192 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII. 



the manner in which ordinary spines and the pedicellariae with 

 their supporting calcareous rods are articulated to the shell. la 

 certain genera of star-fishes, "the very combinations needed to 

 show that the pedicellaria} 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 pedicellaria? and the 

 moveable branches of a spine, is unmistakeable. 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 prehensile 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 pedicellaria. 

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

 the pedicellaria of the star-fishes and the hocks of the Ophiurians, 

 another group of the Echinodermata ; and again between the pedi- 

 cellarise of sea-urchins and the anchors of the Holothuriaj, 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 the 

 head and beak of a vulture in miniature, seated on a neck and cap- 

 able of movement, as is likewise the lower jaw or mandible. In one 

 species observed by me all the avicularia on the same branch often 

 moved simultaneously backwards and forwards, with the lower 

 jaw widely open, through an angle of about 90°. in the course of 

 five seconds; and their movement caused the whole polyzoary to 



