ECHINODERM MORPHOLOGY. 327 
(fig. 9), but without representing the homologous ” oral 
plate with its water-pore. Had he done so he would have 
been obliged to explain that while in the Urchin figure the 
madreporite was represented as seen through the gut, in the 
Crinoid figure the reverse would be the case, viz. the gut 
seen through the perforated oral plate. But nevertheless 
the latter is homologous with the genitals of an Urchin ! 
Again, the positions of the primary actinal and abactinal 
plates, with respect to the oral centre, is not the same in 
the Urchins and Crinoids if the latter be orientirt 
according to Ludwig’s theory. In an Urchin the proximal 
plates to the oral centre are the oculars (radial), and the 
distal series the genitals (interradial). 
In a Crinoid or Ophiurid the interradial orals are proximal 
and the radials distal ; but, nevertheless, the homology of 
the orals and genitals is undoubted, while the interradial 
basals of the Crinoids, which are beyond the radials just as 
the genitals of an Urchin are beyond the oculars, are not 
represented in an Urchin ! Further, Ludwig takes no account 
of the five interradial plates which surround the mouth of 
PalcBostoma, and have the same relation to the water-vascular 
ring, &c., as the orals of Rhizocrinus, a relation which, it is 
almost needless to say, is not characteristic of the genitals of 
an Urchin. 
(3) Another of Ludwig’s arguments is founded upon the 
peculiar relation of the aboral blood-vascular ring of the 
Ophiurids to the mouth* shields. Because it dips down in 
the five interradial spaces, and partially rests upon these 
plates, Ludwig considers them homologous with the genitals 
of the Urchins and Starfishes. But the rest of this blood- 
vascular ring remains in the abactinal position that the 
whole of it occupies in the Urchins and Starfishes, and it is 
from this abactinal position that the principal vessels are 
given off to the genital glands. According to Ludwig 
(p. 352), “Dieses Genitalgefass entspricht sowohl in seinem 
Ursprunge als auch in der Richtung seines Verlaufes 
und in seiner Beziehung zu den Geschlechtsorganen den 
Genitalgefass der Asterien.” On Ludwig’s theory, however, 
one would expect the genital vessels to be given off from 
those parts of the blood -vascular ring which lie nearest the 
ventral surface, since it is in the plates of that ventral surface 
that he recognises the homologues of the genital plates of 
the Starfishes. One would also expect that these ventral 
plates would be occasionally perforated by the openings of 
the genital glands. But is this ever the case? Ludwig 
VOL. XX. NEW SKll. Y 
