Animal Kingdom

BASIS OF CLASSIFICATION

With more than a million animal species described, classification helps in organising and understanding animal diversity

Despite their differences, animals share common features used for classification.
Animals are multicellular, heterotrophic organisms lacking cell walls and chlorophyll.
“Animals are multicellular and eukaryotic organisms.”
“Classification also helps in assigning a systematic position to newly described species.”

“This chapter focuses on the fundamental principles of animal classification, including levels of organisation, body symmetry, and major animal phyla..”

11 major phyla of the animal kingdom

Phylum	Common Example
Porifera	Sponge
Cnidaria (Coelenterata)	Jellyfish
Ctenophora	Comb jelly
Platyhelminthes	Tapeworm
Aschelminthes	Roundworm
Annelida	Earthworm
Arthropoda	Insect (e.g., beetle)
Mollusca	Snail
Echinodermata	Starfish
Hemichordata	Acorn worm
Chordata	Human

11 major phyla of the animal kingdom

1.1.1 Levels of Organisation

Cellular Level: Simple organization with loose cell aggregates.
- e.g. Sponges.
Tissue Level: Cells performing the same function form tissues.
- e.g. Coelenterates & Ctenophora.
Organ Level: Tissues combine to form organs specialised for specific functions.
- e.g. Platyhelminthes.
Organ System Level: Organs work together to form organ systems performing complex body functions.
- e.g. Aschelminthes, Annelids, Arthropods, Molluscs, Echinoderms, and Chordates.
- Examples:
  - Incomplete digestive system (one opening) in Platyhelminthes.
  - Complete digestive system (separate mouth and anus) in more complex animals.
  - Open circulatory system (blood bathes tissues directly).
  - Closed circulatory system (blood circulates through vessels).

1.1.2 Symmetry

Body symmetry in classification refers to the arrangement of an organism’s body parts around a central axis or plane.

Asymmetrical: Body can not be divided in 2 or more equal halves.
- e.g., sponges.
Radial Symmetry: Body can be divided into identical halves by any plane passing through the central axis.
- e.g., coelenterates, ctenophores, echinoderms.
Bilateral Symmetry: Only one plane divides the body into equal left and right halves.
- e.g., annelids, arthropods.

1.1.3 Germinal Layers Organisation

Germinal layers are the primary layers of cells (ectoderm, mesoderm, endoderm) that form during embryonic development and give rise to all tissues and organs.

Diploblastic: Two embryonic layers (ectoderm and endoderm).
- Mesoglea present in between ectoderm & endoderm.
- Mesoglea is a undifferentiated jelly-like layer lacking true tissues.
- e.g., Coelenterates.
Triploblastic: Three embryonic layers (ectoderm, mesoderm, endoderm).
- e.g., Platyhelminthes to chordates.
- Mesoderm develops as a distinct third germinal layer during embryogenesis.

1.1.4 Coelom (body cavity)

A coelom is a fluid-filled body cavity completely lined by mesoderm, found between the digestive tract (gut) and the body wall.
It provides space for the development and suspension of internal organs, allowing for greater complexity in body structure and function.
Based on the nature of their coelom, animals are of three types:

Coelomates: Body cavity completely lined by mesoderm, thus True coelom.
- e.g., Annelids, molluscs, arthropods.
- “Echinoderms, hemichordates, and chordates are also coelomates.”
Pseudocoelomates: Body cavity not fully lined by mesoderm, thus False coelom.
- e.g., Aschelminthes.
- Mesoderm occurs as scattered pouches rather than a continuous lining.
Acoelomates: No body cavity present.
- e.g., Platyhelminthes.
- Body filled with solid mesodermal tissue called parenchyma matrix.

1.1.5 Segmentation (Metamerism)

Metamerism: Body divided into segments (metamers) with serial repetition of at least some organs.
- e.g., Earthworm.
- Segmentation in animals may be external, internal, or both, depending on the species and their evolutionary adaptations.
External segmentation:
- Visible as distinct segments or somites on the outer body surface.
- Facilitates flexibility, movement, and protection.
- e.g., Seen clearly in arthropods.
Internal segmentation:
- Repetition of internal organs like muscles, nerves, and coelomic compartments.
- Enhances coordination and efficiency.
- e.g., Present in chordates as internal segmentation.

