134 BULLETIN OF THE 
plates of the arms is a correct one, and borne out by their mode of for- 
mation as compared with the first ventral plate of the arms, as will be 
explained in a subsequent explanation of the formation of the ventral 
plates. ; 
Second Pair of Adambulacral Plates. — The second pair of adambu- 
lacrals (ad?) arise after the first pair, aborally to the same, and are situated 
more on the interradius than the first pair. These plates also differ from 
the first pair in the possession of appendages in the form of club-shaped 
spines. These spines are free at one extremity. The fact that the second 
pair of adambulacral plates as well as the side plates of the arms have 
spines would incline one to believe that they are homologous to each 
other. The second adambulacral plates are ten in number, in five pairs, 
and with the first adambulacral are among the earliest plates to form. 
In Fig. 17 there will be seen between each pair of terminal plates (tp), 
on the periphery of the disk, two knob-like structures which extend be- 
yond the edge of the disk. These did not escape the attention of Max 
Schultze,* who described them as extending beyond the disk as “keulen- 
formige Fortsiitze.” Ludwigt has also described these bodies, and rec- 
ognized their resemblance to spines. They were purposely omitted by 
him from his figures. I think this omission is unfortunate, as they con- 
firm a theory of the relationship of the second adambulacral plates 
which Ludwig supports, that these plates are homologous to the side 
plates of the arms. He says: { “ Beziiglich der Homologie der Seiten- 
platten am Arme der Ophiuren mit den Adambulacralstiicken der See- 
sterne kann ich auf meine friiheren Ausfiihrungen verweisen und brauche 
wohl kaum zu bemerken, dass diese Homologie auch in den eben erwahlen 
entwickelungsgeschichtlichen Thatsachen eine Stiitze findet.” The posi- 
tion of the second pair of adambulacrals so called, and the club-shaped 
appendage seem to refer these plates to the same category as the side 
plates of the arms, and make them homologous with adambulacral plates 
elsewhere among stellate Echinoderms. 
The club-shaped appendages lose their relatively large size as the 
growth of the arm goes on, and it is only when the terminals are just 
beginning to be pressed out from under the radialia that their projection 
beyond the edge of the disk is noticed. These club-shaped spines can 
be seen by looking at the young Amphiura from the abactinal as well as 
the actinal regions. 
Ambulacral Plates. — Of the plates which have been referred to the 
* Op. cit., Pl. I. Fig. 5c, Fig. 6d, p. 42. t Op. cit., p, 194, 
+ Op. cit., p. 189. 
