- largest phylum of animal kingdom.
- found in every possible habitat of earth.
- multicellular, triploblastic, bilaterally symmetrical, metamerically segmented, system grade of body organization.
- jointed appendages for locomotion, food collection and sensory purposes.
- body externally covered by hard chitinous cuticle.
- body divided into head, thorax and abdomen. In some head and thorax fused to form cephalothorax.
- complete alimentary canal.
- mouth parts are adapted for various modes of feeding like chewing, piercing and sucking.
- respiration through general body surface, gills, trachea or book lungs.
- excretion by malphigian tubules or green glands.
- haemocoelomate i.e. their body cavity is filled with blood.
- open type of circulatory system consisting heart, arteries and blood sinuses.
- well developed sensory organs including simple or compound eyes, antennae, statocyst (balancing organ), ocelli (photoreceptor).
- mostly unisexual and development is indirect.
Image source: thebiologynotes
Arthropods are divided into 4 classes on the basis of locomotory organs. They are:
Class 1: Crustacea
- mostly aquatic.
- body divided into cephalothorax and abdomen.
- head bears two pairs of antennae and stalked compound eyes.
- five pairs of walking legs.
- respiration by gills or general body surface.
- excretion by green glands.
- examples: Cancer (crab), Palaemon (prawn)
Class 2: Myriapoda
- mostly terrestrial.
- body divided into head and trunk or head, thorax and abdomen.
- from each segment arises one or two pairs of walking legs.
- head bears a pair of antennae.
- respiration by trahea and excretion by malphigian tubules.
- examples: Scolopendra (centipede), Julus (millipede)
- mostly terrestrial and some are aerial.
- body divided into head, thorax and abdomen.
- head bears a pair of antennae and a pair of compound eyes.
- from thorax arises three pairs of legs and in some one or two pairs of wings.
- excretory organ malphigian tubules and respiration by trachea.
- examples: Periplaneta americana (cockroach), Pieris (butterfly), Bombyx (moth)
Class 4: Arachnida
- mostly terrestrial, few are aquatic.
- body is divided into cephalothorax (prosoma), and abdomen (ophisthosoma) or prosoma, mesosoma, metasoma.
- head without antennae.
- eyes are simple.
- have four pairs of walking legs, a pair of chelicere and a pair of pedipalp.
- respiration by book-lungs or general body surface.
- examples: Aranea (spider), Palamnaeus (scorpion)
Image source: otosection
Class 5: Onychophora
- small, terrestrial animals found below rocks, crevices, under logs, etc.
- elongated worm like body without external segmentation.
- appendages not joined but lobe like.
- head bears a pair of antennae and simple eyes.
- walking legs are lobe like terminating in claw.
- respiration by trachea and excretion by nephridia.
- examples: Peripatus (walking worm) i.e. connecting link between annelida and arthropoda.
Phylum: Arthropoda, its classification and characteristics