ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 193 



pentagonal, while in others there is a smaller number of 

 actinal plates than usual, but Palaostoma is the only genus 

 in which a regular radiate arrangement is perceptible. The 

 actinal membrane of all Desmosticha, whether it be regularly 

 imbricated or not, bears ten large prominent interradial 

 plates, which are pierced by the ten large buccal tentacles.^ 

 They appear to develop from the continuous plating of lime- 

 stone cells which extends over from the abactinal side so as 

 to cover the whole actinal surface of the young Urchin. 

 They are found in all young Echini^ being the first plates to 

 appear of all those on the actinal membrane," while in many 

 genera they always retain a "greater preponderance." 

 Agassiz-^ regards them as homologous with the five actinal 

 plates of Leskia, and they would therefore represent the five 

 oral plates of the Crinoids. This view is strengthened by 

 the fact that they are pierced by the ten large buccal 

 tentacles, much in the same way as the ocular plates 

 (=radials) are said to be pierced by the odd terminal 

 tentacles of the ambulacra, while the orals of the young 

 Crinoid are also opposite to five pairs of short tubular 

 tentacles placed interradially, though not perforated by them. 

 These orals alternate with five large azygos tentacles, the 

 homologues of which in the Urchins separate each pair of 

 buccal shields, and ultimately become the odd terminal 

 tentacles. 



1 am by no means sure that Agassiz is right in comparing 

 these buccal plates of the Desmosticha to the oral valves of 

 Leskia. In the first place they are paired and perforate, 

 while the latter are single and imperforate. This, however, 

 is a comparatively unimportant difference. The one on 

 which I would lay most stress is the mode of origin of the 

 buccal shields, which is very different from that of the oral 

 plates of Crinoids. Agassiz himself describes them as 

 formed by an extension of the limestone plating from the 

 abactinal over on to the actinal surface. It can, of course, 

 be urged that this be also the origin of the oral valves of 

 Leskia. If so, they cannot be homologous with the oral 

 plates of the Crinoids ; but, as far as mere appearance goes, 

 they resemble these far more than they do the ten perforate 

 buccal shields of the Desmosticha. Loven,* however, gives 

 an entirely different interpretation of these buccal shields. 



» ' Revision of the Echini/ p. 699, plates ix, figs. 2 and 4, and x, 

 fig. 3. 



2 'Revision,' p. 735. 



3 ' Revision,' p. 583. 



" Loc. cit., pp. 27—29. 



