Phylum Arthropoda
Phylum Arthropoda, or joint legged animals as its meaning states, is the most successful
known animal. Being the most successful of animals, they make up nearly seventy five percent
of all living and fossilized animals. It is predicted that many species of this phylum still have yet
to be discovered in tropic rain forests and other ecosystems that support phylum arthropoda.
Ancestral arthropods were the first land animals. The oldest known
animal to have lived on land is the arthropod Pneumodesmus
newmani. Fossil evidence of this species consists of a single
specimen discovered in 2004. Pneumodesmus newmani is classified
as a millipede and thought to have lived 428 million years ago, during
Figure 1Fossil of
pneumodesmus newmani the
fossil evidence of the oldest
animal.
the Late Silurian Period.
Characteristics of Phylum Arthropoda
Listed below are a few characteristics of the phylum arthropoda:
 The body structure of phylum Arthropoda is a body segmented with paired appendages
that typically function as limbs. The body type is known to be so adaptable to its
environment, that phylum Arthopoda can be found in almost any ecosystem.

The inner parts of phylum Arthropoda is also segmented. The nervous system is
composed by anterior ganglia and paired nerve cords.

Phylum Arthopoda has an open circulatory system, moving blood through vessels by
usage of regular contractions. The cavity that the phylum Arthropoda facilitates to pump
blood and assist internal organs is called the hemocoel.

Because of this body cavity, animals of this phylum are coelmates. Coelmates can be
described as a group of animals with a cavity filled with fluid in which the digestive
system is hung from.

Animals in the phylum Arthopoda have a line of symmetry. This symmetry is called bi
lateral (bi= two, latus= side) symmetry. In basic terms, draw a vertical line splitting it in
two and the halves should mirror each other.

When arthropods grow, they must molt their exoskeleton: Since the exoskeleton of an
arthropod is a rigid structure, it must be shed and replaced
periodically to enable growth. This shedding process is
known as molting or ecdysis and consists of a series of four
stages. First, enzymes are secreted that loosen the old
exoskeleton layers. Second, the new exoskeleton layers are
Figure 2 A scorpion "molting" its
exoskeleton
secreted. Third, the old exoskeleton splits and falls off.
Finally, the new exoskeleton hardens and sets.
Examples of Animals in phylum Arthropoda
The phylum Arthropoda has a great number of species. However, to give you an idea of the type
of animals in the phylum here are a few examples of animals listed in the phylum Anthropoda.

