238 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



this is first exposed by the removal of the centrodorsal from the dorsal surface of 

 the radial pentagon which rests upon it, five white rays are visible on a dark back- 

 ground. Unless the plate is immediately removed from the alkaline solution used 

 to effect its separation this distinction in color between the radial and the inter- 

 radial portions of its ventral surface rapidly disappears, owing to the destruction 

 of the pigments contained in the former. 



The development of these basal grooves is not only different in different speci- 

 mens of the various species of comatulids (figs. 229-234, p. 247, 235-242, p. 249, and 

 243-249, p. 251), especially among the Comasteridae, but it varies to a certain extent 

 in the same individual (fig. 248, p. 251). Sometimes one or more of the basal grooves 

 may rapidly diminish in width and end well within the periphery of the centro- 

 dorsal (figs. 243, 248, p. 251). They may gradually diminish (fig. 259, p. 255), or, 

 more rarely, gradually increase (fig. 229, p. 247), from the center to the periphery, 

 or the sides may be quite parallel (figs. 266, p. 257, and 268, 270, p. 259) ; but usually 

 they increase slightly in diameter for a shorter or longer distance, tapering off 

 gradually from this point toward the periphery, thus having, as expressed by 

 Carpenter, a leaflike appearance (figs. 244-249, p. 251). 



Except for very small forms such as Comatilia iridometriformis, Comanihus 

 bennetti and C. pinguis (figs. 171-174, p. 231) are the only species in the Comasteridse 

 in which the centrodorsal develops throughout life and shows but little trace of 

 progressive specialization in the adult stage; in most of the other species the centro- 

 dorsal is disco idal (figs. 160-162, p. 223, 163, p. 225, and 181, 182, p. 233), though it 

 may bo rather thick, with a broad flat polar area and two or three marginal rows 

 of cirrus sockets bearing functional cirri which in some cases, as in Comanthus 

 parvicirra, may be disproportionately small (figs. 160, p. 223, and 182, p. 233) or, as in 

 C. trichoptera, disproportionately slender and thin (fig. 330, p. 281). A number of 

 species commonly have the centrodorsal a very thin disk with a single row of cirrus 

 sockets which may be regularly (as in Comatula purpurea) or irregularly (as in 

 Comanthus parvicirra) incomplete (figs. 79, p. 132, and 182, p. 233); others when 

 adult usually have the centrodorsal without cirri and pentagonal or stellate, but 

 frequently with one or two or even more perfect cirri remaining, as Comanthus 

 annulata, Comanthina schlegelii or Comaster belli (fig. 182, p. 233) ; and a consider- 

 able number always when adult have the centrodorsal small and stellate with never 

 a trace of cirri, as Comatula rotalaria, Comaster typica, CapiTlaster macrobrachius, 

 and Comantheria polycnemis (figs. 153-159, p. 221, 162, p. 223, 164, p. 227, and 166, 

 168-170, p. 229). 



When very young, all the species of the Comasteridas have centrodorsals exactly 

 like those of Antedon, and in all species alike they develop in exactly the same way. 

 The difference in the centrodorsals of the adults is therefore solely a difference in 

 comparative development, demonstrating a fundamental unity, and not a difference 

 in structure, implying a phylogenetic divergence. For instance, the large hemi- 

 spherical centrodorsals of Comanthus bennetti or C. pinguis are merely centrodorsals 

 of the most primitive comasterid type which, though greatly increased in size, 

 are not ontogenetically different from the centrodorsals of the early post-penta- 

 crinoid stage; the centrodorsals of Comactinia or of Comissia, discoidal, with one 



