168 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



ondarily from a single apical plate which was thickened and then divided into many 

 segments by a sort of division or continuous twinning process. Each columnal is 

 thus in itself the equivalent of a single calyx plate, and yet all the columnals col- 

 lectively are also the equivalent of a single calyx plate. Being of secondary deriva- 

 tion, the columnals early come to have an entity of their own, so that all but the 

 very earliest of them are developed as columnals, with little or no hmt as to their 

 pliylogcnetic origin. 



Like the columnals, the brachials are secondarily developed through continu- 

 ous budding, involvmg a mo<lified twinning process, from the distal edge of the 

 radials, which are themselves the llret brachials, and they also preserve scarcely 

 more than a trace of their plate and spicule origin, but appear almost from the fii-st 

 with iill their distinctive characters; indeed so specialized have the brachials become, 

 and so complex are thcii- interrelationships, that we can only consider them as an 

 extraordinary and perfect type of pseudo-vertebrae. 



In the echinoids, except for a small peristome and an equally small or smaller 

 periproctal area, both protected by spicules or small plates and the latter often in 

 addition by a more or less perfect sioranal, the entire body is enclosed within a 

 sohd calcareous test, and a second coronal ring of 10 detached elements, fused with 

 the peristomal edge of the interambulacrals (or secondarily of the ambulacrals) , 

 appears. 



In the crinoids the body is again largely exposed, especially in the later and 

 recent species, this exposure beginning at the anterior end and working posteriorly. 

 The coronal ring has more or less disintegrated, while the arms, derived from the 

 second coronal ring which first appears in the echinoids, are gradually moving 

 inward so that their bases are very near together. 



The holothurians exhibit (1) the ancestral type of a spiculated body covering, 

 imdifTerentiated (or rarely differentiated) into plates; (2) a coronal ring, more or 

 less developed, of five large (interradial) and five small (radial) plates situated in 

 the primitive position about the anterior end of the digestive tube; (3) a longi- 

 tudinal axis determined by the digestive tube which passes through the center of 

 the circle into wliich the longitudinal somatic axis has been resolved, at right angles 

 to its plane. 



Speaking broadly, the echinoid is essentially a holothurian encased in a solid 

 calcareous covermg. A crinoid is . essentially a stalked echinoid. 



In the evolution of the echinoid from the holothurian-like ancestor the body 

 necessarily took on a globular form, this form m a solidly encased organism offering 

 the maximum resistance to fracture and aUomng of a maximum of contents. But 

 the spherical form, quite apart from questions of securing food, etc., is not adapted 

 to a stalked habit. Supported upon a broad more or less flattened area, as in the 

 echinoids, it gives the ma.ximum resistance to external forces; but supported on 

 a very small (apical) area it becomes exceptionally weak. Immediately, therefore, 

 there results a massing and a concentration of the plates about the apical pole 

 to form a platform or a solid cup bound tightly to the summit of the column and 

 making with it practically a single unit upon which the visceral mass, now exposed 

 by the sudden withdrawal of the plates covering its ventral jiortion. rests. This 



