The phylum arthropoda is made up of crustaceans, insects, spiders and their relatives. Antropods have more species than the other phylas put together making them the largest phylum. There are millions of species within the phylum that have not yet been described, it is believed that the tropical forest are home to nearly 9 million species. The arthropods can be found all over the world. The worst famines and plagues known to man have been contributed to members of the phylum while other arthropod species have given us clothing, food, medications and protection from organisms that would otherwise hurt us.
There are some characteristics that the majority of the phylum of arthropods share. The bodies of the arthropods which are bilaterally symmetrical are segmented. This segmentation has an affect on the internal and external structure of the body. Special body sections known as tagmata are formed by the fusing of some segments known as tagmosis. The abdomen, head and thorax are included in these body parts.
An exoskeleton which is made up from chitin which is a polysaccharide in a protein mixture covers the body of arthropods. There are also other proteins, calcium carbonate and lips in the exoskeleton. Each segment of the body has a pair of jointed appendages. Several of the appendages are modified or lost in every living arthropod. The shedding of the exoskeleton is how the arthropod usually grows. The appendages movement is controlled mainly by the muscular system. There are a pair of compound eyes on most arthropods and one or many simple eyes. There are some groups that have fewer or no simple eyes.
Most of arthropods body cavities are open with loose fitting sinuses, tissues and blood. They have an open circulatory system that is made up of the arteries, heart and open spaces in the hemocoel. There is a complete gut on the arthropod. The surface of the body is where the respiration of the arthropod takes place either through the use of a tracheae, book lungs or gills. They have brains and a nerve ring that surrounds the pharynx connecting the brain with a pair of ventral nerve cords. A large number of ganglia can be found in the cords.
The majority of arthropods are dioecious and have pairs of testes and ovaries. While some arthropods have internal fertilization not all of them do. Eggs are laid by most of them with development usually being in the form of metamorphosis.