MUSEUM OF COMPARATIVE ZOOLOGY. 123 



portionally smaller." This fact has possibly a meaning in the comparative 

 time when the suranal (dorsocentral) is formed, as compared with the 

 ocular and genital, as will be spoken of directly. 



In regard to Neumayr's statement that in the young " Glyphostomes " 

 the anal plate is first formed, and that the plates of the genital ring are 

 later detached from it, A. Agassiz* states that undoubtedly the anal 

 plate in young Echinoids is the first plate to appear, and that genital 

 and ocular plates are independently formed around it. 



Our interest in the study of the dorsocentral plate of Amphiura is 

 connected intimately with the origin of the suranal plate of the Echinoids. 

 In Echinoids of some genera, according to the authority mentioned, the 

 abactinal region is covered by a large single plate, suranal, which is pos- 

 sibly the dorsocentral, while in Amphiura a plate believed to be homo- 

 logous, dorsocentral (d), is very small in young stages, and is thought to 

 be developed after the radials. While opinions may be divided as to the 

 homology of the primary radials in Amphiura with ocular and genitals 

 of sea-urchins, it would seem as if a uniformity of opinion might be 

 arrived at in regard to the dorsocentral. If, however, in Echinoids this 

 plate forms before the ocular and genitals, and in Amphiura after the 

 same, one is tempted to ask whether they are homologous. One might, 

 of course, avoid the difficulty by the truism that the relative time of de- 

 velopment is of little consequence, and that the appearance of the plate 

 in Amphiura is simply retarded. Such an escape from the difficulty does 

 not give much satisfaction, even if we remember the abbreviated devel- 

 opment of Amphiura. The theory that the dorsocentral of Amphiura is 

 homologous to the suranal of sea-urchins is believed to be true. As far 

 as any objection based upon the different time of appearance of this struct- 

 ure is concerned, it is first necessary to examine the observations adduced 

 in its support. t While the evidence seems to be against the late forma- 

 tion of the dorsocentral in Echinoids, the author is confident that in one 

 genus of Echinoids, viz., Echinarachnius, no single plate centrally placed, 

 which represents the suranal, is developed before plates regarded as homo- 

 logous to the oculars and genitals. He believes that the suranal, which 



* Report on the Echini, Mem. Mus. Comp. Zool., Vol. X. No. 1, p. 33. 



t It is believed that the difference in relative time of the appearance of the dorso- 

 central, and the ten plates around it, in Stvongylocentrotus and Amphiura cannot be 

 satisfactorily studied except from stages of the former genus much younger than 

 those figured by Loven for this purpose (op. cit., PI. XXI.). There is a call for a 

 study of the young sea-urchin, between these and the pluteus, before we can defi- 

 nitely state whether dorsocentral or the ten surrounding plates first appear, or 

 whether they appear simultaneously or not. 