In humans, metamerism is internally represented by the segmented arrangement of the vertebral column, nerve distribution along spinal segments, and the grouping of muscles into functional units.

“Annelids show both external and internal segmentation.”

“False segmentation or pseudometamerism occurs in tapeworms where segments arise by budding and are called proglottids.”

1.1.6 Notochord

Notochord is a mesodermally derived, flexible, rod-like structure on the dorsal side during embryonic development.

It provides skeletal support in all chordate embryos and in some adult chordates.

Chordates: Animals possessing a Notochord during embryonic development.
- e.g., Vertebrates.
Non-chordates: Animals lacking a notochord.
- e.g., Porifera to echinoderms.

CLASSIFICATION OF ANIMALS

1. Phylum – Porifera

Common Name: Sponges
Habitat and Habit (movement): Aquatic, mostly marine; sedentary; solitary or colonial.
Symmetry: Asymmetrical, few show radial symmetry.
Organization: Cellular level.
Skeleton: Made of spicules or spongin fibers.
Circulatory system: Absent.
Digestive system: Absent, Intracellular digestion.
Reproduction:
- Hermaphrodites (monoecious);
- Asexual reproduction by fragmentation;
- Sexual reproduction by gametes.
Fertilization & Development:
- Internal fertilization.
- Indirect development with a larval stage
- “Larvae are morphologically distinct and free-swimming.”
Unique features:
- Water Canal System:
  - Water enters through ostia (pores), goes to spongocoel (body cavity), exits through osculum (big pore).
- Functions of Water Canal System:
  - Food gathering.
  - Respiratory exchange.
  - Waste removal
- Choanocytes (collar cells):
  - Line the spongocoel and canals
Examples: Sycon (Scypha), Spongilla (Freshwater sponge), Euspongia (Bath sponge), Euplectella (Venus flower basket), Hyalonema (glass rope sponge), and Cliona (boring sponge).

“Three types of canal system occur:

ascon (simple), sycon (moderately complex), and leucon (most complex).”

“Ascon type is seen in Leucosolenia.”

“Sycon type is seen in Sycon.”

“Leucon type is seen in Spongilla.”

2. Phylum – Coelenterata (Cnidaria)

Habitat and Habit (movement): Aquatic, mostly marine; sessile or free-swimming.
- “Few members like Hydra occur in freshwater.”
Symmetry: Radial
Organization: Tissue level
Germ layers: Diploblastic
Coelom: Acoelomate
Digestion: Incomplete, with both extracellular and intracellular digestion.
Respiratory system: Absent
Circulatory system: Absent
Skeleton: Some (e.g., corals) have calcium carbonate (CaCO₃) skeleton
Body Forms:
- Polyp: Sessile and cylindrical (e.g., Hydra, Adamsia)
- Medusa: Umbrella-shaped and free-swimming (e.g., Aurelia)
Reproduction:
- Mostly separate sexes (Dioecious).
- Alternation of generation (Metagenesis) – polyps reproduce asexually to form medusae, medusae reproduce sexually to form polyps.
- “Metagenesis is seen in forms like Obelia.”
Fertilization & Development:
- External fertilization.
- Development is indirect.
Unique features:
- Cnidocytes (Cnidoblasts): Stinging cells present on Tentacles & body surface.
  - Used for anchorage, defence, & prey capture.
  - “Cnidocytes contain nematocysts with poisonous substance called hypnotoxin.”
- The gastro-vascular cavity (coelenteron):
  - Single opening, located on the mouth (hypostome).
  - Serves both for ingestion and egestion.
Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), Meandrina (Brain coral).

“Body wall consists of epidermis, gastrodermis, and non-cellular mesoglea in between.”

“Statocyst, a sense organ for balance, appears for the first time in this phylum.”

