Classification of Animals

The classification of animals is the systematic arrangement of animals into groups based on their similarities, differences, and evolutionary relationships.

• This scientific method of grouping helps in the easy study, identification, and understanding of the vast diversity present in the animal kingdom.
• The animal classification system divides organisms based on shared traits and evolutionary history.

Non- Vertebrates

Non-vertebrates, or invertebrates, are animals that do not possess a vertebral column or backbone. It includes a variety of animals such as porifera, arthropods, Mollusca, annelids, cnidarians, etc. Different characteristics of invertebrates are given below.

• They lack a backbone or vertebral column.
• They show radial symmetry or bilateral symmetry depending on their evolution.
• They are cylindrical, triploblastic, coelomate, or pseudocoelomate animals.
• Respiration is either through gills, trachea or body surface.
• Mode of reproduction can be sexual or asexual.
• Fertilisation is external, though internal fertilisation also occurs in some species.
• An open type of circulatory system is present.

The important characteristic features of the different phyla are described below.

1. Phylum - Porifera

• The organisms belonging to this phylum are usually known as sponges.
• These sponges have the system of water transport system in their body.
• These are usually marine animals, which means most of them live in oceans and seas.
• These are multicellular organisms and have cellular levels.
• The skeleton of these organisms is made up of spicules.
• Reproduction in sponges is done asexually by the process of fragmentation and sexually by the process of gamete formation.
• Fertilisation in these organisms is internal.

Examples: Euspongia and Spongilla.

2. Phylum - Coelenterata (Cnidaria)

• These organisms are more symmetrical when compared to other organisms.
• These are mostly aquatic and free-swimming animals that live in marine environments, seas, and other aquatic places.
• These organisms have Cnidoblasts on their tentacles and bodies.
• These Cnidoblasts are used in their defence mechanism and to capture food.
• Cnidarians have mainly two body forms, known as polyp and medusa.
• Polyp produces medusae using asexual reproduction, and medusae produce polyps using sexual reproduction.

Examples: Gorgonia, Meandrina, and Physalia.

3. Phylum - Ctenophora

• Ctenophores are usually known by names such as Sea walnuts.
• These are diploblastic organisms that live in marine environments and are radially symmetric.
• The organisation in these organisms is of tissue-level organisation.
• The process of digestion takes place both intracellularly and extracellularly.
• In Ctenophores, reproduction takes place only by sexual reproduction.
• Fertilisation takes place externally.

Example: Pleurobrachia and Ctenoplana.

4. Phylum - Platyhelminthes

• They have flattened body-like structures. So, they are called flatworms.
• These organisms are found in many living organisms, including human beings.
• Parasitic worms have hooks and suckers on their body.
• Many of these flatworms absorb nutrients from their host's body directly to survive.
• These organisms have special cells named flame cells, which help in the process of excretion.
• Sexes are not separate in these worms.
• The process of fertilisation occurs internally, and development happens through several larval stages.

Example: Taenia (commonly known as tapeworm) and Fasciola.

5. Phylum - Aschelminthes

• The shape of the body of Aschelminthes is circular.
• These organisms may be aquatic or terrestrial parasitic organisms that live in both plants and animals.
• These organisms choose either plants or other animals as their host and live. They have organs in their body.
• These organisms are bilaterally symmetrical in nature.
• Sexes are separate in these organisms, as we can distinguish male and female in Aschelminthes. Usually, females are longer than males.
• The process of fertilisation takes place internally, and the development of the organism may be direct or indirect.

Example: Ascaris (commonly known as roundworm) and Ancylostoma(Hookworm).

6. Phylum - Annelida

• These organisms can live in both water and on land.
• They can live in both salt and fresh water.
• The organisation of the body is organ level system.
• Their circular muscles and longitudinal muscles help in their locomotion.
• Their body surfaces consist of fragments.
• Nephridia help in excretion in these organisms.
• Reproduction in these organisms is sexual.

Example: Nereis and Pheretima (common name is Earthworm).

