Animal Kingdom

Introduction

When you look around, you will observe different animals with different structures and forms. As over a million species of animals have been described till now, the need for classification becomes all the more important. The classification also helps in assigning a systematic position to newly described species.

4.1 Basis of Classification

Inspite of differences in structure and form of different animals, there are fundamental features common to various individuals in relation to the arrangement of cells, body symmetry, nature of coelom, patterns of digestive, circulatory or reproductive systems.

4.1.1 Levels of Organisation

Though all members of Animalia are multicellular, all of them do not exhibit the same pattern of organisation of cells.

• Cellular Level: In sponges, the cells are arranged as loose cell aggregates. Some division of labour (activities) occurs among the cells.
• Tissue Level: In coelenterates, the arrangement of cells is more complex. Cells performing the same function are arranged into tissues.
• Organ Level: Exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form organs, each specialised for a particular function.
• Organ System Level: In animals like Annelids, Arthropods, Molluscs, Echinoderms and Chordates, organs have associated to form functional systems, each concerned with a specific physiological function.

4.1.2 Symmetry

Animals can be categorised on the basis of their symmetry:

• Asymmetrical: Any plane that passes through the centre does not divide them into equal halves (e.g., Sponges).
• Radial Symmetry: When any plane passing through the central axis of the body divides the organism into two identical halves. This is found in Coelenterates, ctenophores and echinoderms.
• Bilateral Symmetry: Where the body can be divided into identical left and right halves in only one plane (e.g., Annelids, Arthropods, etc.).

4.1.3 Diploblastic and Triploblastic Organisation

• Diploblastic: Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm. An undifferentiated layer, mesoglea, is present in between (e.g., Coelenterates).
• Triploblastic: Animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm (e.g., Platyhelminthes to Chordates).

4.1.4 Coelom (Body Cavity)

Presence or absence of a cavity between the body wall and the gut wall is very important in classification. The body cavity, which is lined by mesoderm, is called coelom.

• Coelomates: Animals possessing coelom (e.g., Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates and Chordates).
• Pseudocoelomates: In some animals, the body cavity is not lined by mesoderm; instead, the mesoderm is present as scattered pouches between the ectoderm and endoderm (e.g., Aschelminthes).
• Acoelomates: Animals in which the body cavity is absent (e.g., Platyhelminthes).

4.1.5 Segmentation

In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs. In earthworm, the body shows this pattern called metameric segmentation and the phenomenon is known as metamerism.

4.1.6 Notochord

Notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.

• Chordates: Animals with notochord.
• Non-chordates: Animals which do not form this structure (e.g., Porifera to Echinoderms).

4.2 Classification of Animals

The broad classification of Animalia, based on common fundamental features, is summarized below:

1. Phylum – Porifera (Sponges)

Commonly known as sponges, these are primitive multicellular animals found mostly in marine habitats.

• Level of Organisation: Cellular level. Cells are arranged as loose cell aggregates.
• Symmetry: Mostly asymmetrical.
• Water Transport / Canal System: Water enters through minute pores (ostia) in the body wall into a central cavity (spongocoel), from where it goes out through the osculum. This pathway is helpful in food gathering, respiratory exchange, and removal of waste.
• Unique Cells: Choanocytes (collar cells) line the spongocoel and the canals.
• Digestion: Intracellular.
• Skeleton: Supported by spicules or spongin fibres.
• Reproduction: Hermaphrodite (sexes are not separate). Reproduce asexually by fragmentation and sexually by formation of gametes. Fertilisation is internal and development is indirect.

2. Phylum – Coelenterata (Cnidaria)

They are aquatic, mostly marine, sessile or free-swimming animals.

• Unique Features: Derived from cnidoblasts or cnidocytes containing stinging capsules (nematocysts) used for anchorage, defense, and capture of prey.
• Symmetry & Level: Radial symmetry; Tissue level of organisation; Diploblastic.
• Body Cavity: Central gastro-vascular cavity with a single opening (mouth) on the hypostome.
• Body Forms:
  – Polyp: Sessile and cylindrical form (e.g., Hydra, Adamsia).
  – Medusa: Umbrella-shaped and free-swimming (e.g., Aurelia or jelly fish).
• Metagenesis: Alternation of generation where polyps produce medusae asexually and medusae form polyps sexually (e.g., Obelia).

