Characteristics of Fish, Amphibians, Reptiles, Birds, and Mammals

Posted on Mar 21, 2025 in Biology

Fish

Body Features

• Fusiform body.
• Fish limbs are fins. Most fish have: 2 pectoral fins, 2 pelvic fins, one caudal fin, one anal fin, and one or more dorsal fins. However, there are exceptions.
• Their body is covered with scales, which are overlapped. Sharks and rays have small denticles instead of flat scales.
• They have a sensory organ, the lateral line, which detects water vibrations.
• Most fish have a natatory bladder, an internal sac filled with air, which they use to control their depth.
• They always have a tail (with the caudal fin).

Functions

• They are poikilotherms.
• They breathe through gills, so they cannot breathe outside the water. In typical fish, gills are protected by a bony piece called the operculum. In sharks, rays, and other fish, there’s no operculum.
• Most are carnivorous, but there are many herbivorous and omnivorous.
• External fertilization.
• They are generally oviparous, but some sharks are ovoviviparous.

Two Major Groups of Fish:

Cartilaginous Fish

• Marine animals only.
• Their skeleton is made of cartilage.
• Their body is covered with small denticles.
• They don’t have an operculum.

Bony Fish

• Marine and freshwater animals.
• Their skeleton is made of bone.
• Their body is covered with overlapping scales.
• They have an operculum.

Amphibians

Body Features

• Amphibians have 4 legs. In frogs and toads, hind legs are adapted to jump.
• Their skin is very thin, smooth, and has no covering. They need to keep it moist because they use it to breathe.
• Frogs and toads don’t have a tail. Salamanders and newts do.

Functions

• They are poikilotherms. They are more abundant in wet, warm areas.
• Their life has two stages: the larva or tadpole and the adult, which are very different. The transition from one to the other takes place by means of a set of transformations, called metamorphosis.
• Adults are terrestrial animals, so they have lungs to breathe, although their moist skin is also useful to absorb oxygen from water. Tadpoles are aquatic and they have external gills.
• Adults are commonly carnivorous and tadpoles are herbivorous.
• Frogs and toads usually have external fertilization. Salamanders and newts have internal fertilization.
• Most are oviparous. Their eggs don’t have a hard shell, so they must be laid in water. Some salamanders are ovoviviparous.

Two Groups of Amphibians

Order Anura

Order Urodela

Order Anura

• Without a tail
• External fertilization
• Oviparous

Order Urodela

• With a tail
• Internal fertilization
• Oviparous and ovoviviparous

Metamorphosis

Tadpole

• Tail
• No legs
• Gills
• Herbivorous

Adult

• No tail
• With legs
• Lungs
• Carnivorous (insects)

Activities

Reptiles

Body Features

• They have 4 legs which are small and laterally projected, so they drag themselves along on their belly. Exceptions: snakes don’t have legs, and sea turtles have modified their legs into paddle-like flippers.
• They always have a tail.
• Their body is covered with hard scales, placed edge to edge without overlapping.
• Some reptiles need to renew or shed their skin to grow. They lose their old skin and replace it with a new one.
• Turtles have a hard, round, bony shell called a carapace.

Functions

• They are poikilotherms, so they can’t autoregulate their body temperature. That’s why they are rare in cold areas.
• They have lungs.
• They are usually carnivorous, although some are herbivorous (e.g., iguanas).
• They are heterosexual and have internal fertilization.
• They are oviparous, but they don’t incubate their eggs. Some snakes are ovoviviparous.

Ophidians or Snakes

Saurians or Lizards

Chelonians or Testudines

Crocodilians

Birds

Body Features

• They have a neck, which is sometimes very long.
• The body shape is aerodynamic.
• 4 limbs: the 2 back limbs are legs, covered with scales and with four fingers; the 2 front limbs are wings.
• They have a horny mandible: the beak or bill and no teeth.
• All birds have a tail.
• Their bones are hollow and light.
• Their breastbone is highly developed, so that strong wing muscles can attach to it. It’s called the keel.
• The body is covered with feathers (except for the legs and beak). Each feather has an axis or rachis, barbs which spread out on each side of it, and a calamus, which joins the feather to the body.

Functions

• They are homeotherms.
• They have lungs connected to air sacs that take up most of their body and reach into their bones. This makes them lighter and refrigerates their body more efficiently.
• Their feeding is diverse and it varies among species. The shape of the bill adapts to the diet.
• They are heterosexual and have internal fertilization.
• They are oviparous and incubate their eggs.

A Feather

Mammals

Body Features

• They have a neck and a tail. Some don’t have the tail (human beings and apes).
• They have many glands, especially mammary glands, which produce milk to feed the newborn.
• They have lips for suckling and teeth.
• 4 limbs, normally legs. Aquatic mammals have fins and bats have 2 wings and 2 legs.
• Their body is covered with fur.

Functions

• They are homeotherms.
• They have lungs. That means they cannot breathe underwater.
• Internal fertilization
• They are viviparous. Exception: Monotremes are oviparous.
• All suckle when they are young. Afterwards, they have a very diverse feeding.

Three Groups of Mammals

Monotremes

Placentals

Marsupials