| OCR Text |
Show 1896.1 OF THE PALEOZOIC OPHIUROIDEA. 1041 The structure is on essentially the same plan as that of Onychaster and it is clearly Streptospondyline in character. The genus Fig. 6. a & b, articular surfaces of the vertebral ossicles of Eucladia, \0-. is therefore to be included among the Streptophiurae, the resemblances to the Euryalidae being homoplastic modifications to suit its mode of life. The Homologies of the Madreporite. The madreporite in Eucladia is certainly dorsal, as Dr. W o o d ward correctly stated ; it has been suggested that this character removes the genus from the Ophiuroidea. In that case Lapworthura and probably Protaster will also have to be excluded from this subclass. But in most Echinoderms the water vascular aperture opens on the aboral surface. According to Bury [2 b. pl. xxxvii. fig. 2, pp. 422-423], the water-pore of the Ophiuroidea originally occupies this position. It is therefore not unreasonable to suppose that in the earliest Ophiurids the water-pore was originally dorsal, and that it subsequently worked round to the ventral side, as it does during the development of the Spatangoida. Hence one cannot use the dorsal position of the madreporite in Palaeozoic Stellerids as a proof that they are not Ophiurids. It follows from this, however, that the madreporite (or plate in which the water-pore opens) of Lapworthura and Eucladia is not homologous with the madreporite of recent Ophiurids, which belongs ontogenetically to the oral system. Carpenter and Bury have both adduced strong reasons to show that the madreporite of Ophiurids is not homologous with that of Asterids. The evidence of the Palaeozoic genera of both groups shows that this plate is not homologous in all the members of even the same subclass. It is certain that in some Ophiurids the madreporite is oral, and that in others it is not. Hence it is quite possible that in those Asterids with a ventral madreporite, the plate may be a member of the oral system. Although, therefore, the madreporite may originate ontogenetically on an oral, it does not do so phylogenetically, and the situation of the water-pore on an oral plate has resulted only from a secondary I modification. |
