Coelomate Metazoa in zvhich the bilateral symmetry of the larva is more or less completely replaced by a radial symmetry. Calcifications of the integument form a mesodermic skeleton generally of great completeness', and a special section of the primitive coelome developes into a water -vascidar system, which has a locomotor and often a respiratory function. The five peripheral vessels of this system define five radii and five intervals or interradii, the ambulacra and interambulacra respectively. There is a metamorphosis.
The typical Echinoderm larva is a bilaterally symmetrical organism with well-marked dorsal and ventral surfaces, the latter the smaller of the two. The mouth is ventral and subterminal at one pole, the anus (= gastrula mouth) ventral and subterminal at the other, and the digestive tract takes an antero-posterior course between the two with a curve convex to the dorsal surface. The right and left peritoneal (= coelomic) sacs unite around it, and the dorsal union between the two walls persists as the mesentery, at the anterior end of which is the water-tube and pore. The Holothurioidea retain some to a greater, others to a less extent these characteristics. Their nervous and water-vascular systems with the longitudinal muscle bands are, however, invariably disposed radially. In all other Echinoderms there is a paramount radial and with few exceptions pentamerous symmetry, and the only trace of the primitive bilateral symmetry is to be found in the mesentery supporting the water-tube and plexiform organ; but though this mesentery is more or less antero-posterior it does not lie in the primitive median dorso-ventral plane.
A secondary plane of bilateral symmetry however is often established, as in irregular Echinoidea and the Crinoidea. Furthermore the dorsal or abactinal surface of the adult corresponds to the right side with more or less of the dorsum of the larva, and the ventral or actinal surface to its left side with more or less of the ventral surface. The mouth of the adult is in the centre of the actinal surface, the anus excentric on the abactinal, though in the course of growth it may travel round interradially to the oral surface. It must be carefully noted, therefore, that the point from which nerves and water-vascular vessels radiate in the Holothurioidea, lies in the larval antero-posterior axis, and that they are parallel both to this axis and the median dorso-ventral plane. But in other Echinoderms the point from which they radiate lies on the left side in a dorso-ventral axis at right angles approximately both to the primitive antero-posterior axis and median dorso-ventral plane.
The calcareous deposits in the integument take the form of spicules or variously shaped plates. The Holothurioidea alone retain a muscular integument. In all other Echinoderms the muscular system is greatly aborted and is only developed in relation with the movements of special parts of their skeleton. In this skeleton there are two important systems of plates, the apical and oral, corresponding respectively to the centres of the dorsal and oral surfaces in the adult. The apical system comprises a dorso-central plate round which are arranged, passing from the centre outwards, at first in a spiral but afterwards in a circle, (i) five basal plates interradial in position; (2) five radial plates radial in position. To these may be added (1) within the circle of basals, a circle of five underbasal plates radially placed, and (2) externally to the radials five primary interradial plates. Underbasals occur only in Asteroidea, Ophiuroidea and Crinoidea; and when they are present the base of the apical system is said to be dicyclic, when absent monocyclic.
Primary interradials occur in Asteroidea, Ophinroidea and Pelmatozoa. The oral system is completely developed only in certain Pelmatozoa. In its typical form, as seen in some Actinocrinidae among Palaeocrinoidea, there is an oro-central surrounded by five interradial oral plates, and these in their turn by circles of oral radials and interradials, the whole forming a dome, beneath which lies the mouth. Apertures pass to the mouth between the orals. As a rule, however, the oro-central is not formed, and the oral plates alone represent the oral system. The region of the body between these two systems of plates is moulded in a fashion characteristic of the various classes. In Echinoidea it is spherical or compressed dorso-ventrally, and the ambulacra and inter-ambulacra extend in parallel lines or meridians from the oral to the apical area. In the Brachiate Echinoderms (Asteroidea, Ophiuroidea, Crinoidea and Cystoidea) the radial meridians are prolonged outwards into arms surrounding a central disc, while the interradial remain more or less undeveloped.
At the same time the ambulacra are confined to the ventral surfaces of the disc and arms, and there is developed outside the apical system an area of dorsal surface known as the anti-ambulacral. The skeletal plates corresponding to these different regions vary much. It is of importance, however, to note whether the radial water-vascular trunks are supported (i) by a system of external, i. e. super-ambulacral, plates alone (Echinoidea), or of internal, i. e. sub-ambulacral, ossicles alone (Asteroidea), or of both combined (Ophiuroidea). Radial anti-ambulacral ossicles attain great importance, and form the arm skeleton in Crinoidea and Cystoidea.
The central nervous, the water-vascular and blood-vascular systems form circumoral rings connected with radial prolongations. The nervous elements consist of parallel fibres with interposed ganglion cells. The ring and nerves lie at the base of the ectoderm cells in Asteroidea and Crinoidea, but in other Echinoderms in the cutis, and then they are surrounded by a cellular sheath. A peripheral sub-ectodermic plexus is well developed in Asteroidea, and a more deeply placed plexus in Echinoidea and Holothurioidea. The Crinoidea possess an anti-ambulacral nervous system. Eyes are found in Echinoidea and Asteroidea. Auditory organs in a few Holothurioidea. The blood-vascular system is apparently mesodermic in origin. Its ring is connected, except in the Holothurioidea, with radial, vessels lying between the radial nerves to the outer, and the water-vascular vessels to the inner side. It is also connected, except in the class named, with a plexiform organ or gland - the so-called heart - which lies in the primitive mesentery supporting the water-tube. The epithelium of the plexiform vessels appears to form, at least in Echinoidea and Asteroidea, blood corpuscles, which are coloured and respiratory in function.