XOETH AMEEICAX STAEFISHES. 103 



foi'ms at once an arch over the arms without expanding, as in other 

 Starfishes, into a flat actinal floor of greater or less extent on the side of 

 the interambulacral plate, from the extent and shape of which the vari- 

 ous families are determined. This obtains its maximum of development 

 in the extreme forms like Culcita and Palmipes ; in one case the actinal 

 and abactinal floors are well separated, in the other closely united hy 

 vertical shafts and walls. The analysis of the structure of Brisinga gives 

 a most satisfiictory explanation of the general homologies existing be- 

 tween Starfishes and Ophiurans, and reduces the gap hitherto unfilled 

 between Starfishes and Ophiurans to a comparatively unimportant method 

 of development. 



As the madreporic body of Starfishes is placed in one of the interbrachial 

 arches, and this arch reduced as it often is to a minimum (Solaster). or lim- 

 ited sometimes to the mesenteric support of the stone canal, we have a ready 

 explanation of its position in the Ophiurans on the homologous plates in 

 the interbrachial spaces, namely, the single plate in the continuation of 

 the line of the interambulacral plates; at the same time the homology 

 between the genital plates and the single interbrachial plate found in 

 some Starfishes at the angle of the arms is fully carried out, as it is 

 well known that it is on each side of the interbrachial arch that the 

 ovarian openings are found. An examination of the base of the arms of 

 Brisinga near its junction with the disk shows already quite a constric- 

 tion, and of course a corresponding reduction in the length of the inter- 

 brachial arch. The great extension of the interambulacral plates across 

 the space covering the ambulacral canal reduces it to its minimiuu at 

 that point. The mode of articulation of the ambulacral plates of joints 

 in the arms of Brisinga has been compared rather with Ophiurans than 

 with Starfishes, but the articulation of the internal skeleton in Ophiurans 

 is not specially different, although somewhat more perfect, from the artic- 

 ulation of the joints of arms in the Starfishes proper, and the homology 

 between the internal skeleton of Ophiurans and the ambulacral system 

 of Starfishes can be clearly established. If we imagine for each primary 

 interambulacral plate but a single row of secondary interambulacral plates 

 composed of a small number of plates, we shall of course have a side arm- 

 plate and an upper arm-plate ; the lower arm-plate being formed of 

 the opposed interbrachial plates, which have become soldered together, 

 and through which the tentacles have pierced their way. 



