The starfish or seastar like animals belong to a group called echinodermata i.e.spiny skinned animals. They are without a backbone and are therefore, considered non-chordates or invertebrates.
Their bodywall has a non-cellular protective covering layer, the cuticle. Some layers of skin in the echinodermates are similar to those in ours viz. the epidermis and the dermis. Beneath the dermis they have a layer each of circular and longitudinal muscle fibers. Under the dermis, is the parietal peritoneum lining their body cavity, the coelom.
Their epidermis consists of live cells and the cells bear cilia – tiny hair like extensions from cell membrane on their free surfaces. The epidermis contains a few mucus secreting glandular cells and some sensory cells. Dermis also contains self-secreted dermal calcareous plates. They form the endoskeleton. Pedicellaria are the tiny pincers on body surface of the echinodermates. They also have the ossicles i.e. tiny bone like structures in their dermis.
Spines are ossicles that extend away from the surface. The spines can be movable or fixed. If movable, spines are equipped with muscles. Most echinodermata have a layer each of circular and longitudinal body wall muscles.
Starfish i.e.the seastar is the most familiar specimen of echinodermata. The other members of echinodermata many people know about are the sea-cucumber, sea-lily, brittle star, and the sea urchin.
Mollusca which are just below the echinodermates on the evolutionary ladder have remarkable skins. Most mollusca have a shell and it is secreted by their mantle,the skin around the visceral mass.
Pearls are formed by the pearl oysters (Pinctada margaratifera Linne, 1758) as also by some mussels, and clams and includes fresh water as awell as marine species. The oysters ( bivalves) can form smooth shelly material called, nacre, on the inside of their shell and cover an offending parasite or a wandering larva of some other animal which they find irritating their soft mantle. The pearl thus is actually formed to reduce the irritation.
The longer the oyster lives, the bigger the pearl gets. Most bivalves however, produce pearls which just serve the basic function of protecting soft mantle and have no pearly sheen and beauty.
In human skin corns are produced much in the same way in response to a particle which has invaded the skin or unusual pressure etc.
Big,spherical,lustrous natural pearls are rare to find and deep sea diving to get them is dangerous.
For inducing pearl formation tiny pieces of mantle are taken from a donor oyster. They are surgically introduced into the recipient oyster’s body along with spherical irritant beads. They are placed between the mantle (mantle is the dorsal epidermis) and the shell.This process is called nucleation.
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