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.
||Crustaceans are divided into five
|Sub Class of Molluscs
||which contains a few small, rare, primitive forms
||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
||the tiny mussel shrimps, are protected by a two-shelled carapace that covers
||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.
||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.
||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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