Ans. The reasons are following:
(1) Have organ system level of organisation.
(2) Bilaterally symmetrical 'segmented' triploblastic encoelomate animals found almost every where in all habits.
(3) Body enclosed by chitinous cuticle (exoskeleton).
(4) They have joined appendages.
(5) They have mouth parts of various types.
(6) Trachea/book lungs or book gills for respiration.