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. Apostolidest also describes five V-shaped plates. 
‘here are really five pazrs 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, sp. 
3. Oral ; mouth-shield ; madreporite, o. 
4, Torus angularis ; jaw-plates, to. 
5. Teeth, ¢. 
Adambulacral 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 (/p) 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 Pair of Adambulacral Plates. — The double origin of the 
“V-shaped” plates has been shown by Ludwig, and is evident from 
the figures of Max Schultze (Figs. 5, 6) and Metschnikoff. Apostolides t 
writes: “Les premieres grandes plaques calcaires qui apparaissent avant 
méme que le bras soit a peine ébauché sont les cing pieces fourchues 
de l’adulte ; il est facile de se convaincre de leur apparition primitive, 
grace & leur forme en V et la disposition des tentacules buccaux que 
* Op. cit., p. 18. The lettering cc, in Metschnikoff’s Figs. 11, 12, 13, 16, and 17, 
Pl. 1V., does not refer to the same structures as cc in Fig. 6, Pl. III. cc in Fig. 16 
is an adambulacral plate ; in Figs. 11, 12, 18, and 17, it is a terminal, and in Fig. 6, 
Pl. III., it is one of the provisional pluteus plates. The rods cc, in Metschnikoffs 
Pl. 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. Comp. Zodl., Vol. 1X. No. 8, Pl. III.): cc, Figs. 9, 10, 11, and 14 
are probably terminals ; in Fig. 13, cc are adambulacral, and not provisional lime- 
stone rods homologues of the pluteus rods. 
+ Op. cit., p. 218. 
