Phylum Arthropoda has over 6 million species and have joint appendages. They have an exoskeleton that is composed of chitin. They serve to protect, anchor muscles, and prevent themselves from drying out.  Some jointed segments in arthropods are fused into regions which are the head, thorax, and abdomen. Insects, arachnids, centipedes, and millipedes contain species adapted to terrestrial life.

Example of body structure of an arthropod:

They have well-developed nervous system, a brain and solid ventral nerve cord. Their compound eye forms many complete visual units and can detect any kind of movement. Some have simple eyes and that has a single lens dispersing image to many receptors. Their exoskeleton is calcified and their body forms are very diverse. Their head has a pair of compound eyes, two pairs of antennae, three pairs of mouthparts. In crayfish, head and thorax are fused into cephalothorax covered by a carapace that covers its gills. Abdominal segments are equipped with swimmers. Their first pair is enlarged in male for passing sperm to female. They have fan-shaped tail ends in uropods and telson. A pair of green glands excrete wastes at the base of the antennae. Coelom is reduced to cavity around reproductive system; hemolymph washes respiratory pigment (hemocyanin) through hemocoel sinuse, an open circulatory system. They have a ventral nerve cord and a ganglia system similar to an earthworm.

Sexes are separate because the gonads are ventral to pericardial cavity and sperm exit at base of fifth leg, ovaries are open at the base of third walking leg, and eggs are attached to swimmerets of the female.

In insects, they have a huge variety of species that include dragonflies, grasshoppers, crickets, beetles, moths, butterflies, flies, bees, wasps, and much more! Insects have three body regions. They have a head, a thorax, and a abdomen. Their head consists of antennae, eyes and mouthparts. The thorax has three pairs of legs, adults have more or two pairs of wings that help them evade enemies, to find food, disperse, and more. Their abdomen contains of internal organs. Insects have an exoskeleton that is lighter with less chitin than other arthropods.