Radially symmetric animals are mostly cylindrical in shape and the central axis is longitudinal in their body. The surface of body bearing mouth is called oral surface and opposite surface is called aboral surface. Most of them are attached to a solid support (substratum) by the aboral surface and are called sessile. Radial symmetry is advantageous for a sessile existence as it helps the animal to gather food from all sides and also to repel enemies from all sides.
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Answers
Echinoderms have a skeleton composed of numerous plates of mineral calcium carbonate (calcite). Part of the body cavity, or coelom, is a water-vascular system, consisting of fluid-filled vessels that are pushed out from the body surface as tube feet, papillae, and other structures that are used in locomotion, feeding, respiration, and sensory perception. The conspicuous five-rayed, or pentamerous, radial symmetry of living echinoderms tends to obliterate their fundamental bilateral symmetry.
Symmetry and body form
Many of the earliest echinoderms either lacked symmetry or were bilaterally symmetrical. Bilateral symmetry occurs in all living groups and is especially marked in the larval stages. A tendency toward radial symmetry (the arrangement of body parts as rays) developed early in echinoderm evolution and eventually became superimposed upon the fundamental bilateral symmetry, often obliterating it. Radial pentamerous symmetry is conspicuous among all groups of living echinoderms. Although the reasons for the success of radial symmetry are not yet completely understood, it has been suggested that a pentamerous arrangement of skeletal parts strengthens an animal’s skeleton more than would, for example, a three-rayed symmetry.
Asteroids have a large central disk from which radiate five or more hollow arms containing parts of the major internal organ systems. The underside (oral surface) of the disk contains a centrally located mouth; the underside of each arm contains five or more bands of tube feet in special grooves called ambulacral furrows. The upper (aboral) surface of the disk has a centrally located anus (often absent) and the sieve plate (madreporite) of the water-vascular system (see below Form and function of internal features). Seven-armed starfish species are not unusual, a deep-sea family has six to 20 arms, and one Antarctic genus may have up to 50 arms. Concentricycloids have a discoid body; the dorsal surface is plated and the ventral surface is naked.
Ophiuroids have a small disk from which five arms radiate. The larger internal organs usually are confined to the disk. The centrally located mouth is on the underside of the disk as are the tube feet, which are not arranged in special grooves. Although most ophiuroids have five arms, a few have six or more, and in one group, the basket stars, the arms are branched to form a complex network.
In echinoids the skeleton forms a rounded, or globular, test of solid plates; tube feet, which emerge through holes in the plates, form five conspicuous bands, or ambulacra. Spaces between bands of feet are called interambulacra. Regular echinoids are roughly spherical in shape, with a centrally located mouth at the junction of the five bands containing tube feet (ambulacra); the anus is located on the side of the body opposite the mouth (aboral). Irregular urchins are elongated or flattened in shape, with the anus on the oral or aboral surface of the body. In regular and some irregular echinoids, the mouth is equipped with five teeth operated by a complex system of plates and muscles called Aristotle’s lantern.
Holothurians are elongated, with mouth and anus at opposite ends of the body. The spaces between the tube feet, which are arranged in five rows, or radii, are known as interradii. The tube feet may be more numerous on the underside of the body than elsewhere, scattered over radii and interradii, or absent. Most holothurians are soft-bodied animals because the skeleton is reduced and the skeletal units, called ossicles or spicules, are microscopic in size. Holothurians usually show bilateral symmetry outside, radial symmetry inside.
Skeleton
The skeleton is dermal but nonetheless conspicuous in echinoderms, with the exception of most holothurians, and forms an effective armour. Each skeletal unit (ossicle) usually consists of two parts, a living tissue (stroma) and a complex lattice (stereom) of mineral calcium carbonate, or calcite, which is derived from the stroma. In living echinoderms, certain properties of calcite are not evident in the stereom because of its latticed structure and the presence of soft stroma. In fossils, however, the stroma may be replaced by secondary calcite (i.e., calcite laid down in continuity with the original skeletal calcite), and recognition of fragments of echinoderm skeletons in fossil strata is easier because no other animal group has the same type of skeleton. Each ossicle is formed from granules in the dermal layer that, after secretion from special lime-secreting cells, enlarge, branch, and fuse to build up a three-dimensional network of calcite. Parts of the skeleton enlarge as an animal grows, and resorption and regeneration of the skeleton may occur.