\v 



30 W. PERCY SLADEN. 



with the first radials of the Crinoid calyx. (The plate marked 

 T in fig. 14, with the plate marked 4 in fig. 10.) This view I 

 hold to be erroneous and base my conclusions upon the following 

 grounds. 



On comparing the Asterid diagram (fig. 14) with the Ophiu- 

 rid diagram (fig. 12), or even the younger stage given in 

 fig. 15 with that in fig. 13, no one, I imagine, would dispute 

 the perfect homology that exists between the plates marked t 

 in both orders, either in the older or in the less developed 

 stages. The anomalous position which would arise from 

 regarding the terminal of the Ophiurid ray as homologous 

 with the first radial of the Crinoid calyx has already been 

 discussed; and the arguments adduced in the case of the 

 Ophiurid apply with equal, if not greater, force in the case of 

 the Asterid. If the terminals in the two groups are not 

 homologous elements, which plates in the one group would 

 represent these plates (the terminals) in the other ? Surely 

 no one would maintain that the terminal in the Asterid (t in 

 fig. 14) is homologous with the first radial in the Ophiurid 

 (4 in fig. 12). Or that the terminal of the Ophiurid (t in 

 fig. 12 was homologous with the first radial in the Asterid 

 (4 in fig. 14). No other conclusions would be possible if the 

 homologies I have indicated be denied ; and the acceptance of 

 either of these contradictory assumptions would land us into 

 an anomalous position, the untenability of which is obvious. 



The comparatively large size of the terminal plate at an 

 early stage of the young Asterid is due, in my opinion, to the 

 coalescence of primitive lateral plates with the primitive or first 

 formed rudiment of the terminal — a circumstance whicli 

 further strengthens my view of the secondary character of the 

 terminal plate. In like manner I would point out that the 

 series of plates (brachials), which extend along the median 

 dorsal line of the ray, diminish in size and age as they proceed 

 outward from the plate which I have spoken of as the first 

 radial. 



Turning now to the actinal surface, we find in the young 

 Starfish at the stage given in fig. 14, that the plate called by 



