MUSEUM OF COMPARATIVE ZOOLOGY. 125 



set of plates (termiuals) 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 1 If we compare the apical region of a sea- 

 urchin and the abactinal hemisome of the young Amphiura, we have iu 

 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 1 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 tlie radial series of Echinoids 1* 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 1 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 ?. 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 dilhculty. 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 

 and the genitals of sea-urchins ; the madreporic body, which in the young Am- 

 phiura is found iu 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 tlie homology of the 

 so-called ambulacral plates, which Ludwig does not find in sea-urchins except in the 

 auriculsB, 



