Biology Sikshya

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   Gr.,arthros=joint; podos=foot

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• They are mostly terrestrial, but some are aquatic.

• They may be herbivores, carnivores, omnivores or may be parasitic in nature.

• Triploblastic animals i.e., body derived from 3 germ layer (ectoderm, mesoderm and endoderm).

• Body is bilaterally symmetrical.

• Body is metamerically segmented.

• True coelom is reduced to form a blood filled cavity called haemocoel. So, these animals are termed as Haemocoelomate.

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• Jointed appendages are found, which have diversified functions. Walking legs are present those helps in locomotion.

• Respiration is direct (through normal body surface) or through gills or tracheae or may be the book lungs.

• Digestive system is complete type.

• Excretion takes place through Malpighian tubules or green glands.

• Blood vascular system is open type. A dorsal heart is present. 

• Nervous system includes a dorsal brain, a ventral primitively paired longitudinal nerve cord with ganglia from which lateral nerves extends from each segment.

• Sense organ may includes simple eyes (oceli and stemmata), compound eyes, tactile or chemo-receptors, balancing and auditory organs.

• Sexual reproduction takes place. Development is mostly indirect i.e., through various larval stages.

• Mostly fertilization is internal, but most of the aquatic species have external fertilization.

• Dioecious animals.

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