136 BULLETIN OF THE 



the spoon-shaped plates are not amhulacral. 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 I 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 tlie arm on the ventral side. In their earliest form 

 they appear as tritid 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 (o) 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, Zoology, Vol. V. pp. 237, 

 238, PI. XXVin. Fig. 6. 



t Ludwig, Ueber den priinaren feteinkanal der Crinoideen, nebst vergleichend- 



