The phylum Arthropoda is the largest and most varied in the animal
kingdom. It includes well over one million described species. This
represents approximately three-quarters of all known biological
organisms, living or extinct. Countless arthropods remain undescribed, and the actual number of living species
could be as high as ten million or more. Some of the more well-known
arthropods include insects, crustaceans, and spiders. Arthropods are found in virtually
every known marine, freshwater, and terrestrial ecosystem, and vary tremendously in their habitats, life
histories, and dietary preferences.
Characteristics of Arthropods
All arthropods possess a stiff exoskeleton composed primarily of chitin
. In some species, lipids, proteins, and calcium carbonate may also
contribute to the exoskeleton. The external skeleton offers organisms
protection as well as support for the body. Its walls provide anchors
for the attachment of muscles. The exoskeleton is incapable of growth,
and is molted repeatedly during the growth of
the animal. This process is called ecdysis. Molting allows for rapid
growth until the newly secreted exoskeleton hardens.
Arthropod bodies are divided into segments. However, a number of
segments are sometimes fused to form integrated body parts known as
tagmata. This process of fusion is called tagmosis. The head, thorax,
and abdomen are examples of tagmata. Arthropods also have appendages
with joints. In early,
primitive anthropods, each body segment was associated with a single
pair of appendages. However, in most species some
appendages have been modified to form other structures, such as
mouthparts, antennae, or reproductive organs.
Some arthropods have highly developed sense organs. Most species have paired compound eyes
, and many also have a number of simpler eyes called ocelli.
Arthropods have an open circulatory system that
consists of a tube that is the heart and an open hemocoel
, the coelom of the animal, in which blood pools. Arthropods also have
a complete gut with two openings, the mouth and the anus.
Gas exchange in the phylum occurs in various ways. Some species have
gills, while others employ tracheae, or book lungs. The tracheal
respiratory system consists of external openings called spiracles that
are linked to a system of branched tubules which allow respiratory gases
to reach internal tissues. Arthropods are characterized by a brain as
well as a nerve ring around the area of the pharynx, in the oral cavity.
A double nerve cord extends backwards
along the ventral surface of the body, and each body
segment is associated with its own ganglion, or mass of nerve cells. In
most arthropod species, the sexes are separate. Fertilization usually
occurs internally, and most species are egg laying. While some species
exhibit direct development, in which eggs hatch as miniature versions of
adults, other species pass through an immature larval stage and undergo
a dramatic metamorphosis before reaching adult form.
Major Groups of Arthropods
Arthropods are divided into four subphyla. These are the Chelicerata,
the Crustacea, the Uniramia, and the Trilobita.
Subphylum Chelicerata
Subphylum Chelicerata comprises a major group within Phylum Arthropoda, including such animals as the arachnids (e.g., spiders and scopions), the extinct eurypterids, and the extant horseshoe crabs that are considered to be living fossils. These arthropods and their ancestral and extinct forms were and are mainly predators. Chelicerates are now predominently terrestrial animals, with most marine chelicerates, including all of the eurypterids, now extinct.
Their
name comes from their chelicerae, pointed appendages that grasp
food, that differ from the chewing
mandibles of most arthropods. Being unable to ingest solid food,
most Chelicerata either drink blood or spit or otherwise inject
digestive enzymes into their prey, and feed on the fluidized result.
Like all arthropods, chelicerates' bodies and appendages are covered
with a tough cuticle primarily composed of chitin and proteins
that chemically harden. Because this exoskeleton cannot stretch,
chelicerates have to molt in order to grow. Thus, they have to
molt the old, and await hardening of the new, during which time
they have reduced mobility and are otherwise more defenseless.
Subphylum Crustacea
Members
of Subphylum Crustacea (the Crustaceans) comprise a large
group of arthropods. The group contains familiar popular marine
food animals such as lobsters, crabs, shrimp. While mainly
found in salt and freshwater environments, there are also
terrestrial Crustacea such as woodlice and isopods. Crustaceans
have three distinct body parts: head, thorax, and abdomen
(also called a pleon). In some, the head and thorax are fused
to form a cephalothorax. They have two pairs of antennae on
the head, compound eyes, three pairs of mouthparts and a telson.
Crustaceans often have a thick carapace on the top (dorsal)
side that makes fossilization more likely; crabs and lobsters,
for example, have a thicker exoskeleton containing calcium
carbonate that is more readily fossilized.
Subphylum Uniramia
Uniramia (uni - one, ramus - branch, i.e. single-branches) is a group within the arthropods. In the past this group included the Onychophora, which are now considered a separate category. The group is currently used in a narrower sense.
Uniramia is one of three subphyla in Arthropoda classification suggested by Sidnie Manton. This classification divided arthropods into a three-phyla polyphyletic group, with phylum Uniramia including the Hexapoda (insects), Myriapoda (centipedes and millipedes) and the Onychophora (velvetworms). The discovery of fossil lobopods, determined to be intermediate between onychophorans and arthropods led to the splintering of the Lobopoda
and Onychophora into separate groups. This redefined the Uniramia as
strictly "true" arthropods with exoskeletons and jointed appendages.
Uniramians have strictly uniramous appendages.
Subphylum Trilobita
The subphylum Trilobita includes only extinct species found in fossil
form. The trilobites were a primitive group of marine species that was
particularly abundant during the Cambrian (570 million years ago) and
Ordovician (505 million years ago) periods. The group became extinct at
the end of the Permian (286 million years ago). Trilobites had
flattened, oval-shaped bodies. Most were a few inches long, although one
species is known to have attained a length of 0.6 meters (2 feet).
By
Reshma P
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