Butterflies

Beetles

Grasshoppers

Spiders

Crabs
Classification of phylum Arthropoda
Anthropod subgroups: Chelicerates (Subphylum Chelicerata) are a group of arthropods
that include spiders, mites, horseshoe crabs, and scorpions. The first evolved about 445 million
years ago during the Late Ordovician Period. Chelicerate bodies are divided into two regions, the
cephalothorax and the opisthosoma.
Crustaceans (Subphylum Crustacea) include about 50,000 species of lobsters, crabs, shrimp,
barnacles, brine shrimp and others. Most crustaceans live in marine or freshwater environments
but there are also some terrestrial species.
Hexapods (Subphylum Hexapoda) are a group of arthropods that includes the insects. The body
of a hexapod is divided into three sections, a head, thorax, and abdomen.
Myriapods (Subphylum Myriapoda) include about 13,000 species of millipedes and centipedes.
Myriapods are terrestrial arthropods that are most abundant in forest habitats. They are notable
for their long bodies and for having many
Figure 3 This is a classification of phylum Arthropoda
Because this phylum is so populated and broad, there are several ways of classifying the
phylum. However the one shown beneath is known to be the most widely accepted classification.
Sub-Phylum
Sub-Classes
Sub-Phylum trilobita
Subphylum Crustacea
Class Malacostraca(crabs, krill, shrimp)
Class Branchiopoda(branchiopods)
Class Ostracoda (ostracods)
Subphylum Hexapoda
Class: Insecta(insects)
Subphylum Myriapoda
Class Chilopoda(centipedes)
Class Diplopoda(millipedes)
Class Pauropoda(pauropods, pauropodans, and
progoneates) Class Symphyla(pseudocentipedes and
symphylans)
Class Arachnida (arachnids)
Class Merostomata(horseshoe crabs)
Class Pycnogonida (sea spiders)
Subphylum Chelicerata
What Makes the phylum Arthropoda so successful
The members of the phylum Arthropoda are so successful for a variety of reasons. One
reason is said to be their defense tactics
against predators. Often the members of the
phylum use methods of camouflage and
mimicry. Camouflage hides the organism
from an unwanted creature (usually a
predator). The hiding creature blends into its
environment and remains unnoticed by
predators. An animal most known to utilize
this method is the butterfly species. Natural
Figure 4 Do you see the butterfly? It is well hidden because of
camoflage
selection, of course, favors the
butterfly born with this defense tactic.
The other tactic mentioned, mimicry, is what it sounds like. Mimicking, a species similar to
another, to protect one species, or even both. It is when one species mimics the behavior of
another to avoid predators. Natural selection also encouraged this among the phylum. Some
arthropods are hermaphrodites, meaning they contain both sexes. It is also believed that animals
of this phylum evolved from three different groups. They evolved separately from worm-like
ancestors with the exception of trilobites.
Arthropods employ a variety of reproductive methods: All terrestrial arthropods
undergo internal fertilization and sperm is usually transferred to the female indirectly. Aquatic
arthropods vary in their reproductive methods, with some species using internal fertilization and
others external fertilization. Most arthropods lay eggs. Scorpions are the exception to this rule,
they incubate their eggs internally and give birth to live young. Some members of this phylum
can also reproduce without sexual reproduction causing a “population explosion”.
Sources:
http://www.biosci.ohiou.edu/introbioslab/bios173/173_7.htm “Lab 7: Phylum Arthropoda.” Ohio
University.http://www.biosci.ohiou.edu/introbioslab/bios173/173_7.htm (accessed May 25,
2011)
http://animaldiversity.ummz.umich.edu/site/accounts/classification/Arthropoda.html#Arthropoda
Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2008. The Animal Diversity
Web (online). Accessed May 24, 2011 at http://animaldiversity.org.
http://www.angelfire.com/mo2/animals1/phylum/arthropod.html
“Arthropods-Phylum Arthropoda.”
AngelFire.http://www.angelfire.com/mo2/animals1/phylum/arthropod.html (accessed May 25,
2011).
http://evolution.berkeley.edu/evolibrary/article//arthropods_04
“Bilateral (left/right) Symmetry.” University of CaliforniaBerkeley.http://evolution.berkeley.edu/evolibrary/article//arthropods_04 (accessed May 25,
2011).
http://biology.clc.uc.edu/courses/bio106/arthrpod.htm Carter, J. Stein. “Phylum Arthropoda.” UC
Clermont Biology Page.http://biology.clc.uc.edu/courses/bio106/arthrpod.htm (accessed May 25,
2011).
http://www.entomology.umn.edu/cues/4015/handouts/Orders.htm “Phylum Arthropoda.”
University of
Minnesota.http://www.entomology.umn.edu/cues/4015/handouts/Orders.htm(accessed May 25,
2011).
http://www.ucmp.berkeley.edu/arthropoda/arthropoda.html Thomas, C.D. “Introduction to the
Arthropoda.” University of California Museum of
Paleontology.http://www.ucmp.berkeley.edu/arthropoda/arthropoda.html (accessed May 25,
2011).
http://www.insectlifeforms.com/Phylums488/JOINTED_LEGGED_ANIMALS_ARTHROPOD
A_6050_488.aspx Deyrup, Mark, Barbra Peckarsky, James A. Cox, and Robert W. Pennak.
“Insect Life Forms - Phylums.” Insect Life
Forms.http://www.insectlifeforms.com/Phylums488/JOINTED_LEGGED_ANIMALS_ARTHR
OPODA_6050_488.aspx(accessed May 25, 2011).