Arthropoda is the largest phylum of Animal Kingdom. It includes about 1,13,40,000 species in all habitats. This constitutes about 83% of all the known animal species on earth. This huge number of species included in this phylum speaks volumes about the success of the species included in Arthropoda. Arthropoda includes spider, scorpions, prawns, crabs, millipedes, centipedes and many other insects.
Arthropoda is characterized by heteronomous metamerism, chitinous exoskeleton and joined appendages. The evolutionary acquisition of these traits is known as arthropodization.
General Characters of Phylum Arthropoda
- Arthropods occur in all types of habitats i.e., marine, fresh water and terrestrial.
- There is an increasing tendency of cephalisation in arthropods.
- In Arthropods, body is metamerically segment; composed of a linear series of segments or somites. Externally endoskeleton is divided into hardened segmental scletites. Internally the nervous system, muscular system and heart chambers are segmentally repeated.
- Metamerism is heteronomous. Segments and appendages are specialized for different functions. Arthropods are characterised by tagmosis. Head, thorax and abdomen are three typical tagmata.
- Appendages are joined. They act as lever systems providing mechanical advantages for locomotion.
- Muscles of arthropods are striated
- Body wall is muscular and consists of chitinous cuticle. The chitinous cuticle or the exoskeleton is one of the key features for the arthropod success. It offers protection, prevents loss of body fluids. It also undergoes moulting to facilitate growth.
- Coelom is reduced in association with a shift from fluid internal skeleton to a solid external skeleton. It is associated with gonads and saccate nephridia. Most of the small coelomic spaces which appear during the development get obliterated with the blastocoel and contribute to haemocoel. Some become excretory organs and gonads. Haemocoel is the functional body cavity and is filled with blood.
- Alimentary canal consists of three regions: foregut which is responsible for ingestion, storage and trituration. Mid gut which helps enzyme secretion and digestion. Hind gut is responsible for the formation of faeces and reabsorption of water.
- In very small crustaceans exchange of the respiratory gases occurs through the general body surface. Large aquatic arthropods respire through gills and book gills, whereas terrestrial forms respire through trachea and book lungs.
- Blood vascular system is of open type. Heart id dorsal in position. Respiratory pigment if present is haemocyanin. It contains copper and is blue in colour. Haemoglobin occurs in few.
- Saccate nephridia (Coxal glands and green glands) are the characteristic excretory organs of the aquatic arthropods. Malphigian tubules are the characteristic of terrestrial arthropods. Some arthropods have both saccate nephridia and malphigian tubules.
- Nervous system consists of nerve ring and a ganglionated double ventral nerve cord.
- Sense organs include compound eye, simple eyes, antennae, statocycts etc.
- Most arthropods are unisexual. Fertilization may be external or internal in aquatic arthropods but internal in all the terrestrial forms.
- Eggs are centrolecithal (yolk in centre). Cleavage is meroblastic and superficial. Development may be direct or indirect. Like annelids, they also show teloblastic growth.
Classification of Phylum Arthropoda
The animals included in phylum Arthropoda have different views concerning their phylogeny. So there is no absolute system of classification for this phylum. The below given classification is the most accepted one. Through seven subphyla are included in this classification, only three subphyla namely Trilobita, Chelicerata and Mandibulata are definitively arthropods.
The following is the classification of phylum Arthropoda:
Sub Phylum I: Trilobita (Gr. tri=three, lobos=lobes)
- This subphylum includes extinct arthropods which were abundant during Paleozoic era.
- These fossil trilobites were exclusively marine bottom dwellers
- Their body can be divided into head, thorax and pygidium.
- A pair of longitudinal axial furrows divided the body into median axial lobe and two lateral pleural lobes
- Head was distinct with one pair of antennae and compound eyes.
- All the post-antennal appendages were biramous and unspecialized.
Examples: Triarthus, Dalmanites
Sub Phylum II: Chelicerata (Gr. chele=claw, keros=horn)
- The body of the animals belonging to this subphylum can be divided into two cephalothorax and abdomen.
- The abdomen is further divided into anterior mesosoma and the posterior metasome with a telson.
