MONOGRAPH OF THE EXISTING CBINOIDS. 161 



At that time I was well aware that the facts of embryology tended to discredit 

 my conclusions, but I hoped later to find some way by which they might be shown 

 to bo in reality in agreement with them; the palseontological evidence and the 

 evidence derived from the study of variants was apparently so clear that I con- 

 sidered myself safe, in relying implicity upon it. 



The recent and later fossil crinoids all have a much more perfect radial pentain- 

 erous symmetry than those of the palaeozoic; but from the facts brought out by 

 a study of the development of Antedon and by a comparative study of each of 

 the various sets of structures which collectively make up the crinoid whole, both 

 in the earlier and in the later types, it becomes evident that the primitive crinoidal 

 arrangement is a perfect pentamerous symmetry, each radial with its post-radial 

 series being exactly like every other, and each iiiterradial area also being exact ly 

 like all the other interradial areas. In other words, the primitive crinoid was 

 as regularly radially symmetrical as the most regular of the urchins. 



Zones of similar skeletal potency. 



One of the results of the assumption of radial symmetry by the crinoids, and 

 by the echinoderms generally, has been the eventual delimitation of concentric 

 zones of similar skeletal potency. This is not by any means a new structural 

 feature, but an adaptation of a very general one in a somewhat new form. 



If we take any crustacean or insect and draw a line around the contour of the 

 animal from the midline of its dorsal surface to the midline of its ventral surface, 

 we find that that line passes over several different thicknesses of dermal covering 

 of which the most dense is the dorsal and the least dense is the ventral, and the 

 same relative proportions are found between the different heights at all points, 

 the degree of morphological differentiation decreasing from the neural (dorsal) to 

 the haemal (ventral) apex in all the radii. A line from the apex of a crinoid, or from 

 the edge of the poriproct in the echinoid, to the edge of the ventral disk in the crinoid 

 and the edge of the peristome in the echinoid, covers exactly the same ground as a 

 line from the middorsal to the midventral line in the bilateral crustaceans or insect-. 



In the echinoids we find in the skeleton forming portion of the body wall two 

 distinct zones, the coronal ring and the area between this ring and the peristome; 

 but in the crinoids the conditions are more complex. Here we have the coronal 

 ring always divided into two separate rings; the first of these, the infrabasal ring, 

 is composed of small plates which, like the oculars of the echinoids which they 

 represent, arc singularly uniform in proportions, and admit of no additions to 

 their number; the second, the basal ring, is composed of larger plates which, like 

 the genitals of the echinoids which they represent, are variable in size, and permit 

 of additions to their number. The radianal is such an addition. 



Any plate added to their number immediately takes on characters identical 

 with those in the original plates of the series. 



Following these arc the plates of the intermediate area (pseudambulacml-' 

 arranged in tandem groups of two each, and beyond them the brachials 



Each of these zones, indicated by (1) the infrabasals, (2) the basals, (3) the 

 pseudambulacrals and (4) the brachials, is a zone of equal growth in which any 



