Echinozoa with bodies flattened dorso-ventr ally, pentagonal in outline or prolonged into arm-like extensions, usually five in number. Arms with a ventral ambidacral furrow lodging the tube-feet which end in discs. The madreporite lies dor sally in an interradius. Caecal extensions of the digestive tract extend into the arms. The anus is dorsal. The genital glands are at the base of the arms and extend more or less into them. Pedicellariaepresent universally.

The ectoderm cells develope a cuticle and are ciliated. There are among them, in some instances at least, gland and sense cells, and a sub-ectodermic plexus of nerve fibres and ganglion cells. A system of connected spaces ramifies through the connective tissue of the integument and appears to communicate with the peri-haemal spaces. The apical system of plates is well defined in the young Asteroid and includes a dorso-central, five basal and radial plates. The radials appear late, and are preceded in point of time by the terminal plate at the apex of the growing arm. This plate is large, and, as in Ophiuroidea, moves outwards with the growth of the arm. It supports the ocular tentacle or first azygos tube-foot, and is connected to the primary radial by a linear series of plates, of which the last formed is, as in Ophiuroidea, next to it. The basals 1 appear early, and the water-pore (madreporite) is frequently but by no means invariably connected to one of the circle. Under-basals are present in the young Asterina gibbosa, and persist in some adult forms, e. g. Zoroaster fidgens, many Goniasteridae, etc. The apical system is generally not traceable in the adult, but it persists in Zoroaster fulgens, many Goniasteridae, etc.

The oral system is represented by five plates which constitute the odontophores of the adult. They are inconspicuous, and, as a rule, hidden by the mouth plates, but in some deep-water species appear in part on the surface. The calcareous structures of the body wall or perisome occur as (1) granular ossicles; (2) a network of ossicles, some of which bear spines; (3) plates variable in size, and mutual closeness; and (4) paxillae or plates bearing spines which spread out at their summits into a number of radiating processes (Astropectinidae). There is often a special series of dorsal and ventral marginal plates bordering the edges of the arms. Spines, when present, are relatively short: they are fixed or free, the latter being especially the case with those at the sides of the ambu-lacral grooves over which they can be closed. The pedicellariae are either sessile or stalked (Asteriadae), but the stalk never contains calcareous supports. The valves or blades are two in number except in Luidia, where there are three. They are either hinged at their base or the bases of the valves cross one another like the blades of a pair of scissors, and they are opened and closed by special muscles.

The length of the valve is greater than its breadth, except in the form known as valvulate pedi-cellariae.

1 The basals are commonly termed genitals, but they do not appear to have any connection with the genital apertures as they have in the majority of Echinoids.

A series of paired vertebral or arnbulacral ossicles, really siib-ambulacral by position, underlies the arnbulacral grooves in their whole extent. The ossicles are narrow rods with flattened sides which are closely fitted to one another, and correspond one to the other on opposite sides of the grooves except in the Palaeozoic Encrinasteriae, in which they alternate. A single series of pores is formed by the juxtaposition of grooves on the flat sides of the rods. These pores are arranged in a straight line in the majority. Sometimes after the first three they are disposed in a zigzag line, i. e. are alternately near and remote from the outer ends of the rods. These ends lie just below the ectoderm at the edges of the arnbulacral groove. The inner ends of each member of a pair which are moveably articulated together, meet dorsally to the radial water-vascular vessel, perihaemal spaces, bloodvessel, and nerve. A muscle runs ventrally from each right to each left ossicle, and the contraction of these transverse muscles deepens the groove. A single series of adambulacral ossicles lies at the outer ends of the (sub)-ambulacrals, each ossicle corresponding to the interval between two of the latter.

And externally to the adambulacrals is a variable number of series of intermediate ambidacral ossicles, intervening between the adambulacrals and the ventral marginal ossicles when present. The members of the first pair of arnbulacral and adambulacral ossicles form the mouth plates ox oral angle pieces. These parts are modified in one of two ways. Either the arnbulacral ossicles are the more prominent while the adambulacral remain small, e. g. in Asterias; or vice versa, as in the majority of Asteroidea. In the first instance the oral angle piece projects across the mouth radially, in the second interradially; and the two types may be .distinguished as arnbulacral and adambulacral respectively. The arms are usually five in number, rarely more, as in Solaster, Archaster, Brisinga. They can be bent ventrally as well as sideways by special muscles, and their tips where the eyes are situated are normally curved slightly upwards. They are extensions of the disc, broad at the base; and the relative proportions of disc to arms is variable. The inter-radial portion of the disc is consequently much extended or much reduced, and in the latter case there are sharp interradial angles to which correspond internal folds.

Brisinga alone has a small disc and long arms sharply marked off from it, and therefore wears an Ophiuroid aspect.