3. Phylum – Ctenophora

Common Name: Sea walnuts or comb jellies
Habit and Habitat: Marine, solitary, and pelagic.
Symmetry: Radial (often described as biradial due to the presence of paired structures).
Organization: Tissue level
Germ layers: Diploblastic
Coelom: Acoelomate
Respiratory system & Circulatory system: Absent
Digestive system: Complete, with both intracellular & extracellular digestion.
Reproduction:
- Only sexual,
- Hermaphrodite,
- External fertilization,
- Indirect development; Cydippid larva.
Unique features:
- Body Features: 8 rows of ciliated comb plates for locomotion.
- Bioluminescence: Can emit light.
- “Special adhesive cells called colloblasts (lasso cells) are present on tentacles for capturing prey.”
Examples: Pleurobrachia, Ctenoplana

4. Phylum – Platyhelminthes

Common Name: Flatworms
Body Features: Dorso-ventrally flattened, thus called flatworms (including tapeworms which are also dorso-ventrally flattened).
Habit and Habitat: Mainly aquatic; mostly endoparasites in animals, including humans.
Symmetry: Bilateral
Organization: Organ-system level.
Germ layers: Triploblastic
Body Cavity: Acoelomate
Digestive system: Incomplete, has a single opening that serves both as the mouth and the anus.
- “Digestive system is reduced or absent in parasitic forms like tapeworms.”
Respiratory system & Circulatory system: Absent
Reproduction & Development:
- asexual (fragmentation/regeneration) and Sexual.
- Hermaphroditic, with internal fertilization.
- Indirect development (multiple larval stages).
Nervous System:
- “Ladder-like nervous system with paired longitudinal nerve cords.”
- “Anterior ganglia may act as a simple brain with sensory organs like eyespots.”
Special Features:
- Hooks and suckers in parasitic forms, aiding attachment and nutrient absorption through body surface.
- “Locomotory and sense organs are reduced or absent in parasitic forms.”
- “Body covering is a thick tegument resistant to host digestive enzymes.”
- High regeneration capacity in some (e.g., Planaria)
- Excretion: By flame cells.
Examples: Taenia (Tapeworm), Fasciola (Liver fluke)

5. Phylum – Aschelminthes (Nematoda)

Common Name: Roundworms
Body: Circular in cross-section, thus called roundworms.
Habit and Habitat: Free-living (aquatic and terrestrial) or parasitic
Symmetry: Bilateral
Organization: Organ-system level
Germ layers: Triploblastic
Body Cavity: Pseudocoelomate
Body Wall: “Body wall consists of a tough cuticle, syncytial epidermis, and longitudinal muscles only.”
Digestive System: Complete with a well-developed muscular pharynx
Respiratory system & Circulatory system: Absent
Reproduction & Development:
- Dioecious (separate sexes), internal fertilization,
- Development can be direct or indirect.
Special Features:
- Males and females are distinct, with females being longer than males.
- “Sense organs include papillae, amphids, and phasmids.”
- Excretion: By Excretory tube and pore
Examples: Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm)

6. Phylum – Annelida

Habitat: Aquatic (marine and freshwater) or terrestrial; free-living or parasitic
Symmetry: Bilateral
Organization: Organ-system level
Layers: Triploblastic
Segmentation: Metamerically segmented, body shows external rings.
Body Cavity: Coelomate
Digestive System: Complete
Respiratory system: Cutaneous (skin) respiration. Some have branchial (gill) respiration.
Circulatory system: Closed, blood circulates within vessels and does not fill body cavities.
Reproduction & Development:
- Sexual reproduction.
- Dioecious in some (e.g., Nereis),
- Monoecious in others (e.g., earthworms, leeches)
- Development may be direct or indirect (trochophore larva).
Special Features:
- Movement: Longitudinal and circular muscles for locomotion,
- Locomotory Organs: Setae (in earthworms) or parapodia in aquatic forms (e.g., Nereis).
  - “Except leeches, setae are usually present.”
- Excretion: By Nephridia
- Nervous System: Paired ganglia and double ventral nerve cord.
- “Sense organs include tactile receptors, photoreceptors, and gustatory receptors.”
Examples: Nereis, Pheretima (Earthworm), Hirudinaria (Blood-sucking leech)

