Among the Urchins proper there are some species, such as the Sphere
Sea-egg, and the one known as Fleming’s, which have a figure not far
from that of a globe. Others are much more depressed, of which the
little Purple-tipped is a notable example. Still the spherical shape
is conspicuous. From this rounded form other species, more and more
flattened, gradually lead to the _Scutella_, which takes the form of
a thin round plate, quite flat beneath, but slightly convex on the
upper surface. The structure is the same as before, but the spines
appear to the naked eye only as very minute hairs; but, when magnified,
are found to be of the most elaborate workmanship, each having a
movable socket-joint. In the genus _Clypeaster_, the round outline
is changed for a five-sided figure, the angles of which in succeeding
species project more and more, and the spaces between become more and
more indented, till we arrive at the Starlets, and at length to the
Cross-fishes (_Asteriadæ_). The rays gradually becoming longer and
more slender, we are brought to those in which they are so lengthened
as to resemble the tails of so many serpents, whence they are named
_Ophiura_. In succeeding genera, such as that called Medusa’s head
(_Gorgonocephalus_), the central part is still further diminished, and
the rays are divided into branches of great length and number. Each
ray, soon after its commencement, separates into two more; these again
into two others, and so on to an astonishing extent. Upwards of 2500
ramifications have been counted on a single specimen, presenting a
living net, by the contraction of which any small animal once touched
would inevitably be detained. The sucker-feet are no longer found,
these animals changing their position by dragging themselves along by
their flexible arms. Finally, we have the Feather-stars, which, as
we have seen, in their infant condition, and the Lily-stars, which
throughout life (as the abundant fossil species in our own land, and
that noble one which still exists in the West Indian seas), consist
of slender-jointed arms, with feather-like filaments, seated at the
free extremity of a tall jointed stalk, also furnished with whorls of
filaments, which is fixed by its base to the solid rock.
Altogether, the series presents us with one of the most instructive
and most marvellous examples of the vast variety of external form
and internal structure which may be assumed by almost insensible
modifications of one plan of organization; and so of the unfathomable
resources of wisdom in the ever-blessed God. For every one of these
links,--and there are multitudes which I have not named, found either
in remote seas or in a fossil condition, that fill up the gaps with
close gradations--displays an essentially common model; and the least
fragment of the stony skeleton of any one would be sufficient to
enable a competent naturalist to decide authoritatively, the instant
he looked at it beneath his microscope, that it belonged to the great
class ECHINODERMATA. The calcareous shell of which the framework is
composed,--a glass made of lime,--is deposited in a fashion which,
while common to all, is found nowhere else throughout the whole animal
kingdom.