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-sliaped 

 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. Tlie second adambulacral plates are ten in number, in five pairs, 

 and with the first adambulacral are among the earliest plates to form. 



In Fig. 1 7 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 Fortsatze." 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 frilheren 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., PI. L Fig. 5 c, Fig. Qd, p. 43. t 0}\ cit., p. 194. 



X Op. cit., p. 189. 



