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

Characteristics of Arthropoda:

(i) Triploblastic, bilaterally symmetrical and metamerically segmented animals.

(ii) Body segments are grouped into two regions - cephalothorax (head and thorax together) and abdomen, or three regions- head, thorax and abdomen. Anterior part of body forms a distinct head, bearing sense organs and brain.

(iii) Exoskeleton of cuticle, containing protein, lipid, chitin and often calcium carbonate is secreted by underlying epidermis and shed (moulted) at intervals.

(iv) Allmotary canal is complete; mouth and anus lie at opposite ends of the body.

(v) Respiration through general body surface, by gills, air tubes (tracheae) or book-lungs.

(vi) True nephridia absent, Excnition by coelomoducts, malphigian tubules or green or coaxl elands

(vii) Sexes are usually seperate, sexual dimorphism is well marked in several forms.

(viii) Fertilisation is usually Internal, oviparous or ovoviviparous and often with metamorphosis.

Classes and their examples:

(i) Crustacea: Palaemon (prawn)

(ii) Myriapoda: Scolopendra (centipede)

(ill) Insects: Leplema (Silver fish)

(iv) Arachnids: Limulus (king crab)