Kingdom Animalia: The Animal Kingdom

Introduction

Kingdom Animalia includes all eukaryotic, multicellular, heterotrophic organisms. They are characterized by the absence of a cell wall and their ability to move, although some are sessile. The study of animals is called zoology.

Basis of Classification

Animals are classified based on several fundamental features:

1. Levels of Organisation

Cellular Level: The body is an aggregation of cells with a rudimentary division of labor. Example: Phylum Porifera.
Tissue Level: Cells performing the same function are arranged into tissues. Example: Phylum Cnidaria and Ctenophora.
Organ Level: Tissues are grouped together to form organs, each specialized for a particular function. Example: Phylum Platyhelminthes.
Organ System Level: Organs are associated to form functional systems, each system concerned with a specific physiological function. Example: Phyla Nematoda to Chordata.

2. Body Plan

Cell Aggregate Plan: The body consists of a cluster of cells with little to no coordination between them. Example: Sponges.
Blind-Sac Plan: The body has a single opening that serves as both mouth and anus. Example: Cnidarians, Platyhelminthes.
Tube-within-Tube Plan: The body has a complete digestive tract with two separate openings, a mouth for ingestion and an anus for egestion. Example: Nematodes to Chordates.

3. Symmetry

Spherical Symmetry: The body can be divided into two identical halves by any plane passing through the center. Example: Volvox.
Radial Symmetry: The body can be divided into two identical halves by any plane passing through the central axis. Example: Cnidarians, Ctenophores, Echinoderms.
Bilateral Symmetry: The body can be divided into identical left and right halves in only one plane. Example: Platyhelminthes to Chordates.

4. Germ Layers

Diploblastic: The body cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm. An undifferentiated layer, mesoglea, is present between the ectoderm and the endoderm. Example: Cnidaria, Ctenophora.
Triploblastic: The body cells are arranged in three embryonic layers: ectoderm, mesoderm, and endoderm. Example: Platyhelminthes to Chordates.

5. Coelom (Body Cavity)

The coelom is the body cavity lined by mesoderm.

Acoelomate: The body cavity is absent. The space between the body wall and the digestive tract is filled with a matrix called parenchyma. Example: Platyhelminthes.
Pseudocoelomate: The body cavity is not lined by mesoderm. The mesoderm is present as scattered pouches in between the ectoderm and endoderm. Example: Nematoda (Aschelminthes).
Coelomate: The body cavity is a true coelom, lined by mesoderm on all sides. Example: Annelida to Chordata.
Haemocoelomate: The coelom is filled with blood (haemolymph). Example: Arthropoda, Mollusca.

6. Segmentation

In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs. This phenomenon is called metamerism. Example: Annelida, Arthropoda, Chordata.

Non-Chordata

Non-chordates are animals that do not possess a notochord.

Phylum Porifera (Sponges)

Level of Organisation: Cellular level.
Symmetry: Mostly asymmetrical.
Canal System: They have a water transport or 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.
Choanocytes: The spongocoel is lined with flagellated cells called choanocytes or collar cells.
Skeleton: The body is supported by a skeleton made up of spicules or spongin fibers.

Examples: Sycon (Scypha), Spongilla (Freshwater sponge).

Phylum Cnidaria (Coelenterata)

Level of Organisation: Tissue level.
Symmetry: Radial symmetry.
Cnidoblasts: They have stinging cells called cnidoblasts or cnidocytes, containing stinging capsules or nematocysts.
Body Forms: They exhibit two basic body forms: a sessile, cylindrical polyp (e.g., Hydra, Adamsia) and a free-swimming, umbrella-shaped medusa (e.g., Aurelia or jellyfish).
Alternation of Generation (Metagenesis): Some cnidarians exhibit alternation of generation, where polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).

A Living Cycle In Metagenesis, the plant-like Polyp (asexual) and the bell-like Medusa (sexual) alternate in the life cycle. This is famously seen in the colonial cnidarian Obelia.

Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone).

Phylum Ctenophora (Sea Walnuts or Comb Jellies)

Level of Organisation: Tissue level.
Symmetry: Radial symmetry.
Comb Plates: The body bears eight external rows of ciliated comb plates, which help in locomotion.
Bioluminescence: They are well-known for their ability to emit light (bioluminescence).
Reproduction: Reproduction takes place only by sexual means. Fertilization is external with indirect development.

Examples: Pleurobrachia, Ctenoplana.

Phylum Platyhelminthes (Flatworms)

Body: They have a dorso-ventrally flattened body.
Symmetry: Bilateral symmetry.
Germ Layers: They are triploblastic and acoelomate.
Excretory System: They have specialized cells called flame cells for osmoregulation and excretion.
Reproduction: They are hermaphrodites. Fertilization is internal and development is through many larval stages.

Examples: Taenia (Tapeworm), Fasciola (Liver fluke).

