These are bilaterally symmetrical, triploblastic, schizocoelomate, segmented animals.
They have an organ system grade of organisation.
They have jointed appendages.
Body is covered by a chitinous exoskeleton. The body is divided into the head, thorax, and abdomen.
Body cavity is filled with colourless blood. It is called haemocoel.
Respiratory organs are gills, book gills, book lungs, trachea.
Open type circulation is seen.
Sense organs are present.
Fertilization is internal, e.g., Limulus and insects.