Phylum - Annelida & Arthopoda - NEET PDF Download

Annelida

Phylum Annelida is a very broad phylum belonging to the kingdom Animalia. The Annelids are found in aquatic as well as terrestrial environments. These are bilaterally symmetrical invertebrate organisms. Their segmented body distinguishes them from any other organism.
Annelida

Characteristics of Annelida

The characteristics of the organisms present in the Phylum Annelida are as follows:

• The Annelids are coelomate and triploblastic.
• They exhibit organ system level organization.
• Their body is segmented.
• They respire through their body surface.
• Nephridia are the excretory organs.
• They have a well-developed circulatory and digestive system.
• Their body contains haemoglobin, which gives them a red colour.
• Regeneration is a very common characteristic of the Annelids.
• Setae help them in movement.
• Most of the Annelids are hermaphrodite, i.e., male and female organs are present in the same body. They reproduce both sexually and asexually. The others reproduce sexually.
• Example, Earthworms, and leeches

Classification of Annelida

Following are the different classification of Annelida:

• Polychaeta
• Oligochaeta
• Hirudinea
• Archiannelida

➤ Polychaeta

• The body is elongated and divided into segments.
• They are found in the marine environment.
• These are true coelomates, bilaterally symmetrical worms.
• They excrete through metanephridia and protonephridia.
• Fertilization is external.
• They have a well-developed nervous system.
• The circulatory system is closed type.
• They are hermaphrodites.
• They might possess fin-like appendages called parapodia.
• The organisms belonging to this group lack clitellum and are dioecious.
• Eg., Nereis, Syllis

➤ Oligochaeta

• They are mostly freshwater and terrestrial organisms.
• The body is segmented metamerically.
• Head, eyes and tentacles are not distinct.
• They are hermaphrodites, but cross-fertilization takes place.
• Fertilization is external.
• Cocoon formation occurs.
• Setae are segmented.
• They do not possess parapodia but clitellum is present.
• The organisms belonging to this class are monoecious.
• They exhibit no free larval stage and the development takes place inside the cocoons.
• Eg., Pheretima, Tubifex

➤ Hirudinea

• Most commonly found in freshwater. Some are marine, terrestrial, and parasitic.
• The body is segmented.
• The tentacles, parapodia, and setae are not present.
• The animals are monoecious.
• The body is dorsoventrally or cylindrically flattened.
• They have an anterior and posterior sucker on the ventral side.
• The organisms lay eggs in cocoons.
• There is no larval stage during the development of the organism.
• The mouth is located ventrally in the anterior sucker, while the anus is present dorsally in the posterior sucker.
• Fertilization is internal.
• They are hermaphrodites.
• Eg., Hirudinaria

➤ Archiannelida

• They are found only in the marine environment.
• The body is elongated without setae and parapodia.
• They are unisexual or hermaphrodite.
• Tentacles are present on the prostomium.
• Eg., Dinophilus, Protodrilus

In conclusion, members of Phylum Annelida have bodies that are segmented, such as leeches and earthworms.

Arthropoda

Arthropoda is the largest phylum with about nine lakh species. They may be aquatic, terrestrial or even parasitic. They have jointed appendages and a chitinous exoskeleton.
This phylum includes several large classes and contains the class Insecta which itself represents a major portion of the animal species in the world. They possess the ability to survive in every habitat.

Arthropoda Characteristics

The arthropoda characteristics are mentioned below:

• The body is triploblastic, segmented, and bilaterally symmetrical.
• They exhibit organ system level of organization.
• The body is divided into head, thorax, and abdomen.
• Their body has jointed appendages which help in locomotion.
• The coelomic cavity is filled with blood.
• They have an open circulatory system.
• The head bears a pair of compound eyes.
• The exoskeleton is made of chitin.
• The terrestrial Arthropods excrete through Malpighian tubules while the aquatic ones excrete through green glands or coaxal glands.
• They are unisexual and fertilization is either external or internal.
• They have a well-developed digestive system.
• They respire through the general body surface or trachea.
• They contain sensory organs like hairs, antennae, simple and compound eyes, auditory organs, and statocysts.

