34 BULLETIN OF THE 



It is a most important thing to know more of the early formed plates 

 of the genera Caulastei', Perr.,* and Ilyaster, D. & K., in which we have 

 appended to the middle of the abactinal zone a short peduncle. It 

 would seem that this peduncle is comparable with the stem of a Cri- 

 noid. In a young Ctenodiscus a protuberance in the same place as the 

 peduncle of Ilyaster has been noticed, but I have never seen it as long 

 as figured in Ilyaster. As Ctenodiscus is a common .starfish off the 

 New England coast, it would present a most instructive genus for the 

 study of the homology of the early formed plates of a starfish with an 

 abactinal prominence. 



If it should be shown that this appendage to the abactinal region 

 of the genus Ilyaster is a remnant of the ancestral Crinoid stem, it 

 might be supposed that the Asteroidea have descended from criuoid- 

 like genera. It may likewise be true that the Crinoids are highly 

 specialized and descended from certain starfishes or Ophiurans. In 

 this case, perhaps, the arrangement of apical plates in the larval 

 starfishes is the most primitive, and may determine the nomenclature 

 of the Echinoids. 



The pedicellariae of Asterias are relatively somewhat larger in the 

 young than in the adult. In their early condition they are short and 

 stunted, clavate, with at least two centres of calcification, which later 

 form the two jaws. Unlike the primary spines, the calcifications in 

 each pedicellaria are not consolidated, but double from the very first. 

 The theory that the pedicellarise are homologous to spines, renders it 

 necessary to compare calcifications which diff'er in shape from the very 

 first, and also to compare a primarily single with a double calcification. 

 Neither of these difficulties is necessarily fatal to the theory, nor does 

 the mode of development of spines and pef^' . .idria^ give w^holly satis- 

 factory proof of the theory. 



The growth of the spines on the calcareous plates of Asterias re- 

 sembles that of the same stinictures in Asterina, as described by Lud- 

 wig. In the case of the spines of the primary plates they arise as 

 separate calcifications, and are not extensions from plates already 

 existing.! 



* Memoire sur les Etoiles de Mer recueilles dans la ^ler des Antilles et le 

 Golfe du Mexique durant les Expeditions de Dragage faites sous la Direction de 

 M. Alexandre Agassiz. Nouvelles Archives du Museum, 2 ser., Vol. VI. 



t The " ambulacral spines " observed by A. Agassiz on the outer edge of the 

 ainbulacral plates were not observed in Asterias. Is it not possible that tiiese 

 spines belong to the " interambulacrals " '? 



