Sea Stars, Sea Urchins, Sand dollars, Serpent stars
Structure and function of body parts
The body wall consists of a thin cuticle, an epidermis consisting of a single layer of cells, a thick dermis formed of connective tissue and a thin coelomic myoepithelial layer, which provides the longitudinal and circular musculature. The dermis contains an endoskeleton ofcalcium carbonate components known as ossicles. These are honeycombed structures composed of calcite microcrystals arranged in a lattice. They vary in form, with some bearing external granules, tubercles and spines, but most are tabular plates that fit neatly together in a tessellated manner and form the main covering of the aboral surface. Some are specialised structures such as the madreporite(the entrance to the water vascular system), pedicellariae and paxillae. Pedicellariae are compound ossicles with forceps-like jaws. They remove debris from the body surface and wave around on flexible stalks in response to physical or chemical stimuli while continually making biting movements. They often form clusters surrounding spines. Paxillae are umbrella-like structures found on starfish that live buried in sediment. The edges of adjacent paxillae meet to form a false cuticle with a water cavity beneath in which the madreporite and delicate gill structures are protected. All the ossicles, including those projecting externally, are covered by the epidermal layer.
Several groups of starfish, including Valvatida and Forcipulatida, possess pedicellariae. In Forcipulatida, such as Asterias and Pisaster, they occur in pompom-like tufts at the base of each spine, whereas in the Goniasteridae, such as Hippasteria phrygiana, the pedicellariae are scattered over the body surface. Some are thought to assist in defence, while others aid in feeding or in the removal of organisms attempting to settle on the starfish's surface. Some species like Labidiaster annulatus, Rathbunaster californicus and Novodinia antillensis use their large pedicellariae to capture small fish and crustaceans.
There may also be papulae, thin-walled protrusions of the body cavity that reach through the body wall and extend into the surrounding water. These serve a respiratory function. The structures are supported by collagen fibres set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the interstices. This arrangement enables both easy flexion of the arms by the starfish and the rapid onset of stiffness and rigidity required for actions performed under stress.
Special features and adaptations
Methods of reproduction and life cycle
Some echinoderms are carnivorous (for example starfish) others are detritus foragers (for example some sea cucumbers) or planktonic feeders (for example basket stars).
Reproduction is carried out by the release of sperm and eggs into the water. Most species produce pelagic (= free floating) planktonic larvae which feed on plankton. These larvae are bilaterally symmetrical, unlike their parents (illustration of a larvae of a sea star below). When they settle to the bottom they change to the typical echinoderm features.
Echinoderms are characterized by radial symmetry, several arms (5 or more, mostly grouped 2 left - 1 middle - 2 right) radiating from a central body (= pentamerous). The body actually consists of five equal segments, each containing a duplicate set of various internal organs. They have no heart, brain, nor eyes, but some brittle stars seem to have light sensitive parts on their arms. Their mouth is situated on the underside and their anus on top (except feather stars, sea cucumbers and some urchins).
Echinoderms have tentacle-like structures called tube feet with suction pads situated at their extremities. These tube feet are hydraulically controlled by a remarkable vascular system. This system supplies water through canals of small muscular tubes to the tube feet (= ambulacral feet). As the tube feet press against a moving object, water is withdrawn from them, resulting in a suction effect. When water returns to the canals, suction is released. The resulting locomotion is generally very slow.
The evolutionary history of the echinoderm dates back to the early Cambrian and they rapidly diversified. Today, only the five classes are present (represented in the graphic at top). Their radial symmetry is only slightly misleading. Genetic studies suggest that echinoderm evolution occurred by means of extensive mutation in regulatory genes inherited from bilateral ancestors. Early theories about the origin of this group included the ‘pentacula’ and ‘dipleurula’ ideas. The former was postulated first and assumes pentaradial symmetry evolved before the formation of the WVS while the latter is based on the earliest recognizable larval stage common to the modern classes of echinoderms.
Crinoids retain the most primitive form of the WVS. Tube feet were believed to originally been used entirely for respiration as well as feeding structures, but fossil eocrinoids have distinct pores between skeletal plates that led Nichols (1972) to conclude that the tube feet system was not very extensive in early in crinoid evolution. But he doesn’t give consideration to other interpretations of the structures, such as openings for internal parts related to locomotion or feeding and unrelated to respiration. Carpoids, such as Gyrocystis and Dendrocystites, found in middle Cambrian sediments are considered basal to the echinoderm lineage due to similarity in skeletal elements and are hypothesized to have soft, protractible internal tentacular structures. The existence of a primitive WVS is inconclusive in the fossil record of carpoids, yet the it is considered to have evolved prior to other echinoderm characteristics. The earliest suggestion of anything like a WVS comes from the Helicoplacoidea found in lower Cambrian deposits (noted by the presence of highly organized rows of ambulacral pores). According to Paul (1977) no modern class has diverged from the carpoids, which has been extinct since the Mesozoic. Helicoplacoids, with triradial symmetry, diverged very early from the carpoids and the rest of the echinoderm lineage evolved upon the Helicoplacoid bauplan.