CHAP. VII.] THEORY OF NATURAL SELECTION. 179 



cellari;ie, one resembling those of Echinus, and the other those of 

 Spatangus ; and such cases are always interesting as affording 

 the means of apparently sudden transitions, through the abortion 

 of one of the two states of an organ. 



With respect to the steps by which these curious organs have 

 been evolved, Mr. Agassiz infers from his own researches and 

 those of Mtiller, that both in star-fishes and sea-urchins the 

 pedicellariae must undoubtedly be looked at as modified spines. 

 This may be inferred from their manner 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 pedicellari#\ 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, moveable branches, articulated to near their 

 bases ; and higher up, on the same spine, three other moveable 

 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 the pedi- 

 cellariaa and the moveable 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 moveable 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 pre- 

 hensile 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 

 moveable. I wish I had space here to give a fuller abstract of Mr. 

 Agassiz's interesting observations on the development of the pedi. 

 cellariae. 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 been 



7* 



