Updated by: Arabinda Naik
(Arthron — foot) include the first and simplest segmented animals. These are commonly called as jointed legged animals. It is the largest group of animals that include about 1,000,000 insects, 102. 248 spiders and scorpion species. 103,248 arachnoid species and 47000 crustacean species representing about of total known animal species.
Habitat and Habit They may be aquatic or terrestrial. They may occur as free-living parasitic forms also. e.g., Bed bugs, ticks, mosquitoes etc.
Body parts Body is segmented externally. It has distinct head. thorax and abdomen. Head bears many fused segments and sense organs.
Symmetry and Body Organisation Arthropods are bilaterally symmetrical. They are triploblastic with organ system level of organisation.
Appendages They have jointed. paired appendages, which are present in some or all somites or segments. These perform various functions like walking. clinging, jumping. feeding. etc.
Exoskeleton The exoskeleton is formed of thick, tough, and non-living chitinous cuticle. Exoskeleton protects animal from mechanical and chemical injuries, prevents desiccation and helps in attachment.
Locomotion Jointed appendages help in locomotion. Cilia arc found to be absent in arthropod.
Digestive System Alimentary canal is complete and well developed. Divided into foregut, midgut, and hindgut.
Respiration It occurs through general body surface, gills (e.g. Prawn), book lungs (e.g. Scorpion and spider), trachea (e.g. cockroach) and book gills King crab).
Circulation The circulatory system is open type, i.e., blood flows in the haemocoel instead of blood vessels.
Excretion The excretory organs are Malpighian tubules (opening into duct) or green glands opening directly to the exterior.
Nervous system: The nervous system consists of a nerve ring and a double ganglionated ventral nerve cord.
Sense Organs: Arthropods have simple eyes called ocelli and compound eyes. Aquatic forms have Statocysts for balancing.
Reproduction: Arthropoda reproduce by sexual methods. Fertilisation is internal type. They are generally oviparous, but few are viviparous. i.e., scorpion. Development may be direct or indirect involving metamorphosis.