7. Phylum – Arthropoda

Largest Phylum: Includes insects, over two-thirds of all known species.
Habitat: Cosmopolitan (found in all environments).
Symmetry: Bilateral
Organization: Organ-system level
Germ layers: Triploblastic
Segmentation: Metamerically segmented.
Body Cavity: Coelomate
- “True coelom is reduced and body cavity functions as haemocoel.”
Digestive System: Complete
Respiratory system: Gills, book gills, book lungs, or tracheal system.
Circulatory system: Open type, blood (hemolymph) is not confined to vessels and flows freely through body cavities (haemocoel).
Reproduction & Development:
- Sexual reproduction.
- Mostly dioecious and usually internal fertilization.
- Mostly oviparous.
- Development can be direct or indirect.
Special Features:
- Body Structure:
  - Chitinous exoskeleton (cuticle)
  - Body parts: head, thorax, abdomen (sometimes cephalothorax and abdomen).
  - Jointed appendages
- Sensory Organs: Antennae, compound and simple eyes, statocysts (balancing organs)
Excretion: By Malpighian tubules
Examples:
- Economically important: Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect).
- Vectors: Anopheles, Culex, Aedes (Mosquitoes).
- Gregarious pest: Locusta (Locust)
- Living fossil: Limulus (King crab)

“Representative classes include Crustacea (Palaemon), Chilopoda (Scolopendra), Diplopoda (Julus), Insecta, and Arachnida.”

8. Phylum – Mollusca

Second Largest Phylum
Habitat: Terrestrial & aquatic (marine and freshwater).
Symmetry: Bilateral
Organization: Organ-system level
Germ layers: Triploblastic
Segmentation: Unsegmented body
Body Cavity: Coelomate
Digestive System: Complete
Respiratory system: Gills (ctenidia) in aqueous forms and mantle cavity or lung in terrestrial forms.
Circulatory system: Open type.
Reproduction & Development:
- Sexual reproduction.
- Mostly dioecious (but some are monoecious)
- Usually internal fertilization.
- Mostly oviparous.
- Development may be direct or indirect.
Special Features:
- Calcareous shell
- Distinct head, muscular foot, visceral hump
- Mantle layer covering visceral hump
- Mantle cavity with feather-like gills for respiration & excretion.
- Feeding organ: Radula (a file-like rasping organ).
- Excretion: occurs through one or two pairs of kidneys (organ of Bojanus).
- “Gastropods show torsion during development leading to asymmetry.”
Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea-hare), Dentalium (Tusk shell), Chaetopleura (Chiton)

9. Phylum – Echinodermata

Habitat: Exclusively marine.
Symmetry: Adults: radial symmetry, larvae: bilateral symmetry
Organization: Organ-system level
Germ layers: Triploblastic
Segmentation: Unsegmented body
Body Cavity: Coelomate
Digestive System: Complete, with mouth on ventral side and anus on dorsal side
Respiratory system: Skin gills (dermal branchiae) and water vascular system.
Circulatory system: Open type.
Reproduction & Development:
- Sexual reproduction.
- Mostly dioecious.
- External fertilization.
- Mostly oviparous.
- Development is indirect, with ciliated free-swimming larva.
Special Features:
- Endoskeleton of calcareous ossicles (spiny-bodied).
- Head absent.
- Water vascular system for respiration, locomotion, and capture and transport of food.
- “Water vascular system consists of madreporite, stone canal, ring canal, radial canals, and tube feet.”
- Excretory system absent.
- “Some echinoderms (e.g. , Starfish) show autotomy and regeneration.”
Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber), Ophiura (Brittle star)

“Echinodermata is divided into 5 classes: Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea, and Crinoidea.”

