Basis Of Classification

Classification of animals is based on fundamental features common to various individuals. These features provide a framework for organizing animals into distinct groups.

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Levels Of Organisation

All animals are multicellular, but the way their cells are organized varies:

• **Cellular level of organisation:** Cells are arranged as loose cell aggregates. There is some division of labour among cells, but no tissues are formed. Example: **Porifera** (Sponges).
• **Tissue level of organisation:** Cells performing the same function are organized into tissues. Example: **Coelenterata** and **Ctenophora**.
• **Organ level of organisation:** Tissues are grouped together to form organs, each specialized for a particular function. Example: **Platyhelminthes**.
• **Organ system level of organisation:** Organs are associated to form functional organ systems, each performing a specific physiological function (e.g., digestive system, circulatory system). This is the highest level of organisation. Example: **Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata**.

Organ systems show varying complexity across different animal groups. For instance, the digestive system can be **incomplete** (single opening serving as both mouth and anus, e.g., Platyhelminthes) or **complete** (two openings, mouth and anus, e.g., Aschelminthes to Chordates). The circulatory system can be **open** (blood pumped out of the heart, bathing cells/tissues directly, e.g., Arthropoda, Mollusca) or **closed** (blood circulated through vessels, e.g., Annelida, Chordata).

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Symmetry

Symmetry describes how the body of an animal is arranged around a central axis or plane. Animals can be categorized based on their symmetry:

• **Asymmetrical:** Any plane passing through the center does not divide the body into equal halves. Example: **Porifera** (Sponges).
• **Radial symmetry:** Any plane passing through the central axis of the body divides the organism into two identical halves. Example: **Coelenterata, Ctenophora**. Some **Echinodermata** (adults) also show radial symmetry.

• **Bilateral symmetry:** The body can be divided into identical left and right halves in only one plane. Example: **Annelida, Arthropoda, Platyhelminthes, Chordata** (and many others).

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Diploblastic And Triploblastic Organisation

Classification can be based on the number of embryonic germ layers from which the body tissues develop:

• **Diploblastic organisation:** Cells are arranged in **two embryonic layers**: an external **ectoderm** and an internal **endoderm**. An undifferentiated layer called **mesoglea** is present between these two layers. Example: **Coelenterata, Ctenophora**.

• **Triploblastic organisation:** The developing embryo has a third germinal layer, the **mesoderm**, located between the ectoderm and the endoderm. Example: **Platyhelminthes to Chordates**.

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Coelom

The presence or absence of a body cavity between the body wall and the gut wall is an important criterion. This body cavity, lined by mesoderm, is called the **coelom**. Animals are classified based on the nature of their coelom:

• **Coelomates:** Animals possessing a **true coelom**, where the body cavity is lined by mesoderm on both sides. Example: **Annelida, Mollusca, Arthropoda, Echinodermata, Hemichordata, Chordata**.

• **Pseudocoelomates:** Animals with a body cavity that is **not lined by mesoderm**. Instead, the mesoderm is present as scattered pouches between the ectoderm and endoderm. This body cavity is called a pseudocoelom. Example: **Aschelminthes**.

• **Acoelomates:** Animals in which the body cavity is **absent**. Example: **Platyhelminthes**.

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Segmentation

In some animals, the body is externally and internally divided into segments, with a serial repetition of at least some organs. This pattern is called **metameric segmentation**, and the phenomenon is known as **metamerism**. Example: **Annelida** (Earthworm, Nereis).

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Notochord

The **notochord** is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals. Its presence is a key distinguishing feature:

• **Chordates:** Animals possessing a notochord. Example: **Phylum Chordata**.
• **Non-chordates:** Animals that do not form a notochord. Example: **Phyla Porifera to Hemichordata**.

Classification Of Animals

Based on the fundamental features discussed (levels of organisation, symmetry, germ layers, coelom, segmentation, notochord), the animal kingdom (Animalia) is broadly classified into various phyla. The classification presented here is based on these features.

