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 (F) 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 plates. 



Ventral Plates. — The ventral arm-plate or 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 (F) 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 aiid 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 Lymau,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 ot plates on either side of th6 

 middle line of the arm. 



♦ Op. dt., p. 189. 



t Ophiuridse and Astrophytidse, Old and New, Bidl. Mus. Comp. Zool., 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. 

 (Seep. 145.) 



