132 BULLETIN OF THE 



lowed them back to this larva, from a study of my figures and a com- 

 parison with Metschnikoff's Fig. 16, there seems abundant evidence 

 that they precede all other plates of the actinal region of the disk. 

 There is also evidence that they antedate the formation of the terminals, 

 although they are probably formed after the radialia or primary radials. 



Metschnikoff,* in speaking of the plates of the actinal region, says that 

 the five parts of the skeleton on the ventral surface of the embryo repre- 

 sent the future jaws. Apostolides t also describes five V-shaped plates. 

 Tliere are really five pairs of these plates in early stages, and ten pairs in 

 later conditions when both pairs of adambulacral plates are developed. 



The following plates will be considered in our discussion of the mouth 

 skeleton : 



1. Adambulacral ; first and second pairs, ad} and ad"^. 



2. " Ambulacral ; " spoon-shaped plates, os, spl. 



3. Oral ; mouth-shield ; madreporite, o. 



4. Torus angularis ; jaw-plates, to. 



5. Teeth, t. 



Adambidacral Plates. — There are two pairs of adambulacral to be 

 considered in this connection, known after Ludwig as the first {ad^) and 

 second {ad"^) pairs. They are both regarded as homologous to the side 

 plates (Ip) of the arms, which will be later described. The adambulacrals 

 originate in pairs, five in number. The first pair [ad^) support the torus 

 (to). Both pairs are early formed in the development of the actinal 

 hemisome. 



First Fair of Adambulacral Flates. — The double origin of the 

 "V-shaped" plates has been shown by Ludwig, and is evident from 

 the figures of Max Schultze (Figs. 5, G) and Metschnikofi". Apostolides t 

 writes : " Les premieres grandes plaques calcaires qui apparaissent avant 

 meme que le bras soit a peine ebauche sont les cinq pieces fourchues 

 de I'adulte ; il est facile de se convaincre de leur apparition primitive, 

 grace a leur forme en V et la disposition des tentacules buccaux que 



* 0}}. cit., p. 18. The lettering cc, in Metschnikoffs Figs. 11, 12, 13, 16, and 17, 

 PI. IV., does not refer to the same structures as cc in Fig. 6, PI. III. cc in Fig. 16 

 is an adambuhicral plate ; in Figs. 11, 12, 13, and 17, it is a terminal, and in Fig. 6, 

 PI. III., it is one of the provisional pluteus plates. The rods cc, in MetschnikofTs 

 PI. IV. Fig. 16, are probably the first and second adambulacral. The following cor- 

 rections may be made in " Selections from Embryological Monographs, Echinoder- 

 mata" (Mem. Mus. Camp. Zoo!., Vol. IX. No. 3, PI. III.): cc, Figs. 9, 10, 11, and U 

 are probably terminals ; in Fig. 13, cc are adambulacral, and not provisional lime- 

 stone rods homologues of the pluteus rods. 



+ Op. cit., p. 213. 



