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 Ampliiura 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-i;rchin 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 Echinoid 

 it would seem as if a non-ocular (without pigment-spot) plate {rj)) 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. 



