THEORY OF NATURAL SELECTION. 227 



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 whicli 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 pedicel- 

 lariae are only modified branching spines'' may be found. 

 Thus we have fixed spines, with three equi-distant, 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 pedicellariae, 

 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 generally 

 admitted that the ordinary spines serve as a protection; 

 and if so, there can be no reason to doubt that those fur- 

 nished 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 

 prehensile or snapping apparatus. Thus every gradation, 

 from an ordinary fixed spine to a fixed pedicellariae, 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 inter- 

 esting observations on the development of the pedicellarite. 

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

 between 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. 



Certain compound animals, or zoophytes, as they have 



