I \l \M.Mi;i\i S Mil' l 69 



and Crinoid, but the ultimate result of the developmenl of the plates ol 

 this central area is \<t\ differenl in the Sea-urchins and Crinoids from 

 what it is in the Starfishes and Ophiurans, in which the central plate remains 

 Bingle.* 



And again we have in the embryo Echi lerms the radially placed ocular 



plates dI' Echini, terminal plates of Ophiurans and of Starfishes, and t 1m- 

 radials of the Crinoids, which form in these (except in the Ophiurans) the 

 distal ring annmd the central plate. lint hen- again modifications are 

 introduced during growth in the different orders which complicate the 

 question of the homology of these parts. In the young Sea-urchin the cen. 

 tral area is — no matter what its position may he in the adult, which is 

 covered by a greater or smaller number of plates — occupied by the anal 

 system, and in a large number of genera the anal system is covered by 

 a single plate. 



In the Crinoids the central area is occupied by a stem or its repre- 

 sentative, and it seems to me more natural to homologize this central area 

 of the Echini and of the Crinoids than to attempt, as has been done, to 

 pick out a single anal plate of the Echini which does not exist in many 

 recent families, and probably not in many fossil types, and homologize 

 it with the central plate of Starfishes or of Ophiurans and the terminal 

 plate of the stem of the larval Comatulse, as has been done by Carpenter. 



In the genital ring even, there are difficulties in the way of a strict 

 homology of the basals of Crinoids and of the genitals of Starfishes and 

 Ophiurans. 



Carpenter! has figured diagram matically the disks of a large number of 

 Ophiurans. in which he thinks he has discovered the infrabasals of the dicy- 

 clic Crinoid. The regular arrangement of the plates of the central disk of 

 Ophiurans has been observed by all writers who have dissected Ophiurans. 

 and they have spoken of the apical rosette, and of its similarity to the 

 arrangement of the plates of the crinoidal calyx. Sladen has also carried 

 the comparison of the abactinal plates of the Starfish somewhat further 

 than his predecessor, Loven, and looks upon the ring of plates between the 



* Ludwig truly says (Zur Entwickelungsgeschichte des Ophiurenskelettes). " Erscheinen liberal] 

 im Bereiche des dorsaleu Scheibenperisoms neue Intermediarplatten zwischen und neben den einmal 

 gebildeten sich anlegen zu konnen." Ludwig was the first to show that the terminalia in Ophiu- 

 rans became separated, as those of the Starfishes, from the central disk by the intercalation of addi- 

 tional radial plates. Some of these Carpenter interpreted as representing the infrabasals of Crinoids. 



t Quart. Journ. Mier. Sei . Vol. XXIV p. 1. January. 1884. 



