. 

 The character of this reticulation is best seen either in very thin sections of any 

 part of the skeleton, or hi that curiously inflected cribriform lamina which I have 

 termed the rosette. This is the only part of the skeleton of the adult Antedon in 

 which the reticulation lies all in one plane; but * * * even its most solid por- 

 tions * * * make then- first appearance in the same form of cribif orm lamellae ; 

 and whilst these lamellae increase in superficial dimensions by the extension of the 

 reticulation from their margins, they are augmented in thickness also by an exten- 

 sion of the reticulation from their inner surfaces into the animal basis in which 

 they are embedded. When a portion of the skeleton, either from a fresh or from 

 a spirit specimen, is subjected to the action of dilute nitric or hydrochloric acid, 

 by which the calcareous network is dissolved away, a continuous film of pellucid 

 sarcodic substance is left, presenting no other trace of structure than in being 

 studded at regular intervals with minute granular spots." 



In the young of certain comatulids, as, for instance, in the young of Thaumato- 

 crinus (figs. 115-118, p. 183), the disk becomes invested with a pavement of large 

 plates, which become resorbed and disappear before or shortly after the loss of the 

 larval column. These plates are entirely different from the secondary perisomic 

 plates which are developed at a much later stage, and represent the condition from 

 which the enormously specialized dome of the Camerata was developed. These 

 should be regarded as primary plates, though not always occurring in the young; 

 if present at all they appear and disappear again in a very short space of time. 



