" Paradoxical
as this may appear to Mr. Mivart, tridactyle forcepses, immovably fixed at
the base, but capable of a snapping action, certainly exist on some star-
fishes; and this is intelligible if they serve, at least in part, as a
means of defence. Mr. Agassiz, to whose great kindness I am indebted for
much information on the subject, informs me that there are other star-
fishes, in which one of the three arms of the forceps is reduced to a
support for the other two; and again, other genera in which the third arm
is completely lost. In Echinoneus, the shell is described by M. Perrier as
bearing two kinds of pedicellariae, one resembling those of Echinus, and
the other those of Spatangus; and such cases are always interesting as
affording the means of apparently sudden transitions, through the abortion
of one of the two states of an organ.
With respect to the steps by which these curious organs have been evolved,
Mr. Agassiz infers from his own researches and those of Mr. Muller, that
both in star-fishes and sea-urchins the pedicellariae must undoubtedly be
looked at as modified spines. This may be inferred from their manner of
development in the individual, as well as from a long and perfect series of
gradations in different species and genera, from simple granules to
ordinary spines, to perfect tridactyle pedicellariae. The gradation
extends even to the manner in which ordinary spines and the pedicellariae,
with their supporting calcareous rods, are articulated to the shell.
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