The largest, most diverse phylum of animals is the Arthropoda. The name means jointed legs and all the animals in this group share that characteristic along with an exoskeleton. There are some other common Arthropod characteristics as well. They are all coelomate, meaning that they have a proper body cavity (coelom), although this may be greatly reduced, and they are protostomes, which means that in the embryological development of the egg, the blastopore becomes a mouth. Cleavage is sometimes spiral in primitive forms or highly modified in others. The animal that forms is bilaterally symmetrical.
Another common arthropod characteristic is segmentation. In early stages all segments have appendages but later some may be lost and many are specialised for different functions. Classification of the group is based in part on the nature of the specialised appendages. The outer covering of arthropods, the cuticle, is a hard exoskeleton made of chitin that provides protection but must be shed regularly in order for the animal to grow. This exoskeleton design also determines the upper size limit of arthropods which can never get as big as animals with an endoskeleton. Because of this hard exoskeleton, cilia are completely missing and movement is by muscular manipulation of the jointed appendages.
Internally, arthropods have a well developed contractile heart and circulatory system. The nervous system has a highly differentiated brain and a double ventral nerve trunk. The gut is also highly developed, a one way system leading from mouth to anus and with specialised modifications in different species for different lifestyles.
There are three major groups of Arthropods or subphyla: the extinct Trilobita, the Chelicerata and the Mandibulata. Trilobites once ruled the world but gradually fell behind and a couple of great extinction events finally took them all out. Only their hard parts have been preserved so we know little about their internal anatomy. They are different enough in external structure to be immediately recognisable and are so different from the other two groups that they warrant their own subphylum.
The other two groups are differentiated by their mouthparts. The chelicerates have 'chelicerae' which are appendages modified into jaws and appear to be homologous with the first antennae of mandibulates. Spiders, ticks and mites are all chelicerates as are scorpions, horseshoe crabs, sea spiders, whip scorpions, sun spiders and pseudoscorpions. The main characteristics of this group are: the head and thorax are joined in one body part with a separate abdomen so in general these animals appear to have two body parts. Chelicerates have no antennae and the first pair of appendages are the jaw-like chelicerae. These are followed by leg-like pedipalps which are often sensitive to touch and vibrations. The next four somites of the cephalothorax have a pair of walking legs each so these animals are usually 8 legged.
The mandibulates are all the rest of the arthropods and make up the biggest, most diverse group. They have mandibles which develop from a different set of appendages than the chelicerae and usually three body parts instead of two, with the head and thorax being separate. All insects and crustaceans are mandibulates along with the myriapods (centipedes and millipedes) The insects and myriapods dominate on land while the crustaceans reign over aquatic and marine habitats.
Aquatic mandibulates, the crustaceans, have the following common characteristics. First, because they are aquatic they have gills for respiration. On their heads, they have five pairs of appendages: first and second entennae, mandibles and then two pairs of maxillae. The body usually has a recognisable head, thorax and abdomen. The last segment, called the telson, contains the anus and has no appendages. There is a heart and is a typical arthropod circulatory system. There are a pair of eyes and a median eye as well. The sexes are usually separate and there is a nauplius larval stage. The legs are biramous (split in two) and this further separates them from the terrestrial mandibulates which always have uniramous legs.
Terrestrial mandibulates, the insects and myriapods, have lungs instead of gills for breathing. The millipedes and centipedes have a short head and long trunk with uniramous legs on every trunk segment. Insects have three body parts: head, thorax and abdomen, with the sensory organs, mandibles and antennae on the head and uniramous legs or wing appendages on the thorax. The abdomen is segmented and contains the gut and reproductive organs. The sexes are usually separate in all of these animals.
The number of species of arthropods in the world can only be estimated as many are still undiscovered and undescribed. Sadly, with the destruction of forests worldwide and the threats to coral reefs and other marine ecosystems plus the pollution of aquatic and terrestrial ecosystems, many species may become extinct before they are even noticed. It is estimated that in the Amazon alone a hundred species are becoming extinct every day because of clearing. There may be as many as 4-5 million Arthropod species worldwide and many must be considered critically endangered.
References: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press
Borror and Delong. Introduction to the Study of Insects. 5th Ed.