368 p. HERBERT CARPENTER. 



shown that in the young Aster aeanthion there is an apical 

 system developed upon the abactinal surface, which is pre- 

 cisely similar to that of the young Urchin. There is a cen- 

 tral plate (fig. VII, 1) as to the ultimate fate of which we 

 are still somewhat in the dark, though it remains distinct up 

 to the age of three years. 



Fig. VII. — Apical System of Asterias glacialis (young ; after Loveu). 1 . 

 Central disc. 3. Genital plates = Costals (Loveu). 4. Ocular 

 plates = Hadials (Loven). p. First rudiments of perisomatic 

 system. 



Around this are two rings of plates, a proximal series, the 

 interbrachials (fig. vii, 3, 3), and a distal series, the bra- 

 chial plates (fig. VII, 4, 4) . The former, if not resorbed, 

 persist as the genital plates of the adult, while the original 

 radial arm plates are gradually " pushed away from the 

 body, by the addition of new spines formed at the base of 

 the ray," but remain perfectly well defined at the extremity 

 of each ray, in connection with the odd terminal tentacle. 

 The same is the case in the Ophiurids as shown by Miiller,^ 

 and since by Agassiz^ and MetschnikofF.^ The terminal 

 plates of the arms are the first to appear, and are carried out- 

 wards by the development of new plates between them and 

 the body. 



Thus, then, the terminal arm plates of the starfishes (fig. 

 VII, 4, 4) are homologous with the ocular plates of the 

 Echini (fig. ii, 4, 4), and, therefore, with the (first) radials 

 of the Crinoids, (figs, i, iv, v, 4, 4) which remain fixed, 

 as in the Urchins, in their original positions. 



' " Ueber die Ophiurenlarven des Adriatischen Meeres," ' Abhandlungen 

 der Berlin Akadcmie,' 1852, p. 14. "Ueber den Bau der Echinodermen," 

 ' Abhandlungen der Berlin Akademie,' ISSi, p. 46. 



' 'Embryology of Eclunoderms,' loo. cit., dgs. 29, 32. 



» Loo. cit., PI. iv, fig. 17. 