Here are the characteristic features of the different phyla and classes within the animal kingdom:

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Phylum – Porifera

Commonly known as **sponges**. Mostly **marine** and **asymmetrical**. Primitive multicellular animals with **cellular level of organisation**. Distinctive feature is the **water transport or canal system**. Water enters through small pores (ostia) into a central cavity (spongocoel) and exits through a large opening (osculum). This system aids in food gathering, respiration, and waste removal. Spongocoel and canals are lined by **choanocytes** (collar cells). Digestion is **intracellular**. Body supported by a skeleton of **spicules or spongin fibres**. Sexes are not separate (hermaphrodite). Reproduction is asexual (fragmentation) and sexual (gametes). Fertilisation is internal; development is indirect (involves a larval stage different from the adult). Examples: *Sycon, Spongilla* (freshwater), *Euspongia* (bath sponge).

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Phylum – Coelenterata (Cnidaria)

Aquatic, mostly marine, sessile or free-swimming, **radially symmetrical**. Exhibit **tissue level of organisation** and are **diploblastic**. Name Cnidaria from **cnidoblasts** (stinging cells with nematocysts) on tentacles/body, used for anchorage, defense, prey capture. Possess a central **gastro-vascular cavity** with a single opening (mouth on hypostome). Digestion is extracellular and intracellular. Some (corals) have calcium carbonate skeleton. Exhibit two body forms: **polyp** (sessile, cylindrical, e.g., *Hydra, Adamsia*) and **medusa** (umbrella-shaped, free-swimming, e.g., *Aurelia*). Those existing in both forms show **alternation of generation (Metagenesis)**: polyps produce medusae asexually, medusae form polyps sexually (e.g., *Obelia*). Examples: *Physalia, Adamsia, Pennatula, Gorgonia, Meandrina*.

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Phylum – Ctenophora

Commonly known as **sea walnuts or comb jellies**. Exclusively **marine**, **radially symmetrical**, **diploblastic** with **tissue level of organisation**. Body bears eight external rows of ciliated comb plates for locomotion. Digestion is extracellular and intracellular. **Bioluminescence** is well-marked. Sexes are not separate. Reproduction is only sexual. Fertilisation is external with indirect development. Examples: *Pleurobrachia, Ctenoplana*.

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Phylum – Platyhelminthes

Have a dorso-ventrally flattened body, called **flatworms**. Mostly **endoparasites** in animals (including humans). **Bilaterally symmetrical**, **triploblastic**, and **acoelomate** with **organ level of organisation**. Hooks and suckers in parasitic forms; some absorb host nutrients directly. Specialised **flame cells** for osmoregulation and excretion. Sexes are not separate. Fertilisation is internal; development through many larval stages. *Planaria* possesses high regeneration capacity. Examples: *Taenia* (Tapeworm), *Fasciola* (Liver fluke).

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Phylum – Aschelminthes

Body is circular in cross-section, called **roundworms**. May be free-living (aquatic/terrestrial) or parasitic (plants/animals). **Organ-system level of body organisation**, **bilaterally symmetrical**, **triploblastic**, and **pseudocoelomate**. **Complete alimentary canal** with muscular pharynx. Excretory tube removes waste via excretory pore. Sexes are separate (dioecious), females often longer than males. Fertilisation is internal; development may be direct or indirect. Examples: *Ascaris* (Roundworm), *Wuchereria* (Filaria worm), *Ancylostoma* (Hookworm).

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Phylum – Annelida

Aquatic (marine/freshwater) or terrestrial; free-living or sometimes parasitic. **Organ-system level of body organisation**, **bilateral symmetry**, **triploblastic**, **metamerically segmented**, and **coelomate**. Body surface distinctly marked into segments (metameres), hence the name (Latin annulus: little ring). Possess longitudinal and circular muscles for locomotion. Aquatic forms (Nereis) have lateral appendages (parapodia) for swimming. **Closed circulatory system**. **Nephridia** (sing. nephridium) for osmoregulation and excretion. Neural system with paired ganglia and a double ventral nerve cord. Nereis is dioecious; earthworms and leeches are monoecious. Reproduction is sexual. Examples: *Nereis, Pheretima* (Earthworm), *Hirudinaria* (Blood sucking leech).

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Phylum – Arthropoda

The **largest phylum of Animalia**, including insects. Over two-thirds of all named species. **Organ-system level of organisation**, **bilaterally symmetrical**, **triploblastic**, **segmented**, and **coelomate**. Body covered by a **chitinous exoskeleton**. Body divided into head, thorax, and abdomen. Characterised by **jointed appendages**. Respiratory organs are gills, book gills, book lungs, or tracheal system. Circulatory system is **open type**. Sensory organs (antennae, eyes - compound/simple, statocysts) present. Excretion through **malpighian tubules**. Mostly dioecious and oviparous. Development may be direct or indirect. Examples: *Apis* (Honey bee), *Bombyx* (Silkworm), *Laccifer* (Lac insect), *Anopheles*/*Culex*/*Aedes* (Mosquitoes), *Locusta* (Locust), *Limulus* (King crab - living fossil).

