Small sea urchin from the Philippines, suitable for airplants.

Sea urchins (Echinoidea) are spiny aquatic animals belonging to the phylum echinoderms. Other well-known members of this tribe are the starfish and sea cucumbers. There are about 850 species of sea urchins that are common in tropical waters and coral reefs, as well as in the Netherlands and Europe. They are also known as sea apples, sea tangles and sand dollars. Some sea urchins are poisonous, but even without poison it is annoying to accidentally step on a sea urchin. The hollow spines are difficult to remove and can lead to inflammation. Sea urchins are usually between three and six centimeters in size. The smallest sea urchin is 6 millimeters, the largest has a diameter of 36 centimeters. The marine animals can be divided into regular sea urchins (Regularia) and irregular sea urchins (Irregularia).