These are bilaterally symmetrical, triploblastic, schizocoelomate, segmented animals.
- They have organ system grade of organisation.
- They have jointed appendages
- Body is covered by chitinous exoskeleton. Body is divided into 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.
