When it is desired to preserve starfishes for future study, immersion in diluted spirit or a solution of formaldehyde will answer all purposes, the soft parts being thus preserved as well as the harder structures; but it is usual to preserve them in a dry state when they are required merely for purposes of identification, as is usually the case with the specimens in an ordinary museum collection. In the latter case it is advisable to put the starfishes in strong spirit for a few days, changing the spirit if several specimens are put together, and then drying them as quickly as possible in the open air.
Fig. 108.—Section of the Spine of a Sea Urchin
We have now to consider the Sea Urchins or Sea Eggs, which are readily known by the hedgehog-like covering of hard spines. Externally they appear as globular or heart-shaped bodies, the surface entirely hidden by spines except, perhaps, the mouth on the under side, which is provided with an apparatus for mastication. If alive, and in the water, one may notice that the animal creeps along the bottom, mouth downwards, moving itself either by means of its moveable spines, or by soft tube-feet resembling those of starfishes, that are protruded between the spines, or by both combined; and the movements of its masticating organ may be seen by observing the animal through the side or bottom of a glass vessel of sea water. The last-named organ is surrounded by an area of soft skin, and is not present in all species.
A closer examination of the common globular urchin will show that it is wonderfully constructed. Even the spines, which are in themselves uninteresting objects to the naked eye, are most beautifully formed, a transverse section revealing a radiate or reticulated structure when viewed through the microscope. Each spine has a concave base which fits on a little tubercle of the calcareous shell or test that covers the body of the animal, forming a perfect ball-and-socket joint, and is capable of being moved in any direction by means of small muscular bands.
Fig. 109.—Sea Urchin with Spines Removed on one side