



93 



with the genus Solaster. Still neither of these genera exhibits a disc reduced in any thing 

 like the same degree as that of theBrisinga; and even in those species of the genera Asterias 

 (A. glacialis) and Pedicellaster which are furnished with a relatively very small disc, we 

 shall yet find that a rather considerable number of vertebrae have entered into the formation 

 of the disc; while the number of such vertebrae belonging to the disc in the Brisinga is 

 properly speaking zero. It is this which properly determines the peculiar exterior habitus, 

 so different from that of other known star-fishes, that distinguishes the genus Brisinga, and 

 gives to it at first glance a certain habitual similarity to the Ophiurae. 



7. 



Relation of the genus Brisinga to extinct star-fishes. 



Asbj0rnsen has already (1. c.) drawn attention to the habitual conformity of the genus 

 Brisinga with the oldest known fossile star-fish Protaster. The small round ophiura-like 

 disc, with the sharply defined arms issuing from it, in the Protaster, reminds us decidedly 

 of the Brisinga; as also the skeleton of the arms, by its shape and composition of ambu- 

 lacral and adambulacral plates only, exhibits in both forms considerable resemblance. We 

 shall however find, on more minute examination, that the body even in this ancient form, is 

 already considerably more centralised than in the Brisinga; a considerably greater number 

 of vertebrae enters into the composition of the disc, notwithstanding its small dimensions. 

 On the whole we shall find in the Protaster evident indications of the course of develop- 

 ment diverging from the star-fish type and tending towards the Ophiurae. type, which is by 

 no means the case in the Brisinga. Antecedently to this divergent development we must 

 naturally presuppose a number of more indifferent stages, wherein the nature of the deve- 

 lopment would not yet be clearly indicated, in other words, wherein the cormus would shew 

 a slighter degree of centralisation; and these stages must certainly be supposed to have been 

 in some degree more corresponding to the Brisinga. If we admit the theory above noticed 

 as to the composite individuality of star-fishes, or their original value as colonies of worm- 

 like persons, we have actually in the now living Brisinga a still more primitive form than 

 even in the Protaster, wherein this colony has already attained to a considerable degree 

 of centralisation, and in which the number of persons (arms) has become permanently the 

 typical number for the Echinoderms, namely 5. The Brisinga may henceforward be assumed 

 to exhibit a still greater conformity with the supposed ancestors of the genus Protaster 