3. Phylum – Ctenophora (Comb Jellies)

Exclusively marine, commonly known as sea walnuts or comb jellies.

• Symmetry & Level: Radially symmetrical, diploblastic, tissue level.
• Locomotion: The body bears eight external rows of ciliated comb plates.
• Digestion: Both extracellular and intracellular.
• Unique Feature: Bioluminescence (the property to emit light) is well-marked.
• Reproduction: Sexual means only; sexes not separate; external fertilisation with indirect development.

4. Phylum – Platyhelminthes (Flatworms)

They have a dorso-ventrally flattened body, hence called flatworms.

• Lifestyle: Mostly endoparasites found in animals including human beings.
• Symmetry & Level: Bilaterally symmetrical, triploblastic, and acoelomate; Organ level of organisation.
• Parasitic Adaptations: Presence of hooks and suckers. Some absorb nutrients directly through the body surface.
• Excretion: Specialised cells called flame cells help in osmoregulation and excretion.
• Features: Internal fertilisation; high regeneration capacity (e.g., Planaria).

5. Phylum – Aschelminthes (Roundworms)

The body is circular in cross-section, hence the name roundworms.

• Symmetry & Level: Bilateral, triploblastic, and pseudocoelomate; Organ-system level.
• Digestive System: Alimentary canal is complete with a well-developed muscular pharynx.
• Reproduction: Dioecious (sexes are separate). Often females are longer than males. Internal fertilisation.

6. Phylum – Annelida (Segmented Worms)

Named after Latin annulus (little ring) because their body is metamerically segmented.

• Symmetry & Level: Bilateral, triploblastic, coelomate; Organ-system level.
• Locomotion: Longitudinal and circular muscles; aquatic annelids like Nereis have lateral appendages called parapodia for swimming.
• Circulation: Closed circulatory system.
• Excretion: Nephridia for osmoregulation and excretion.

7. Phylum – Arthropoda (Largest Phylum)

This is the largest phylum of Animalia which includes insects. Over two-thirds of all named species are arthropods.

• Body Parts: Head, thorax, and abdomen.
• Unique Feature: Jointed appendages (arthros = joint, poda = appendages).
• Exoskeleton: Covered by a chitinous exoskeleton.
• Respiration: Gills, book gills, book lungs, or tracheal system.
• Circulation & Excretion: Open circulatory system; excretion through Malpighian tubules.

8. Phylum – Mollusca (Second Largest)

Molluscs are terrestrial or aquatic (marine or fresh water).

• Body Parts: Head, muscular foot, and visceral hump.
• Features: Unsegmented body covered by a calcareous shell. A soft layer of skin forms a mantle over the visceral hump.
• Mantle Cavity: Space between hump and mantle; contains feather-like gills for respiration/excretion.
• Feeding: Mouth contains a file-like rasping organ called radula.

9. Phylum – Echinodermata (Spiny Bodied)

Exclusively marine animals with an endoskeleton of calcareous ossicles.

• Symmetry: Adults are radially symmetrical but larvae are bilaterally symmetrical.
• Most Distinctive Feature: Presence of a water vascular system which helps in locomotion, capture/transport of food, and respiration.
• Features: Complete digestive system (ventral mouth, dorsal anus); excretory system is absent.

10. Phylum – Hemichordata

A small group of worm-like marine animals, previously considered a sub-phylum of Chordata.

• Body Structure: Cylindrical body consisting of an anterior proboscis, a collar, and a long trunk.
• Features: Open circulatory system; respiration through gills; excretory organ is the proboscis gland.

11. Phylum – Chordata

Fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits.

• General Features: Bilaterally symmetrical, triploblastic, coelomate; possess a post-anal tail and a closed circulatory system.
• Subphyla:
  1. Urochordata: Notochord present only in larval tail (e.g., Ascidia).
  2. Cephalochordata: Notochord extends head to tail and is persistent (e.g., Branchiostoma).
  3. Vertebrata: Notochord replaced by a vertebral column.