196 THE ORIGIN OF SPECIES 



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 backward and forward, with the lower 

 jaw widely open, through an angle of about 90 degrees, 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 sup- 

 posed difficulty of organs, namely the avicularia of the Polyzoa 

 and the pedicellariae of the Echinodermata, which he considers as 

 "essentially similar," having been developed through natural selec- 

 tion in widely distinct divisions of the animal kingdom. But, as far 

 as structure is concerned, I can see no similarity between tridactyle 

 pedicellariae and avicularia. The latter resembles somewhat more 

 closely the chelae or pincers of Crustaceans; and Mr. Mivart might 

 have adduced, with equal appropriateness, this resemblance as a 

 special difficulty, or 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 corre- 

 sponding with the lower and movable mandible 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 

 conjecture 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 avicu- 

 laria 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 

 penultimate segment, or against one whole side, and is thus en- 

 abled to catch hold of an object, but the limb still serves as an 

 organ of locomotion. We next find one corner of the broad penul- 

 timate segment slightly prominent, sometimes furnished with irreg- 

 ular 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. 



