Echinoids come in a variety of different shapes, sizes, and color, but like all echinoderms, they have pentamerous radial symmetry (with the exception of irregular echinoids that have bilateral symmetry) and spiny skin. One of the smallest recorded urchins, Echinocyamus scaber, has a test diameter of only 0.2 in (0.5 cm), whereas the largest, Sperosoma giganteum, a deep-water species, has a test diameter up to 15 in (38 cm).
Echinoids have a hard calcareous shell made up of a skeleton of tightly packed or fused plates called a test. The skeleton is designed to prevent cracks from spreading if damaged. All urchins are covered with moveable spines that articulate like a ball and socket joint. However, their morphology is highly variable between species. Some range from the thick blunt spines of the pencil urchin, Eucidaris tribuloides, to the long poisonous spines of Diadema antillarum. Spines are primarily used for locomotion and defense against predators, although some species cover them with shell fragments, algae, or encrusting organisms to camouflage themselves from visual predators or, alternatively, to provide shade from direct sunlight.
The echinoid test has tiny pincer-like structures called pedicellariae that, in some species, are poisonous. These species tend to have fewer spines and use their pedicellariae in defense, while others use them to clear away settling microorganisms, unwanted parasites, and detritus. Sometimes they are visible to the naked eye, e.g., the stalked globular-shaped pedicellariae of the West Indian sea egg, Lytechinus variegatus.
As in other echinoderms, echinoids posses rows of tube feet that go from the very top of the anus down to the top of the mouth and they use them to trap food such as detritus and algae, assist in locomotion, prey capture, adhere to substrata and kelp stipes, and even respiration. On the dorsal (upper) surface of the test, a small sieve-like or perforated plate, madreportite, allows seawater into the water vascular system,
which is a canal of tubes running through the body to the tube feet. Following muscular contraction, seawater is drawn into the canal under pressure and directed at the tube feet, which then extend under its force. In most species, the anus is located on the dorsal surface. In some species the distinctive coloration of the anus is used by biologists and naturalist to identify differences between species. Diadema steosum, for example, has an orange ring around its anus, which is the only visible distinction from D. savignyi, which has an iridescent blue ring.
In regular urchins, the mouth is located on the aboral (opposite to the mouth) surface and consists of an array of very tough calcium carbonate plates embedded in tissue and five teeth arranged symmetrically. The entire feeding apparatus has a characteristic pattern has given rise to the name "Aris totle's lantern," a term used because of the resemblance to a five-sided Greek lantern and because urchins and sand dollars were a favorite animal of Aristotle. Some species such as the rock boring urchin, Echinostrephus aciculatus, use the teeth to rasp away at rock to form a cup-shaped hiding place. Urchins have a well-developed digestive system because they process a large amount of indigestible material such as sand, stones, and plant matter.
The test shape of regular urchins tends to be globular and pentaradiate, whereas that of irregular urchins tends to have a more flattened, oval, and bilateral symmetry. Their test shape generally reflects their mode of life. Irregular urchins, for example, are flattened for burrowing efficiently into sediment, and are covered in very short spines to aid feeding when beneath the seabed surface. Moreover, some species (e.g., sand dollar, Leodia sexiesperforata) have a series of slits or perforations called lunules. These may prevent it from being washed out of the sediment by the pressure of a passing wave surge. The internal organization of irregular urchins is generally similar to regular echinoids. The exception is that regular urchins have five pairs of gonads, whereas irregular urchins have between two and five.
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