ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 367 



below the radials in the Tessellate Crinoids (figs, iv and 

 VI, 2, 2, S, 3). ' Das Verhaltniss des ausseren zuin inneren 

 Basalkreis des Encrinus entspricht der Unterscheidung von 

 Parabasis and Basis bei den Crinoidea Tessellata ; jedoch 

 wiirde die uebertragung dieser terminologischen Ausdriicke 

 auf die entsprechenden Theile des Encrmus vermieden, weil 

 sie dahin fuhren wiirde bei nachst verwandten Gattun- 

 gen. wie Encrmus und Apiocrinus, Gleichwerthiges mit 

 ungleichen Bennengungen zu belegen.' In this opinion 

 I entirely agree, for I believe that the parabasals of the 

 Tessellata (fig. vi, 3, 3) are truly homologous with the 

 basals of Encriiius (fig. iv, 3, 3), the second or under- 

 basals of which (fig. iv, 2, 2) represent the plates that 

 have been hitherto called basals (fig. vi, 2, 2) in such 

 Tessellata as possess parabasals (fig. vi, 3, 3). 



Loven, regarding the radial plates of the first ring in Mar- 

 supites as the true parabasals (fig. iii, 2, 2), states that the 

 " second parabasals " (fig. iii, 3, 3), which rest upon and 

 alternate with them, are unrepresented in the Urchins and 

 in most Crinoids. It will be evident from the reasoning 

 given above that this assertion is incorrect. The interradial 

 '' second parabasals " of Marsupites represent an essential 

 element in the Apical system of the other Echinoderms, 

 namely, the interradial abactinal plates. They are homo- 

 logous with the genital plates of the Urchins (fig. ii, 3, 3), 

 the basals of the Articulate Crinoids (figs, i, iv, v, 3, 3), and 

 the parabasals of most Tessellata (fig. vi, 3, 3) ; while it is 

 the so-called basals of the latter, and the under basals of 

 Eficrmus (figs, iv, vi, 2, 2), which are homologous with the 

 radial "first parabasals" of Marsiipites (fig. iii, 2, 2), and 

 are unrepresented in the Urchins and in most Articulate 

 Crinoids. 



It is now, I think, a view universally accepted that the 

 Starfishes, both Asterids and Ophiurids, have an apical sys- 

 tem homologous with that of the Echini. L. Agassiz and 

 Miiller long ago pointed out, on purely anatomical grounds, 

 the correspondence between the so-called ocular (Intergenital, 

 Miiller) plates of the Echini (fig. ii, 4, 4) and the terminal 

 plates (fig. vii, 4, 4) of the five arms of an Asterid, which 

 support the eyespot and the odd terminal tentacle. The 

 study of the development of the Urchins and Starfishes has 

 proved this correspondence to be a true homology. The 

 beautiful observations of A. Agassiz^ and Metschnikoff ^ have 



1 ' North American Starfishes,' p. 48, PI. vi, fig. 10. 



2 " Studien iiber die Eutwickeluug der Echinodermea und Nemertinen," 

 * Memoires de I' Academic Imperiale de St. Petersbourg/ viie serie, tome 

 xir. No. 8, PI. xii, fig. 1. 



VOL. XVIII. NEW SEtt. B B 



