Subclass SarsostracaClass Remipedia
Order NectiopodaClass Cephalocarida
Order BrachypodaClass Maxillopoda
Subclass PodocopaClass Malacostraca
The crustaceans (Crustacea) are a large group of arthropods
(55,000 species), usually treated as a subphylum. They
include various familiar animals, such as lobsters
, crabs, shrimp and barnacles. They are variously found
in marine and freshwater, with a few terrestrial members
(such as woodlice).
The scientific study of crustaceans is known as carcinology.
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. They have two pairs of antennae
on the head, compound eyes, three pairs of mouthparts
and a telson. Smaller crustaceans respire through their
body surface by diffusion and larger crustaceans respire
with gills. Crustaceans typically have a thick carapace
on the dorsal side of their body. Their appendages are
typically biramous, including the second pair of antennae
(but not the first).
Most crustaceans have separate sexes and are distinguished
by appendages on the abdomen called swimmerets or, more
technically, pleopods. The first (and sometimes the second)
pair of pleopods are larger on the male than on the female.
Terrestrial crabs (such as the Christmas Island red crab)
mate seasonally and return to the sea to release the eggs.
Crabs' eggs are retained by the females until they hatch
into free-swimming larvae.
The formal classification of crustaceans varies somewhat.
In general, because of the large number of species, taxonomists
have made extensive use of subordinate taxonomic categories
(suborders, superfamilies and so forth), and the status
of different groupings is frequently controversial; this
can make taxonomic references hard to follow. Evolutionary
relationships between the different groups are not entirely
clear, making the exact definition of larger groups difficult.
Some authorities have treated the entire group of crustaceans
as a class, in which case the classes escribed in the
taxonomic table are treated as subclasses. Other authors
omit some or all of the classes listed here, in which
case some of the groups given here as subclasses are promoted
to full class rank. This listings given at the end of
this article and in the summary table at the right are
those recommended by ITIS, and as such probably represent
a consensus of modern opinion. However, good practice
in describing crustaceans must clearly be to include descriptions
at several taxonomic levels, to ensure that readers can
link the information to others' schemes.
Less formally, we can state that the most important groups
of crustaceans are barnacles (infraclass Cirripedia),
branchiopods, copepods and Malacostraca (crabs, lobsters,
shrimps and krill). There are around 1,220 barnacle species,
1,000 branchiopods, 13,000 copepods, and 30,000 Malacostraca.
Although crustaceans are rarer as fossils than trilobites
are, a number of different types of crustaceans are common
in the rocks of the Cretaceous period as well as those
of the Caenozoic era. Most of the smaller crustaceans,
such as shrimp, have an exoskeleton which is somewhat
delicate and for this reason their fossil record is incomplete.
However, crustaceans such as crabs and lobsters have a
thicker exoskeleton which is reinforced with calcium carbonate
and so their fossil record is much better. The fossil
record of barnacles is scarce and little is known of their
history prior to the Mesozoic era. Well preserved specimens
are known from the rocks of the Cretaceous period and
the Caenozoic era.
The most well known crustaceans, the malacostraca (crabs,
lobsters, crayfish, shrimp), although widespread today
are only found sporadically as fossils. Most of the known
fossil crabs are of forms which lived on the sea-floor
or in a reef environment. In rocks such as the Gault clay
from the Cretaceous period and the London clay from the
Eocene period in England fossil crabs may be found. The
'Lobster Bed' of the Greensand formation from the Cretaceous
period which occurs at Atherfield on the Isle of Wight
in England contains many well preserved examples of the
small glypheoid lobster Mecochirus magna. The lithographic
limestones from the Jurassic period of Solnhofen in Germany
have long been famed for the many exceptionally preserved
crab, lobster and shrimp fossils they have yielded (such
as Aeger, Eryon, and Pseudastacus).
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