10. Phylum – Hemichordata

Earlier Considered a sub-phylum of Chordata, now placed as a separate phylum under non-chordates.”
Habitat: Exclusively marine.
Symmetry: Bilateral.
Organization: Organ-system level
Germ layers: Triploblastic
Segmentation: Unsegmented body
Body Cavity: Coelomate
Digestive System: Complete, “Mostly ciliary feeders.”
Respiratory System: Gills or pharyngeal slits.
Circulatory system: Open type, “Respiratory pigment vanadium is present in blood.”
Reproduction & Development:
- Sexual reproduction.
- Mostly dioecious.
- External fertilization.
- Mostly oviparous.
- Indirect development.
  - “Larval stage is tornaria larva, resembling bipinnaria larva of echinoderms.”
Nervous system: “Nerve ring-like brain similar to non-chordates.”
Special Features:
- Worm-like marine animals
- Cylindrical body divided into three regions: anterior proboscis, collar, and long trunk.
- Collar bears a structure called the stomochord.
- Stomochord is a rudimentary structure similar to notochord (found in chordates), but not homologous to it..
- Excretion: Proboscis gland
Examples: Balanoglossus (acorn worm / tongue worm), Saccoglossus

“Hemichordates are considered a connecting link between non-chordates and chordates.”

11. Phylum – Chordata

Key Features:
- Notochord: A solid, flexible, rod-shaped structure on the dorsal side that provides support.
- Dorsal hollow nerve cord: Located above the notochord.
- Paired pharyngeal gill slits: Openings in the pharynx, that are present at some stage of development.
- Post-anal tail: Extending beyond the anus, present at some stage of development.
- Bilateral symmetry, triploblastic, coelomate
- Organ-system level of organisation
- Closed circulatory system: Blood flows within vessels.

Differences between Chordata and Non-Chordata

Feature	Chordata	Non-Chordata
Notochord	Present during embryonic stage (may be retained in some adult forms)	Absent
Central Nervous System	Dorsal, hollow, and single	Ventral, solid, and double
Pharyngeal Gill Slits	Present at some stage of development	Absent
Heart Position	Ventral (on the front or belly side)	Dorsal (on the back side, if present)
Post-anal Tail	Present in some form at some stage of development	Absent
Examples	Vertebrates (mammals, birds, reptiles, amphibians, fish), Tunicates, Lancelets	Invertebrates (arthropods, mollusks, annelids, echinoderms, etc.)

Differences between Chordata and Non-Chordata

Phylum Chordata is further classified into 3 subphyla:

Subphyla:
1. Urochordata (Tunicata):
  - Notochord: Present only in the larval tail.
  - Body covering: Covered by a tunic made up of tunicin.
  - Habitat: Exclusively marine.
  - Reproduction: Hermaphroditic (having both male and female reproductive organs).
  - Examples: Ascidia, Salpa, Doliolum.
2. Cephalochordata:
  - Notochord: Extends from head to tail, present throughout life.
  - Body structure: Fish-like body.
  - Habitat: Exclusively marine.
  - Reproduction: Sexes are separate (dioecious).
  - Examples: Branchiostoma (commonly known as Amphioxus or Lancelet).
3. Vertebrata (Craniata):
  - Notochord: Present during the embryonic period, replaced by a vertebral column in adults.
  - Additional features:
    - Ventral muscular heart,
    - kidneys for excretion and osmoregulation,
    - Paired appendages (fins or limbs).
  - Called “Craniata” because these animals possess a cranium (skull) that encases and protects the brain.
  - Examples: This subphylum includes all vertebrates like fishes, amphibians, reptiles, birds, and mammals.

“Urochordates and cephalochordates are collectively called protochordates.”

“All vertebrates are chordates, but all chordates are not vertebrates.”

Further Classification of Vertebrata

Pisces:
• Aquatic forms bearing fins.

Gnathostomata (jawed vertebrates):
• Chondrichthyes
• Osteichthyes

Tetrapoda (limbs present):
• Amphibia
• Reptilia
• Aves
• Mammalia

Classes within Vertebrata

1. Class – Cyclostomata

Features:
- Ectoparasites on fish
- Elongated body, 6-15 pairs of gill slits
- Sucking, circular mouth without jaws (jawless fishes).
- No scales and no paired fins.
- “Skin is smooth and soft with unicellular mucous glands.”
- Cartilaginous cranium and vertebral column
- Marine, but migrate to freshwater for spawning.
- Die after spawning; larvae (ammocoetes) return to the ocean after metamorphosis.
- “Circulatory system is closed with a two-chambered heart (one auricle and one ventricle).”
- “Kidneys are protonephric or mesonephric and open through urinogenital papilla.”
Examples: Petromyzon (Lamprey), Myxine (Hagfish)

