SEA-URCHIN. -581 
of the Radiates, such as the sea-urchin, the whole body is. encrusted with a 
chalky coat, while in others it is as soft and easily torn as if it were composed 
of mere structureless gelatine. ; 
The mode of walking, or rather creeping, which is practised by these 
beings, is very interesting, and may be easily seen by watching the proceed- 
ings of a common star-fish when placed in a vessel of sea-water. At first it 
will be quite still, and lie as if dead, but by degrees the tips of the arms will 
be seen to curve slightly, and then the creature slides forward without any 
perceptible means of locomotion. If, however, it be suddenly taken from the 
water and reversed, the mystery is at once solved, and the walking apparatus 
is seen to consist of a vast number of tiny tentacles, each with a little round 
transparent head, and all moving slowly but continually from side to side, 
sometimes being thrust out to a considerable distance, and sometimes being 
withdrawn almost wholly within the shell. These are the “ambulacre,” or 
walking apparatus, and are among the most extraordinary means of progres- 
sion in the animal kingdom. Each of these innumerable organs acts as a 
sucker, its soft head being applied to any hard substance, and adhering 
thereto with tolerable firmness, until the pressure is relaxed and the sucker 
released. The suckers continually move forward, seize upon the ground, 
draw the body gently along, and then search for a new hold. As there are 
nearly two thousand suckers continually at work, some being protruded, 
others relaxed, and others still feeling for a holding-place, the progress of the 
creature is very.regular and gliding, and hardly seems to be produced by 
voluntary motion. 
We will now proceed to some examples of these curious beings. 
WE first take a beautiful family of this order, called Echinida, because 
they are covered with spines like the quills of the hedgehog. Popularly they 
are known by the name of SEA-URCHINS, or SEA-EGGS. 
In all these curious beings the upper parts are protected by a kind of shell, 
always more or less dome-shaped, but extremely variable in form. The shell 
is one of the most marvellous structures in the animal kingdom, and the 
mechanical difficulties which are overcome in its 
formation are of no ordinary kind. In the case 
of the common SEA-EGG, which is shown on the 
following illustration, the shell is nearly globular. 
Now, this shell increases in size with the age of \\ 
the animal; and how a hollow spherical shell WN 
can increase regularly in size, not materially 3 
altering its shape, is a problem of extreme 
difficulty. It is, however, solved in the following 
manner. The shell is composed of a vast number 
of separate pieces, whose junction is evident when 
the interior of the shell is examined, but is almost : 
entirely hidden by the projections upon the outer SEAPURSHIN. techni: 
surface. These pieces are of a hexagonal or pen- phere) 
tagonal shape, with a slight curve, and having mostly two opposite sides 
much longer than the others. As the animal grows, fresh deposits of 
chalky matter are made upon the edges of each plate, so that the plate 
increases regularly in size, still keeping its shape, and in consequence 
the dimensions of the whole shell increase, while the globular shape is 
preserved. If a fresh and perfect specimen be examined, the surface is 
seen to be covered with short sharp spines set so thickly that the substance 
of the shell can hardly be seen through them, The structure of these 
spines is very remarkable, and under the microscope they present some 
most interesting details, Moreover, each spine is movable at the will of the 
