Pedicellaria and papulae of Asterias forbesi 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 of calcium carbonate components known as ossicles. These are honeycombed structures composed of calcite microcrystals arranged in a lattice.
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. 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.
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. These serve a respiratory function. 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. Water enters the system through the madreporite , a porous, often conspicuous, sieve-like ossicle on the aboral surface.
It is linked through a stone canal, often lined with calcareous material, to a ring canal around the mouth opening. A set of radial canals leads off this; one radial canal runs along the ambulacral groove in each arm.
There are short lateral canals branching off alternately to either side of the radial canal, each ending in an ampulla. These bulb-shaped organs are joined to tube feet podia on the exterior of the animal by short linking canals that pass through ossicles in the ambulacral groove. There are usually two rows of tube feet but in some species, the lateral canals are alternately long and short and there appear to be four rows.
The interior of the whole canal system is lined with cilia. These extend to contact the substrate. Although the tube feet resemble suction cups in appearance, the gripping action is a function of adhesive chemicals rather than suction.
The tube feet latch on to surfaces and move in a wave, with one arm section attaching to the surface as another releases. When crawling, some arms act as the leading arms, while others trail behind. The sand star Luidia foliolata can travel at a speed of 2. The water vascular system serves to transport oxygen from, and carbon dioxide to, the tube feet and also nutrients from the gut to the muscles involved in locomotion.
Fluid movement is bidirectional and initiated by cilia. Oxygen is transferred from these to the coelomic fluid , which acts as the transport medium for gasses. Oxygen dissolved in the water is distributed through the body mainly by the fluid in the main body cavity; the circulatory system may also play a minor role. Intestine and anus, 3. Ambulacral ridge The gut of a starfish occupies most of the disc and extends into the arms. The mouth is located in the centre of the oral surface, where it is surrounded by a tough peristomial membrane and closed with a sphincter.
The mouth opens through a short oesophagus into a stomach divided by a constriction into a larger, eversible cardiac portion and a smaller pyloric portion. The cardiac stomach is glandular and pouched, and is supported by ligaments attached to ossicles in the arms so it can be pulled back into position after it has been everted.
The pyloric stomach has two extensions into each arm: These are elongated, branched hollow tubes that are lined by a series of glands, which secrete digestive enzymes and absorb nutrients from the food. A short intestine and rectum run from the pyloric stomach to open at a small anus at the apex of the aboral surface of the disc. Shell valves and other inedible materials are ejected through their mouths.
The semi-digested fluid is passed into their pyloric stomachs and caeca where digestion continues and absorption ensues. When the prey is a clam or other bivalve , the starfish pulls with its tube feet to separate the two valves slightly, and inserts a small section of its stomach, which releases enzymes to digest the prey.
The stomach and the partially digested prey are later retracted into the disc. Here the food is passed on to the pyloric stomach, which always remains inside the disc. Their diets include clams and oysters , arthropods , small fish and gastropod molluscs. Some starfish are not pure carnivores , supplementing their diets with algae or organic detritus. Some of these species are grazers , but others trap food particles from the water in sticky mucus strands that are swept towards the mouth along ciliated grooves.
Starfish have no distinct excretory organs; waste ammonia is removed by diffusion through the tube feet and papulae. These cells engulf waste material, and eventually migrate to the tips of the papulae, where a portion of body wall is nipped off and ejected into the surrounding water.
Some waste may also be excreted by the pyloric glands and voided with the faeces. Although some species can tolerate relatively low salinity , the lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in many estuarine environments.
The tube feet, spines and pedicellariae are sensitive to touch. The tube feet, especially those at the tips of the rays, are also sensitive to chemicals, enabling the starfish to detect odour sources such as food.
These are composed of pigmented epithelial cells that respond to light and are covered by a thick, transparent cuticle that both protects the ocelli and acts to focus light.