2. Class – Chondrichthyes

Features:
- Marine fishes with streamlined body.
- Cartilaginous endoskeleton
- Ventral mouth; notochord persistent throughout life.
- Separate gill slits (5–7 pairs), no operculum (gill cover).
- Tough skin with placoid scales
- Powerful jaws, predaceous habit.
- No air bladder, must swim to avoid sinking
- Two-chambered heart (one auricle and one ventricle).
- Electric organs in some (e.g., Torpedo), poison sting in some (e.g., Trygon).
- Cold-blooded (ectothermic), separate sexes.
- In males, pelvic fins bear claspers for mating (internal fertilization).
- Internal fertilization; many are viviparous (give birth to live young).
- “Teeth are modified placoid scales directed backwards.”
- “Intestine contains scroll valve to increase absorptive surface.”
Examples: Scoliodon (Dogfish), Pristis (Sawfish), Carcharodon (Great white shark), Trygon (Stingray)

3. Class – Osteichthyes

Features:
- Marine and freshwater fishes
- Bony endoskeleton, streamlined body
- Mouth mostly terminal.
- Four pairs of gills covered by an operculum.
- Skin with cycloid or ctenoid scales
- Air bladder for buoyancy
- Two-chambered heart
- Cold-blooded (ectothermic), separate sexes.
- External fertilisation, mostly oviparous
- Direct development
- “Placoid scales are absent.”
- “Scroll valve in intestine is absent.”
- “Adult kidneys are mesonephric.”
Examples:
- Marine: Exocoetus (Flying fish), Hippocampus (Sea horse)
- Freshwater: Labeo (Rohu), Catla (Katla), Clarias (Magur)
- Aquarium: Betta (Fighting fish), Pterophyllum (Angel fish)

4. Class – Amphibia

Features:
- Live in both water and on land (terrestrial) habitats and need water for breeding, hence called amphibians.
- Two pairs of limbs (except limbless forms).
- Body divided into head and trunk; tail may be present
- Moist skin without scales
- Eyes with eyelids, tympanum represents the ear.
- Common chamber for digestive, urinary, and reproductive tracts (i.e. cloaca)
- Respiration: gills (in larva), lungs, and skin
- 3-chambered heart (two auricles, one ventricle)
- Cold-blooded (Poikilotherms), separate sexes
- External fertilisation, oviparous, indirect development (tadpole larva).
- “Red blood cells are nucleated.”
- “Excretion is ureotelic.”
- “Amphibians are divided into 3 orders: Apoda, Urodela, and Anura.”
Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibian)

5. Class – Reptilia

Features:
- Mostly terrestrial, with creeping or crawling mode of movement.
- Dry, cornified skin with epidermal scales or scutes.
- Shed scales periodically as skin cast.
- No external ear openings, tympanum represents the ear.
- Two pairs of limbs (when present)
- Heart usually 3-chambered (4-chambered in crocodiles)
- Cold-blooded (ectothermic).
- Respiration by lungs throughout life.
- Separate sexes, internal fertilisation
- Oviparous, direct development
- “Kidneys are metanephric.”
- “Excretion varies: crocodiles are ammonotelic, turtles ureotelic, lizards and snakes uricotelic.”
- “Brain shows better development of cerebrum compared to amphibians.”
- “Twelve pairs of cranial nerves are present.”
- “Lateral line system is absent.”
- “Jacobson’s organ is present in the roof of the buccal cavity.”
Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator), Hemidactylus (Wall lizard), Naja (Cobra), Bangarus (Krait), Vipera (Viper)

