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Crustacean

2007 Schools Wikipedia Selection. Related subjects: Organisms

                    iCrustacea
   Abludomelita obtusata, an isopod
   Abludomelita obtusata, an isopod
            Scientific classification

   Kingdom:   Animalia
   Phylum:    Arthropoda
   Subphylum: Crustacea
              Brünnich, 1772

                            Classes & Subclasses

   Branchiopoda

          Phyllopoda
          Sarsostraca

   Remipedia
   Cephalocarida
   Maxillopoda

          Thecostraca
          Tantulocarida
          Branchiura
          Pentastomida
          Mystacocarida
          Copepoda

   Ostracoda

          Myodocopa
          Podocopa

   Malacostraca

          Phyllocarida
          Hoplocarida
          Eumalacostraca

   The crustaceans (Crustacea) are a large group of arthropods, comprising
   approximately 52,000 described species , and are usually treated as a
   subphylum .They include various familiar animals, such as lobsters,
   crabs, shrimp and barnacles. The majority are aquatic, living in either
   fresh water or marine environments, but a few groups have adapted to
   terrestrial life, such as terrestrial crabs, terrestrial hermit crabs
   and woodlice. The majority are motile, moving about independently,
   although a few taxa are parasitic and live attached to their hosts
   (including sea lice, fish lice, whale lice, tongue worms, and Cymothoa
   exigua), and adult barnacles live a sessile life - they are attached
   head-first to the substrate and cannot move independently.

   The scientific study of crustaceans is known as carcinology. Other
   names for carcinology are malacostracology, crustaceology and
   crustalogy, and a scientist who works in carcinology is a
   carcinologist, crustaceologist or crustalogist.

Structure of crustaceans

   Crustaceans have three distinct body parts: head, thorax, and abdomen
   (or pleon), although the head and thorax may fuse to form a
   cephalothorax. The head bears two pairs of antennae, one pair of
   compound eyes and three pairs of mouthparts. The thorax and pleon bear
   a number of lateral appendages, including the gills, and the tail ends
   with a telson. Smaller crustaceans respire through their body surface
   by diffusion , and larger crustaceans respire with gills or, as shown
   by Birgus latro, with abdominal lungs . Both systems (diffusion and
   gills) were being used by various crustaceans as early as the Middle
   Cambrian .

   In common with other arthropods, crustaceans have a stiff exoskeleton
   which must be shed to allow the animal to grow ( ecdysis). Various
   parts of the exoskeleton may be fused together; this is particularly
   noticeable in the carapace, the thick dorsal shield seen on many
   crustaceans. Crustacean appendages are typically biramous, meaning they
   are divided into two parts; this includes the second pair of antennae,
   but not the first, which is uniramous. There is some doubt whether this
   is an advanced state, as had been traditionally assumed, or whether it
   may be a primitive state, with the branching of the limbs being lost in
   all extant arthoropod groups except the crustaceans. One piece of
   evidence supporting the latter view is the biramous nature of trilobite
   limbs .
   The nauplius larva of a dendrobranchiate
   Enlarge
   The nauplius larva of a dendrobranchiate

   Despite their diversity of form, crustaceans are united by the special
   larval form known as the nauplius.
   Porcellio scaber, the common rough woodlouse, a terrestrial crustacean
   Enlarge
   Porcellio scaber, the common rough woodlouse, a terrestrial crustacean

   Although a few are hermaphroditic, most crustaceans have separate
   sexes, which are distinguished by appendages on the abdomen called
   swimmerets or, more technically, pleopods. The first (and sometimes the
   second) pair of pleopods are specialised in the male for sperm
   transfer. Many terrestrial crustaceans (such as the Christmas Island
   red crab) mate seasonally and return to the sea to release the eggs.
   Others, such as woodlice lay their eggs on land, albeit in damp
   conditions. In many decapods, the eggs are retained by the females
   until they hatch into free-swimming larvae.

Taxonomy

   Pollicipes polymerus, the gooseneck barnacle
   Enlarge
   Pollicipes polymerus, the gooseneck barnacle

   Although the classification of crustaceans has been quite variable, the
   system used by Martin & Davis  is the most authoritative, and largely
   supersedes earlier works.

   Six classes of crustaceans are generally recognised:
     * Branchiopoda - including brine shrimp (Artemia) and Triops
       (Notostraca)
     * Remipedia - a small class restricted to deep caves connected to
       salt water, called anchialine caves
     * Cephalocarida - horseshoe shrimp
     * Maxillopoda - various groups, including barnacles and copepods. It
       contains Mystacocarida and Branchiura, which are sometimes treated
       as their own classes.
     * Ostracoda - small animals with bivalve shells
     * Malacostraca - the largest class, with the largest and most
       familiar animals, such as crabs, lobsters, shrimp, krill and
       woodlice.

   The exact relationships of the Crustacea to other taxa are not yet
   entirely clear. Under the Pancrustacea hypothesis , Crustacea and
   Hexapoda (insects and allies) are sister groups. Studies using DNA
   sequences tend to show a paraphyletic Crustacea, with the insects (but
   not necessarily other hexapods) nested within that clade.

Geological history

   Glyphea pseudastacus, a fossil glypheoid
   Enlarge
   Glyphea pseudastacus, a fossil glypheoid

   Those crustaceans that have hard exoskeletons reinforced with calcium
   carbonate, such as crabs and lobsters tend to preserve well as fossils,
   but many crustaceans have only thin exoskeletons. Most of the fossils
   known are from coral reef or shallow sea-floor environments, but many
   crustaceans live in open seas, on deep sea-floors or in burrows.
   Crustaceans tend, therefore, to be rarer in the fossil record than
   trilobites. Some crustaceans are reasonably common in Cretaceous and
   Caenozoic rocks, but barnacles have a particularly poor fossil record,
   with very few specimens from before the Mesozoic era.

   The Late Jurassic lithographic limestone of Solnhofen, Bavaria, which
   are famous as the home of Archaeopteryx, are relatively rich in decapod
   crustaceans, such as Eryon (an eryonoid), Aeger (a prawn) or
   Pseudastacus (a lobster). The "lobster bed" of the Greensand formation
   from the Cretaceous period which occurs at Atherfield on the Isle of
   Wight contains many well preserved examples of the small glypheoid
   lobster Mecochirus magna. Crabs have been found at a number of sites,
   such as the Cretacoues Gault clay and the Eocene London clay.
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