er Agassiz has seen an Echinus or sea-urchin
rapidly passing particles of excrement from forceps to forceps down
certain lines of its body, in order that its shell should not be fouled.
But there is no doubt that besides removing dirt of all kinds, they
subserve other functions; and one of these apparently is defence.

With respect to these organs, Mr. Mivart, as on so many previous
occasions, asks: "What would be the utility of the FIRST RUDIMENTARY
BEGINNINGS of such structures, and how could such insipient buddings
have ever preserved the life of a single Echinus?" He adds, "not even
the SUDDEN development of the snapping action would have been beneficial
without the freely movable stalk, nor could the latter have been
efficient without the snapping jaws, yet no minute, nearly indefinite
variations could simultaneously evolve these complex co-ordinations of
structure; to deny this seems to do no less than to affirm a startling
paradox." 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. In cert