OBJECTIONS TO THE THEORY OF NATURAL SELECTION 195 



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

 of Mr. Miiller, that both in star-fishes and sea-urchins the pedi- 

 cellarise must undoubtedly be looked at as modified spines. This 

 may be inferred from their manner of development in the indi- 

 vidual, 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, 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 the pedicellariae and the 

 movable branches of a spine, is unmistakable. It is generally ad- 

 mitted 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 as by meeting 

 together they acted as a prehensible or snapping apparatus. Thus 

 every gradation, from an ordinary fixed spine to a fixed pedicel- 

 laria, 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 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 

 termed, namely the Polyzoa, are provided with curious organs 

 called avicularia. These differ much in structure in the different 

 species. In their most perfect condition they curiously resemble 



