

"^ 



PRIMARY LARVAL PLATES OF BRACHIATE ECHINODERMS. 31 , /> 



h! 



A iguier ^ the odontophore, has almost, if not quite, attained its | r 



ultimate position. On studying a series of the earlier stages, 

 we learn that this plate has followed, during the process of 

 development, a path similar to that taken by the mouth-shield 

 of the Ophiuran, with which I consider it to be homologous, 

 and consequently homologous with the oral plate of the 

 Crinoid, as previously held byLudvrig^ and still maintained by 

 Carpenter,^ although Ludwig^ has altered his opinion. I shall 

 presently revert to this subject and mention other points in 

 support of the same view. 



The madreporite or water-pore in Asterids usually punc- 

 tures a basal plate (3 in fig. 14), although in some species 

 it permanently lies external to this plate; and in the early stages 

 of even those in which the basal plate is actually punctured -> 



the pore is probably always more or less marginal and is "^ 

 probably at first quite disconnected from the plate. 



In the earlier stages of the Asterid larva — and I would 

 here refer with admiration to the careful and most ex- 

 cellent memoir of Ludwig ^ — the first-formed plates in the 

 viviparous or abbreviated larva are the primitive elements of 

 the terminal plates, the basal plates, and the dorsocentral 

 plate. These become well-developed before any traces of the 

 radial plates make their appearance. Concurrent with the 

 radial plates the lateral plates (interambulacral plates) are 

 developed. The diagram given in fig. 15 represents this stage 

 of growth. 



Referring to the splendid and now classical memoir of 

 A. Agassiz^ on the "Embryology of Echinoderms,^' we find a 

 similar order of appearance of plates obtaining in Asterids 

 which pass through a brachiolarian stage; and indications of 



' " Auatomie comparee du squelette des Stellerides," 'Archives de ZooL 

 exper. et gen.,' t. vii, p. 33 — 250. 



» 'MorphoL Studien/ i, p. 263 ; ii, pp. 3, 16. 



' "Notes, V," 'Quart. Journ. Micr. Sci.,' vol. xxii, 



* "Entwickl. der Asterina gibbosa," ' Morphol. Studien,' ii, p, 181. 



5 " Entwieklungsgeschichte der Asterina gibbosa," ' Zeitschr. f. wissensch . 

 Zool.,' Bd. xxxvii; (' Morphol. Studien,' Bd. ii). 



6 ' Contrib. Nat. Hist. U.S.,' vol. v. 



> 



