(Echinus.) A kind of shell-fish or sea-egg covered with prickles like a chestnut bur, and closely related to the star-fish. The shells are of the thickness of egg-shells, and have rows of dots or knobs with lace-work between, and are made up of hundreds of plates joined like mosaic work. The animals do not cast their shells like crabs, but the flesh secretes lime from sea water, and deposits it round the plates, which increase uniformly. The spines are beautifully carved columns with ball-and-socket joints on the knobs. There are also protruding through holes double rows of tube-like feet, which are supplied with water by a tube opening at the top of the shell. In the centre of the shell is a tube-like stomach opening to the top of the shell. The sea-urchin has also a mouth, intestines, heart, and five teeth. There are many kinds.