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13.29
Level IV life: Crustaceans
 
  The name Crustacean is the common name used to describe simple species organisms found principally in water with jaws and two pairs of antennae, such as the crab, lobster, and shrimp. They are among the most successful animals, dominating the sea much as insects dominate the land. The majority of individual animals in the world are marine crustaceans that belong to the copepod subclass of the crustacean subphylum. Crustaceans are also successful in fresh water; a few, such as woodlice, are also abundant in moist land environments. Although most crustaceans are small, they have a wide range of body forms and habits, and the class includes larger invertebrates such 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 35,000 known species.  
13.29.1 Structure  
  Crustaceans are divided into five categories:  
 
Sub Class of Molluscs Examples
Cephalocarida which contains a few small, rare, primitive forms
Branchiopoda has four orders of usually small animals that feed on suspended matter in fresh water; an exception is Artemia, the brine shrimp, which lives in saline lakes and ponds
Ostracoda, the tiny mussel shrimps, are protected by a two-shelled carapace that covers the body
Maxillopoda has five subclasses. The subclass Copepoda, the copepods, consists of small animals of simplified structure that are abundant in both marine and fresh water; many copepods are parasites. The subclass Branchiura consists of ectoparasites on marine and freshwater fish. The marine subclass Cirripedia consists of the barnacles and a few allied animals; some are parasites, but the most abundant ones capture food with their limbs. As adults they are immobile and highly modified.
Malacostraca with three subclasses, all members of which have eight segments in the thorax and six or seven in the abdomen. The subclass Hoplocarida has one order, Hoplocaridea, which consists of the mantis shrimps, which are predatory and often large animals. The subclass Eumalacostraea has four super orders. The super order Peracarida consists of malacostracans, usually of moderate size, that brood their young in a pouch formed from projections of the legs. Two of the orders of the super order Peracarida are abundant and diverse: the order Isopoda consists of the woodlice, or sow bugs, and their allies; the order Amphipoda, of beach hoppers and their allies. The super order Eucarida has two orders. The order Euphausiocea consists of shrimp-like animals that are abundant in the sea and form the krill upon which many whales feed. The most familiar order is Decapoda ("ten-footed"), the name of which is derived from its species' five pairs of thoracic legs; the carapace of these animals is fused to the body to form a protected chamber for the gills. In scientific classification, the names shrimp, lobster, and crayfish do not refer to definite decapod groups. In popular terminology, "shrimp" is a term applied indiscriminately to small crustaceans, whereas lobsters are thought of as large ones, and crayfishes are freshwater animals. Because the zoological classification has little to do with size or habitat, common names are hard to reconcile with scientific ones. The task is somewhat easier with crabs, which are shortened and broadened and have a reduced abdomen. Even so, the term "crab" designates two distinct zoological groups: the swimming forms, including the most commonly eaten shrimps and prawns, and the crawling forms, of which the elongated ones are generally considered lobsters and the shorter ones crabs. The spiny lobsters, which do not have large crushing claws, are placed in the section (infra order) Palinura. The lobsters in the narrow sense (Homarus and Nephrops) are placed in the section Astacidea, together with the crayfishes and mud shrimps. Nephrops, the Norwegian lobster, is often confused with shrimps because of its small size; it is also called scampi. The hermit crabs and certain other crab-like animals form the section Anomura. Crabs in the narrow sense all belong to the section Brachyura.
 
  Like all insects(arthropods), crustaceans have an external skeleton (exoskeleton) and a body made up of a series of segments; each of these generally bears a pair of two-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, reproductive organs, and other structures, or may be simplified or lost.  
  The head is usually fused with a number of segments behind it to form a region called the cephalothorax, which is followed by an 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), behind which are two other pairs of mouthparts (maxillae). The head is usually equipped with a pair of compound eyes, or an unpaired eye, or both.  
  The cephalothorax generally bears limbs used in locomotion and respiration. Often the carapace provides a protective cover for the gills, which are part of the limbs. Some of the limbs may form pincers (chelae). Abdominal appendages may be used in locomotion but frequently have other functions, such as respiration, and they tend to be reduced in size. A tail portion (telson) that bears the anus is occasionally used in swimming.  
  The main body cavity is an expanded circulatory system through which blood is pumped by a dorsal heart. The gut is basically a straight tube, often with a gizzard-like gastric mill used in breaking down the food, and a pair of digestive glands that not only secrete digestive fluids but also absorb food. Structures that serve as kidneys are located near the antennae. A brain exists in the form of ganglia near the sense organs, and a collection of ganglia and major nerves is found below the gut.  
13.29.2 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; some parasites and most barnacles, however, which have difficulty obtaining mates, are simultaneous hermaphrodites (that is, male and female at the same time). This increases the number of possible partners and may allow self fertilization- as a last resort. A number of crustaceans also change sex as they get older. Many crustaceans exhibit elaborate courtship behaviour, and the males may fight for the chance to mate.  
  In marine crustaceans, the young generally pass through one or more larval stages in which they are quite unlike the adult. Often the larvae swim in open water, thereby finding a place to live. Freshwater and terrestrial crustaceans miss the larval stage, except for those that return to the sea to spawn. After fertilization the developing eggs are generally cared for by the mother until they have reached the larval or post-larval condition; otherwise, little parental care exists among crustaceans. Some live in male-female pairs or are gregarious, but they do not form well-organized societies. The smallest crustaceans live for just a few days, but the largest ones may live for decades.  
 
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