Many starfish also possess individual photoreceptor cells in other parts of their bodies and respond to light even when their eyespots are covered. Whether they advance or retreat depends on the species. The peripheral nerve system consists of two nerve nets: Neurons passing through the dermis connect the two. The starfish does not have the capacity to plan its actions. If one arm detects an attractive odour, it becomes dominant and temporarily over-rides the other arms to initiate movement towards the prey.
The mechanism for this is not fully understood. The vessels form three rings: The heart beats about six times a minute and is at the apex of a vertical channel the axial vessel that connects the three rings. At the base of each arm are paired gonads ; a lateral vessel extends from the genital ring past the gonads to the tip of the arm.
This vessel has a blind end and there is no continuous circulation of the fluid within it. This liquid does not contain a pigment and has little or no respiratory function but is probably used to transport nutrients around the body.
The steroids are mostly saponins , known as asterosaponins, and their sulphated derivatives. They vary between species and are typically formed from up to six sugar molecules usually glucose and galactose connected by up to three glycosidic chains.
Long-chain fatty acid amides of sphingosine occur frequently and some of them have known pharmacological activity. Various ceramides are also known from starfish and a small number of alkaloids have also been identified. The functions of these chemicals in the starfish have not been fully investigated but most have roles in defence and communication. Some are feeding deterrents used by the starfish to discourage predation. Others are antifoulants and supplement the pedicellariae to prevent other organisms from settling on the starfish's aboral surface.
Some are alarm pheromones and escape-eliciting chemicals, the release of which trigger responses in conspecific starfish but often produce escape responses in potential prey.
These are usually not distinguishable externally as the gonads cannot be seen, but their sex is apparent when they spawn.
Some species are simultaneous hermaphrodites , producing eggs and sperm at the same time and in a few of these, the same gonad, called an ovotestis , produces both eggs and sperm. Protandrous individuals of species like Asterina gibbosa start life as males before changing sex into females as they grow older. In some species such as Nepanthia belcheri , a large female can split in half and the resulting offspring are males.
When these grow large enough they change back into females. Fertilization is generally external but in a few species, internal fertilization takes place. In most species, the buoyant eggs and sperm are simply released into the water free spawning and the resulting embryos and larvae live as part of the plankton. In others, the eggs may be stuck to the undersides of rocks. Brooding may be done in pockets on the starfish's aboral surface,   inside the pyloric stomach Leptasterias tenera  or even in the interior of the gonads themselves.
In Parvulastra parvivipara , an intragonadal brooder, the young starfish obtain nutrients by eating other eggs and embryos in the brood pouch. Spawning takes place at any time of year, each species having its own characteristic breeding season. The first individual of a species to spawn may release a pheromone that serves to attract other starfish to aggregate and to release their gametes synchronously.
When she releases eggs into the water, he is induced to spawn. In some species, mature females produce chemicals to attract sperm in the sea water. Painted by Ernst Haeckel Most starfish embryos hatch at the blastula stage. The original ball of cells develops a lateral pouch, the archenteron. The entrance to this is known as the blastopore and it will later develop into the anus.
Another invagination of the surface will fuse with the tip of the archenteron as the mouth while the interior section will become the gut. At the same time, a band of cilia develops on the exterior. This enlarges and extends around the surface and eventually onto two developing arm-like outgrowths. At this stage the larva is known as a bipinnaria.
The cilia are used for locomotion and feeding, their rhythmic beat wafting phytoplankton towards the mouth. These are at the anterior end, surround a sucker and have adhesive cells at their tips. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. When fully developed, the brachiolaria settles on the seabed and attaches itself with a short stalk formed from the ventral arms and sucker. Metamorphosis now takes place with a radical rearrangement of tissues.
The left side of the larval body becomes the oral surface of the juvenile and the right side the aboral surface. Part of the gut is retained but the mouth and anus move to new positions.
Some of the body cavities degenerate but others become the water vascular system and the visceral coelom. The starfish is now pentaradially symmetrical.