828 THE ORIGIN OF SPECIES 



even their resemblance to the head and beak of a bird. 

 The avicularia are believed by Mr, Busk, Dr. Smitt, and 

 Dr. Nitsche — naturalists who have carefully studied this 

 group — to be homologous with the zooids and their cells 

 which compose the zoophyte; the movable lip or lid of 

 the cell corresponding with the lower and movable man- 

 dible of the avicularium. Mr. Busk, however, does not 

 know of any gradations now existing between a zooid 

 and an avicularium. It is therefore impossible to con- | 

 jecture by what serviceable gradations the one could 

 have been converted into the other; but it by no means 

 follows from this that such gradations have not existed. 



As the chelae of Crustaceans resemble in some degree 

 the avicularia of Polyzoa, both serving as pincers, it may 

 be worth while to show that with the former a long 

 series of serviceable gradations still exists. In the first 

 and simplest stage, the terminal segment of a limb shuts 

 down either on the square summit of the broad penulti- 

 mate segment, or against one whole side; and is thus 

 enabled to catch hold of an object; but the limb still 

 serves as an organ of locomotion. We next find one 

 corner of the broad penultimate segment slightly promi- 

 nent, sometimes furnished with irregular teeth; and 

 against these the terminal segment shuts down. By an 

 increase in the size of this projection, with its shape, as 

 well as that of the terminal segment, slightly modified 

 and improved, the pincers are rendered more and more; 

 perfect, until we have at last an instrument as efficient 

 as the chelae of a lobster; and all these gradations can 

 be actually traced. 



Besides the avicularia, the Polyzoa possess curious 

 organs called vibracula. These generally consist of long 



