MUSEUM OF COMPARATIVE ZOOLOGY. 143 
In a discussion of the relationship of the genus Brisinga to the Ophi- 
urans, A. Agassiz * has sought to show that it is an intermediate genus, 
connecting starfishes and Ophiurans. He finds the homologues of the 
ventral plates of the Ophiuran in the fusion of the interambulacral plates 
along the median line of the arm. He says: ‘“‘In the case of the Ophi- 
urans ... the lower arm-plate is formed by the junction of opposing 
spurs of the interambulacral plates, as can readily be imagined from a 
comparison with Brisinga, where we find a spur from the interambulacral 
plates extending nearly two thirds across the arms.” That there are 
anatomical, perhaps embryological, grounds upon which Brisinga may be 
regarded as reducing the “gap hitherto unfilled between starfishes and 
Ophiurans,” is not doubted; but it may well be questioned whether the 
ventral plate of Amphiura, originating as it does on the median line and 
later than the corresponding lateral plates, is homologous to any part of 
the interambulacral plates of Brisinga. 
A. Agassizt states that “‘a row of limestone cells extending along the 
median line separates the base of the suckers,” and that the embryo 
starfish has no trace of any interambulacral system. He calls attention 
(p. 53) to the absence of a well-defined interambulacral system of plates 
in a young starfish (Pl. VIII. Fig. 9), in which the rays are well devel- 
oped, and considers the young starfish as still eminently Ophiuroid in 
most important embryonic features. He shows a distinct row of “ me- 
dian ambulacral spines (w!)” on the abactinal side of the arms. These 
plates with spines are those supposed from A. Agassiz’s description to 
be formed as follows: The radial plates of the abactinal system of the 
“dorsal part of the arms gradually extend towards the edge of and 
down on to the actinal side, enclosing the water-system little by little, 
and finally, as has been described, covering the ambulacral tube,” ete. 
The median plates are later, according to A. Agassiz,t+ absorbed along the 
median line in Asteracanthion. It would appear then that from the 
unabsorbed end of these plates, homologous with the adambulacral 
plates, grow the ambulacral plates above, or on the abactinal side of 
the water-system into the position which they eventually have in the 
adult. In the Ophiurans the median plate of the starfish before absorp- 
tion of the plate is represented, according to him, by the lower arm- 
plates (ventrals). The Ophiurans are regarded as remaining in an 
embryonic condition so far as these plates are concerned. 
Aside from the fact already commented upon, that in Amphiura the 
* Mem. Mus. Comp. Zoil., Vol. VI. p. 102. 
+ Embryology of Starfish, p. 47. + Op: ctt., p.'92- 
