48 BULLETIN OF THE 



the odontophores, we certainly cannot also believe that the genitals of 

 the starfish, interradials of which the madreporic body may be one, are 

 also homologous with orals. There is a manifest impossibility that orals 

 can be the same as genitals, and odontophores at the same time. There 

 must be some mistake somewhere if they are compared to both. 



If we examine the observations in regard to the fate of the primary 

 interradial plates (orals) in Amphiura as recorded, pi - oof seems to be 

 wanting in observation that they do form the orals. We may readily 

 concede that the madreporic plate may form interradially, and that it 

 may grow down and form an oral, but is it not a leap at a conclusion 

 that the other plates in other interradii do the same thing ] Can we 

 not suppose, then, that the madreporic plate is morphologically differ- 

 ent from so called genital plates 1 Are we forced to place it in the 

 same category as other genitals 1 It seems to me that at present we 

 may say that it is possible that the madreporic plate of Asterias is a 

 modified homologue of an odontophore which has become consolidated 

 through the stone canal with a genital, and that it is the same as that 

 of the oral Amphiura. The orals of Arnphiura are, then, the same 

 as the odontophores of starfishes.* 



Dorsals. — The dorsals of Asterias are thought to be homologous 

 with the dorsals of Amphiura. f They originate in the same relative 

 positiou, have the same sequence in development, and to all appearances 

 are identical. While in Asterias they bear spines and in Amphiura 

 are destitute of these structures, this fact does not seem of great impor- 

 tance in showing a want of homology of the two. As the Asterias 

 matures, the relative predominance in size as compared with other 

 plates is diminished, while in Amphiura it is increased. It is thus 

 brought about that in the older stages of Asterias it is more difficult 

 to recognize the dorsal plates. This results both from the relatively 

 small development of the dorsals and the appearance of dorsolaterals 

 and connectives, neither of which are thought to be represented in 

 Amphiura. 



* The mode of growth of the odontophore of Brisinga, as recorded by Perrier, 

 seems to differ from that of Asterias, — a fact which does not seem surprising con- 

 sidering the other important differences in the two genera. My observations on 

 Asterias support Sladen and others, that the odontophores are formed on the 

 actinal hemisome. 



t The dorsals in the young Amphiura were first figured in my paper on the 

 development of the calcareous plates of Amphiura (PI. III. fig. 19). They are 

 not represented in any of Ludwig's figures, although I believe that they will later 

 be found in stages younger than his fig. 21, as already pointed out. 



