250 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



diminishing the size of the opening of the central cavity. In some extreme cases, as 

 in Psatht/rometra (figs. 208-213, p. 241, 2S6, p. 202) or in Aidea-invs (figs. 123, p. 192, 

 124, 125, p. 193, and 300, p. 264), the centrodorsal is but a tliin shell surrounding the 

 chambered organ and associated structures, but usually the walls are moderately 

 thick; in genera containing species in which the centrodorsal is proportionately large 

 and broad, as IleUometra (figs. 292, 293, p. 263), Solanometra (fig. 295, p. 263), Pro- 

 machocrinus (fig. 294, p. 263), and Antedon (figs. 280, 281, 283, p. 261), the central 

 cavity, though in reality relatively as large as in the others, may appear small by 

 comparison. 



In the groups previously treated, all of which belong to the Oligophreata, the 

 centrodorsal in the adult stage has undergone more or less resorption at the dorsal 

 pole and along the lateral faces which has resulted, owmg to its hemispherical or 

 conical shape (the latter a derivation from the more primitive hemispherical shape 

 by a process of lateral radial resorption), in a progressive proportionate broadening 

 of the dorsal polo with an elimination of the earlier formed cirrus sockets. New 

 cirri are only developed between the topmost (proximal) row of cirri present and 

 the proxunal rim of the centrodorsal, and never, except by regeneration, anj'where 

 else; hence in these groups wo have lost a valuable aid in tracing out the phylogeny, 

 for the earlier and more primitive portion of the centrodorsal, and with it the earlier 

 cirri, has been lost by resorption. There is commonly an incomplete row of cirrus 

 sockets below those bearmg the typical cirri, which may be more or less obliterated 

 or may bear cirri with fewer segments than the others; but these less perfect cirri 

 arc so nearly like the perfect type, or so obviously degenerate, as to furnish no basis 

 for any phylogenetic speculation. 



In the Macrophreata, however, the dorsal tip of the centrodorsal is usually 

 subject to comparatively little resorption, except in the larger species, and even 

 there this is rarely vcr}' extensive. Below the rows of perfect adult cirri there are 

 rows of sockets of diminishing size which may show progressive obliteration, or may 

 bear cirri of an entirely chflerent character from the more adult, and of a more 

 primitive tj^ic, these two types being connected by intergradcs of all stages (figs. 

 310, 311, p. 269). In the adults of the species of Antedon, for instance (though in 

 this genus there is rather more resorption of the dorsal pole than is usual in the 

 group), about the dorsal polo there are usually to be found several verj' small cirri, 

 with all the mature characters but "svith fewer and proportionately longer segments 

 than the others (figs. 312, 313, p. 271), resembling the cirri seen in the earlier free 

 stages of the animal which, indeed, they are. It is thus possible to trace in a 

 single adult Antedon all the progressive changes in the cirri from the earliest to the 

 perfected type, and it is easy to see that the earliest typo found in Antedon 

 resembles the adult type in the species of more primitive genera. In Antedon, 

 however, the difference between these polar cirri (the "small mature cirri" of P. li. 

 Caqienter) and the adult cirri is comparatively small, as tho cirri do not alter to 

 any appreciable extent during the whole life of the animal, and the most primitive 

 cirri are cut off by resorption; but in some of the species of Nanometra (fig. 310, 

 p. 269) or of Ilathrometm tho polar cirri are only one-fourth the length of the others 

 and consist of only one-third as many segments, all of which arc very slender and 



