■largest phylum (two-third of recognized animals) ■bilaterally symmetrical ■covered by chitinous exoskeleton ■ body consists of head, thorax and abdomen ■ have jointed appendages ■Respiratory organs: gills, book gills, book lungs or tracheal system ■ Sensory organs: antennae, eyes, statocysts or balancing organs are present. ■ Excretion - malpighian tubule ■ Mostly dioecious. ■Fertilisation is internal ■ mostly oviparous
