118 Prof. P. M. Duncan aad Mr. W. P. Sladen oa 



L. Agassiz did not mean to imply that the pLate was on 

 the anus, although not having seen a recent Salenia he did 

 not distinguish between the orifice of the anal tube — the anus 

 proper — and the periproctal ring, which gives attachment to 

 the membrane which has the anal passage in it. He meant 

 vis-h-vi's to the periproctal ring. The name of the plate in 

 question has been subjected to several alterations, and its 

 morphological meaning and homologies have been repeatedly 

 discussed. 



Alex. Agassiz * in 1864 pointed out that in young 

 examples of StrongTjlocentrotus the central area of the apical 

 or abactinal system is closed by a single large circular plate 

 which occupies the whole of the space within the ring of the 

 genital or basal plates. During the succeeding stages of 

 growth this plate does not increase in size, but is more or 

 less resorbed. Numerous supplementary plates are subse- 

 quently formed on the periproctal membrane, from which, at 

 the adult stage, the original primary plate is often scarcely 

 distinguishable. 



The same naturalist, when studying, at a later date, a recent 

 Salenia^ showed that the sur-anal pUite (called by him sub- 

 anal) is the homologue of the first-formed central plate of 

 young Echini, and remarked f that " the abactinal system of 

 Salenia is entirely homologous with the abactinal system of 

 the other Echinoids," the original central plate retaining a 

 greater preponderance than is the case in other genera. 



Lovenjis very definite in his views regarding the homology 

 of the sur-anal plate of Salenia and the primary central 

 abactinal plate of young Echinidie, and he terms both the 

 *' central disk " of the dorso-ccntral system. 



In his remarks upon \\\&&.\)\(zdX&yA[Q,xn oi Stronyylocentrotusy 

 after referring to the discoveries of Alex. Agassiz, lie observes 

 that the " central disk " is developed before the anal opening 

 of the intestinal canal is formed, and that the arrangement of 

 the parts is such that the opening cannot occur, except after 

 the putting aside or partial resorption of the disk. Conse- 

 quently the central disk, far from being a simple protecting 

 appendage of the anal opening, must be considered as a " pi^ce 

 ^ elle " and independent — an integral part of the dorso-central 

 system having a special morphological value. He then pro- 

 ceeds to state that ^'Salenia furnishes a conclusive proof of 

 this, as Mr. Alex. Agassiz has remarked. In this genus the 



• " Oh tlie Embryology of Ecliiiioderms," Mem. Amer. Acad. 1864, 

 vol. ix. p. 12. 



t ' Revision of the Echini,' p. 259. 



I ' Etudes sur les Echiiioideef,' p. 60 et seq. 



