Sea Urchin
2 of the 7 encyclopedias on this shelf carry an entry for Sea Urchin. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
a marine relative of the starfish. A hollow apple with a mouth at the stem end and a second opening at the blossom end, surrounded by a rigid shell covered with spines like a chestnut bur, is a loose, rambling description of the sea-urchin. Like the starfish it moves by muscular water tubes controlled from a central reservoir. It can climb steep surfaces and hold to timbers or the faces of rocks. In addition to being a genuine curiosity, the sea-urchin is an animal of first importance to scientific investigators. Its eggs and spermatozoa lend themselves very well to the investigation of laws of animal development. In Italy the eggs of the sea-urchin are eaten and considered a delicacy at table. See Starfish Sea Water. It is well known that the water of the ocean is much denser than fresh water, owing to minerals in solution. Few people have any idea, however, of the large amount of mineral matter that has been washed out of the lands by rains and carried by streams to the sea. The ocean covers about 145,000,000 square miles. The average depth is thought to be not far from two miles. In that case, the cubical content of the ocean is not far from 290,000,000 cubic miles. The following statistics relating to the contents of a cubic mile of sea water are gleaned from an interesting article contributed by F. E. Mariner to Popular Mechanics: Cubic feet 146,197,952,000 Weight, tons 4,460,544,000 Gold content at 1 grain per ton $178,421,700 Common salt, tons 117,089,280 Value, at $5 per ton $585,446,400 Magnesium chloride, tons 14,050,713 Value $280,000,000 Potassium chloride, tons 5,520,285 Value $220,811,400 Gypsum, tons 7,181,475 Value $20,000,000 Bromide of sodium, tons 1,717,209 Value $858,604,500 Total mineral value of cu. mile $2,143,484,000
Collier's New Encyclopedia (1921)
one of the Echinoids, or Echinidæ, an order of Echinodermata. They have the body covered with a calcareous crust or shell, of an extremely porous structure, in polygonal plates nicely adapted to each other, and increasing by additions to the edges of each plate, so that the shell may enlarge with the enlargement of the animal, while new plates are also added around the superior orifice. The shell is pierced with rows of holes for the ambulacra, and is externally covered in a living state with a membrane, sometimes very delicate, sometimes thick and spongy, and unites the bases of all the spines. The spines differ very much in the different genera and species . They are attached to tubercles on the surface of the shell by cup-like bases capable of working upon the tubercles in the manner of a ball-and -socket joint; and they are moved by means of the connecting membrane so as to be employed in locomotion. By means of the spines, some can walk even on dry ground; others employ them in burying themselves in the sand. The mouth of the Echinidæ is situated at the lower orifice of the shell, and is generally furnished with five flat calcareous teeth moved by a very complex apparatus of bony sockets and muscles. Their food is supposed to consist of small crustaceans and mollusks. They abound in all seas.