ORAL AND APICAL SYSTEMS OF THE ECHINODERMS, 197 



auibulacral skeleton of the Critioids and of the other Echino- 

 dern;s has already been mentioned. Underlying the ambu- 

 lacra on the disc of Pentacriniis, Miiller' found a series of 

 median azygos plates, which he termed suhamhulacral, and he 

 compared them to similar plates found by Roemer beneath 

 the ambulacra of the Biastoids. I pointed out above that they 

 are also represented in the Palaeocrinoidea. Miiller rightly 

 regarded them as corresponding in their position with respect 

 to the water vessel with the radial ossicles of the oral calca- 

 reous ring in the HolothurianSj and with the rotulaj in the 

 lantern of EcJiini. Simrotlr has shown that there are good 

 reasons for regarding the rotulae (together with the radii) 

 and the auricles as respectively representing the first and 

 second vertebral ossicles of the Starfish arms. Had Miiller 

 continued to hold his original views, which are now generally 

 accepted, as to the homology of the radial pieces of the oral ring 

 in the Holothurians with the auricles of the Ech'mi, and with 

 the vertebrse of the Starfish arms, he would, no doubt, have 

 also described these last as suhamhulacral.^ The principal 

 component of the Crinoid skeleton being anti-ambulacral 

 (or abactinal) is, of course, not to be found in the Urchins, 

 and is only imperfectly represented in the Starfishes. 



It is merely an extensive development in a radial direction 

 of the primitive abactinal or apical system, situated at the 

 dorsal pole of the larva, which is of extreme importance in 

 Echinoderm morphology, for, as shown by Agassiz,* it is the 

 foundation of the whole skeleton, whether anti-, sub-, or 

 super-ambulacral. " In fact, the external limestone plates 

 forming the test of a Sea-urchin, the reticulated network of 

 the actinal and abactinal surface of a Starfish, together with 

 the ambulacral and interambulacral plates and the plates 

 forming the disc of an Ophiuran, the upper, lower, and side 

 arm-plates, as well as internal skeleton, are all directly 

 derived from the simple system of limestone plates of the 

 abactinal surface of the Echinoderm embryo." 



In the Crinoids the abactinal or antiambulacral system 

 remains most nearly in its primitive condition, extending but 

 very slightly towards the actinal side. But in the other 

 Echinoderms the radial plates of the abactinal system, 

 situated round the dorsal pole of the embryo, gradually 

 extend towards the edge of and down on to the actinal side, so 



'^ Pentacrinus, loc. cit., p. 49, and 'Bau der Echiuodermen,' pp. 57, 58. 

 - " Anatomie uud Schizogonie der Ophiadis Virens," Sars,, Zweiter Theil, 

 ' Zeitschr. fiir Wiss. Zool.,' xxviii, pp. 511, 512. 

 3 See Addenda, No. 6, p. 205. 

 ■» ' North American Starlishes,' pp. 91—93. 