Classification of Phylum Arthropoda

The classification of phylum arthropoda are as follows:

➤ Crustacea

• They are aquatic, terrestrial, or parasitic.
• The head is fused with the thorax region known as the cephalothorax.
• Respiration occurs through gills or general body surface.
• The body is covered by a single large carapace.
• They possess two pairs of antennae and five pairs of appendages.
• They excrete through green glands or antennal glands.
• They have a pair of compound eyes and gonopores.
• Development is indirect. Larval stage is present.
• Eg., Daphnia, Palaemon

The subphylum Crustacea is divided into six classes-

• Branchiopoda
• Remipedia
• Chephlocarida
• Maxillopoda
• Ostracoda
• Malacostraca

➤ Myriapoda

• These are mostly terrestrial.
• The body is elongated with numerous segments.
• The head is provided with antennae, two pairs of jaws, and a pair of simple eyes.
• They contain numerous legs.
• The upper lip of the mouth contains epistome and labrum, and the lower lip contains a pair of maxillae.
• A pair of mandibles is present inside the mouth.
• They respire by trachea and excretion occurs by Malpighian tubules.
• Eg., Julus, Scolopendra

The subphylum Myriapoda is divided into the following classes:

• Chilopoda
• Diplopoda
• Pauropoda
• Symphyla

➤ Hexapoda

• They are mostly terrestrial.
• The body is differentiated into head, thorax, and abdomen.
• Head bears a pre-segmental acron.
• The thorax is divided into three segments.
• The abdomen has 7-11 segments.
• They have three pair of appendages.
• It has a pair of compound eyes
• They respire through gills and trachea.
• Malpighian tubules are the excretory organ.
• Development is indirect, and the larval stage is present.
• Eg., Tabernus, Mosquitoes, Ants.

The subphylum Hexapoda is divided into two classes:

• Insecta
• Entognatha

➤ Chelicerata

• They are mostly found on land.
• The body is differentiated into cephalothorax and abdomen.
• Antennae are absent.
• The abdomen is divided into 13 segments.
• It has four pairs of interior appendages.
• They respire through trachea or gills.
• The Malpighian tubules help in excretion.
• Eg., Aramea, Limulus

The subphylum Chelicerata is divided into the following classes:

• Arachnida
• Merostomata
• Pycnogonida

➤ Onychophora

• These are small-sized, terrestrial arthropods.
• The body is divided into segments.
• Excretion occurs through nephridia.
• They respire through the trachea.
• Eg., Paripatus

➤ Trilobitomorpha

• These are primitive arthropods and are extinct.
• They were found in abundance during the Paleozoic era.
• The body was divided into three lobes- one median and two lateral lobes.
• Head bore a pair of compound eyes and a pair of antennae.
• There was no structural differentiation of the body parts.
• The body was divided into head, thorax and pygidium.
• Appendages are biramous.

The subphylum had only one class- Trilobita

Frequently Asked Questions

Why is the name arthropods given? 
The term “arthropod” means “jointed legs”. The phylum included invertebrates with an exoskeleton and jointed legs, hence named, arthropoda.

What are the important characteristics of arthropoda?
The important characteristics of arthropoda include:

• They possess an exoskeleton.
• They have jointed appendages.
• Their body is segmented.
• They are bilaterally symmetrical.
• They possess an open circulatory system.

Which animals fall under the phylum arthropoda?
Spiders, centipedes, mites, crabs, ticks, lobsters, scorpions, shrimp, are some of the animals found in phylum arthropoda. 

How did the arthropods evolve?
It is believed that the arthropods evolved from the same root as the annelids. The three lineages of arthropods are believed to have evolved independently from a common ancestor. 

How did the arthropods adapt to land?
The early arthropods evolved adaptations such as tracheae for breathing. They also developed exoskeleton that prevents the skin of the animals from drying and provides support in case of buoyant water. 

When did the first arthropods evolve?
According to fossil records, the first arthropods are believed to have evolved 545 million years ago. Scorpions, centipedes and millipedes were the first arthropods to have adapted to dry land. 

Why is arthropoda the largest phylum?
Phylum arthropoda includes 80% species of animals. These species evolved certain adaptive features over the years to survive the changing climatic conditions and form the largest phylum in the animal kingdom.

Name the largest arthropod.
The coconut crab, Birgus latro, is the largest arthropod on land. It is also a crustacean.