Scientific classification: The subphylum Crustacea is placed in the phylum Arthropoda. The major classes in the subphylum Crustacea are the Branchiopoda (fairy shrimps, brine shrimps), Ostracoda (mussel, or seed, shrimps), Copepoda (copepods), Cirripedia (barnacles), and Malacostraca (crabs, lobsters, shrimps, crayfish, krill, sow bugs, beach hoppers).
Introduction
Crustacean are common name for any of a group of mainly aquatic arthropod invertebrates, including the crab, lobster, and shrimp. They are among the most successful animals, abundant in the sea much as insects are on land. The majority of animals in the world are marine crustaceans that belong to the copepod subclass of the crustacean subphylum. Crustaceans are also successful in fresh water, and a few, such as sow bugs, are common in moist land environments, what is a crustacean. Although most crustaceans are small, they vary widely in form and include such large members as lobsters up to 60 cm (24 in) long and a spider crab with a leg span of 3.6 m (12 ft). The subphylum contains about 26,000 known species.
Description
Like all arthropods, crustaceans have an external skeleton (exoskeleton) and a body made up of a series of segments. Each of these segments generally bears a pair of branched limbs. In the course of evolution, the segments and other parts of the body have become specialized. The limbs, used in respiration, swimming, crawling, and feeding, may be highly modified as jaws, what is a crustacean reproductive organs, and other structures, or they may be simplified or lost.
The head is usually fused with several segments behind it, forming a region called the cephalothorax, which is followed by the abdomen. Commonly an outgrowth of the head, called the carapace, covers much or even all of the body. On the head are two pairs of sensory organs (antennae) and a pair of jaws (mandibles), what is a crustacean behind which are two other pairs of mouthparts (maxillae). The head is usually equipped with a pair of compound eyes, an unpaired eye, or both.
Reproduction
Reproduction is primarily sexual in crustaceans. The only kind of asexual reproduction is parthenogenesis (development from unfertilized eggs), but this is rare. Usually the sexes are separate, but some parasites and most barnacles, what is a crustacean which have difficulty obtaining mates, are simultaneous hermaphrodites (male and female at the same time). This increases the number of possible partners and may allow self-fertilization as a last resort. Some crustaceans switch sex as they get older. Many crustaceans exhibit elaborate courtship behavior, and the males may fight for the chance to mate.
Habitat
Crustaceans are modified for a wide variety of habitats and ways of life. They are important in the food chain, partly because so many of them eat small plants and animals. Many strain small particles of food from the water, but the larger crustaceans, such as shrimp and crabs, are often omnivores, scavengers, or predators, what is a crustacean. Several species are also parasites. Crustaceans are consumed by many animals, including humans, and are rich in protein.
Different species
The crustacean subphylum is large and diverse. The brine shrimp, a branchiopod, lives in saline lakes and ponds. Ostracods, or ostracodes, the tiny mussel, or seed, shrimps, are protected by a two-shelled carapace that covers the body.