Anatomical Nomenclature of Echinoderms. 8 



cation wliicli " arises in tlie walls of tlie water tube," thus 

 giving a third meaning to the same term, wlnle Agassiz, as 

 we have seen, lias used it in yet another sense. Is it too 

 much to ask on behalf of the student of the future that it be 

 enii)]ojed in one sense oidy? In the following pages it will 

 be used to denote the madreporic or stone-canal. 



2. Dorsocentral and Gentro-dorsal. 



These two names are frequently used as if they were 

 synonj-mous, though in reality they denote plates of very 

 different morjihological characters. 



The term " dorsocentral " a])pears to have been first used 

 by the Messrs. Austin * for that part of a Crinoid which was 

 called the pelvis by Miller, i. e. the ring of plates which rest 

 upon the top stem-joint. In some cases five separate plates 

 may be distinguished, in others only three, while in others 

 there seems to be but one undivided plate with a stem-facet oa 

 its lower surface ; and even this facet is absent on the central 

 plate of Mnrsupites. Owing to the rapid spread of the 

 Miillerian terminology, in which the lowest plates of the 

 Crinoidal calyx were designated basals, the collective name 

 " dorsoceiitral " applied to them by Austin never found 

 general acceptance. But in LovcJin's classical work f on the 

 Echini the term " dorsocentral system " is used to denote 

 the central plate in the apex of a young Urchin, together with 

 the two rings of genital and ocular plates around it. He 

 regarded the central plate of Marsupites as homologous with 

 that of the Urchin, and also compared the ocular plates of the 

 latter to the radials of Marsupites^ two determinations which 

 I fully accej)ted when writing on the subject in 1878 J, 

 though I could not follow Loven in the other homologies 

 which he proposed, nor in his views respecting the primitively 

 compound nature of the dorsocentral plate. I suggested at 

 the same time that the homologue of the latter was to be 

 found in the terminal plate at the base of the stem in the 

 stalked larva of Comatula, which I carefully distinguished 

 from the enlarged upper stem-joint or centro-dorsal piece. 

 Sladen § adopted this view in 1884, since which time the 



• " Descriptions of several new Genera and Species of Criuoidea," 

 Ann. & Mag. Nat. Hist. 1843, vol. xi. p. 190. 



t " Etudes sur les fichinoid^es," Kongl. Sveuska Vetenskaps-Aka- 

 demiens Haudliugar, 1874, Bd. xi. no. 7, p. 05. 



t " On the Oral and Apical Systems of the Echinoderms," Quart. 

 Joum. Micr. Sci. 1878, vol. xviii. p. 359. 



§ "On the Homologies of the Piimary Larval Plates in the Test of 

 Brachiate Echinoderms," Quart. Jouru. Micr. Sci. 1884, vol. xxiv. p. 25. 



1* 



