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 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.
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