Animal Diversity and Physiology

This topic is classification with physiology attached. The test asks which group an animal belongs to and what that group does differently, so the useful unit of study is the group rather than the animal.

Invertebrates

No backbone, and the great majority of animal species.

Arthropoda is the largest phylum in the animal kingdom, and insects are the largest group within it. If a question asks which group has the most species, that is the answer.

An exoskeleton is on the outside, which is why arthropods must molt to grow - the shell does not expand.

Vertebrates

A backbone, and the five classes the test asks about.

Amphibians are the class defined by the change. They begin in water with gills and move to land with lungs, which is what "amphibian" means and why a frog tadpole is the standard example.

Mammals are defined by mammary glands and hair, not by live birth. The platypus lays eggs and is still a mammal, which is a favorite question.

Warm-blooded and cold-blooded

The proper terms are endothermic and ectothermic.

Endothermic (warm-blooded): generates its own body heat, keeps a steady temperature, needs far more food. Birds and mammals only.

Ectothermic (cold-blooded): takes its temperature from the surroundings, needs much less food, becomes sluggish when cold.

Cold-blooded does not mean cold. A lizard in the sun is warmer than you are. It means the temperature is not internally regulated, which is what the test is checking.

Symmetry and body plans

Bilateral symmetry goes with a head end, and a head end goes with concentrated sense organs and a brain. Animals that move in a direction are bilateral; animals that sit still or drift are often radial.

Metamorphosis

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Complete metamorphosis: egg, larva, pupa, adult. The larva looks nothing like the adult. Butterflies, beetles, flies and bees.

Incomplete metamorphosis: egg, nymph, adult. The nymph resembles a smaller adult and grows through molts. Grasshoppers, cockroaches and dragonflies.

The pupa is the difference. If there is a pupal or chrysalis stage, the metamorphosis is complete; if the young looks like a small adult, it is incomplete.

Amphibians undergo metamorphosis too - a tadpole to a frog - though the insect terms are what the test usually asks about.

How other animals do the basics

Breathing. Gills extract oxygen from water. Lungs extract it from air. Insects use tracheae, a network of tubes carrying air directly to the tissues, which is why they have no need to carry oxygen in their blood. Earthworms breathe through their moist skin.

Circulation. An open circulatory system pumps blood into body cavities rather than staying in vessels, which is what most arthropods and mollusks have. A closed system keeps blood in vessels and is found in vertebrates, annelids and cephalopods. Closed systems move blood faster and support higher activity.

Fish hearts have two chambers, amphibian and most reptile hearts three, and bird and mammal hearts four. The four-chamber arrangement keeps oxygen-rich and oxygen-poor blood completely separate, which supports the high metabolism that being warm-blooded requires.

What you can skip

Across all 2,104 General Science questions in our bank:

• The Latin phylum names beyond the table - Porifera for sponges, nematodes, flatworms and roundworms as groups - never appear.
• Mammal subgroups. Marsupials, monotremes and placental mammals are not asked. What makes a mammal a mammal is.
• Cephalopods and tracheae by name do not appear. The open-against-closed circulation contrast is the idea worth keeping from that section.

Where people lose points

Saying all mammals give birth to live young. The platypus and the echidna lay eggs.

Thinking cold-blooded animals are cold.

Putting a pupa in incomplete metamorphosis.

Saying insects breathe through lungs. They use tracheae.

Confusing exoskeleton with endoskeleton.

Calling a starfish a fish. It is an echinoderm, and it has no backbone.

Work one in under a minute

An animal has dry scaly skin, lays leathery eggs on land, breathes with lungs throughout its life, and its body temperature follows the air temperature. What class is it?

Dry scales and land eggs rule out amphibians, which need moisture. Lungs from birth rule out amphibians again. Not warm-blooded rules out birds and mammals.

A reptile.

Each clue eliminated something. That is how these questions are built and how they should be answered.

Where this leads

The classes here fill the phyla from the classification lesson, and the circulatory comparisons are the human systems seen from outside.