Describe salient features of phylum Arthropoda.

Arthropoda (Arthros: Joint, Podos: leg): Arthropoda forms the largest phylum of kingdom Animalia. 

Characteristics of Arthropoda: 

a. Habitat: Arthropods are omnipresent. 

b. Forms: Solitary or colonial, most of them are free-living. Barnacles are sedentary. Few are parasitic and sanguivorous, (e.g. Female mosquito, bed bug.) 

c. Body symmetry: Body is bilaterally symmetrical. 

d. Germ layers: They are triploblastic. 

e. Body cavity: Arthropods are eucoelomates. 

f. Body plan: They show tube within tube body plan. 

g. Level of body organization: They show organ system level of organization. 

h. Special features: The members of this phylum have jointed appendages. Hence, they are known as arthropods. Some insects like honey bee, ants, termites, etc. exhibit polymorphism. 

i. Exoskeleton: Body is covered by a tough, non – living chitinous exoskeleton. As the exoskeleton does not allow body growth, arthropods shed off their exoskeleton periodically during growth. This process is called moulting or ecdysis. 

j. Body division: Body is divided into head, thorax and abdomen. 

k. Segmentation: Body shows metameric segmentation. 

l. Digestion: Digestive system is complete and divided into foregut, midgut and hindgut. 

m. Circulation: Circulatory system is of open type wherein, blood flows through body cavity called haemocoel. 

n. Respiration: Respiration occurs through respiratory organs like gills, trachea, book lungs or book gills. 

o. Excretion: Excretion takes place by green glands, Malpighian tubules or coxal glands. 

p. Nervous system: Nervous system consists of nerve ring and double, ventral ganglionated nerve cord. 

q. Sense organs: Arthropods have well developed sense organs in the form of antennae, simple or compound eye and various receptors. 

r. Sexual reproduction: Sexes are generally separate in arthropods with distinct sexual dimorphism. 

s. Significance: Beneficial arthropods: Some arthropods are of economic importance. For example, Honey bees (Apis) are important for their honey and wax, silk worms for the production of silk. Lobsters, prawns, crabs are edible. Harmful arthropods: Some arthropods are harmful and act as vectors to spread various diseases, e.g., Mosquitoes. Locusta (locust) is a gregarious pest. Limulus (King crab) is a living fossil. Other examples: Cockroach (Periplaneta), butterfly, scorpion (Hottentotta) and millipede (Archispirostreptus) prawn.