Sea Urchin Anatomy: Labeled Diagrams and Interactive 3D Model

A sea urchin is an echinoderm, a relative of sea stars and sea cucumbers. Its body is a hollow globe with the mouth in the middle of the flat underside (the oral surface) and the anus at the top (the aboral surface). Except for a thin skin, all of its soft anatomy is inside the test (Fox).^{[1, 30]}

Everything comes in fives: five double rows of tube feet, five teeth, five gonads, five radial canals and five radial nerves.^{[1, 3]}

This page describes the purple sea urchin, Strongylocentrotus purpuratus, which lives on rocky shores of the Pacific coast of North America and is a standard animal in embryology classes. Its anatomy is typical of the "regular" urchins. Where the green urchin (S. droebachiensis), Arbacia, Diadema, pencil urchins (cidaroids) or the irregular sand dollars and heart urchins differ, the text says so. Irregular urchins are flattened, have short spines and a superficial bilateral symmetry, and most have a reduced lantern (Fox; UCMP).^{[26, 21, 1, 30]}

Two directions matter: oral (towards the mouth) and aboral (towards the anus). The five ambulacra, the zones with tube feet, run from mouth to apex like lines of longitude; the five interambulacra lie between them. Biologists number them from the madreporite, which sits in interambulacrum 2.^{[3, 6]}

A sea urchin test is an internal skeleton of thin calcite plates fused into a rigid shell. In a regular urchin the plates form ten double columns from mouth to apex: five ambulacral zones pierced by pore pairs for the tube feet, and five interambulacral zones without pores. A thin living skin covers it (Fox; NHM Echinoid Directory).^{[1, 3]}

At the top, a ring of ten plates called the apical disc surrounds the periproct, the flexible membrane that holds the anus. Five large genital plates sit on the interambulacra, each with a gonopore; one of them, genital plate 2, is enlarged and sieve-like and forms the madreporite. Five small ocular plates sit at the tops of the ambulacra; each has a tiny pore for the end of a radial canal (NHM Echinoid Directory). New ambulacral plates are added just below the ocular plates as the urchin grows (Turner, 2026).^{[6, 19]}

On the underside, the peristome is a soft membrane covering a large hole in the test, with the mouth at its centre. It carries five pairs of small buccal plates, each with a tube foot. Around its edge, inside the test, the perignathic girdle forms five tall arches called auricles that anchor the muscles of the lantern (NHM Echinoid Directory; Fox).^{[7, 1]}

Sea urchin spines defend the animal and help it move and wedge itself into rock; pedicellariae are tiny stalked pincers between the spines that keep the surface clean and attack small intruders. Each spine sits on a ball-shaped tubercle and is moved by a ring of muscle, while collagen fibres can lock it rigidly in place (Fox; NHM Echinoid Directory).^{[1, 7]}

Purple urchin spines are short, rarely more than 2.5 cm long (Walla Walla University). Adults have bright purple spines, while juveniles under about 30 mm are often pale green (UCSC MARINe). Broken spines regrow (ASU Ask A Biologist). Pedicellariae have three jaws on a stalk. Urchins have four main types: globiferous pedicellariae with venom glands, ophicephalous, tridentate and triphyllous; all four can occur on the same animal (Ehlert-Flaskämper et al., 2025). Small glossy sphaeridia on the ambulacral plates are thought to sense balance (Thompson et al., 2021).^{[26, 28, 24, 14, 18]}

Sea urchin tube feet are hydraulic. Fluid thought to enter through the madreporite runs down the stone canal to a ring canal on top of Aristotle's lantern, then along five radial canals up the ambulacra. Squeezing the flat ampulla inside the test pushes fluid into its tube foot and extends it (Fox; University of Washington).^{[1, 21]}

Each tube foot connects to its ampulla through two pores in the test, which is why the pores come in pairs. Urchin ampullae are flat, standing on edge like the pages of a book, rather than round bulbs as in a sea star (Fox). The suckers are stiffened by tiny calcite plates, and extended tube feet are usually longer than the spines. Strongylocentrotus has Tiedemann's bodies on its ring canal but no Polian vesicles (Fox).^{[1, 2]}

Aristotle's lantern is the sea urchin's jaw apparatus: five interlocking jaws called pyramids, each holding one long tooth, surrounding the mouth inside the test. Muscles push the lantern out to scrape algae off rock and pull it back in. Aristotle described it in his History of Animals, and its shape recalls a five-sided horn lantern (ASU Ask A Biologist).^[25]

Each pyramid is two wedge-shaped hemipyramids joined in the middle, with a notch, the foramen magnum, at the top of their joint. Small axe-shaped epiphyses cap each pyramid, a tongue-shaped rotula lies between neighbouring pyramids, and a narrow compass rests on each rotula (NHM Echinoid Directory). Sources count the pieces differently: the NHM gives about 50 skeletal elements and 60 muscles, while Fox gives 40 ossicles (five jaws of eight) and 60 muscles.^{[4, 1]}

The teeth wear down at the tip and grow continuously from a soft upper end held in a dental sac (Fox). In camarodont urchins, the group that includes the purple urchin, about 1.5 mm of tooth is replaced each week; the tooth is self-sharpening, and its "stone" part has the most magnesium (Stock, 2014). Purple urchins use their teeth, and to a lesser extent their spines, to grind shelter pits in rock (Troha et al., 2024). Strongylocentrotus has a camarodont lantern, with a deep V-shaped foramen magnum, enlarged epiphyses that meet above it, and T-shaped teeth (NHM Echinoid Directory).^{[1, 12, 13, 5]}

