300 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII. 



In certain genera of star-fishes these organs, instead 

 of being fixed or borne on an immoveable 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 ab- 

 stract of Mr. Agassiz's interesting observations on the de- 

 velopment of the pedicellariffi. All possible gradations, 

 as he adds, may likewise be found between the pedi- 

 cellarise of the star-fishes and the hooks of the Ophiuri- 

 ans, another group of Bchinodermata; and again be- 

 tween 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 the head and beak of 

 a vulture in miniature, seated on a neck and capable 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 tremble. 

 When the Jaws are touched with a needle they seize it 

 so firmly that the branch can thus be shaken. 



Mr. Mivart adduces this case, chiefly on account of 

 the supposed difficulty of organs, namely the avicularia 

 of the Polyzoa and the pedicellaris of the Echinoder- 



