826 THE ORIQIN OF SPECIES 



spines and the pedicellariae with their supporting calcare- 

 ous rods are articulated to the shell. In certain genera 

 of star-fishes, "the very combinations needed to show 

 that the pedicellarisB are only modified branching spines" 

 may be found. Thus we have fixed spines, with three 

 equidistant, serrated, movable branches, articulated to 

 near their bases; and higher up, on the same spine, 

 three other movable branches. Now when the latter 

 arise from the summit of a spine they form in fact 

 a rude tridactyle pedicellaria, and such may be seen on 

 the same spine together with the three lower branches. 

 In this case the identity in nature between the arms of j 

 the pedicellariae and the movable branches of a spine is 

 unmistakable. It is generally admitted that the ordinary 

 spines serve as a protection; and if so, there can be no 

 reason to doubt that those furnished with serrated and 

 movable branches likewise serve for the same purpose; 

 and they would thus serve still more effectively as soon 

 Hs by meeting together they acted as a prehensile or 

 snapping apparatus. Thus every gradation, from an ordi 

 nary fixed spine to a fixed pedicellaria, would be of 

 service. 



In certain genera of star-fishes these organs, instead 

 of being fixed or borne on an immovable support, are 

 placed on the summit of a flexible and muscular, though 

 short, stem; and in this case they probably subserve some 

 additional function besides defence. In the sea-urchins 

 the steps can be followed by which a fixed spine be- 

 comes articulated to the shell, and is thus rendered mov- 

 able. I wish I had space here to give a fuller abstract 

 of Mr, Agassiz's interesting observations on the develop- 

 ment of the pedicellariae. All possible gradations, as he 



