NOTES ON ECHINODERM MORPHOLOGY. 305 



not pushed out to the extremity of the rays, but always remain 

 in the disk, aud that another set of plates (terminals) do suffer 

 this change, we have this difficulty in a comparison of the 

 young Echinoid with the young Amphiura. The terminals 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 7" 



I must confess that I do not quite understand what " new " 

 phase this question has assumed since I wrote on the subject 

 in 1882. All the facts to which Fewkes refers in the above 

 passage were then known, having been discovered by Ludwig 

 in the previous year; and it was these very facts which led me 

 to homologise the primary radials of Amphiura and of the 

 Ophiurids generally with the oculars of an Urchin. Sladen 

 has accepted this view, and I am not aware of its ever having 

 been disputed. 1 I fail to see therefore what the new phase of 

 the question may be to which Fewkes refers ; for the fact 

 which he mentions, that the radials of Ophiurids are not per- 

 forated for the eye-spots, like the oculars of an Urchin, is 

 familiar to all Echinoderm students. 



The radials of a Crinoid are equally devoid of any relation 

 to the ambulacral structures, and yet they have been always 

 regarded as homologous with the oculars of an Urchin by 

 every writer who has dealt with the subject ; and Fewkes 2 

 arrives at the same conclusion as Sladen and myself had pre- 

 viously done, that u it seems more natural to compare radials 

 in Amphiura with oculars iu sea-urchins, notwithstanding the 



1 Fewkes refers, somewhat unnecessarily, " to those who compare the ter- 

 minals of starfishes and brittle stars without pluteus with the ocular plates 

 of the sea-urchin." I know of no writer who has done so since the publication 

 of Ludwig's aud Sladen's discoveries respecting the development of both radials 

 and terminals in Ophiurids and Asterids. Neither has anyone, that 1 am 

 aware of, suggested that either terminals or radials of Amphiura are com- 

 parable to the genitals of the Urchins, though Fewkes takes some trouble to 

 point out the very obvious impossibility of such an homology (p. 125). 



2 Loc. cit., p. 126. 



