144 BULLETIN OF THE 



ventral plate is a median actinal deposit, and not, as in the starfish, joined 

 by the growth of plates to the median line to form the embryonic 

 median plate, the author finds this difficulty in the homology indicated 

 above. It would seem from the description that both ambulacral and 

 adambulacral were derived from the same calcification in Asteracanthion, 

 a calcification beginning on the abactinal and growing down on each side 

 to the actinal median coalescence. In Amphiura ambulacral and adam- 

 bulacral plates are from the first separate and distinct calcifications. 



In the abbreviated development of Asterina as given by Ludwig there 

 seems to be a wide diff"erence in the growth of the plates from that of 

 Asteracanthion recorded by A. Agassiz. Adambulacral and ambulacral 

 plates are recorded by the former from the very first, and in the oldest 

 stage figured no embryonic median actinal row of spiniferous plates is 

 figured in the arm. Asterias glacialis seems also, according to Loven's 

 figures, to differ from Asteracanthion in this particular. The plates which 

 appear to correspond with the lateral ambulacra of A. Agassiz's account 

 seem to be separated along the median line of the arm on the actinal 

 side, and median plates below the water-vessel or its position are not 

 figured. Perhaps no genus of starfishes can better serve to explain these 

 discrepancies than Pteraster. The embryology of this starfish is very 

 much needed, and from the interesting fact that the young is carried in 

 a pouch on the abactinal surface of the body, and is therefore probably 

 without a brachiolaria, we may expect interesting revelations from its 

 study. 



If one cannot accept the theory that the ventrals of Amphiura are 

 homologous with the embryonic median plates of the young starfish, 

 Asteracanthion, and cannot regard them as formed from the adambulacral 

 plates by coalescence along the median line, it may be asked, What are 

 they, and to what plates in the starfish are they homologous 1 In answer 

 to this question we might ask another, Why is it necessary to suppose 

 that they are represented in the starfish ] We know that there are plates 

 on the dorsal hemisome of Amphiura which seem not to be represented 

 in the starfish. Why not suppose that the ventrals are unrepresented 1 

 Perhaps they belong neither to ambulacral nor interambulacral systems, 

 but are special plates for the protection of the nerve and water system of 

 the arms. Perhaps also similar coverings of the Echinoid are also not 

 referable to either ambulacral * or interambulacral systems, as we under- 



* Ludwig regards the ambulacral plates of the starfish as unrepresented in the 

 sea-urchin, or highly modified into the auricles. This homology of what are com- 

 monly called the ambulacrals in the sea-urchin with the adambulacral of starfishes 



