Phylum: Arthropoda, its classification and characteristics

• 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

Classification

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)

Class3: Insecta

• 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.

References: 

i) https://manoa.hawaii.edu/exploringourfluidearth/biological/invertebrates/phylum-arthropoda

ii) https://courses.lumenlearning.com/wm-biology2/chapter/phylum-arthropoda/

Phylum: Arthropoda, its classification and characteristics