166 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The coronal plates of the urchins are definite and distinct, five large, in the 

 center of the five somatic divisions, and five smaller, situated between them. The 

 five large stand at the base of the interambulacral series, and the five small cover 

 the bases of two adjacent marginal series. These coronal plates always main- 

 tain the same relationship with the other plates. They increase in size more or 

 less by accretion, but necessarily this accretion occurs only, or at least chiefly, on 

 their free inner edges. In a circlet of alternating large and small plates the large 

 plates will possess, through the dominance of excess growth, the more nearly perfect 

 shape. Thus, the lateral borders of the larger plates will not be directed straight 

 toward the center of the periproctal area, but will be mutually more convergent; 

 and so, as the larger plates grow proportionately faster than the smaller ones, they 

 tend to come into contact behind the smaller ones, cutting these off one by one from 

 the periproctal area, though without in any way altering their original interrela- 

 tionships or their relationships with the columns of plates arising from them. 



In the crinoids the primitive arrangement of the coronal ring has been altered 

 by the segregation of the plates into two rings, the larger plates forming a closed 

 circlet surrounding the closed circlet composed of the smaller. The central plate, 

 formed during the echinoid stage by the assembling of the calcareous elements in 

 the periproctal area within the coronal ring, and by no means a constant feature in 

 echinoid morphology, has now become fixed and permanent, increased enormously 

 in size, and become reduplicated so that it typically forms a long and solid column. 

 The enclosure of the small plates of the coronal ring within the closed circlet 

 formed by the larger resulted in separating the small plates from the columns of 

 plates arising from them; these thereupon ceased abruptly to develop, and became 

 segregated and metamorphosed into the division series. 



The internal ring of the holothurians came to the surface and moved to the 

 posterior end of the body in the echinoids. But in the latter the elements, 10 in 

 number, of another ring surrounding the anterior portion of the digestive tube 

 appeared and, in many forms, became greatly multiplied and developed. These 

 fused with the plates of the body wall on their peristomal border, forming the 

 auricles, in the more specialized types surmounted by apophyses, and connected 

 with complicated dental pyramids. 



In the crinoids the original coronal ring has become greatly reduced and more 

 or less degenerate, the small plates becoming frequently reduced to three, or absent 

 altogether in the adults, and the larger also becoming often reduced to three, or 

 entirely metamorphosed or absent in the adults. The second coronal ring, con- 

 sisting of the auricles and apophyses in the echinoids, has in the crinoids followed 

 the same course as the first; it has become external, the 10 elements having fused 

 into 5, through lateral apposition with then 1 fellows in the adjacent somatic 

 areas, which have become produced as long intersomatic arms borne upon a basal 

 structure formed of fused and metamorphosed body plates (radials) corresponding 

 with the somatic marginals of the echinoids (ambulacrals). 



In the more specialized comatulids the first circlet of coronal plates (infra- 

 basals) is only represented in the early larva of a few species, and the second is 

 almost completely altered in early postlarval life, moving inward so as to form an 