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Phylum – Mollusca

The **second largest animal phylum**. Terrestrial or aquatic (marine/freshwater). **Organ-system level of organisation**, **bilaterally symmetrical**, **triploblastic**, and **coelomate**. Body covered by a **calcareous shell** (usually). Body is **unsegmented** with a distinct head, muscular foot, and visceral hump. Skin forms a soft, spongy layer (mantle) over the visceral hump. **Mantle cavity** between hump and mantle contains feather-like gills for respiration and excretion. Anterior head region has sensory tentacles. Mouth has a file-like rasping organ (radula) for feeding. Usually dioecious and oviparous with indirect development. Examples: *Pila* (Apple snail), *Pinctada* (Pearl oyster), *Sepia* (Cuttlefish), *Loligo* (Squid), *Octopus* (Devil fish).

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Phylum – Echinodermata

Have an **endoskeleton of calcareous ossicles**, hence named (Spiny bodied). All are **marine** with **organ-system level of organisation**. Adults are **radially symmetrical**; larvae are bilaterally symmetrical. **Triploblastic** and **coelomate**. Complete digestive system (mouth ventral, anus dorsal). Distinctive feature is the **water vascular system**, helping in locomotion, food capture/transport, and respiration. Excretory system absent. Sexes separate. Reproduction sexual. Fertilisation usually external. Development is indirect with free-swimming larva. Examples: *Asterias* (Star fish), *Echinus* (Sea urchin), *Antedon* (Sea lily).

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Phylum – Hemichordata

Earlier sub-phylum of Chordata, now a separate phylum under non-chordata. Have a rudimentary notochord-like structure (stomochord) in the collar region. Small group of **worm-like marine animals**. **Organ-system level of organisation**, **bilaterally symmetrical**, **triploblastic**, and **coelomate**. Body cylindrical with proboscis, collar, and trunk. Circulatory system is **open type**. Respiration through gills. Excretory organ is proboscis gland. Sexes separate. Fertilisation external. Development is indirect. Examples: *Balanoglossus, Saccoglossus*.

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Phylum – Chordata

Characterised by presence of a **notochord** (at least during embryonic development), a **dorsal hollow nerve cord**, and **paired pharyngeal gill slits**. **Bilaterally symmetrical**, **triploblastic**, **coelomate** with **organ-system level of organisation**. Possess a post-anal tail and a **closed circulatory system**. Table 4.1 compares chordates and non-chordates.

Phylum Chordata is divided into three subphyla: Urochordata (Tunicata), Cephalochordata, and Vertebrata. Urochordata and Cephalochordata (protochordates) are exclusively marine. In Urochordata, notochord is only in larval tail; in Cephalochordata, it extends from head to tail and is persistent. Examples: Urochordata – *Ascidia*; Cephalochordata – *Branchiostoma* (Amphioxus).

Members of subphylum **Vertebrata** have notochord during embryonic period, replaced by a cartilaginous or bony **vertebral column** in the adult. Thus, all vertebrates are chordates, but not all chordates are vertebrates. Vertebrates also have a ventral muscular heart, kidneys, and paired appendages (fins or limbs). Vertebrata is divided into divisions Agnatha (lacks jaw) and Gnathostomata (bears jaw).

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Class – Cyclostomata (under Agnatha)

All living members are **ectoparasites** on some fishes. Elongated body (6-15 pairs of gill slits). **Sucking and circular mouth without jaws**. Body lacks scales and paired fins. Cranium and vertebral column cartilaginous. Closed circulation. Marine but migrate to fresh water for spawning, then die. Larvae return to ocean after metamorphosis. Examples: *Petromyzon* (Lamprey), *Myxine* (Hagfish).

