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 Opliiuran 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. Agassiz t 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 (PI. 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 (u^) " on the abactinal side of the arras. These 

 plates witli 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," etc. 

 The median plates are later, according to A. Agassiz,:}: 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 fi\ct already commented upon, that in Amphiura the 



* Mem. Mils. Comp. Zool, Vol. VI. p. 102. 



t Embryology of Starfish, p. 47. J 0/^. cU., p. 92. 



