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 difference 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 Lovén’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? 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 ? 
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 
