PRIMARY LARVAL PLATES OF BRACHIATE ECHINODERMS. 37 



tured by the water-pore, Ludwig has been led to consider that 

 the ventral area of the Crinoid is comparable to the dorsal area 

 of Asterids and Echinoids. This view has been ably contro- 

 verted by Carpenter^ from the Crinoid standpoint ; and it is 

 only necessary to remark here that if there be any validity 

 whatever in the homologies of calyx plates, which have been 

 considered in the preceding pages, as occurring in Asterids and 

 Ophiurids, the supposition of Ludwig's must fall to the ground. 

 In the case of the Echinoids Ludwig^s theory^ is to my mind 

 altogether untenable, for the genital plates at the apical pole 

 are homologated to the oral plates of the Crinoid, notwith- 

 standing that the larval central plate is regarded by him as the O 

 homologue of the dorsocentral plate of the group or the root- 

 disk at the end of the Crinoid stem, as maintained by Carpenter. 



A further outcome of this hypothesis is that the basal plates 

 of Crinoids have no representatives in the Echinoid test. It is 

 hard to imagine a more anomalous position than that in which 

 plates developed on the left peritoneal diverticulum of Crinoids 

 (the orals) are homologated with plates developed on the right 

 peritoneal diverticulum of Echinoids (the genitals) and made to 

 surround the homologue of the plate (the dorsocentral), which 

 in the Crinoid is unquestionably developed at the end of the 

 right peritoneal diverticulum. The fact that the basal plates 

 of the Crinoid, the Asterid, the Ophiurid and the Echinoid are 

 all developed on the right peritoneal diverticulum or enterocoel 

 and that the oral plates and their representatives are, on the "] 

 other hand, all primitively formed on the left enterocoel, is, in 

 my opinion, conclusive evidence that the dorsal or abactiual 

 area of the Crinoid is comparable with the dorsal area of the 

 Asterid, the Ophiurid and the Echinoid, as Carpenter^ has 

 justly maintained. 



Before leaving the subject of the madreporite I would here 

 add a few remarks on the orientation of the Echiuoderm larvae 



' " Notes on Echiu. Morphology," No. v, p. 12, 'Quart. Journ. Micr. Sci.,' 

 vol. xxii. 



2 ' Morphol. Studien,' Bd. ii, pp. 42, 43. 



3 'Notes, V,' p. 16. 



