Phylum Arthropoda

Animals are those organisms that are eukaryotic, multicellular, and heterotrophic in their mode of nutrition. Animal cells do not have cell walls. Except for a few, most animals are mobile. Multicellular animals are often called metazoa which has been divided into two branches: Parazoa and Eumetazoa. In Parazoa, the animal’s body is formed of loosely aggregated cells. These animals have poorly differentiated tissues and have no organ or digestive cavity (e.g., sponges). However, in Eumetazoa, the cells are properly arranged into tissues and organs. Their digestive tract is the mouth but with or without an anus. Eumetazoa is further classified on the basis of 

• the number of germ layers present in the embryo; 
• the symmetry of the body of the organism; and 
• the mode of origin of mouth. 

Those animals which have two germ layers in the embryo are called diploblastic animals and those having three germ layers are called triploblastic animals. Further classification of animals is on symmetry, on basis of mouth origin, and based on the presence or absence of body cavity or coelom.

Animals do not have cell walls and chlorophyll. The central vacuole is absent. They store food as glycogen and fat. Animals can transmit impulses due to the presence of nerve cells. Sponges do not have nerve cells. Centrioles are present. 

Phylum Arthropoda

This is the largest phylum of Animalia. Hereunder, are the distinguishing features of Arthropoda:

1. The body consists of the head, thorax, and abdomen. There is a distinct head.
2. They have jointed appendages.
3. The body is covered with a thick, touch and non-living chitinous cuticle, which forms the exoskeleton. 
4. The true coelom is greatly reduced in adults, and is only represented by the cavities of the reproductive and excretory organs. The body cavity is a haemocoel viz., cavity filled with blood.
5. Digestive tract is complete. The alimentary canal consists of the stomodaeum(fore gut), mesenteron(mid gut), and proctodaeum(hind gut).
6. The blood vascular system is of open type viz, Blood does not flow in definite vessels. There are present irregular spaces, known as lacunae or sinuses, filled with blood to form the haemocoel.
7. The respiratory organs are gills or book gills in aquatic forms and tracheae or book-lungs in terrestrial forms. The exchange of gases is also carried on by the general body surface in some forms.
8. The excretory organs are either green glands or Malpighian tubules. In some forms, coxal glands are excretory organs.
9. The annelids type of nervous system is present,viz., it consist of a nerve ring and a double ganglionated ventral nerve cord.
10. In many arthropods, compound eyes are present, in which mosaic vision is developed.
11. An important feature of the arthropods is the complete absence of the cilia.
12. Endocrine glands are absent.
13. Sexes are separate and the sexual dimorphism is observed in many forms. The development of the animal may involve metamorphosis.
14. Parental care is often seen in many arthropods.
15. Fertilization is usually internal, oviparous, or ovoviviparous.
16. Arthropods form the largest phylum of the Animal kingdom. About 900,000 species are known.
17. Examples: Economically important insects- Apis(Honey bee), Bombyx(silkworm), Laccifer(lac insect).

Classification

Class 1 Onychophora

• The body is elongated, segmented, and covered with a thin cuticle. 
• Peripatus is an important example of the class Onychophora which has characters of phylum Annelida and Arthropoda. Hence it is called connecting link.

Class 2 Crustacea

• The body is divisible into cephalothorax(head+thorax) and abdomen. 
• Dorsally, the cephalothorax is covered by a thick exoskeleton carapace. 
• There are present two pairs of antennae and a pair of stalked compound eyes. 
• Respiration is carried out either by body surface or by gills, e.g., Palaemon(prawn), Astacus(crayfish), Cancer(crab), etc.

Class 3 Insecta 

• The body is divisible into the distinct head, thorax, and abdomen.
• The abdomen lacks appendages, the head has a pair of antennae and mouth parts and the thorax has three pairs of legs and two pairs of wings.
• Respiration by tracheae. Examples: Lepisma(silverfish), Melanopus(grasshopper), Pierces(butterfly),etc.

Class 4 Arachnida 

• Body divisible into cephalothorax and abdomen.
• Antennae and true jaws are absent. 
• Respiration by tracheae, book-lungs, or book gills. 
• Examples: Limulus(king-crab), Aranea(spider), etc.

Class 5 Merostomata. 

• Limulus, (the king or Horseshoe crab) is a good example of class Merostomata which respires through book gills. The king’s crabs are called living fossils. A living fossil is a living animal of ancient origin with many primitive characters.

Class 6 Chilopoda 

• The body is divisible into the head and trunk. 
• The first pair of legs are modified into poison claws.
• There is a single pair of antennae. 
• Respiration occurs by tracheae. 
• Excretion takes place by Malpighian tubules, e.g., Scolopendra(Centipede).

Class 7 Diplopoda 

• Body is divisible into the head, thorax, and abdomen. 
• There is a single pair of antennae.
• Respiration occurs by tracheae. 
• Excretion takes place by Malpighian tubules, e.g., Julus(millipede).

Chilopoda and Diplopoda were earlier included in a single class Myriopoda.

• The body is elongated and metamerically segmented with each segment having one or two pairs of legs. 
• Respiration by tracheae. Examples: Scolopendra(centipede), Julus(millepede).

Differences between Annelids and Arthropods

• The appendages of Annelids are unjointed whereas those of Arthropods are jointed.
• Annelids have a well-developed true coelom but Arthropods have a small coelom. Instead, they have blood filled with cavities called hemocoel.
• A chitinous exoskeleton is absent in Annelids and present in Arthropods.
• The sensory system is less developed in Annelids whereas well developed in Arthropods.
• Annelids’ locomotory organs include parapodia and chaetae but legs and wings are those of Arthropods.

FAQs on Arthropoda

Question:1  Define the largest phylum of Animalia.

Answer:

Phylum Arthropoda- The Animals with Jointed Feet is the largest phylum of Arthropoda. The body consists of the head, thorax, and abdomen. There is a distinct head. They have jointed appendages.

Question:2 What are the five main classes of arthropods?

Answer: 

Onychophora, Crustacea, Myriapoda, Insecta, and Arachnida are the main classes of Arthropoda.

Question:3 Where do most arthropods live?

Answer:

Arthropods are found in virtually every known marine (ocean-based), freshwater, and terrestrial (land-based) ecosystem, and vary tremendously in their habitats, life histories, and dietary preferences.

Question:4 What is the study of Arthropods called?

Answer:

Arthropodology is known as the study of Arthropods.

Question:5 Give examples of Arthropods.

Answer:

Examples of arthropods are lobsters, crabs, spiders, mites, insects, centipedes, and millipedes.

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