ain genera of star-fishes, "the very
combinations needed to show that the pedicellariae are only modified
branching spines" may be found. Thus we have fixed spines, with three
equi-distant, serrated, movable branches, articulated to near their
bases; and higher up, on the same spine, three other movable branches.
Now when the latter arise from the summit of a spine they form, in fact,
a rude tridactyle pedicellariae, and such may be seen on the same spine
together with the three lower branches. In this case the identity in
nature between the arms of the pedicellariae and the movable branches
of a spine, is unmistakable. It is generally admitted that the ordinary
spines serve as a protection; and if so, there can be no reason to doubt
that those furnished with serrated and movable branches likewise serve
for the same purpose; and they would thus serve still more effectively
as soon as by meeting together they acted as a prehensile or snapping
apparatus. Thus every gradation, from an ordinary fixed spine to a fixed
pedicellariae, would be of service.

In certain genera of star-fishes these organs, instead of being fixed
or borne on an immovable support, are placed on the summit of a flexible
and muscular, though short, stem; and in this case they probably
subserve some additional function besides defence. In the sea-urchins
the steps can be followed by which a fixed spine becomes articulated to
the shell, and is thus rendered movable. I wish I had space here to
give a fuller abstract of Mr. Agassiz's interesting observations on the
development of the pedicellariae. All possible gradations, as he adds,
may likewise be found between the pedicellariae of the star-fishes and
the hooks of the Ophiurians, another group of the Echinodermata; and
again between the pedicellariae of sea-urchins and the anchors of the
Holothuriae, also belonging to the same great class.

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,
i