Phylum Arthropoda (General characteristics and classification)

Salient features:

  • Phylum Arthropoda is the largest group of the animal kingdom, surpasses all others both in number and ecological diversity of organisms.
  • They are mostly terrestrial but few are aquatic forms.
  • They are bilaterally symmetrical, metamerically segmented Their metamers are not alike but tare specialized and their number is generally fixed.
  • They have jointed appendages with variable function.
  • Exoskeleton is made up of chitinous cuticle that is shed at intervals. The process of casting off the skin or integument is known as ecdysis or moulting.
  • Body is divisible into head, thorax and abdomen in Insecta whereas into cephalothorax (head+ thorax) and abdomen in Crustacea and Arachnida.
  • Tube within a tube body plan is present.
  • Alimentary canal is complete which consists of foregut, midgut and hindgut.
  • Mouth parts are adapted for various modes of feeding (biting, cutting, chewing, sucking and piercing etc.) and hepatopancreatic gland is present.
  • Circulatory system is open type, with a dorsal heart, arteries and blood sinus.
  • The blood is colorless and contains WBC only.
  • The body cavity is full of haemolymph (blood). They are haemocoeloate, i.e. the true coelom is restricted to gonads.
  • Respiration takes place through general body surface or gills in crustaceans, through trachea in insects and Myriapoda, through book lungs in Arachnida and book gills in king
  • Excretion is brought about by green glands in aquatic forms and malphighian tubules or coxal gland in terrestrial arthropods.
  • Aquatic arthropods are ammonotelic whereas terrestrial forms are uicotelic.
  • Central nervous system is composed of paired preoral ganglia connected by commisures to a ventral nerve cord. The ventral nerve cord is double, solid with segmental ganglia and nerves.
  • Sense organs comprise of simple or compound eyes, antennae, chemoreceptor, statacyst
  • Unit of simple eye is called ocelli and of compound eyes is called ommatidia. Vision is mosaic.
  • Sexes are separate and are oviparous animals.
  • Fertilization is internal with direct or indirect development.


Phylum Arthropoda is divided into seven classes but modern classification includes Chilopoda and Diplopoda under a single class called Myriapoda.

Class: Onychophora

  • It is a connecting link between annelids and arthropods and are mostly terrestrial.
  • Body is without external segmentation.
  • Annelidan characteristics:
    • The integument is thin, soft and without exoskeleton.
    • Parapodia like appendages are present which are unjointed.
    • Excretory organs are nephridia.
  • Arthropodan characteristics:
    • Tracheal gills are present for respiration.
    • Appendages terminate into claws.
    • They have salivary gland and their coelom is reduced.
    • Head has a pair of simple eyes, a pair of antennae, a pair of oral papillae and a pair of jaws.
    • e.g. Peripatus

Class: Crustacea

  • Chitinous cuticle covers the body which is divisible into cephalothorax and abdomen.
  • Cephalothorax is covered by a carapace which bears spiny rostrum.
  • A cephalothorax bear two pairs of antennae, two pairs of maxillae, one pair of mandibles, generally a median eye and a pair of compound eyes.
  • They are ammonotelic and excretion is by green glands or antennary gland.
  • The appendages are modified for swimming, respiration and reproduction.
  • Respiration is carried out either by body surface or by gills.
  • Heart is arterial, i.e only oxygenated blood is present.
  • Sexes are usually separate with sexual dimorphism.
  • Development is indirect with Zoea larva.
  • Most of them are edible, constitute the food for aquatic or terrestrial life.
  • e.g. Palaemon (prawn), Cancer (crab)

Class: Myriapoda

  • They are mostly terrestrial.
  • Body is long and divisible into head and trunk.
  • Head consists a pair of antennae, a pair of simple eyes and two to three pairs of jaws.
  • Trunk consists of many segments with each segment having one to two pairs of legs.
  • Respiration occurs with the help of trachea.
  • Excretion takes place by malphighian tubules.
  • Development is direct.
  • e.g. Scolopendra (Centipede), Julus (Millipede)

Class: Insecta

  • They are mostly terrestrial and rarely aquatic.
  • Body is bilaterally symmetrical and segmented. It is divided into head, thorax and
  • Head bears a pair of compound eyes, a pair of antennae and different types of mouth parts (chewing and biting, piercing and sucking).
  • Thorax consists of three segments (prothorax, mesothorax and metathorax).
  • Wings arise from meso- and metathorax while legs arise from each thoracic segment. Abdomen neither consists of legs nor wings.
  • Respiration takes place by trachea or gills.
  • Heart is tubular and divided into chambers (in cockroach- 13 chambered).
  • Malphighian tubules help in excretion and are uricotelic.
  • Sexes are separate.
  • e.g. Periplaneta (Cockroach), Butterfly, Mantis, Housefly etc.

Class: Merostomata

  • They are exclusively marine forms.
  • Body is divided into prosoma (cephalothorax) and opisthosoma (abdomen).
  • Cephalothorax is covered by a carapace and bears a pair of simple eyes, a pair of large compound eyes, a pair of chelicerae, a pair of pedipalp (mouth parts) and five pairs of appendages (legs).
  • The anus ends in a sharp telson.
  • Respiration occurs by book gills and development is indirect.
  • e.g. Limulus (King crab)

Class: Arachnida

  • They are mostly terrestrial and rarely aquatic.
  • Body is divisible into prosoma (cephalothorax) and opisthosoma (abdomena).
  • Prosoma bears simple eyes, one pair of chelicerae, one pair of of pedipalp and four pairs of legs.
  • Antennae and true jaws are absent. Opisthosoma is usually without appendages.
  • Respiration takes place by book lungs.
  • They are unisexual and usually without sexual dimorphism.
  • Development is generally direct.
  • e.g Aramea (Spider), Palamnaeus (Scorpion)

Phylum Arthropoda (General characteristics and classification)