544 
THE SEA-URCHINS, OR SEA-EGGS. 
We now come to the Sea Citcumbek, which has received its generic name from its great 
resemblance to that vegetable. The smaller species are appropriately named Sea Gherkins. 
The food of all these animals consists of marine mollusks and other small inhabitants of the 
sea. The complete but empty shells of several small mollusks have been found within the 
stomach of dissected specimens, proving that the creature must have swallowed the shell 
entire, and dissolved out its inhabitant by the process of digestion. 
It may as well be mentioned that the only vestige of a skeleton in these creatures is a ring 
of chalky substance surrounding the beginning of the intestinal canal, and formed of ten 
pieces, five large and as many small. To this curious ring are attached the longitudinal 
muscles of the body, by which the creature can lengthen or shorten itself at will, the expan- 
sion and contraction of the body being due to a series of transverse muscular fibres. The 
longitudinal muscles are ten in number, and are arranged in five pairs. 
Another example of the Cucumarise is termed Cucumaria lyatina. It is remarkable for 
its beautiful mouth, which is adorned with a crown of tentacles. 
A Cucumaria called Synapta is a more singular being. It derives its name from a Greek 
word signifying to seize hold of anything. This name is given to it because, when the hand is 
drawn over its surface, the skin is slightly arrested by some invisible agency. 
On taking off part of the skin of the Synapta and placing it under a microscope, a most 
wonderful sight is disclosed. The skin is furnished with a number of little tubercles on which 
are set numbers of tiny spicules, which look as if they were anchors for a fairy fleet. They 
are of extremely minute dimensions, and are quite invisible without the aid of a microscope, 
but never fail to excite admiration when they are well exhibited. Perhaps the best method 
of bringing out their beautiful shapes is by using a parabolic condenser or a spotted lens, as 
then their translucent glassy forms shine out against a dark background. 
These little objects are of exactly the same shape as the classic anchors of ancient times, 
and were it not for their extreme minuteness, the person who sees them for the first time is 
tempted to think that they have been manufactured by some ingenious impostor. But the 
hand of man is quite incapable of making these beautiful little objects, with their long shanks, 
their gracefully curved arms, and their sharply-pointed and regularly-serrated flukes. 
Nor are the anchors the only wonders which so appropriately deck the skin of a marine 
animal. If the little prominences can be neatly placed under the microscope without being 
rubbed, each anchor is found to be affixed by the end of the shank to the end of a curiously- 
formed shield, made of the same translucent substance as the anchor itself, and pierced with a 
perfectly regular pattern like ladies’ “ cut- work” embroidery. These shields hold the anchor 
in such a way that, as the shield lies flatly upon the skin, the flukes of the anchor are held in 
the air. The object of this remarkable arrangement is not known. 
There are several species of Synapta, all with the anchors and shields, but the pattern 
upon the shields is different in the various species, as is the shape of the anchor. These 
remarkable appendages have been compared by some authors to the little hooks on the calyx 
of the well-known burdock. Synaptas are abundant on the Reef, and one or more are found 
on the shores of New England. 
We now come to a new and beautiful family of this order, called Echinidse, because they 
are covered with spines like the quills of the hedgehog. Popularly, they are known by the 
name of Sea-urchixs, or Sea-eggs. The general shape of these curious beings can be best 
learned by reference to our colored illustration, which in every respect is most true to nature. 
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, as will be seen in the illustrations. 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, the shell is nearly globular. Now, this shell increases in size with the age 
of the animal ; and how a hollow spherical shell can increase regularly in size, not materially 
altering its shape, is a problem of extreme difficulty. It is, however, solved in the fol- 
lowing manner ; — 
