1896.] OF THE PALHOZOIC OPHIUROIDEA, 1041 
The structure is on essentially the same plan as that of Ony- 
chaster and it is clearly Streptospondyline in character. The genus 
Fig. 6. 
a b 
a & b, articular surfaces of the vertebral ossicles of Hucladia, 4,9. 
is therefore to be included among the Streptophiure, the resem- 
blances to the Euryalide being homoplastic modifications to suit its 
mode of life. 
The Homologies of the Madreporite. 
The madreporite in Zucladia 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- 
worthura 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. 
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 
Paleozoic 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 Hucladia 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 Paleozoic 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, 
67* 
