CHAPTER VII - MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION
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 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 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 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 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 as the chelae of a lobster; and all these
gradations can be actually traced.
Besides the avicularia, the Polyzoa possess curious organs called
vibracula. These generally consist of long bristles, capable of
movement and easily excited. In one species examined by me the
vibracula were slightly curved and serrated along the outer margin;
and all of them on the same polyzoary often moved simultaneously; so
that, acting like long oars, they swept a branch rapidly across the
object-glass of my microscope. When a branch was placed on its face,
the vibracula became entangled, and they made violent efforts to
free themselves. They are supposed to serve as a defence, and may be
seen, as Mr. Busk remarks, "to sweep slowly and carefully over the
surface of the polyzoary, removing what might be noxious to the
delicate inhabitants of the cells when their tentacula are protruded."
The avicularia, like the vibracula, probably serve for defence, but
they also catch and kill small living animals, which it is believed
are afterwards swept by the currents within reach of the tentacula
of the zooids. Some species are provided with avicularia and
vibracula; some with avicularia alone, and a few with vibracula alone.
It is not easy to imagine two objects more widely different in
appearance than a bristle or vibraculum, and an avicularium like the
head of a bird; yet they are almost certainly homologous and have been
developed from the same common source, namely a zooid with its cell.
Hence we can understand how it is that these organs graduate in some
cases, as I am informed by Mr. Busk, into each other. Thus with the
avicularia of several species of Lepralia, the moveable mandible is so
much produced and is so like a bristle, that the presence of the upper
or fixed beak alone serves to determine even its avicularian nature.
The vibracula may have been directly developed from the lips of the
cells, without having passed through the avicularian stage; but it
seems more probable that they have passed through this stage, as
during the early stages of the transformation, the other parts of
the cell with the included zooid could hardly have disappeared at
once. In many cases the vibracula have a grooved support at the
base, which seems to represent the fixed beak; though this support
in some species is quite absent. This view of the development of the
vibracula, if trustworthy, is interesting; for supposing that all
the species provided with avicularia had become extinct, no one with
the most vivid imagination would ever have thought that the
vibracula had originally existed as part of an organ, resembling a
bird's head or an irregular box or hood. It is interesting to see
two such widely different organs developed from a common origin; and
as the moveable lip of the cell serves as a protection to the zooid,
there is no difficulty in believing that all the gradations, by
which the lip became converted first into the lower mandible of an
avicularium and then into an elongated bristle, likewise served as a
protection in different ways and under different circumstances.
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