ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 371 



Comatula, though they are important constituents of the 

 Apical system of the other Echinoderms. This is contrary 

 to fact, for they are present as the basals (Allman, Wyville 

 Thomson, Dr. Carpenter, M. Sars, Gotte). 



2. The interradial plates of the young Pentacrinus repre- 

 sent the genital plates of Echini, but the basal circlet of the 

 adult which is developed out of them is homologous with 

 the subanal plate. This means that the homology of these 

 plates changes during their development, which is, of 

 course, impossible. 



There is one fact which brings out very clearly the homo- 

 logy of the basals of the young Crinoid (figs, i, v, 3, 3) 

 with the interradial abactinal plates of the young Starfish 

 or Sea Urchin (figs, ii, vii, 3, 3). 



It is worth mentioning here, because it tells very strongly 

 against the view held by Agassiz and Loven, that the basis 

 of the Crinoids represents the central disc in the apical sys- 

 tem of the other Echinoderms. Both the basals of the Crinoid 

 (figs. I, IV, V, 3, 3), and the genital plates of the Urchin 

 or Starfish (figs, ii, vii, 3, 3), are developed in the form of 

 a spiral around the right peritoneal sac or water tube of the 

 larva. Agassiz^ first demonstrated this in the Echini and 

 Starfishes, and Gotte^ has shown it to be equally true in 

 Comatula, pointing out at the same time the homologies of 

 the two sets of plates, and also that the basals appear before 

 the radials in both Asterids and Crinoids. 



Apart from this important identity in the mode of deve- 

 lopment of the two sets of plates, it appears to me that 

 Agassiz and Loven are by no means justified in assuming 

 that the single central abactinal plate of the young Starfish 

 or Sea Urchin (figs, ii, vii, 1) is in any way homologous to 

 the " basis " of the Crinoids (figs, i, iv, v, 3, 3). 



All our knowledge of the development of this element 

 of the Crinoid skeleton goes to show that it primitively con- 

 sists of five pieces, which first appear quite independently 

 of one another, although they subsequently become firmly 

 united by Synostosis. The mere fact that in some cases the 

 sutures between them become ultimately obliterated does not 

 appear to me to be a sufficient argument for considering the 

 apparently simple piece that is formed by their unusually 

 close union, as homologous with the central abactinal plate 

 of the young Starfish, which is single and undivided from 

 the very first (fig. vii, 1). 



1 'North American Starfishes,' pp. 37, 38. 



- " Vergleichende Entwickelungsgeschiehte der Comatula Mediterranea," 

 ' Archiv fiir Mikroscopiscbe Anatomic,' Band xii, pp. 595, 620. 



