NOTES ON ECHINODERM MORPHOLOGY. o 



are the plates which are generally called " primaries " in 

 specific descriptions, and their real nature has only lately 

 become apparent. The terminals, on the other hand, are an 

 additional set of plates altogether, which are carried out- 

 wards from the disc at the ends of the growing arms (PI. I, 

 figs. 12, 13; t). 



In describing the young Asterid, however, Ludwig followed 

 his distinguished predecessors Loven and A. Agassiz, and 

 spoke of the terminal plates as radials.^ A difference was thus 

 established between the Asterids and Ophiurids respectively, 

 which could not easily be explained, affecting as it would not 

 merely the fate of the radial abactinal plates, but also the 

 nature of the arras. In a later phase of Asterid develop- 

 ment^ Ludwig found five radially situated plates (PI. I, 

 fig. 15 ; 4) occupying an intermediate position between the 

 terminals and the primary interradials (genitals); and he 

 spoke of them as the forerunners of a system of intermediate 

 plates which form the greater part of the dorsal skeleton of 

 the disc and rays. In the diagram ^ which I gave of the apical 

 system of the larval Asterid I followed Ludwig's nomen- 

 clature ; but Sladen now suggests that these plates {im of the 

 diagrams given by Ludwig and myself) are the true radials. 

 They would thus be homologous with those of an Ophiurid 

 and Crinoid, and should therefore have been marked 4 in my 

 diagram, as they are in Sladen's revised edition of it (PI. I, 

 fig. 15) ; while the plates marked 4 in my figure, and called 

 radials, or preferably *' Terminalia " by Ludwig, are really 

 terminals corresponding to those of Ophiurids, and should have 

 been lettered t, as in fig. 1 on p. 379 of my fifth note (PI, I, 

 fig. 13). Sladen's reasons for this important suggestion are 

 set forth in his own paper a few pages farther on ; but I wish 

 to say that I entirely acquiesce in it, as also in his recognition 

 of the presence in Asterids of plates homologous with the 



' ' Morph. Stud.,' ii, p. 160. 

 ' Ibid., p. ISO. 



3 This Journal, vol. xxii, 1882, p. .382, (5-. 4. This is reproduced with 

 slightly different lettering on PI. I, (\g. 15. 



