Reptiles, birds and mammals

Class Reptilia: creeping animals

The name comes from the way many of them move: by creeping or crawling. Reptiles are mostly land animals.

• The skin is dry and cornified, covered with epidermal scales or plates called scutes.
• There is no external ear opening. A tympanum serves as the ear.
• Limbs, when present, are two pairs. Snakes have none.
• The heart usually has three chambers, but in crocodiles it has four.
• Reptiles are cold-blooded, or poikilotherms.
• Snakes and lizards shed their scales as a whole skin cast.

The sexes are separate. Fertilisation is internal. Reptiles are oviparous, and development is direct: the young hatch looking like small adults.

Examples: Chelone, the turtle, Testudo, the tortoise, Chameleon, the tree lizard, Calotes, the garden lizard, Crocodilus, the crocodile, Alligator, and Hemidactylus, the wall lizard. Three poisonous snakes: Naja, the cobra, Bangarus, the krait, and Vipera, the viper.

Class Aves: the birds

The mark of a bird is the feather. Most birds can fly. The flightless birds, such as the ostrich, cannot.

• They have a beak, and the forelimbs are wings.
• The hind limbs usually carry scales. They are built for walking, swimming or gripping branches.
• The skin is dry, with no glands except the oil gland at the base of the tail.
• The skeleton is fully bony. The long bones are hollow and filled with air: they are pneumatic, which makes the body light.
• The gut has two extra chambers, the crop and the gizzard.
• The heart is completely four-chambered.
• Birds are warm-blooded, or homoiothermous: they keep a constant body temperature.
• They breathe with lungs, helped by air sacs joined to the lungs.

The sexes are separate. Fertilisation is internal. Birds are oviparous, and development is direct.

Examples: Corvus, the crow, Columba, the pigeon, Psittacula, the parrot, Struthio, the ostrich, Pavo, the peacock, Aptenodytes, the penguin, and Neophron, the vulture.

Class Mammalia: the mammals

Mammals live almost everywhere: on polar ice, in deserts, on mountains, in forests and grasslands, and in dark caves. Some fly, and some live in water.

• Their most special feature is the mammary glands, which make milk to feed the young.
• They have two pairs of limbs, built for walking, running, climbing, burrowing, swimming or flying.
• The skin has hair.
• They have external ears, the pinnae.
• The teeth are of different kinds: incisors, canines, premolars and molars.
• The heart has four chambers.
• Mammals are homoiothermous, and they breathe with lungs.

The sexes are separate, and fertilisation is internal. Mammals are viviparous, with a few exceptions, and development is direct.

The exception to remember is Ornithorhynchus, the platypus, which lays eggs. The viviparous examples are many.

• Macropus, the kangaroo, and Pteropus, the flying fox.
• Camelus, the camel, Macaca, the monkey, and Rattus, the rat.
• Canis, the dog, Felis, the cat, Elephas, the elephant, and Equus, the horse.
• Delphinus, the common dolphin, and Balaenoptera, the blue whale.
• Panthera tigris, the tiger, and Panthera leo, the lion.

A flying fox flies, but it has hair and feeds its young on milk, so it is a mammal. A dolphin swims, but it breathes with lungs, so it is not a fish.

Hearts and body heat

• Skin. Cyclostomes have no scales. Sharks have placoid scales, and bony fishes cycloid or ctenoid ones. Amphibians have moist skin with no scales. Reptiles have dry skin with scales or scutes. Birds have feathers, and mammals have hair.
• Breathing. Fishes use gills. Amphibians use gills, lungs and skin. Reptiles and mammals use lungs, and birds use lungs with air sacs.

Chambers of the heart

Move the slider from a fish to a mammal. Watch the atria and the ventricles, and the warm-blooded readout.

Try it in Python

The program lists the heart chambers of six vertebrates, says whether each is warm-blooded, and counts the four-chambered hearts.

Practise with new numbers

Common mistakes

• "All reptiles have a three-chambered heart." Crocodiles have four chambers.
• "A four-chambered heart makes an animal warm-blooded." Crocodiles have four chambers and are cold-blooded.
• "A bat is a bird, and a whale is a fish." Both have hair and feed their young on milk. Both are mammals.
• "Every mammal gives birth to live young." Ornithorhynchus, the platypus, lays eggs.
• "Bird skin is full of glands." It has only one, the oil gland at the base of the tail.

Remember these

• Reptilia: dry cornified skin with scales or scutes, a tympanum, a heart of three chambers (four in crocodiles), internal fertilisation, eggs.
• Aves: feathers, beak, wings, pneumatic bones, crop and gizzard, air sacs, a four-chambered heart, warm-blooded.
• Mammalia: mammary glands, hair, pinnae, different kinds of teeth, a four-chambered heart, warm-blooded, mostly viviparous.
• Only birds and mammals are homoiothermous. All the other classes are poikilothermous.
• Ornithorhynchus is the egg-laying mammal.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A wall lizard leaves behind a thin, scaly skin. What has it done, and which class does this point to?

2. Name three features of a bird that make its body light or help it breathe in flight.

3. Which feature gives the mammals their name?

4. Which reptile has a four-chambered heart, and is it warm-blooded?

5. Explain why a flying fox is a mammal and not a bird.