The majority of starfishes are pentamerous, but a number of species have more arms.
Basically, the body consists of a central disc with the mouth in the middle of the under-surface and the anus in the centre of the upper-surface. The arms project laterally. The delimitation of arms and disc is not clearly marked.
All asteroids have a skeleton of internal ossicles, in the shape of rods, crosses, or plates. The ossicles are arranged in such a way that they form a lattice network and are bound together by connective tissue. The amount of space between ossicles varies, depending on the form of the ossicles (# asteroid ossicles).
Spines and tubercles are also part of the skeleton. They either consist of separate pieces resting on the deeper dermal ossicles, or represent extensions of the dermal ossicles that project to the outer surface. Regularly arranged groups of spines are named paxillae.
The arms have an internal skeleton of paired plates, the ambulacral plates. On the under side of the arm there is an open furrow. This ambulacral furrow is limited on each side by a series of distinct adambulacral plates which correspond to each ambulacral plate. Lateral to the adambulacrals come the plates forming the side of the arm: the inferior and superior marginals and, if present, the dorso-laterals. The midline of the aboral surface, if present, is formed by the carinals (plaat???).
In the bottom of the furrow on the underside, the tube-feet are found. They are mostly arranged in two series and originate from between the ambulacral plates. Along the edges of the ambulacral furrow, on the adambulacral plates, some regularly arranged adambulacral or furrow spines or papillae are found. These spines are generally somewhat larger than the other spines on the underside.
At the tip of the arm there is an unpaired tentacle, carrying on its underside a red pigment-spot provided with a sort of lens, forming a rather well developed eye. Above this tentacle is a large unpaired plate, the terminal plate.
The marginal plates along the edge of the disc are mostly covered with fine granules or spines of varying sizes. Other structures which can be found on the skin are papulae, or gills, and small pincers, the pedicellariae.
The mouth is on the underside and leads into a vertical alimentary canal (# asteroid internal). The stomach consists of two parts: the cardiac stomach (cs), followed by a smaller pyloric stomach (ps). From the pyloric stomach two hepatic caeca (hc) enter into each arm. Each caecum is heavily sacculated and here digestion is continued and absorption takes place. A short rectum rises from the pyloric stomach and ends into the anus (an). However, in some burrowers, the gut may end blindly.
The general body cavity is not divided; the perivisceral coelom (pc) of the disc is a single cavity and is continuous with those in the arms. The papulae (pa) emerge from the body wall and are partly formed by the coelomic epithelia. They have functions in the gaseous exchange between sea-water and perivisceral coelomic fluid and in the excretion.
The tubular coelomic systems, consist of the water vascular system (wvs), haemal system (hs) and perihaemal system (phs), and are well developed.
The water vascular system supplies the fluid necessary for the hydraulically operated tube-feet. The madreporite (ma) leads down into the stone canal (sc), which opens into the circum-oral water ring (wr). From the wall of this ring, five pairs of interradial Tiedemann's bodies (tb) arise and in some species, this ring bears the polian vesicles. The Tiedemann's bodies are thought to secrete some of the coelomocytes, while the polian vesicles maintain turgor in the system. From the water ring the radial water canals arise, one to each arm. On alternate sides of the radial canal, branches are given off to the tube-feet.
The haemal system (hs) has a number of ring elements round the gut and radial components leading off into each arm. The uppermost aboral ring gives off two branches to each arm, leading to the gonads (go). In asteroids, it is not possible to distinguish a separate axial organ (ao); it seems to be part of the haemal system. Large parts of the haemal system are surrounded by the perihaemal system (phs).