what is their real function. But, at last, Agassiz—the younger Agassiz, I believe—discovered that these curious organs, called pedicillariæ, are for keeping the spines clean.
The mouth of the sea-urchin is provided with five pointed teeth, which shut together on a common centre; and these teeth can all be removed together, and, thus removed, they present quite a curious appearance, and are known among naturalists as "Aristotle's lantern."
The shell, which is composed of hundreds of pieces, presents a very beautiful sight when the spines are removed. It is made up of ten segments, radiating from the mouth, and converging to a central region on the top (Fig. 27). Every alternate segment is perforated for the numerous locomotive suckers to pass out, the intermediate segments being imperforate, and more prominently marked with tubercles, on which spines are borne. At the termination of the five perforated segments there is a triangular plate with a minute opening; here the eye is situated. Alternating with these five plates are five larger ones, each with a hole, through which the eggs are laid. The largest of these plates is the madreporic body, corresponding perfectly to that seen on the starfish, already spoken of, and which doubtless acts as a sieve or water-filter.
As sea-urchins do not shed the shell, as do crabs and lobsters, the inquiring mind will naturally ask how the animal can continue to enlarge when once it is invested with a hard shell. The answer is, that every piece of the shell grows at the same time, and in this way the whole shell enlarges together, and in a perfectly symmetrical manner.
As already indicated, the sea-urchin moves by means of its locomotive suckers. Extending these beyond the spines, it lays hold of the surface of the rock or sea-weed, and then, contracting the suckers, pulls itself along. And these suckers can be extended quite a distance beyond the spines. For example, a sea-urchin can extend a sucker from near the top of the shell, and bend it over, and lay hold of the surface upon which the animal is resting.
The sizes and forms of sea-urchins are very numerous. The ordinary kinds are two or three inches in diameter; some of the elongated kinds in the tropics have a diameter of five inches or more. Some are nearly hemispherical; others rise in the centre so as to be almost a cone; others are flat, and are known as cake-urchins (Fig. 28); others are more or less heart-shaped, etc. Some of the cake-urchins are curiously modified, as seen in Fig. 29.
On the coast of Maine especially, but also along both shores of the Atlantic, as well as in most other parts of the world, the sea-shore visitor will find the "sea-cucumber" appearing not very unlike a cucumber while it still retains the blossom upon its end. On the coast of Asia it is known as trepang, and is the animal which the Chinese use so extensively for food. Aristotle called it Holothuria, but for what reason he does not tell us, and we can only conjecture. The dead
