35f) V. HERBERT CARPENTER. 



the centre of the disc to form a low pyramid covering the 

 mouth, and further states that "these oral plates are mter- 

 radial, and the spaces between them radial corresponding 

 with the arm grooves." 



From the descriptions of Duchassaing^ it is possible that 

 they occur in Pentacrinus asteria, though this is doubtful. 

 According to Wyville Thomson" there is no trace of them in 

 P. 3MIeri. 



The importance of the Oral system in the morphology of 

 the Crinoids, and of most, if not all the other Echinoderms, 

 has hitherto been but little noticed, and 1 propose in another 

 paper to attempt to determine its homologies in the EcJmii 

 and Holothurians, though not in the Starfishes, in which it 

 is apparently undeveloped. For the present, however, we 

 will confine our attention to the Abactinal or Apical System. 



These names were first used by A. Agassiz^^ for the 

 assemblage of plates at the dorsal pole of the test of the 

 Sea-urchins. It has been called the *' dorsocentral system " 

 by Loven,'* and consists of the so-called genital and ocular 

 plates, disposed symmetrically around a central group, the 

 anal system of Agassiz, which is composed of a series of 

 polygonal plates, arranged with more or less irregularity 

 around the anal opening. 



The essential element in this anal system is a single plate 

 which is, as a rule, most distinct in the young Echini, as it 

 undergoes very important modifications during the passage 

 from the young state to that of the full-grown animal. 

 Loven, recognising its morphological importance as an inte- 

 gral part of the dorsocentral system, has named it the 

 central disc. 



As shown by Agassiz,^ this central disc appears at a very 

 early period in the development of the young Urchin, before 

 any traces of either the genital or the ocular plates become 

 visible. In its primitive condition it has a fairly regular 



' Quoted by De Koniuck, ' lleclierches, &c.,' p. 53. See the * Cauadiau 

 Naturalist' for 1S6S, p. -lil. Liitken states that Loven, who had ex- 

 amined Duchassaing's original specimen iu the Micheliu coUectiou at Paris, 

 told him that it did not siiow any oral valves because it had no peristome 

 at all. This, of course, proves nothing either way. 



- ' Phil. Trans.,' vol. civ, j). 512. 



^ " Revision of the Eciiini," p. G35, ' Illustrated Catalogue of the 

 Museuni of Comparative Zoology at Harvard College,' No. 7- 



* " Etudes sur Ics Echinoidces," ' Kongl. Svenska Vetenskaps Akade- 

 miens Handlingar," Band ii, No. 7, p. <)5. 



^ " Embryology of the Echinoderms," 'Memoirs of the American Academy,' 

 ix, 18G4, p. 12, fig. 2S. " Contributions to the Fauna of the Gulfstream, 

 &c.," pp. 381, 284, 285. " Revision of the Echini," pp. 280, 286, 300, 

 G83. - 



