THEORY OF NATURAL SELECTION. 213 



With respect to the steps by which these curious orgaus 

 have been evolved, Mr. Agassiz infers from his own re- 

 searches and those of Mr. Miiller, that both in star-fishes 

 and sea-urchins the pedicellariae must undoubtedly be looked 

 at as modified spines. This may be inferred from their man- 

 ner of development in the individual, as well as from a long 

 and perfect series of gradations in different species and 

 genera, from simple granules to ordinary spines, to perfect 

 tridactyle pedicellariae. The gradation extends even to the 

 manner in which ordinary spines and the pedicellariae, with 

 their supporting calcareous rods, are articulated to the shell. 

 In certain genera of star-fishes, "the very combinations 

 needed to show that the pedicellariae are only modified 

 branching spines" may be found. Thus we have fixed spines, 

 with three equi-distant, serrated, movable branches, articu- 

 lated 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 tridac- 

 tyle 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 the pedicellariae and 

 the movable branches of a spine, is unmistakable. It is gen- 

 erally admitted that the ordinary spines serve as a protec- 

 tion ; 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 as by meeting together they acted as a 

 prehensible or snapping apparatus. Thus every gradation, 

 from an ordinary 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 becomes articulated to 

 the shell, and is thus rendered movable. I wish I had space 

 here to give a fuller abstract of Mr. Agassiz's interesting 

 observations on the development of the pedicellariae. All 

 possible gradations, as he adds, may likewise be found be- 

 tween the pedicellariae of the star-fishes and the hooks of 

 the Ophiurians, another group of the Echinodermata; and 

 again between the pedicellariae of sea-urchins and the anchors 

 of the Holothuriae, also belonging to the same great class. 



