Crustacea

• WordNet 3.6
  • n Crustacea class of mandibulate arthropods including: lobsters; crabs; shrimps; woodlice; barnacles; decapods; water fleas
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• • n. pl Crustacea (Zoöl) One of the classes of the arthropods, including lobsters and crabs; -- so called from the crustlike shell with which they are covered.☞ The body usually consists of an anterior part, made up of the head and thorax combined, called the cephalothorax, and of a posterior jointed part called the abdomen postabdomen, and (improperlytail. They breathe by means of gills variously attached to some of the limbs or to the sides the body, according to the group. They are divisible into two subclasses, Entomostraca and Malacostraca, each of which includes several orders.
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• • crustacea A class of Arthropoda; one of the prime divisions of articulated animals with articulated legs, as distinguished from Insecta, Myriapoda, and Arachnida respectively. They are mostly aquatic arthropods with (generally) two pairs of antennæ and numerous thoracic as well as (usually) abdominal articulated appendages, and breathing by means of branchiæ. The body is covered with a hard chitinous test or crust, whence the name. It is segmented into head, thorax, and abdomen, the two former of which are more or less completely united into a cephalothorax, shielded with a continuous carapace; the abdomen is usually segmented and mobile, presenting the appearance of a tail. A typical segment or somite of the body consists, at least theoretically, of a dorsal portion or tergite of two pieces, a ventral portion or sternite, also of two pieces, an epimeron on each side above, and an episternum on each side below. The shell sends inward sundry hard processes or partitions called apodemata. The typical number of segments in the higher Crustacea is 21, actually or theoretically. The crustaceans shed their shells (exoskeletons), in some cases with extraordinary frequency, and they possess great reparatory powers in the reproduction of lost parts. Most of them pass through several larval stages, the best-marked of which are those of the forms called the nauplius, zoea, and megalopa. The crustaceans include all kinds of crabs and lobsters, shrimps, prawns, crawfish, etc., among the higher forms; and among the lower, a great variety of creatures known as sand-hoppers, beach-fleas, wood-lice, fish-lice, barnacles, etc. Leading types, in more technical terms, are the thoracostracan, podophthalmic, or stalk-eyed crustaceans, as crabs and crawfish; the edriophthalmous or sessile-eyed crustaceans, as læmodipods, amphipods, and isopods (all the foregoing being sometimes grouped together as malacostracous crustaceans); the entomostracous crustaceans, as the copepods, ostracodes, cladocerans, phyllopods, etc., the trilobites and their related forms being often brought under this division; the epizoans, ichthyophthirians, or fish-lice; and finally, the cirripeds. Great as is the difference between extremes in any of these forms, they are closely related by connecting forms, and naturalists are by no means agreed upon the formal division of the class. The older divisions which have been made are now mostly superseded, and even the modern ones are seldom exactly conterminous. A series of subclasses sometimes now adopted is: Cirripedia or Pectostraca, with three or four orders; Epizoa or Ichthyophthiria; Entomostraca, with such orders as Copepoda, Ostracoda, Cladocera, Phyllopoda, Xiphura, Trilobita, Eurypterida; Edriophthalma, with Læmodipoda, Amphipoda, and Isopoda; Podophthalma, with Stomatopoda and Decapoda; to which some add Podosomata, often considered to be arachnidans. The fourth and fifth of these are often united as one subclass, Malacostraca. The trilobites with the eurypterygians and king-crabs sometimes constitute one prime division called Gigantostraca. Haeckel uses Carides as a substitute for Crustacea.
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• • n.pl Crustacea krus-tā′shi-a a large class of Arthropod animals, almost all aquatic, including crabs, lobsters, shrimps, sand-hoppers, wood-lice, water-fleas, barnacles, acorn-shells, &c
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