External structures
In the sea-urchins, the skeletal elements form a rigid theca or test. In the more primitive regularia, the globular test is smooth and radially symmetrical. The more advanced irregularia have secondary bilateral symmetry superimposed on the basic radial plan, giving them distinct anterior and posterior ends.

Test
The test is composed of interlocking plates radiating in rows from the apex round to the mouth in the centre of the under surface. The test is divided into 10 regular areas: alternating double columns of ambulacra and interambulacra. The ambulacral plates are distinguished by having pairs of pores through which the canals leading to the tube-feet pass (# echinoidea test). Echinoidea have a closed ambulacrum, so that the radial water vessels and the other radial structures are all internal to the skeleton. Generally, the interambulacra are conspicuously broader than the ambulacra ones. In each area there is usually a double series of primary tubercles. These tubercles carry the primary spines. In several forms the plates of the ambulacra have fused, forming compound plates. These compound plates carry as many pairs of tube-feet as there are small plates (primary plates) contained. Forms with 3 pairs of pores in each compound plate are designated "oligoporous" (# oligoporous plate), those with more than 3 pairs of pores in each compound plate as "polyporous" (# polyporous plate).
In the middle on the upper side of the test of the regularia is a small, distinctly limited area situated, the apical system (# apical regularia). This system consists of 10 plates: 5 situated radially, off the ambulacra (the ocular plates or oculars), and 5 situated interradially, off the interambulacra (the genital plates which carry the openings of the genital organs). One of the genital plates also functions as madreporite. Inside the 10 apical plates, the anal area is situated. In the irregularia, the posterior genital plate is lacking. The madreporite is usually prolonged backwards, separating the other plates of the apical system (# apical irregularia). The anal area has moved outside the apical area to the posterior edge of the test or even down to the underside, close to the mouth.
The mouth is on the under side of the test. In regular species it is situated in the centre, in a small, round, soft-skinned area, the mouth area or peristome. Off each ambulacral area is a pair of buccal plates, which carry a tube foot. In some of the irregularia, the peristome has moved towards the anterior end of the test. It is usually transverse-oval, wholly covered by small plates, none of which carry tube-feet.
The test is covered with a thin, ciliated skin.

Appendages
To the test and peristome various appendages are attached: spines, pedicellariae, sphaeridiae, tube-feet and gills.
The spines are articulated to the tubercles and are moveable in all directions. In the irregularia the spines are partly transformed into a flat, spade-shaped digging apparatus, while others are very small, club-shaped, close-set, forming peculiar bands: the fascioles. While in the irregularia the spines are small, very numerous and crowded without definite order; they are more structured in the regularia.
The pedicellariae are small and delicate seizing organs. They are scattered among the spines and articulate to small tubercles on the test. They consist of a head and a stalk. The head consists of usually 3 blades, more rarely 2 or 4, or even more. Each blade has its very characteristic calcareous skeleton (# pedicellaria blades). The 3 blades are connected with each other by strong muscles, and are thus able to open and close. On the inner side of each blade are some sensory hairs. If these hairs are touched, the blades close instantly. The pedicellariae are a sort of weapons for defence, but play also a rule in removal of small impurities. There are five different types of pedicellariae: triphyllous, globiferous, ophicephalous, tridentate and rostrate pedicellariae (# pedicellariae).
The sphaeridia are quite small, glassy, spherical or oval bodies, which are thought to have the function of static or of taste organs. They are attached in the middle of the ambulacra or near the tube-feet, from the edge of the peristome to some way upwards on the test (# sphaeridia). In some groups they are concealed by grooves in the test.
The tube-feet in the regular Echinoidea ends in a sucking disc. As a rule, they contain more or less numerous spicules of various forms. To each tube foot, except for the small, simple ones in Clypeastroids and Spatangoids, correspond two pores in the test.
The gills are bush-shaped outgrows from the peristomial membrane. There are 5 pairs of them, one pair at each interambulacrum. In the irregular Echinoids, gills are lacking.

Mouth
The mouth is on the under side of the test. In the regular Echinoids and Clypeastroides, it has with five strong teeth, which are fastened in a very complicated organ: the Lantern of Aristotle (# Lantern of Aristotle). The teeth, which consist of a hard, enamel-like substance, are constantly growing at their upper ends, and are pushed out steadily as the points wear off. In the Spatangoidae the mouth is placed anteriorly. In this order there is no dental apparatus, but the mouth is partly covered by a prominent lip.

Internal structures
In the regular Echinoids the rasped off food passes into an oesophagus, which leads to the intestine. The intestine runs right round the inside of the body to its starting-point, doubles back on itself and runs halfway round again and ascends to the anus in the apical disc (# Echinoidea internal). In the irregular Echinoids there is a large blind gut issuing from the intestine. In this group the anus is placed posteriorly.
The perivisceral coelom in the Echinoidea is more extensive than in the other classes. It is subdivided into cavities around the anus, perianal coelom, around the lantern, the peri-oesophageal coelom, and outside these cavities, the periproctal coelom.
The circum-oral ring vessels of the tubular coelomic systems lie just above the lantern. The radial water vessels pass down the sides of the lantern to the inside mid-line of the ambulacra. The polian vesicles (pv) are represented by some small bushy outgrowths from the ring canal. The stone canal (sc) ascends directly through the body cavity to the madreporite (ma).
The radial haemal strands pass down the inside of the lantern. The oral haemal ring gives off a branch leading to the extensive lacunar system in the gut wall. The aboral haemal ring is surrounded by a perihaemal ring and gives off branches to the gonads. The circum-oral perihaemal ring does not appear to be in communication with the radial perihaemal canals. The axial part of the haemal system seems to be distinct from the axial organ (ao).
The gonads (go) lie in a circle round the apical system. The regular Echinoids have 5 gonads, which coalesce in some forms. In the irregular Echinoids the posterior gonad has disappeared, and the number of gonads and genital pores may be reduced to three or even two.
The main ring of the nervous system (ns) is situated inside the Lantern, and the radial nerves leave it just below the radial haemal strands. Because the nerve tracts lie beneath the skeleton (sk), contact with the sub-epithelial tissues is only possible through the ambulacral pores.