136 BULLETIN OF THE 
the spoon-shaped plates are not ambulacral. If the spoon-shaped plates 
are ambulacral, they are highly modified. 
The manner of growth of the ambulacral plates has been carefully de- 
scribed and figured by Ludwig, and IJ have little to add to his account. 
They originate in pairs one on each side of the median line of the arm 
in a deeper region of the arm on the ventral side. In their earliest form 
they appear as trifid spicules or small parallel bars. These two members 
of a single joint unite over the median line, forming by coalescence the 
solid arm-joint. The union of the separate calcifications has been well 
figured by Ludwig. The union takes place at first on the adoral and 
aboral ends, so that in a half-formed arm-joint a median unclosed opening 
remains after the junction of the two ends. The body thus formed is at 
first much longer than broad ; later it becomes discoid, when the consoli- 
dation is complete. 
_ The distinction between ambulacral and lateral arm-plates is recogniz- 
able from very early conditions, and the former are well consolidated 
before union with the lateral plates is effected. 
That the ambulacral plates of Ophiurans are spurs of the side plates 
was questioned by Lyman,* from his study of certain lower genera of 
deep-sea Ophiurans, Ophiothelia and Ophiohelus. The separation of the 
two members which compose one of these “arm-bones” even close to the 
tip of the arm affords a difficulty in accepting the theory of some natural- 
ists that they originally formed as spurs from the small lateral plates. 
The development of the plates in Amphiura shows that the arm-joints, 
“ambulacra,” and lateral arm-plates not only originate in two separate 
calcifications, but also that they have great similarity to the permanent 
plates in such a genus as Ophiohelus. Attention has been called by 
Ludwig to the resemblance of the unconsolidated ambulacral plates of 
Amphiura and the same plates of the deep-sea Ophiuran Ophiohelus. 
Lyman first suggested the embryonic nature of the unjoined arm-joints 
of Ophiohelus and other genera. 
Orals, or Oral Shields. —The position of the first-formed orals (0) is 
an interesting one ; and a morphological interpretation of their relationship 
to plates in other Echinoderms is beset with many difficulties, which others 
have discussed.t 
* A Structural Feature hitherto Unknown among Echinodermata, Ann. Mem. 
Bost. Soc. Nat. Hist., 1880, pp. 3-7, Pl. I. Fig. 12. Report on the Ophiuroidea 
dredged by H. M. S. Challenger during the years 1873-76, Zodlogy, Vol. V. pp. 287, 
238, Pl. XXVIII. Fig. 6. 
+ Ludwig, Ueber den primiren Steinkanal der Crinoideen, nebst vergleichend- 