- Cephalothorax has six pairs of appendages of which the first pair is chelicerae
- Antennae are absent.
Subphylum Chelicerata is further divided into the following two classes,
Class I: Xiphosura (Gr. xiphos=sword, oura=tail)
- This class included horseshoe crabs. All the genera of this class are extinct except three
- Animals of this class are marine in nature
- Cephalothorax bears one pair of chelicerae, four pairs of walking legs and one pair of pusher legs.
- Mesosomal appendages are modified into a genital operculum and five pairs of book gills
- They have median ocelli and lateral compound eyes
- Excretory organs are coxal glands
- Development is indirect and includes trilobite larva.
Class II: Arachnida (Gr. Arachne =spider)
- This class includes scorpions, ticks, mites and spiders.
- Mostly these animals are terrestrial
- Cephalothorax has one pair or preoral chelicerae, one pair of postoral pedipalps and four pairs of winged legs.
- The spiders bear fangs with poisonous glands on each chelicera
- Abdominal appendages are modified into book lungs, spinnerets in spiders, pectin in scorpions etc.
- Telson is usually absent but is present as sting in scorpions
- Excretory glands are coxal glands and Malphigian tubules
- Development is direct.
Examples: Palamnaeus, Aranea
Subphylum III: Mandibulata (L. mandibula=mandible, ata=bearing)
- Mandibles are the first pair of mouth parts
- The first pair of appendages are antennae
This subphylum is further divided into the following four classes:
Class I: Crustaceae (L. Crusta =shell)
- This class includes prawns, crabs, lobsters, crab fishes etc.
- They are mostly marine water dwellers. Few also exist as freshwater forms.
- Few of these animals are terrestrial but they are not well adapted to terrestrial life.
- In most of the species the head and thorax fuse to form cephalothorax.
- Cephalic appendages are five pairs namely First antennae, Second antennae, Mandibles, First maxillae, Second maxillae
- Thoracic appendages are biramous
- Respiration takes place with the help of gills or general body surface in small forms
- Excretion is through green glands
- Sensory organs include statocysts, compound eyes and antennae
- Gonopores are paired.
- Development is direct or indirect involving several larval stages, The basic larva is nauplius.
Examples: Palaemon, Balanus
Class II: Chilopoda (Gr. chelios =lips, podos=foot)
- This class includes centipedes.
- These animals are terrestrial and carnivorous.
- The body of these animals is divisible into head and trunk.
- They are trignathic with mandibles, first maxillae and second maxillae
- Each segment of the trunk bears one pair of clawed edges.
- Appendages of the first trunk segment bears poisonous claws
- Respiration is through trachea
- Excretory organs are Malphigian tubules
- Centipedes are ophisthogoneate
- Development is direct or indirect
Examples: Scolopendra, Scutigera
Class III: Diplopoda (Gr. diplos =double, podos=foot)
- This class includes millipedes
- They are terrestrial and detritivorous
- The body is divisible into head and trunk.
- They are dignathic with mandibles and gnathochilarium.
- Most of the trunk segments are diplosegments, formed by the fusion of two segments during development
- Each diplosegments, formed by the fusion of two pairs of legs and two pairs of spiracles
- Respiratory organs are tracheae
- Excretory organs are Malphigian tubules. They are progoneate
- Development is direct
Examples: Spirobolus, Julus
Class IV: Hexopoda (Gr. Hex =six, podos=foot)
- This class is also known as Insecta as it includes insects
- These insects are present in all habitats except the marine habitat.
- The body of the insects is divided into head, thorax and abdomen.
- Thorax bears three pairs of joined legs hence the name Hexapoda.
- Respiratory organs are tracheae
- Excretory organs are Malphigian tubules
- Insects are uricotelic. And this is a water conservation adaptation
- Development includes metamorphosis.
Examples: Musca, Lepisma
- Enumerate the general features of Phylum Arthropoda?
- Chart the classification of phylum Arthropoda.
- Write abbout the sub phylum Trilobita and the animals included in this group.
- What is the meaning of the term Hexapoda and Diplopoda?
- Give the examples of the class Crustaceae.
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