142 BULLETIN OF THE 
rather than real; for the ventral plate V belongs to the first pair of 
adambulacral plates (ad!), which are themselves homologous with side 
plates. Although the first ventral (V) is formed before the first side 
plates, it appears after the first adambulacral pair to which it belongs. 
Considering these facts, the law pointed out by Ludwig,* that the side 
plates precede the ventral in time of formation, holds likewise for the 
first adambulacral, and is true of the side plates if we give them their 
true interpretation as adambulacral piates. 
Ventral Plates. — The ventral arm-plate cr under arm-plate is early 
developed in the young Amphiura. The first ventral (V) is already well 
formed when the terminal plate has just begun to push its way axially 
from the periphery of the disk. The first ventral plate (Figs. 16, 17) is 
formed before the first side plates of the arm, but after the first adam- 
bulacral. Ludwig* states: ‘ Erst nachdem die Seitenplatten sich an- 
gelegt haben, beginnen auch die Dorsal- und Ventralplatten aufzutreten 
und zwar sind wenigstens bei Amphiura squamata die Ventralplatten 
den Dorsalplatten immer ein wenig voraus.” I have already spoken of 
this sequence in my account of the side plates. It is here for the first 
time suggested that the plate Vv is the ventral plate of the first or second 
pair of adambulacrals. If it (V) is a ventral plate of the true side 
plates, it is more rapid in its growth, as compared with the true side 
plates, than the other and subsequently formed ventrals. 
The ventrals originate on the median ventral line, and are unpaired. 
They first appear as a trifid spicule (Fig. 20, v), one prong of which is 
adoral, the others aboral and lateral. In early formation they are free, 
and do not arise from the lateral arm-plates. They precede, in time of 
formation, the corresponding dorsal of the same joint ; and while in most 
particulars the growth of the new plates resembles the growth of those of 
a repaired arm of an Ophiuran as described by Lyman,t the sequence of 
dorsals and ventrals does not seem to agree with the descriptions by the 
last-mentioned author, who says that in a repaired arm the upper (dorsal) 
plate is first formed. 
Ventrals never arise by coalescence of plates on either side of the 
middle line of the arm. 
* Op. cit., p. 189. 
+ Ophiuride and Astrophytide, Old and New, Bull. Mus. Comp. Zoél., p. 258. 
“Then appear on the central point of juncture, above, clusters of grains which in 
time grow into upper arm-plates, and a similar process follows for the lower arm- 
plates.” The fact that the dorsal arm-plate in Amphiura originates as a single 
spicule after the ventral has a meaning as compared with certain embryonic genera. 
(See p. 145.) 
