MUSEUM OF COMPARATIVE ZOOLOGY. 125 
set of plates (terminals) do suffer the change, we have this difficulty in a 
comparison of the young Echinoid with the young Amphiura. The ter- | 
minals of Amphiura are independent centres of calcification from the 
radials. If terminals and radials in Amphiura lie in the same radius, 
how can the one or the other, especially the former, be the same as the 
oculars of the sea-urchin? If we compare the apical region of a sea- 
urchin and the abactinal hemisome of the young Amphiura, we have in 
Amphiura the terminals, plates which are supposed to be the same as 
oculars of the sea-urchins. If that is so, what plates in sea-urchins can 
be found to represent the radials of Amphiura, plates which are separate 
calcifications in both Ophiurans (Amphiura) and Asterina between termi- 
nals and dorsocentral? None exist. If, on the other hand, we say the 
oculars of sea-urchins are the homologues of the radials of Amphiura, 
they are not the same as another definite calcification, the terminals situ- 
ated at the tip of the arm. Is it not more logical, from embryological 
grounds, if we compare the apical system of young Echinoids with the 
abactinal hemisome of Amphiura, not to suppose the first-formed plate in 
an ambulacral radius is an ocular homologous to a terminal, but an oc- 
ular homologous to a radial; provided, of course, we compare the radial 
series of Amphiura with the radial series of Echinoidst* Is it possible 
that what we call the ocular of the sea-urchin is in reality a consolida- 
tion of the radial and the terminal, or that a plate homologous to the 
radial is never developed? Either of these conditions would be a pos- 
sibility, and more probable than that the eye-plate of the sea-urchin 
* By the “ radial series” of plates in Amphiura the author means the series which 
lies in the radius extending from the centre of the dorsocentral through the middle 
of the primary radials and terminals. By the radial series of the sea-urchins, the 
author means those plates which lie in a radius extending from the centre of the dor- 
socentral through the ocular. The above remarks in relation to radials and oculars 
of Ophiurans and sea-urchins apply to those who compare the terminals of starfishes 
and brittle stars without pluteus, with the ocular plates of the sea-urchin. Those, on 
the other hand, who compare the terminals and radials (primary) of Amphiura are 
believed to have this difficulty. If the terminal of Amphiura is compared with the 
genital of a sea-urchin, the madreporic opening of the Echinoid, which lies in the 
same interradius as a genital, ought to lie in the same radius as a terminal in Am- 
phiura. The same objection would hold in a comparison of the radialia of Amphiura 
aud the genitals of sea-urchins; the madreporic body, which in the young Am- 
phiura is found in a plate called the oral, in the interradius would be found in the 
radius. This, of course, supposes the fact that the genitals and the madreporic 
body, since they lie in interradii, are comparable, and waives the homology of the 
so-called ambulacral plates, which Ludwig does not find in sea-urchins except in the 
auricule, 