6. Class – Aves

Features:
- Feathers, ability to fly (except flightless birds like ostrich)
- Beak present; teeth absent.
- Forelimbs modified into wings.
- Hind limbs have scales and are adapted for walking, swimming, or perching.
- Dry skin without glands (except uropygial oil gland at the base of the tail)
  - “Oil gland is absent in ostrich and parrot.”
- Bony endoskeleton, with hollow bones containing air cavities (pneumatic bones).
- Sternum bears a median keel for attachment of flight muscles.
- Clavicle and interclavicle fuse to form furcula (wishbone).
- Digestive tract with crop and gizzard
- 4-chambered heart
- Warm-blooded (homeothermic).
- Respiration by lungs, supplemented by air sacs
- Voice produced by syrinx.
- Separate sexes, internal fertilisation
- Only left ovary functional; urinary bladder absent.
- Oviparous, direct development
Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture)

7. Class – Mammalia

Features:
- Found in diverse habitats: polar, desert, mountains, forests, grasslands, caves
- Some adapted for flying (bats) or living in water (whales, dolphins).
- Milk-producing glands (mammary glands) present for nourishment of young ones.
- Two pairs of limbs adapted for various activities
- Skin with hair, external ears (pinnae) present.
- Middle ear contains three ossicles: malleus, incus, and stapes.
- Different types of teeth (heterodont dentition), deep embedded teeth (thecodont), & two successive sets of teeth (diphyodont) in lifetime.
- Muscular diaphragm separates thorax and abdomen.
- 4-chambered heart
- Warm-blooded (homeothermic).
- Respiration by lungs
- Separate sexes, internal fertilisation
- Mostly viviparous (give birth to live young), with some exceptions (e.g., oviparous monotremes like the platypus).
- Direct development
- Placenta present in higher mammals.
- Parental care is well developed.
Examples:
- Oviparous: Ornithorhynchus (Platypus)
- Viviparous: Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel), Macaca (Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant), Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale), Panthera tigris (Tiger), Panthera leo (Lion)

• “Living mammals are divided into two subclasses: Prototheria and Theria.”
• “Theria is further divided into Metatheria (marsupials) and Eutheria (placental mammals).”

Chapter Summary:

Basic Features for Classification:
- Level of organisation
- Symmetry
- Cell organisation
- Coelom
- Segmentation
- Notochord
Porifera:
- Multicellular animals
- Cellular level of organisation
- Flagellated choanocytes (collar cells) present
Coelenterates:
- Tentacles with cnidoblasts (cnidocytes)
- Mostly aquatic, sessile or free-floating
Ctenophores:
- Exclusively marine animals
- Eight rows of comb plates for locomotion
Platyhelminths:
- Dorsoventrally flattened body
- Bilateral symmetry
- Many parasitic forms with suckers and hooks
Aschelminthes:
- Pseudocoelomates
- Parasitic and free-living roundworms
Annelids:
- Metamerically segmented animals
- True coelom present
Arthropods:
- Most abundant animal phylum
- Jointed appendages and chitinous exoskeleton
Molluscs:
- Soft body with calcareous shell
- Mantle secretes calcareous shell
Echinoderms:
- Exclusively marine animals
- Spiny skin
- Water vascular system
Hemichordates:
- Worm-like marine animals
- Cylindrical body divided into proboscis, collar, and trunk
Phylum Chordata:
- Notochord present throughout life or during embryonic stage
- Dorsal, hollow nerve cord
- Paired pharyngeal gill slits
Agnatha (Class Cyclostomata):
- Primitive jawless chordates
- Ectoparasites on fishes
Gnathostomata:
- Two superclasses: Pisces and Tetrapoda
Pisces:
- Classes Chondrichthyes and Osteichthyes
- Have fins for locomotion
Chondrichthyes:
- Fishes with cartilaginous endoskeleton
- Marine fishes
Tetrapoda:
- Classes Amphibia, Reptilia, Aves, and Mammalia
- Two pairs of limbs (with modifications)
Amphibia:
- Adapted to live on land and water
Reptilia:
- Dry and cornified skin
- Limbs absent in snakes
Poikilothermous Animals:
- Cold-blooded
- Fishes, amphibians, and reptiles
Aves:
- Warm-blooded animals
- Feathers and wings present
- Hind limbs adapted for various activities like walking, swimming, or perching
Mammalia:
- Mammary glands and hair present
- Usually viviparous

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