Phylum Nematoda (Aschelminthes) (Roundworms)

Body: The body is circular in cross-section.
Symmetry: Bilateral symmetry.
Germ Layers and Coelom: They are triploblastic and pseudocoelomate.
Digestive System: They have a complete digestive system with a well-developed muscular pharynx.
Reproduction: Sexes are separate (dioecious). Females are often longer than males.

Examples: Ascaris (Roundworm), Wuchereria (Filaria worm).

Phylum Annelida (Segmented Worms)

Metamerism: Their body is metamerically segmented.
Locomotion: They have longitudinal and circular muscles which help in locomotion. Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.
Circulatory System: They have a closed circulatory system.
Excretory System: Nephridia help in osmoregulation and excretion.
Nervous System: The nervous system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.

Examples: Nereis, Pheretima (Earthworm).

Phylum Mollusca (Soft-bodied Animals)

Body: The body is unsegmented and covered by a calcareous shell. It is divisible into a distinct head, muscular foot, and visceral hump.
Mantle: A soft and spongy layer of skin forms a mantle over the visceral hump.
Radula: The mouth contains a file-like rasping organ for feeding, called a radula.
Gills: They have feather-like gills for respiration and excretion.
Circulatory System: The circulatory system is of open type.

Examples: Pila (Apple snail), Octopus (Devil fish).

Phylum Arthropoda (Animals with Jointed Appendages)

Exoskeleton: The body is covered by a chitinous exoskeleton.
Body Divisions: The body consists of a head, thorax, and abdomen.
Jointed Appendages: They have jointed appendages.
Respiratory Organs: Respiratory organs are gills, book gills, book lungs, or tracheal system.
Circulatory System: The circulatory system is of open type.

Examples: Apis (Honey bee), Anopheles (Mosquito).

Phylum Echinodermata (Spiny-skinned Animals)

Endoskeleton: They have an endoskeleton of calcareous ossicles.
Symmetry: The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical.
Water Vascular System: They have a characteristic water vascular system which helps in locomotion, capture and transport of food, and respiration.
Digestive System: The digestive system is complete with a mouth on the lower (ventral) side and anus on the upper (dorsal) side.
Excretory System: An excretory system is absent.

Examples: Asterias (Starfish), Echinus (Sea urchin).

Phylum Hemichordata

Body: The body is worm-like, cylindrical and is composed of an anterior proboscis, a collar, and a long trunk.
Stomochord: They have a rudimentary structure in the collar region called the stomochord, which is similar to a notochord.
Circulatory System: The circulatory system is of open type.
Respiration: Respiration takes place through gills.
Excretory Organ: The excretory organ is the proboscis gland.

Examples: Balanoglossus, Saccoglossus.

Chordata

Chordates are animals that possess a notochord at some stage of their life.

Subphylum Urochordata (Tunicata)

Notochord: Notochord is present only in the larval tail.
Body: The body is covered by a test or tunic.
Examples: Ascidia, Salpa.

Subphylum Cephalochordata

Notochord: Notochord extends from head to tail region and is persistent throughout their life.
Body: The body is fish-like.
Examples: Branchiostoma (Amphioxus or Lancelet).

Subphylum Vertebrata

Vertebrates possess a notochord during the embryonic period, which is replaced by a cartilaginous or bony vertebral column in the adult.

Class Cyclostomata

Mouth: They have a sucking and circular mouth without jaws.
Fins: Paired fins are absent.
Scales: The body is devoid of scales.

Examples: Petromyzon (Lamprey), Myxine (Hagfish).

Class Chondrichthyes (Cartilaginous Fishes)

Endoskeleton: The endoskeleton is cartilaginous.
Gill Slits: Gill slits are separate and without an operculum (gill cover).
Placoid Scales: The skin is tough, containing minute placoid scales.

Examples: Scoliodon (Dogfish), Pristis (Sawfish).

Class Osteichthyes (Bony Fishes)

Endoskeleton: The endoskeleton is bony.
Gill Slits: They have four pairs of gill slits which are covered by an operculum on each side.
Air Bladder: They have an air bladder which regulates buoyancy.

Examples: Exocoetus (Flying fish), Hippocampus (Sea horse).

Class Amphibia

Habitat: They can live in aquatic as well as terrestrial habitats.
Skin: The skin is moist and without scales.
Tympanum: A tympanum represents the ear.

Examples: Bufo (Toad), Rana (Frog).

Class Reptilia

Skin: The body is covered by dry and cornified skin, epidermal scales or scutes.
Limbs: They have two pairs of limbs, when present.
Heart: The heart is usually three-chambered, but four-chambered in crocodiles.

Examples: Chelone (Turtle), Naja (Cobra).

Class Aves (Birds)

Feathers: The body is covered by feathers.
Forelimbs: The forelimbs are modified into wings.
Bones: The long bones are hollow with air cavities (pneumatic).

Examples: Corvus (Crow), Columba (Pigeon).

Class Mammalia

Mammary Glands: They have mammary glands to produce milk to nourish their young ones.
Hair: The skin is unique in possessing hair.
Ear: External ears or pinnae are present.

Examples: Homo sapiens (Human), Panthera tigris (Tiger).