List three important distinguishing characters of phylum Arthropoda.

Explanation

(i)The body of arthropods are externally segmented. The segments are not separated internally by septa as in annelids.
(ii) The symmetry is bilateral in arthropods. They have three germ layers. Hence, they are triploblastic.
(iii) There is organ-system level of organisation.
(iv) Anterior parts of the body form a distinct head. It bears various types of mouth parts and sense organs.
(v) Arthropods bear jointed appendages. This character gives the phylum its name.
(vi) The body wall consists of a thick, chitinous cuticle and a single layered epidermis. The hard, cuticle shed periodically. The process is called ecdysis or moulting
(vii) The digestive tract is complete. Mouth bears movable appendages, the mouth parts. Digestion is intercellular. Alimentary canal is divisible into foregut, midgut and hindgut.
(viii) Respiration may occur through general body surface. Various respiratory structures are gills, tracheae, book lungs etc. are present.
(ix) The circulatory system is open type, i.e., the blood flows in open spaces (in haemocoel). The excretions occur with the help of green glands or malpighian tubules. The excretory product is ammonia in aquatic form and uric acid in land form.
(xi) The nervous system comprise of central, peripheral, and sympathetic parts.
(xii) Arthropods are unisexual. Fertilization is external in few species but it is internal in majority of species. Asexual reproduction is absent.