this covering in a process calledmolting.
It is the largest phylum in the Animal Kingdom with more than a million described species
making up more than 80% of all described living species. They are found commonly through-
outmarine, freshwater, terrestrial, andevenaerialenvironments, inadditiontovariousforms
that areparasiticandsymbiotic. They range in size from microscopic plankton (approxi-
mately ¼ mm) up to the largest living arthropod, the Japanese spider crab, with a leg span
up to 12 feet (3.5 m).
Aquatic arthropods use gills to exchange gases. These gills have an extensive surface area
in contact with the surrounding water. Terrestrial arthropods have internal surfaces that
are specialized for gas exchange. Insects and most other terrestrial species have atracheal
system,whereairsacsleadintothebodyfromporesintheexoskeleton,foroxygenexchange.
Others usebook lungs, or gills modified for breathing air, as seen in species like the coconut
crab. Someareasofthelegsofsoldiercrabsarecoveredwithanoxygenabsorbingmembrane.
Terrestrial crabs sometimes have two different structures: one that is gilled, which is used
for breathing underwater, and another adapted to take up oxygen from the air.
Arthropods have an open circulatory system withhaemolymph, a blood-like fluid, which
is propelled by a series of hearts into the body cavity where it comes in direct contact with
the tissues. Arthropods have a complete digestive system with a mouth and anus.
Crustaceans
The crustaceans are a large group of arthropods, consisting of almost 52,000 species. The
majority of them are aquatic, living in either marine or freshwater habitats. A few groups
haveadaptedtolivingonland,suchasterrestrialcrabs,terrestrialhermitcrabs,andwoodlice
(Figure12.24).
Crustaceans are among the most successful animals and are as abundant in the oceans as
insects are on land. The majority of crustaceans are motile, although a few groups are
parasitic and live attached to their hosts. Adult barnacles live a sessile life, where they are
attached headfirst to the substrate and cannot move independently.
Various parts of the crustacean exoskeleton may be fused together, such as in thecarapace,
the thick dorsal shield seen in many crustaceans that often forms a protective chamber for
the gills. The main body cavity is an expanded circulatory system, through which blood
is pumped by a heart located near the dorsal surface. The digestive system consists of a
straight tube that often has a gizzard-like gastric millfor grinding food and a pair of
digestive glands that absorb food.
Structures that function as kidneys are located near the antennae. A brain exists in the form
ofganglia(connections between nerve cells) close to the antennae and a collection of major
ganglia below the gut. Most crustaceans have separate sexes. Many terrestrial crustaceans,
such as the Christmas Island red crab, mate seasonally and return to the sea to release the