PHLUM ARTHROPODA
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(arthros- joint; poda-appendages)
They are the animals with jointed legs and appendages.
Introduced by Von Siebold.
It is the largest phylum in the animal kingdom.
It includes 2/3 of the total named species of the animal world on earth.
They are evolved about 600 million years ago. Trilobites are fossil arthropods that existed about 600 million years ago.
TRILOBITES
The horse-shoe crab, an ancient relative of the Arachinds is found living in warm estuarine zones like the Sunderbans, in Bengal.
Important characteristic features.
• They are cosmopolitan in distribution. They are found in everywhere, in freshwater, in marine water, on land and in the air.
• They are joint footed animals. They possess an organ system level of body organisation.
• Bilaterally symmetrical.
• They are triploblastic animals.
• They are segmented animals.
• They are coelomates. True coelom is reduced. The body cavity is filled with blood and is called haemocoel.
• The segmented body is divided into 3 regions- head, thorax and abdomen, e.g, insects. In some cases head and thorax fuse to form a single region called cephalothorax
eg: Arachinds, Crustaceans, etc.
In some arthropods the body is divided into head and trunk only eg: myriapods.
• Exoskeleton: the whole body is covered by chitinous jointed exoskeleton. The exoskeleton is light weight, tough and composed of proteins and a polysaccharide known as chitin, resulting in a chitinous cuticle. Casting of old exoskeleton is known as moulting or ecdysis.
• Podomeres: They have jointed appendages which are differentially modified to perform different functions. Each body segment bears a pair of segmented appendages. Their segments are called podomeres.
• Circulatory system: It is open type with a dorsal heart.
• Digestive system: It is well developed and complete. The mouth is surrounded by mouth parts which are modified into different types according to the feeding habits of different arthropods.
• Respiratory system: Respiratory organs are gills (eg: prawn) or gill books (eg: horse shoe crab) in aquatic forms, and book lungs (eg: scorpion) or tracheal system (eg: insects)in terrestrial forms.
• Excretion: Excretory organs are green glands or malphigian tubules. Nephridia are absent.
• Cilia is absent.
• Brain is present.
• Sensory organs are well developed. Eyes may be simple or compound.
Compound eye is made up of a number of independent visual units called ommatidia each with its own lens.
Sound receptors are present in chirping crickets and cicadas.
Heads of many arthropods have feelers called antennae for perceiving odour and sensation of touch.
Insects also communicate by chemicals called pheromones.
• Reproduction: they are mostly dioecious. Fertilisation is external in aquatic arthropods and internal in land arthropods.
• Arthropods are generally oviparous. Some are viviparous. In such arthropods the eggs hatch within the female and the young ones are born.
• In many arthropods the development is direct and the young ones resemble the adult.
In several groups of insects the development is indirect. In such arthropods the egg hatches into an independent larva which does not resemble the adult.
The common arthropods are: Araneus (garden spider), Limulus (king crab), Eupagurus (hermit crab), Cancer, Macrobrachium (prawn), Lepisma (silver fish), Periplanata (cockroach), Apis (bee), Anopheles (mosquito), Musca (housefly), Triops (tadpolefish), Daphnia (water flea), Xensopsylla (rat flea), Cyclops, Squilla, Astacus (cray fish), Barnacle (Lepas and Balanus)
Classification of phylum Arthropoda:
Class I: Crustacea-Mostly aquatic with gills as respiratory organ (eg: prawn, crab, hermit crab, barnacles, etc.)
Class II: Myriaopoda- terrestrial, air breathing and many legged (eg : millipede, centipede, etc)
classIII: Arachnida – air breathing with 4 pairs of legs (eg: spider, scorpion, king crab, etc.)
Class IV: Insecta – air breathing with 3 pairs of legs (eg: cockroach, butterfly, Lepisma, Oryctes, Leptocorisa, Musca, Apis silkworm, etc.)
Class V: Onychopora – terrestrial, air breathing with claw-bearing legs (eg: Peripatus, a living fossil)
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