120 Prof. P. M. Duncan and Mr. W. P. Sladen on 



logy with the single large anal plate of the earlv stages of young 

 Echini belonging to other families " (' Blake ' Echini, p. 18). 

 He states : — " But in all the young stages, even when not 

 measuring more than 1*5 millim. in diameter, the arrange- 

 ment of tlie plates of the abactinal system does not differ from 

 that of the older specimens, the sur-anal plate being only 

 proportionally somewhat smaller." He states that, in the 

 youngest stage of Salenia examined, the anal system is dis- 

 tinctly pentagonal and covered by eight large triangular 

 plates. 



From the foregoing observations it may be deduced that the 

 central plate of the abactinal system is a primary embryonic 

 plate, which in Salenia and its allies grows pari passu with 

 the growth of the test, and by this means remains contingent 

 through life, along the greater portion of its circumference, with 

 the basal plates. It thus lies outside the periproctal ring, of 

 the margin of wdiich it contributes to form a part, and the 

 supplementary plates which are formed on the periproctal 

 membrane are prevented from inserting themselves between 

 it and the neighbouring basal plates. 



In the Echinidge, on the other hand, the primary central 

 plate does not grow as the test grows, but may even be 

 diminished by a greater or less amount of resorption. Con- 

 sequently it always lies inside the periproctal ring, and the 

 supplementary plates which are formed upon the periproctal 

 membrane insert themselves between the rudiments of the 

 primary central plate and the basal plates ; and, finally, in the 

 adult stage, the primary central plate may have become so 

 insignificant as to be scarcely distinguishable from the supple- 

 mentary or so-called " anal " plates. Although thus masked 

 and diminished in size, the significance of the plate from a 

 phylogenetic point of view is in no way lessened. 



The persistence of a sur-anal plate ever since the age of 

 the Lias (an age not greatly removed from that in which the 

 Perischoechinidje alone were represented) in Acrosalenia, 

 Peltastes, and finally in Salenia, shows that the plate has a 

 very great significance, and this has impressed every natu- 

 ralist who has studied the structural resemblances of the great 

 groups of the Echinodermata. 



As a sequel to these remarks upon the sur-anal plate, it is 

 extremely desirable to fix its proper terminology, ibr neither 

 of the names given to it by the elder Agassiz, and by A. Agassiz, 

 nor that suggested by Lovdn is free from objection. Sur-anal, 

 sub-anal, and central plate bring the structure too much into 

 relation with the anus, with which it has really nothing 

 to do. 



