182 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Thus the radials, originally only the basal plates of the arms, gradually become 

 incorporated into the calyx and, increasing at the expense of the other plates, finally 

 become ])racticnlly the whole calyx in themselves. 



In certain crinoids, as in the comatulids and in the pentacrinites, the calyx has 

 become so reduced that it serves merely as a jilatform upon which the central part 

 of the visceral mass rests, this being chiefly suj)portcd by the arm bases. (Compare 

 fig. 145, p. 209, with 85, p. 139, 89, p. 147, 92, p. 151, 113, p. 181, and 119, p. 185; see 

 fig. 74, p. 127). In these forms there is no differentiation of the anal intcrradius or 

 of the right posterior ray so far as the calyx is concerned, though the anal area on 

 the disk is always eidarged, sometimes, as in certain comastcrids, occupying prac- 

 tically its entire surface. The calyx plates, here reduced to a small platform sup- 

 porting merely the central portion of the almost completely exposed ^-isce^al mass, 

 are no longer subject to any stress from the pressure exerted by the constant 

 movements of the distal end of the digestive tube, these being compensated, as in 

 the holothurians, by the pliant body wall; and therefore those in and to the right 

 of the posterior interradius, obeying the reductive influence whicli, as a result of 

 the radial symmetry, is exactly equal in all the radii, are reduced to exactly the 

 same degree as are all the others. 



It has already been remarked that in a radially symmetrical animal divided 

 by lines of weakness the body would naturall}' be expected to consist of an uneven 

 number of segments so that none of the lines of weakness will pass directly through 

 the animal in the same plane. The number five represents the optimum number of 

 divisions for such an animal. It was probably the coincidence of this number 

 with the five segments usually incorporated in the crustacean thorax which originally 

 permitted the formation of the ecliinoderms from the primitive crustacean ancestors. 



I have noticed that in the dead and shghtly shrunken embryos of a species 

 of salamander {Amblystoma j)unctatum) which came under my observation the body 

 wall on the convex (unpigmented) side was cracked, and that the cracks were more 

 or less regularly arranged, so that there were formed one subpentagonal central 

 area surrounded by five subequal similar areas, the general appearance bemg the 

 same as that of Marsupitis viewed dorsally. This could have been nothing but 

 the result of mechanical processes. 



In a spicule forming skeleton like that of the echinoderms mechanical con- 

 siderations will sometuncs produce radical changes in the shape and arrangement 

 of the plates even after they have become, through long existence as phylogenetic 

 entities, of ))rlmar\' importance, and may result in their more or less permanent 

 disuitcgration in certaiji groups or sections of groups, so that they may never 

 appear in the ontogeny or in the perfect animal except as a mass of smaller plates 

 or of scattered spicules. 



Such conditions obtam in those crinoids which possess three instead of the 

 more common five basals or infrabasals; these three basals or infrabasals are col- 

 lectively the equivalent of the usual five; but, except in particular cases, 'we are not 

 justified in saymg or assinning that any one of these three is the exact equivalent 

 of any one or two of the pentamerous series. 



