372 p. HERBERT CARPENTER. 



Agassiz, Claus,^ and apparently also Gotte, regard this 

 plate as homologous with the centrodorsal piece of Comatvla 

 (fig. I, cd). 



Gotte's statements^ with regard to the origin of the cen- 

 trodorsal piece are not in accordance with those of his prede- 

 cessors. His view of it, as originating in small rods which 

 appear on the lower edges of the primitively independent 

 basal plates, and simultaneously with them, involves the 

 homology of an originally single plate, the central abactinal 

 plate of the other Echinoderms, with another which results 

 from the gradual union of five primitively independent 

 elements. As I have pointed out above, in reference to the 

 views of Agassiz and Loven, comparisons such as these do 

 not appear to me to be correct. 



I have shown elsewhere^ that it is very probable that 

 Gotte is mistaken upon this point, and that the generally re- 

 ceived view that the centrodorsal piece of Comatula is simply 

 the enlarged uppermost stem segment is the correct one. 

 From its appearance in the adult Cotnatula, particularly in 

 Acti?iO)netra, in which it is nearly always a simple flat disc, 

 it would seem quite natural to regard it, as Claus and Agassiz 

 do, as homologous with the central abactinal plate of the 

 young Starfish (fig. vii, 1). 



There appears to me, however, to be one serious objection 

 to this view. In the embryos of both Starfishes and Sea 

 Urchins, the central plate of the abactinal system is a simple 

 plate from the very first, developed at the centre of thepos- 

 terior end of the right peritoneal sac. According to Agassiz,* 

 it is formed in the Starfish as a small cluster of polygonal 

 limestone cells " round the rod placed in the verj/ cejitre of the 

 abactinal area," and its earliest condition in the young Urchin 

 {Toxoiyneu&tesf is that of " a single large plate covering 

 the opening of the anus, which leads out on one side of it." 

 On the other hand, the future centrodorsal piece of the em- 

 bryo Crinoid is merely the uppermost of a series of calcareous 

 rings developed around the elongated hinder portion of the 

 right peritoneal sac (fig. i, cd)> Wyville Thomson^ speaks 

 of an irregular calcareous ring which is early formed imme- 

 diately beneath the basal plates, and is considerably wider 

 and broader than the ordinary rings of the stem. 



1 Loc. cit., pp. 597, 598. 



- ' Grundziige de Zoologie,' Dritte Auflagc, p. 289. 



^ See my memoir ou Aciinometra, now in course of publication in the 

 ' Transactions of the Linnean Society.' 



' ' North American Starfishes,' p. 48. 



* ' Embryology of Echinoderms,' p. 12. 



6 ' Phil. Trans.,' vol. civ, p. 539. 



