Arthropoda

Arthron→ Jointed

Podos → Legs

Von Siebold coined the term Arthropoda.

It is the largest group in an animal kingdom.

General characteristics of phylum Arthropoda:

• This is the largest phylum. The animal of this group is found in all habitats.
• They are triploblastic and bilaterally symmetrical.
• Body consist of jointed appendages or legs.
• The body is covered externally by a hard chitinous exoskeleton.It moulds periodically and replaces it by a new exoskeleton. It is called moulting.
• The body is divisible into head, thorax, and abdomen. In some head and thorax is often fused to form cephalothorax as in prawn.
• Cephalisation( A true head is present). It bears a pair of compound eyes, antenna and feeding apparatus.
• It is a haemo-coelom type.
• Locomotion takes place by paired lateral jointed appendages.
• The alimentary canal is straight and well developed. The mouth bears mouth parts for ingestion of food. Mouths part are modified for biting, chewing, sponging, piercing, siphoning.
• Respiration takes place by trachea or book lungs or gills.
• The circulatory system is open type consisting of perivisceral and pericardial sinuses. Blood vessels are present.
• Excretory organs take place by malpighan tubules, green glands or coxal glands. Voluntary muscles are present.
• The nervous system consists of a brain and double solid nerve cord like Annelida.
• Sense organs include antennae, compound eye, statocysts etc. The compound eye consists of many ommatidia to form several images.
• They are unisexual i.e. male and female sexes are separate. Fertilization is internal. They are either oviparous or ovoviviparous.
• Development is direct or indirect. In some species, parthenogenesis is seen.

Classification:

It is classified into four classes on the basis of the number of a locomotory organ. Around 9 lakh species are present.

Crustacea:

• Five pairs of walking legs.
• The body is divided into cephalothorax and abdomen.
• The body is externally covered by a chitinous cuticle.
• Head with two pairs of antennae.
• Respiration by gills.
• Example: Prawn, Crab, cyclops etc.

Myriapoda:

• Many pairs of walking legs.
• The body is divided into head and many segmented trunks.
• Head with one pair of antenna.
• Respiration by trachea.
• Example: Julus, Scolopendra.

Insecta or Hexapoda:

• Three pairs of walking legs.
• The body is divided into head, thorax, and abdomen.
• Head with one pair of antenna, compound eye and mouth parts.
• Respiration by trachea.
• Example: Periplaneta, Pieris.

Arachnida:

• Four pairs of walking legs.
• The body is divided into persoma, mesosoma, and metasoma.
• The head is without an antenna. It bears chelicerae and pedipalpi for capturing and tearing the prey.
• Respiration by book lung and trachea.
• Example: spider, scorpion.