92 HOMOLOGIES OF ECHIXODEEMS. 



is passed through by Ophiurans and Echini as well as Starfishes, the only 

 difference in the subsequent development being that the Ophiurans always 

 remain in an embryonic condition, closely resembling the one just described. 

 In the Starfishes the actinal plates formed by the bridges separating suc- 

 cessive pairs of tentacles become resorbed along the central line, the edges 

 forming inwardly by spurs the true ambulacral plates, and the plates which 

 little by little develop so as to form the edge of the arms are likewise 

 formed from the plates originally a part of the abactinal system. Those 

 which are on the outside of the tentacles become the interambulacral 

 plates, but differ in no way from the plates forming the sides of the arms. 

 In the case of the Starfishes these side arm-plates are often very numerous ; 

 in the case of the Ophiurans they are reduced to a minimum, the upper 

 arm-plate being, as in young Starfishes, very prominent and distinct, while 

 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 one third across the arms. We must only remember that in Ophiu- 

 rans the lower arm-plates represent the original plates derived from the 

 abactinal side extending across the tentacles, while in Brisinga and Star- 

 fishes the median part of the plate has become resorbed, so that the 

 tentacles passing between the ambulacral plates are inside of the inter- 

 ambulacral plates, while in Ophiurans they pierce the connected inter- 

 ambulacral plates (or the lower arm-plate). 



Something analogous to what takes place in Ophiurans occurs in Echini. 

 The plates which cover the water-system never become resorbed (as in 

 Starfishes) ; there is no internal ambulacral system of plates developed, 

 from the fact that new plates in the Echini are always developed near 

 the basal plate (the apical system), while new plates in Ojihiurans and 

 Starfishes are invariably formed at the extremity of the arms. In Echini, 

 therefore, the extremity of the water-system (the ocular tentacle) remaining 

 connected with the original apical system, the water-system thus forms a 

 loop, one end of Avhich is attached to the so-called ocular plate, while the 

 other connects with the circular canal at the mouth, and hence, both ends 

 being fixed, the new plates must necessarily cover the water-system, while 

 in Ophiurans and Starfishes, one end alone being fixed, it is possible, as in 

 the case of Starfishes, for the water-system, owing to the resorption of the 

 central part, to appear in a peculiar position. But in spite of this sunilarity 