Inside a sea urchin, the large body cavity (the perivisceral coelom) is filled with fluid. Aristotle's lantern sits in the centre of the floor, the stomach and intestine loop around the inner wall, five gonads hang under the top of the test, and the axial complex runs from the madreporite down to the lantern (Ziegler et al., 2008; Fox).^{[8, 1]}

Food takes a long route. It passes from the mouth up the pharynx inside the lantern, then along the oesophagus to the wall of the test. There the gut widens into the stomach, which makes one full circle around the lower part of the test, counterclockwise when seen from above. At its end the gut reverses as the intestine, which circles back clockwise, higher on the wall, and finally the rectum rises to the anus (Fox). A thin tube, the siphon, bypasses the stomach and probably carries off swallowed water so it does not dilute the digestive enzymes (Fox; Holland & Ghiselin, 2009).^{[1, 10]}

A sea urchin has no heart like ours, but it has a small pulsating vessel at the top of its axial organ, just under the madreporite. Colourless haemal fluid runs in a ring around the top of the lantern and in vessels along the gut; most transport is done by the body-cavity fluid (Fox; Smith & Hill, 2025).^{[1, 16]}

The axial organ is a soft brown spindle wrapped around the stone canal. Its function has long been debated: it has been called a heart, a contractile vessel and a source of immune cells. Echinoderms are generally said to have no excretory organs (Fox), but genes of the filtering cells of vertebrate kidneys are active only in the urchin axial organ, so Smith & Hill (2025) propose that it filters waste, which then leaves through the gut. The pulsating vessel beats about five times a minute according to Fox; in S. purpuratus it has been described as two contractile chambers (cited by Ziegler et al., 2009).^{[1, 16, 9]}

A sea urchin has no brain. Its nervous system is a nerve ring around the pharynx at the base of Aristotle's lantern, from which five ribbon-like radial nerves run up the inside of the ambulacra, between the radial canals and the test (Fox; University of Washington). It has no eyes either, but its tube feet sense light.^{[1, 21, 11]}

In the purple urchin, each tube foot has up to 140 light-sensitive cells at the rim of its sucker and at its base. The basal cells sit in a cup of the opaque skeleton, which shades them from one side, so Ullrich-Lüter et al. (2011) proposed that the whole urchin works like a huge compound eye. A newer gene-expression study of juvenile urchins found that most of the body has a "head-like" identity and describes sea urchins as all-brain rather than no-brain (Paganos et al., 2025).^{[11, 15]}

Sea urchins breathe mainly through their tube feet. Oxygen crosses the thin walls of the feet into the water vascular system, and the flat ampullae inside the test pass it on to the body-cavity fluid (Fox; Walla Walla University). Regular urchins also have five pairs of bushy gills around the peristome.^{[1, 26]}

Sources stress different jobs for these peristomial gills. Fox describes them as the respiratory organs of the lantern coelom, supplying oxygen to the hard-working lantern muscles. The NHM Echinoid Directory explains them as outpocketings that buffer the change in volume inside the rigid test when the lantern moves in and out; pencil urchins (cidaroids) lack them.^{[1, 7]}

Male and female purple sea urchins look the same from outside (UCSC MARINe). Each has five gonads, ovaries or testes, against the interambulacra, and each gonad opens through a gonoduct to a gonopore on top of the test. Fertilisation happens in the water: females release orange eggs and males release milky white sperm (Fox; University of Washington).^{[28, 1, 23]}

In Strongylocentrotus both ovaries and testes look orangish; only the gametes inside differ (Fox). In the breeding season the gonads can fill up to 80% of the space inside the test (University of Washington), and most purple urchins spawn from January to March (Walla Walla University; Animal Diversity Web). Each gonad is a mass of small sacs filled with nutritive cells that store food reserves before the eggs or sperm develop; these gonads are the seafood sold as uni (Rodríguez & Olmos, 2026). The larva, the echinopluteus, is bilaterally symmetrical and turns into a five-sided juvenile at metamorphosis (University of Washington; Fox).^{[1, 22, 21, 26, 27, 17]}

Sea urchins and starfish share the echinoderm body plan: five-part symmetry, a water vascular system with a madreporite, tube feet and a calcite skeleton in the skin. In an urchin the ambulacra have spread up over the body until they enclose almost all of it, leaving only the small periproct at the top (Fox).^{[1, 2]}

The urchin's plates are also fused into a rigid test instead of a flexible grid (UCMP). Compare with our 3D starfish model, or see how other body plans differ in comparative anatomy.^[30]

Sea urchin facts in this table come from Fox's Strongylocentrotus exercise, starfish facts from his Asterias exercise.^{[1, 2]}

The purple sea urchin's test is usually about 5 cm across and at most about 10 cm (Walla Walla University; Animal Diversity Web; UCSC MARINe); the Monterey Bay Aquarium gives up to 7 cm. It lives from Alaska to Baja California, from the low intertidal to 160 m (Walla Walla University; UCSC MARINe), often in pits ground into rock.^{[26, 27, 28, 29]}

Purple urchins eat algae, especially drifting giant kelp, but hungry urchins graze living kelp down to the holdfast and can turn a kelp forest into an "urchin barren" (UCSC MARINe; Monterey Bay Aquarium). Sea otters, sunflower stars and California sheephead eat them (Monterey Bay Aquarium). They grow slowly, reaching large size in about 10 years (Walla Walla University); Animal Diversity Web gives an average of 20 years in captivity, and the related red sea urchin can live for more than a century (Amir et al., 2020).^{[28, 29, 26, 27, 20]}

The text version of the 3D model: every labeled structure of the purple sea urchin, grouped by body system, with what it does.