40 W. PEECY SLADEN. 



the actinal liemisome, and consequently formed on the left 

 euterocoel, one might almost be so bold as to assert that the 

 method of origin suggested by Perrier was theoretically 

 impossible, involving as it does the assumption that a plate 

 primitively located on the abactiual hemisome, and developed 

 upon the right enterocoel should, during the process of growth, 

 pass away from that position to become metamorphosed into, 

 and assume the form, position, and functions of, a plate 

 belonging to the actinal hemisome and developed upon the 

 left enterocoel, which normally exists independently. As I 

 have not had the opportunity of examining any examples of 

 Brisinga as young as those which Perrier has studied, I am 

 unable to speak personally of the phases of the development 

 of the plates in question in that form; but I trust that M. 

 Perrier will acquit me of discourteous scepticism if I suggest 

 on the theoretic grounds above enunciated, that possibly 

 some misinterpretation may have arisen, owing to the intricacy 

 of the subject and the difficulty of investigating such a rare 

 organism in its earliest stages. 



I have previously stated my belief that the odontophores of 

 Asterids are homologous with the mouth-shields of Ophiurids 

 and the orals of Crinoids. This view was originally held by 

 Ludwig,^ but has subsequently been rejected by him^ in favour 

 of their recognition as odd interambulacral plates homologous 

 with the "^ plaques peristomiennes^^ of Loven^ in Spatangoids and 

 Clypeastroids. From the embryological standpoint I am still 

 disposed to maintain Ludwig's previous position. It is true 

 that the homology between the odontophore of the Asterid and 

 the mouth-shield of the Ophiurid is at first sight somewhat 

 obscured by the difi'erence of position; the mouth-shield being 

 a superficial plate externally visible on the actinal surface, 

 whilst the odontophore of Asterids is usually situated at a much 

 deeper level aud is masked by the mouth-plates. This con- 



1 'Morphol. Studien,' Bd. i, p. 2G3 ; Bd.ii, p. 3. 

 =" ' Morphol. Studien,' Bd. ii, p. 181. 



3 " £tudes sur les ilchiaoidees," ' Kongl. Svenbka Vet. Akad. Handl./ 

 Bd. ii, No. 7. 