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Class – Chondrichthyes (under Gnathostomata/Pisces)

**Marine fishes** with **cartilaginous endoskeleton**. Streamlined body. Mouth ventral. Notochord persistent. Gill slits separate, **without operculum** (gill cover). Skin tough with placoid scales. Teeth modified placoid scales. Powerful jaws. Predaceous. Lack air bladder, swim constantly to avoid sinking. Heart two-chambered. **Cold-blooded**. Sexes separate. Pelvic fins bear claspers in males. **Internal fertilisation**, many are **viviparous** (give birth to live young). Some have electric organs (*Torpedo*), some possess poison sting (*Trygon*). Examples: *Scoliodon* (Dog fish), *Pristis* (Saw fish), *Carcharodon* (Great white shark), *Trygon* (Sting ray).

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Class – Osteichthyes (under Gnathostomata/Pisces)

Includes marine and freshwater fishes with **bony endoskeleton**. Streamlined body, mouth mostly terminal. **Four pairs of gills covered by an operculum** on each side. Skin covered with cycloid/ctenoid scales. **Air bladder present** (regulates buoyancy). Heart two-chambered. **Cold-blooded**. Sexes separate. Fertilisation usually external. Mostly **oviparous** (lay eggs) and development is direct. Examples: Marine – *Exocoetus* (Flying fish), *Hippocampus* (Sea horse); Freshwater – *Labeo* (Rohu), *Catla* (Katla); Aquarium – *Betta* (Fighting fish), *Pterophyllum* (Angel fish).

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Class – Amphibia (under Gnathostomata/Tetrapoda)

(Gr., Amphi: dual, bios: life) Can live in aquatic and terrestrial habitats. Most have two pairs of limbs. Body divisible into head and trunk (tail present in some). Skin **moist (without scales)**. Eyes have eyelids. Tympanum represents ear. Alimentary, urinary, and reproductive tracts open into common chamber (cloaca). Respiration by gills (larval), lungs (adult), and through skin. Heart **three-chambered** (two auricles, one ventricle). **Cold-blooded**. Sexes separate. Fertilisation external. **Oviparous**, development indirect (metamorphosis). Examples: *Bufo* (Toad), *Rana* (Frog), *Hyla* (Tree frog), *Salamandra* (Salamander).

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Class – Reptilia (under Gnathostomata/Tetrapoda)

(Latin, repere or reptum, to creep or crawl) Mostly terrestrial. Body covered by **dry and cornified skin, epidermal scales or scutes**. Lack external ear openings (tympanum represents ear). Limbs two pairs (when present; absent in snakes). Heart usually three-chambered (except four-chambered in crocodiles). **Cold-blooded**. Sexes separate. Fertilisation internal. **Oviparous**, development direct. Examples: *Chelone* (Turtle), *Testudo* (Tortoise), *Chameleon* (Tree lizard), *Calotes* (Garden lizard), *Crocodilus* (Crocodile), *Naja* (Cobra), *Vipera* (Viper).

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Class – Aves (under Gnathostomata/Tetrapoda)

Commonly known as **birds**. Characterised by **feathers** and ability to fly (except flightless birds like Ostrich). Possess beak. Forelimbs modified into **wings**. Hind limbs adapted for walking, swimming, etc. Skin dry without glands (except oil gland at tail base). Endoskeleton bony, long bones **pneumatic** (hollow with air cavities) for flight. Digestive tract has crop and gizzard. Heart completely **four-chambered**. **Warm-blooded**. Respiration by lungs with air sacs. Sexes separate. Fertilisation internal. **Oviparous**, development direct. Examples: *Corvus* (Crow), *Columba* (Pigeon), *Psittacula* (Parrot), *Struthio* (Ostrich), *Pavo* (Peacock).

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Class – Mammalia (under Gnathostomata/Tetrapoda)

Found in diverse habitats. Most unique characteristic is presence of **mammary glands** for nourishing young. Two pairs of limbs for various adaptations. Skin unique with **hair**. External ears (pinnae) present. Different types of teeth. Heart completely **four-chambered**. **Warm-blooded**. Respiration by lungs. Sexes separate. Fertilisation internal. Mostly **viviparous** (give birth to live young, few exceptions). Development direct. Examples: Oviparous – *Ornithorhynchus* (Platypus); Viviparous – *Macropus* (Kangaroo), *Pteropus* (Flying fox), *Canis* (Dog), *Felis* (Cat), *Balaenoptera* (Blue whale), *Panthera tigris* (Tiger).

Table 4.2 provides a comprehensive summary of salient features for each phylum in the Animal Kingdom, comparing their levels of organisation, symmetry, coelom, segmentation, and various organ systems.

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