214 MISCELLANEOUS OBJECTIONS TO THE 



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 minia- 

 ture, seated on a neck and capable of movement, as is like- 

 wise the lower jaw or mandible. In one species observed by 

 me, all the avicularia on the same branch often moved simul- 

 taneously 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 polyzo- 

 ary 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 "essentially 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 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 corresponding 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 con- 

 verted into the other, but it by no means follows from this 

 that sueh 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 locomo- 

 &}uu- \ye next find one corner of the broad penultimate 



