The reasons are following:
(a) Have organ system level of organisation.
(b) Bilaterally symmetrical ‘segmented’ triploblastic coelomate animals found almost every where in all habits.
© Body enclosed by chitinous cuticle (exoskeleton).
(d) They have joined appendages.
(e) They have mouth parts of various types.
(f) Trachea/book lungs or book gills for respiration.