🦴 Introduction

Vertebrates (Subphylum Vertebrata) possess a vertebral column replacing the notochord, a cranium enclosing the brain, and a highly organized organ-system level of body design. They show progressive adaptation from aquatic to terrestrial life.

Subphylum Vertebrata is divided into two Divisions: Agnatha (jawless) and Gnathostomata (jawed).

Agnatha Division

Class 1. Cyclostomata (Jawless Fishes)

  • All living members are ectoparasites on some fishes.
  • Body is elongated, jawless, with a sucking and circular mouth.
  • Skin is devoid of scales and paired fins.
  • Endoskeleton is cartilaginous. Cranium and vertebral column are cartilaginous.
  • Circulation is of closed type.
  • Heart is two-chambered.
  • Respiration: 6-15 pairs of gill slits.
  • Marine, but migrate to freshwater for spawning (anadromous migration). After spawning, they die. Their larvae, after metamorphosis, return to the ocean.
  • Examples: Petromyzon (Lamprey), Myxine (Hagfish).

Division Gnathostomata (Bear Jaws)

This division is further divided into two more divisions: Pisces and Tetrapoda

Superclass Pisces (Bear Fins)

Class 2. 🐟 Chondrichthyes (Cartilaginous Fishes)

  • Marine animals with streamlined bodies.
  • Endoskeleton is entirely cartilaginous.
  • Mouth is located ventrally.
  • Notochord is persistent throughout life.
  • Gill slits are separate and without an operculum (gill cover), usually 5-7 pairs.
  • Skin is tough, containing minute placoid scales (tooth-like).
  • Teeth are modified placoid scales, backwardly directed.
  • Air bladder is absent, so they have to swim constantly to avoid sinking.
  • Heart is two-chambered (one auricle, one ventricle).
  • Some possess electric organs (Torpedo) or poison sting (Trygon).
  • Poikilothermous (cold-blooded) - lack the capacity to regulate body temperature.
  • Sexes are separate. Males have pelvic fins bearing claspers.
  • Fertilisation is internal, and many are viviparous (give birth to live young).
  • Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray), Torpedo (Electric ray).

Class 3. 🐟 Osteichthyes (Bony Fishes)

  • Includes both marine and freshwater fishes.
  • Streamlined body.
  • Endoskeleton is bony.
  • Mouth is mostly terminal.
  • Four pairs of gill slits covered by an operculum on each side.
  • Skin is covered with cycloid/ctenoid scales.
  • Air bladder is present, which regulates buoyancy.
  • Heart is two-chambered.
  • Poikilothermous (cold-blooded).
  • Sexes are separate. Fertilisation is usually external. Mostly oviparous (egg-laying).
  • Development is direct.
  • Examples:
  • Marine: Exocoetus (Flying fish), Hippocampus (Sea horse).
  • Freshwater: Labeo (Rohu), Catla (Katla), Clarias (Magur).
  • Aquarium: Betta (Fighting fish), Pterophyllum (Angel fish).

Superclass Tetrapoda (Bear Limbs)

🐸 Class Amphibia — Amphibians

  • Can live in aquatic as well as terrestrial habitats ('amphibious'). Need water for reproduction.
  • Most have two pairs of limbs.
  • Body divisible into head and trunk; tail may be present in some.
  • Skin is moist, without scales. Eyes have eyelids. A tympanum represents the ear.
  • Alimentary canal, urinary, and reproductive tracts open into a common chamber called the cloaca, which opens externally.
  • Respiration: By gills (larvae), lungs, and through the skin (adults).
  • Heart is three-chambered (two auricles, one ventricle).
  • Poikilothermous (cold-blooded).
  • Sexes are separate. Fertilisation is external. Oviparous. Development is indirect (tadpole larva).
  • Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).

🐍 Class Reptilia — Reptiles

  • Characterized by creeping or crawling mode of locomotion (Latin, repere or reptum, to creep).
  • Mostly terrestrial. Body covered by dry, cornified skin with epidermal scales or scutes.
  • Do not have external ear openings; tympanum represents the ear.
  • Limbs, when present, are two pairs.
  • Heart is usually three-chambered, but four-chambered in crocodiles.
  • Poikilothermous (cold-blooded).
  • Snakes and lizards shed their scales as skin cast.
  • Sexes are separate. Fertilisation is internal. Oviparous. Development is direct.
  • Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator. Snakes: Naja (Cobra), Bangarus (Krait), Vipera (Viper).

🕊️ Class Aves — Birds

  • Presence of feathers and most can fly (except flightless birds like Ostrich).
  • Possess a beak. Forelimbs are modified into wings. Hind limbs generally have scales and are modified for walking, swimming or clasping.
  • Skin is dry without glands except the oil gland at the base of the tail.
  • Endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic).
  • Digestive tract has additional chambers: the crop and gizzard.
  • Heart is completely four-chambered.
  • Homeothermous (warm-blooded) - able to maintain constant body temperature.
  • Respiration: By lungs. Air sacs connected to lungs supplement respiration.
  • Sexes are separate. Fertilisation is internal. Oviparous. Development is direct.
  • Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

🦌 Class Mammalia — Mammals

  • Found in a variety of habitats. Some adapted to fly (Bat) or live in water (Whale, Dolphin).
  • Most unique characteristic is the presence of mammary glands (milk-producing glands) to nourish young ones.
  • Have two pairs of limbs.
  • Skin possesses hair.
  • External ears (pinnae) are present.
  • Different types of teeth (heterodont) are present in the jaw.
  • Heart is four-chambered.
  • Homeothermous (warm-blooded).
  • Respiration: By lungs.
  • Sexes are separate. Fertilisation is internal. Mostly viviparous (giving birth to live young), with few exceptions (e.g., Platypus is oviparous).
  • Development is direct.
  • 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), Homo sapiens (Man).

🧠 Summary Table

Class Body Covering Heart Chambers Temperature Fertilization Examples
Pisces Scales 2 Cold External Rohu, Shark
Amphibia Moist skin 3 Cold External Frog
Reptilia Scales 3–4 Cold Internal Lizard, Crocodile
Aves Feathers 4 Warm Internal Pigeon
Mammalia Hair 4 Warm Internal Human

💡 Questions and Answers

Q1. Name the fish having a cartilaginous skeleton.

A1. Scoliodon (dogfish) and Torpedo.

Q2. Which reptile possesses a four-chambered heart?

A2. Crocodile.

Q3. Why are birds called homeothermic?

A3. They maintain constant body temperature irrespective of surroundings.

Q4. Which mammal lays eggs?

A4. Platypus (Monotreme).

Q5. Write one difference between Amphibia and Reptilia.

A5. Amphibians have moist skin and external fertilization, reptiles have dry scaly skin and internal fertilization.

Q6. Differentiate between Chondrichthyes and Osteichthyes.

A6.

Feature Chondrichthyes (Cartilaginous) Osteichthyes (Bony)
Skeleton Cartilaginous Bony
Mouth Position Ventral Mostly Terminal
Gill Slits Separate, no operculum 4 pairs, covered by operculum
Scales Placoid Cycloid / Ctenoid
Air Bladder Absent Present
Fertilisation Internal Usually External
Development Many viviparous Mostly Oviparous
Claspers (Male) Present Absent

Q7. What are the key characteristics unique to Mammalia?**

A7. The most unique defining characteristics of mammals are:

  1. Presence of mammary glands for producing milk to nourish young.
  2. Presence of hair on the skin. Other features common to mammals (though sometimes shared with birds) include being warm-blooded (homeothermous), having a four-chambered heart, and possessing a diaphragm for respiration. They also typically have external ears (pinnae) and heterodont dentition.