What a wonderful piece of mechanism is a Sea-Urchin! Accustomed as I
am to the multitudinous contrivances and compensations that present
themselves at every turn to the philosophic naturalist, often as
surprising and unexpected as they are beautifully effective, I am yet
struck with admiration at the structure of an _Echinus_ whenever I
examine it anew. A globular hollow box has to be made, of some three
inches in diameter, the walls of which shall be scarcely thicker than
a wafer, formed of unyielding limestone, yet fitted to hold the soft
tender parts of an animal, which quite fill the concavity at all ages.
But in infancy the animal (and, of course, its box, as this must be
full) is not so big as a pea; and it has to grow till it attain its
adult dimensions. The box is never to be cast off, and replaced by a
new one; the same box must hold the infant and the veteran Urchin. The
limestone, not being a living tissue, but an inert earth, can grow
only by being deposited. Now the vascular tissues are within, and the
particles they deposit must be on the interior walls. This would indeed
augment the amount of limestone in the box, but it would be at the
expense of the contained space. The thicker the walls, if thickened
from within, the less room in the cavity; but what is wanted is _more_
room, ever more, and more. The growing animal feels its tissues
swelling day by day, by the assimilation of food, and its cry is, “Give
me space! a larger house, or else I die.”
How is this problem solved? Ah! there is no difficulty. The
inexhaustible wisdom of Jehovah the Creator has invented a beautiful
contrivance for the emergency. The box is not made in one piece, nor
in ten, nor in a hundred; six hundred distinct pieces go to make up
the hollow case; all so accurately fitted together, that the perfect
symmetry of the outline is not broken; and yet, thin as their substance
is, they retain their relative position with unchanging exactness, and
the slight brittle box possesses all requisite strength and firmness.
Each of these symmetrical pieces of shell is enveloped by a layer of
living flesh, a vascular tissue of exceeding thinness, which passes
up between the joints where one meets another, on every side, and not
only so, but actually spreads itself over the whole exterior surface.
So that when you take up an Urchin into your hand, and having rubbed
a small space clear of spines, look on it; you have not, as you may
suppose you have, exposed the surface of the shelly box, but only the
flesh that covers it; yet this is so transparent and colourless, so
inconceivably thin, so absolutely adherent at every point, that its
presence will not be discernible to feeling or sight, without the aid
of high microscopic powers.
This being so, the glands of the investing fleshy tissue secrete
lime from the sea-water, which holds it in solution, and constantly
deposits it, after a determinate and orderly pattern, on every part
of the surface of each shelly piece; the inner face, the outer face,
and each of the sides and angles of the polyhedron grow together, and
all so evenly, that while the dimensions increase, both of thickness
and superficies, the form characteristic of that individual piece is
maintained with immutable mathematical precision. Thus the volume and
capacity of the box grow with the growth of the individual segments,
and it ever keeps the globose shape at first imposed upon it.