228 MISCELLANEOUS OBJECTIONS TO TEE 



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 ob- 

 served 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 

 supposed difficulty of organs, namely the avicularia of 

 the Polyzoa and the pedicellariae of the Echinodermata, 

 which lie considers as ^^ essentially similar,^' having been 

 developed through natural selection in widely distinct 

 divisions of the animal kingdom. But, as far as struct- 

 ure is concerned, I can see no similarity between tridac- 

 tyle pedicellariae and avicularia. The latter resembles 

 somewhat more closely the chelae or pincers of Crusta- 

 ceans; 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 movable mandible of 

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

 any gradations now existing between a zooid and an avicu- 

 larium. 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 



