126 BULLETIN OF THE 
is homologous at the same time to the radials and the terminals which 
originate in both Amphiura and Asterina in two separate calcifications. 
A comparison of the young sea-urchin with the young Amphiura reveals 
the following fact. The radials (rp) of Amphiura occupy a position, as 
regards the dorsocentral, similar to that which the ocular plates of the 
young sea-urchin hold to its anal plate, which is regarded as the dorso- 
central. In the sea-urchin the ocular plate is perforated for the eye-spot. 
The eye has not been found in the radials of any Ophiuran. In one 
case the plate is an ocular, or eye-plate ; in the other it has no eye. Nor 
is the eye known in the terminal in the Amphiura. Is a plate with an 
eye homologous to one without an eye? This of course opens the ques- 
tion whether the ‘‘ eye” of the starfish and the “ eye” of the Echinus are 
homologous. I believe an answer to this question can only be given by 
a histological study of the eye and its relation to the water-tube in 
Echinoids. A. Agassiz has shown the relationship of the eye-spot to 
the unpaired tentacle of Asteracanthion, and it remains to be seen 
whether in Echinoids the eye-spot is similarly formed. When it has 
been shown that the eye-spot of Echinoids is homologous to the eye-spot 
at the end of the arm of the starfish, it may be asserted that the termi- 
nal plate of the starfish is homologous to the ocular of the urchin. 
The adult form of the starfish and of Amphiura would imply that the ter- 
minal plate or the plate over the eye-spot of the starfish is homologous 
to the terminal plate of Amphiura, notwithstanding an eye-spot has not 
been described in any of the Ophiurans. If an association of the eye-spot 
with a plate means homology of those plates, the oculars of the urchins 
and the terminals of the starfishes (Ophiurans) are homologous ; but if 
homologous, what plate in the young sea-urchin corresponds with the 
radial (rp) of Amphiura ? 
From a comparison of the young Amphiura with the young Kchinoid 
it would seem as if a non-ocular (without pigment-spot) plate (7p) in 
the former occupies the same relative position to the dorsocentral as an 
oculated plate in the latter. Are they not then homologous? From a 
study of the adult Amphiura and the adult starfish it would seem that 
the plates (tp) at the tip of the ray adjacent in the latter to an eye-spot 
are homologous. If the terminal plate of the starfish with the eye-spot 
is homologous to the terminal of an Amphiura, and the plate with the 
eye-spot is homologous in starfishes and urchins, we are led to suppose 
the radials of Amphiura to be plates unrepresented in urchins. It 
seems more natural to compare radials in Amphiura with oculars in sea- 
urchins, notwithstanding the position of the eye-spot. 