7. Phylum - Arthropoda

• The largest phylum among the Animalia is Arthropoda.
• This phylum includes insects. Almost 66 per cent of species on Earth belong to the phylum of Arthropoda.
• The organisms in this phylum have organs in their body.
• These organisms are bilaterally symmetrical. In these kinds of organisms, excretion takes place through Malpighian tubes.
• Their bodies consist of the head, eyes, thorax, abdomen, etc.
• Respiration in Arthropods takes place through the lungs and gills.
• Fertilisation is mostly internal, and development happens either directly or indirectly.

Examples: Apis(bee), Bombyx, Limulus, Locusta, and Anopheles.

8. Phylum - Mollusca

• The second largest phylum after Arthropoda in the whole animal kingdom is the Mollusca phylum.
• The organisms that belong to this phylum are both aquatic and terrestrial animals.
• They can live in both fresh water and marine water.
• The body of these organisms is covered with a shell called a calcareous shell.
• With a muscular foot, visceral mass, and head, its body is unsegmented.
• The space between the mantle and visceral mass is usually known as the mantle cavity.
• They have gill-like structures to breathe. These are mostly unisexual.

Example: Octopus, Loligo, and pila

9. Phylum - Echinodermata

• Mostly, organisms belonging to the phylum Echinodermata are marine animals. They live in salt water.
• Fully grown echinoderms are radially symmetrical. But in the larva stage, they are bilaterally symmetrical.
• They possess a water vascular system for locomotion, the process of respiration, and capturing food.
• There are no specially defined excretory organs in these organisms.
• The nervous system and sense organs are poorly developed in echinoderms, and the brain is absent.
• Sexes are mostly separate, and the fertilisation process is external.

Example: Antedon, Cucumaria, and Ophiura

10. Phylum - Hemichordata

• The phylum Hemichordata was initially placed in the subphylum of Chordata. Now it is considered a separate phylum under non-chordates.
• These organisms are worm-like structures and mostly aquatic animals.
• They mostly live in marine water.
• Their body is in a cylindrical structure.
• Respiration in these creatures is done through gills that open into the pharynx. They have a dorsal heart.

Example: Saccoglossus and Balanoglossus

11. Phylum - Chordata

Animals belonging to the phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. These are bilaterally symmetrical, triploblastic, coelomate with an organ-system level of organisation. They possess a post-anal tail and a closed circulatory system.

Chordata vs Non-Chordata

Chordates are further classified into three subphyla:

1. Subphylum - Urochordata or Tunicata

The organisms in the subphylum Urochordata are marine organisms, and we can find them from the surface to depths in the ocean. Their body is composed of a tunic, which is formed by cellulose, and their body is unsegmented. The circulatory system in their body is of a circular type. Activities like free swimming develop in their tadpoles. They have a ventral heart, which reverses the flow of blood. Example: Salpa, Pyrosoma, and Doliolum

2. Subphylum - Cephalochordata

Cephalochordates are aquatic animals. We can find these animals mostly in marine water. They possess a notochord, pharyngeal slits, and tubular nerve chords. Respiration occurs through their body surface. The circulatory system in their body is a closed type. Excretion in Cephalochordata is done through solenocytes. The process of fertilisation is external. Example: Branchiostoma

3. Subphylum - Vertebrata

The Subphylum Vertebrata is divided into many classes:

• Amphibia
• Reptilia
• Aves
• Mammalia
• Pisces

Vertebrates

Vertebrates are a subphylum of chordates that are characterised by the presence of a vertebral column or backbone. It encloses and protects the spinal cord. It includes animals such as fish, amphibians, reptiles, birds, and mammals. Different characteristics of vertebrates are given below.

• They are characterised by the presence of a vertebral column or backbone. It encloses and protects the spinal cord.
• They show bilateral symmetry.
• Vertebrates have an internal skeleton made of bone or cartilage, providing structural support and protection for internal organs.
• They are triploblastic and coelomate with the organ-system level of organisation.
• The body has a closed circulatory system.
• The notochord is present only in the larval tail, but in some, it is present throughout their life from head to tail region.