Vertebrate Classification: Mammals, Fish, Amphibians, Reptiles, and Birds

Key Biological Terms

• Notochord: A flexible rod to which muscles are attached.
• Agnathans: The most primitive fishes that lack a jaw.
• Air bladder: An internal sac that fills with gas to regulate buoyancy and depth.
• Tadpoles: Aquatic larvae that emerge from eggs laid in the water.

Mammals

Mammals are endothermic vertebrates. While many live on land, some are aquatic or adapted for flight.

• Body: Covered with hair for temperature regulation and camouflage.
• Mammary glands: Produce milk for offspring.
• Limbs: Share anatomical patterns adapted to their specific environment.
• Physiology: Possess teeth and breathe using lungs.
• Reproduction: Internal fertilization.

Types of Mammals

• Monotremes (e.g., platypuses): Oviparous, possess a leathery beak, and lack teeth.
• Marsupials (e.g., kangaroos, koalas, possums): Viviparous. Babies are born underdeveloped and complete growth in an external pouch called a marsupium.
• Placentals: Viviparous. Babies are born in an advanced stage of development, having been nourished in the uterus via a placenta.

Fish

Fish are ectothermic aquatic vertebrates. The most primitive known are agnathans (jawless fish).

• Long, fusiform body
• Air bladder
• Lateral line

Amphibians

Amphibians are ectothermic vertebrates that inhabit both terrestrial and aquatic environments.

• Characteristics: Tetrapods (as adults), utilize lungs and subcutaneous respiration, internal fertilization, and undergo metamorphosis.
• Tadpoles: Possess a long tail and no limbs.

Amphibian Orders

• Urodela (salamanders and newts): Long body composed of a head, neck, torso, and four short limbs.
• Anura (frogs and toads): Lack a tail in the adult stage. They possess an extendable sticky tongue, produce sounds, and some secrete toxic substances through their skin for protection.

Reptiles

Reptiles are ectothermic vertebrates that are mostly terrestrial, though some are adapted to aquatic environments.

• Skin: Covered with hard, horny scales to prevent dehydration.
• Physiology: Four short limbs, two lungs, and main sense organs located in the head.
• Reproduction: Internal fertilization; eggs have a hard, porous covering to prevent the embryo from drying out.
• Habitat: Prefer hot climates due to their ectothermic nature.

Reptile Groups

• Chelonia (turtles and tortoises): Possess a hard shell.
• Squamata (Ophidia - snakes): Lack limbs and shed their skin to grow.
• Squamata (Sauria - lizards): Possess five toes with claws and mobile eyelids.
• Crocodilia: Bodies covered with a thick layer of bony plates.

Birds

Birds are terrestrial endothermic vertebrates adapted for flight.

• Body: Fusiform and aerodynamic to facilitate movement in the air.
• Feathers: Protect against water and cold. Parts include the rachis, barb, and quill.
• Anatomy: Hollow bones and lungs with air sacs to reduce weight and oxygenate muscles during flight.
• Digestion: They do not chew food. It is stored and softened in the crop (part of the esophagus) and ground in the muscular gizzard within the stomach.
• Reproduction: Internal fertilization.