General Characteristics: The phylum contains more species than any other phylum. They are commonly called arthropods (arthron = joint +pods = feet). Insects (cockroaches, grasshoppers, butterflies, mosquitoes) are most common arthropods on the earth.
The body is segmented. Each segment is attached to its neighbor by means of a modified portion of cuticle which is thin and flexible, they possess jointed appendages. These appendages have been modified for specialized functions.
These are belived to have common origin with annelids because both have some common characteristics such as segmented body, appendages and cuticle.
Arthropods have exploited every type of habitat on land and in water. The aquatic species include both freshwater and marine. Many of these can fly, therefore visit air periodically.
Arthropods are variable structurally. Some are worm-like centipedes while the others are flying insects with the body divided into distinct regions, the head, thorax and abdomen. The body is covered with waterproof chitinous cuticle secreted by the epidermis.
The coelom is not present as the main body cavity. Instead a haemocoel has developed. It is reduced coelom and communicates with blood vascular system.
The digestive system is in the form of alimentary canal with two openings, the mouth and anus. It is divided into different parts each performing a specific function. The food comprises of small plants and animals.
A well developed excretory system comprising of Malpighian tubules is present in arthropods. The nitrogenous wastes are excreted in the form of solid uric acid.
A highly developed nervous system is present. It consists of paired ganglia (simple brain) connected to a ventral double nerve cord. A ganglion is present in each segment. Nerves arise from these ganglia. The sensory organs are usually a pair of compound eyes and antennae etc.
Most arthropods possess an extensive tracheal system formed of air tubes called tracheae for the exchange of gases. Main tubes open to the exterior through paired openings, called spiracles. Aquatic arthropods respire through gills and book lungs.
The blood circulatory system in arthropods is unique. It is open circulatory system. The blood flows in the body cavity bathing the tissues of the body. However, there is a primitive heart and a main blood vessel situated dorsally. Blood is colorless as it is without hemoglobin. The skeleton is external, i.e., exoskeleton. It is in the form of an outer covering, the cuticle which is light in weight; and is formed chiefly of chitin. It provides surface for the attachment of muscles which help in locomotion.
The arthropods exhibit active and swift movements. They swim, crawl or fly depending upon the habitat they occupy. The organs of locomotion are paired appendages and in some cases paired wings also.