1896.] OF THB PALiEOZOlC OPHIUROIDBA, 1041 



The structure is on essentially the same plan ns that of Ony- 

 cliasier and it is clearly Streptospondyline in character. The genus 



Pig. 6. 



a h 



a &b, articular surfncea of the vertebral ossicles of Eucladia. ■>/. 



is therefore to be included among the Streptophiurfe, the resem- 

 blances to the Euryalida) being homoplastic modifications to suit its 

 mode oC life. 



The Uomdlocjies of the Madreponte. 



The madreporite in Eucladia is certainly dorsal, as Dr. Wood- 

 ward correctly stated ; it has been suggested that this character 

 removes the genus from the Ophiuroidea. In that case Lap- 

 ivorihura 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 6. 

 pi. 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 

 Palseozoic 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 Palseozoic 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 ontogeneti- 

 cally 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 

 modification, 



