180 MISCELLANEOUS OBJECTIONS TO THE [CHAP. VII. 



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 

 pedicellariae of the Echinodermata, which he considers as " essen- 

 tially similar," having been developed through natural selection 

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

 as structure is concerned, I can see no similarity between tridac- 

 tyle pedicellariae and avicularia. The latter resemble 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 moveable lip or lid of the cell 

 corresponding with the lower and moveable 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 

 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 penultimate 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 prominent, 

 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 







