This is the largest phylum with more than 80% of the living animals falling into this category. All members of this phylum have and external skeleton made of chitin ( the principal component of the exoskeletons of arthropods and of the bodies of fungi.) This provides protection for the organism. The body of the arthropods is composed of numerous segments called Somites. They have a dorsal heart, as well as a ventral nervous system. Arthropods have joint appendages , that are modified into organs for walking, eating and grasping purposes.
Major groups
Myriapada (millipedes)
Various wormlike arthropods of the class Diplopoda, having a long body composed of many narrow segments. Most have two pairs of legs.
Closely related to but smaller than the lobsters.
Insecta (insects)
Any animal of the class Insecta, comprising small, air-breathing arthropods having the body divided into three parts (head, thorax, and abdomen), and having three pairs of legs and usually two pairs of wings.
Arachnidia (spiders)
any of numerous predaceous arachnids of the order Araneae, most of which spin webs that serve as nests and as traps for prey.
Ecological roles
They are of such importance because of their numbers, and diversity (more than 874,000 living.) Insects play an important role in the pollination of vegetation, and some are also a nutritious source of food. More than a thousand Arthropods have not been discovered and named. Most of these insects come from the rain forest, which has not yet been well explored.
References
http://www.entomology.umn.edu/cues/4015/handouts/Orders.htm
http://www.scienceclarified.com/Al-As/Arthropods.html
http://www.dictionary.com
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