PRTxMARY LAR7AL PLATES OP BBAOEIATE EOHINODEEMS. 35 



nasteria carinifera, in various species of Pentaceros, and 

 a large number of the Goniasteridse. 



The presence of under-basals in Asterids and Ophiurids 

 (first pointed out in the latter group by Carpenter^) apart 

 altogether from their presence in numerous Crinoids, goes 

 very strongly, in my opinion, against the views of A. Agassiz, 

 Loven, Wachsmuth, and other writers, who consider that the 

 dorsocentral plate of Echinoderms is the solidified homologue 

 of the five under-basals (or basals of earlier writers^ of a 

 Crinoid; and that a support which almost amounts to con- 

 firmation is given to Carpenter^s ^ opinion that the dorsocentral 

 plate, which is simple from its earliest embryonic commencement, 

 is indeed the homologue of a single and simple plate. This plate 

 Carpenter^ homologates with the radical plate or root-disk at 

 the extremity of the Crinoid stem. This view I endorse 

 entirely, and fail to see how five plates, even when ankylosed, 

 can be homologated with an embryonically single and simple 

 plate. 



It seems to me that Ludwig places too much importance upon 

 the puncture of the water-pore (the madreporite) as furnishing 

 tlie index for determining the homologies of plates.^ Because 

 the oral plate in Crinoids bears this puncture, the presence of 

 the water-pore in any plate whatever is held by him as indicating 

 that the plate thus furnished is the homologue of the oral 

 plate of the Crinoid, irrespective altogether of whether the plate 

 be developed on the right or the left peritoneal diverticulum — a 

 question undoubtedly of far greater morphological importance. 



All that we know about the early embryonic stages of Starfishes 

 goes to show that primarily the water-pore is independent from 

 any plate whatever, and that it is only at a later period that it 

 becomes engulfed as it were, or surrounded, by the extensions 



1 'Notes ou Echiiioderni Morphology,' No. v, p. 10 (separate copy). 



2 " Oral aud Apical Systems of Echinoderms," ' Quart. Journ. Micr. Sci.,* 

 vol. xviii, p. 371, et siq. 



^ Oj). cit., p. 37i. 



* Cf. Carpenter, 'Quart. Journ. Micr. Sci.,' vol. xx, p. 322; also F. M. 

 Balfour, ' Comparative Embryology,' vol. i, p. 4:77 (note). 



