MONOGRAPH OF THE EXISTING CRINOIDS. 333 



Crinoids are fundamentally and primarily regular!}- pentamerous. In endo- 

 cyclic forms the movement of the posterior part of the digestive tube exerts a 

 constant or intermittent force the direction of wliich is upward and toward the 

 right (fig. 20, p. 69). This force, operating in the posterior interradius, tends to 

 keep separate the two posterior radials and to prevent the right posterior radial 

 from slipping downward and coming into contact along its proximal border with 

 the distal borders of the two subjacent basals. 



Therefore there persists between the two posterior radials, long after its counter- 

 parts have disappeared from between the other radials, the primitive interradial, 

 now known as the anal; and there persists beneath the right posterior radial, long 

 after similar plates have disappeared from beneath all of the other radials, the 

 primitive subradial, now known as the radianal. 



In the later fossil and in all the recent forms regular pentamerous symmetry 

 again occurs as the result of the progressive reduction of the calyx plates whereby 

 the visceral mass comes to be largely exposed and thereby able to accommodate 

 the constant motion of the digestive tube through temporary and transient move- 

 ments and changes in its perisomic covering. 



In exocyclic forms movement of the posterior part of the digestive tube 

 (fig. 21, p. 69) operates to shove the marginal mouth to the right, with the effect 

 of making the originally left posterior a true posterior ray, different in character 

 from the other four. As the calyx plates have become metamorphosed into a 

 small flat platform before the commencement of the transition of the digestive tube 

 from the endocyclic to the exocyclic type no effect is produced upon them. 



The subradial plates of the crinoids, of wliich the radianal, itself only appealing 

 hi the very young of the recent forms, is the last remnant, are all that remain in the 

 crinoid organism of the ambulacral series of the urchins with the exception of the 

 radials, which represent the first ambulacrals formed, those situated about the 

 border of the peristome. 



W. B. Carpenter says that in Antedon Mfida for some little time after the 

 appearance of the arms the relation of the skeleton of the calyx to the visceral 

 mass it includes undergoes but little change, the chief difference consisting in the 

 more compact condition it now comes to present in consequence of the advanced 

 development of its component pieces. The five basals now possess a regularly 

 trapezoidal form, the lower part of each being an acute-angled triangle with its 

 apex pointing downward, and its upper part an obtuse-angled triangle with its apex 

 directed upward. The sides of the lower triangle are bordered by a somewhat 

 tliickened edge of solid transparent calcareous substance, the presence of which 

 signifies that the plate has received its full increase in that direction. The adjacent 

 borders of these plates, however, do not come into actual contact, a thin lamina of 

 sarcode being interposed between them, anil there is also a passage left at the 

 truncated apex of the inverted pyramid formed by their junction through wliich 

 the axial sarcodic cord of the stem is continued into the calyx. The upper margins 

 of the basals have no distinct border and seem to be still in process of growth. The 

 radials, with the radianal intercalated between two of them, now form a nearly 

 complete circle resting upon the basals and separating them entirely from the 



