28 



W. PEEOX' SLADEN. 



The plates marked 4, 4, in the diagram given in fig. 12, are 

 called " primary radials " by Ludwig/ and he also remarks : — 

 *^ Was nun die fiinf primaren Radialia des Scheibenriickens 

 anbelangt, so sind dieselben nichtnur bei Amphiura squamata, 

 sondern auch bei anderen Arten von friiheren Forschern bereits 

 ofters erwahnt worden." Carpenter ^ has clearly asserted their 

 homology with the first radials of a Crinoid. In further 

 support of this view it should be borne in mind that not only 

 these radial plates, but also companion plates homologous with 

 the other elements of the Crinoid calyx^are persistent throughout 

 the adult stage of a very large number of Ophiurans. Upon 

 this subject Carpenter has prosecuted some very interesting in- 

 vestigations, which are published in another page of this 

 Journal (ante, pp. 4 — 11). 



On the strength of these considerations it seems to me 

 irrational to regard the terminal plates t, t, as the homologues of 

 the first radials of the Crinoid calyx, for we should then be com- 

 pelled to call the plates 4, 4, in fig. 12, second radials, and we 

 should be placed in the anomalous position of having a ray in 

 which the second radial was proximal in relation to the first 

 radial ; a state of things which in the adult would be further 

 emphasised by the separation of those two plates by the whole 

 series of brachial plates. I have alluded to this subject at 

 somewhat greater length than might perhaps be thought 

 necessary for reasons which will be apparent when we discuss 

 the homologies of the plates in the next group. 



In the Ophiuran at the stage given in fig. 12, the plates 

 which ultimately become the mouth-shields are entirely on the 

 actinal hemisome, but have not } et reached their permanent 

 position. These plates are homologous with the oral plates of 

 a Crinoid. One of the mouth-shields (orals) bears tlie water- 

 pore. 



Turning now to the earlier stages of the development of the 

 Ophiurid under notice (Amphiura squamata), Ave find 

 that the first plates formed on the ubactinal hemisome are 



' Op. cit., pp. 104, 105. 



- " Notes, V," op. cit., pp. 9, 10, (igs. i and ii. 



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