Question 12: Explain the general characters of phylum Arthropoda. Give one example of following classes (i) Crustacea; (ii) Myriapoda; (iii) Insecta; (iv) Arachnida.

General characteristics of phylum Arthropoda: 

(a) They are triploblastic, bilaterally symmetrical and metamerically segmented animals.
(b) Body segments are grouped into two regions - cephalothorax and abdomen or three regions - head, thorax and abdomen. 
(c)Body cavity is haemocoel and the coelom is reduced to the spaces of the genital and excretory organs.
(d) They have a complete alimentary canal, with mouth and anus lying opposite each other.
(e) Sexes are separate. Sexual dimorphism is well marked in several forms. Fertilisation is usually internal, oviparous or ovoviviparous and often with metamorphosis.
(f) They have gills, trachea or book-lungs for respiration.
(g) They lack true nephridia and excretion occurs via coelomoducts, malpighian tubules or green or coxal glands.
(h) Circulatory system is open with dorsal heart, arteries and haemocoel but without capillaries and veins.
(i) Their exoskeleton comprises cuticle containing protein, lipid, chitin. Often, calcium carbonate is secreted by underlying epidermis and shed at intervals.

Examples:
(i) Class Crustacea: Palaemon
(ii) Class Myriapoda: Scolopendra
(iii) Class Insecta: Lepisma
(iv) Class Arachnida: Sarcoptes