13.2 Classifying Living Things & Comparing Animal Organ Systems
Key Takeaways
- The Linnaean hierarchy runs domain, kingdom, phylum, class, order, family, genus, species; organisms that share more ranks are more closely related.
- Binomial nomenclature gives each species a two-part italicized name (genus capitalized, species epithet lowercase), such as Alligator mississippiensis.
- The five vertebrate classes (fish, amphibians, reptiles, birds, mammals) differ in body covering, breathing, body-temperature regulation, and reproduction.
- Dichotomous keys identify organisms through a series of paired, observable choices.
- Animals meet the same needs with different structures: gills, tracheae, air-sac lungs, and alveolar lungs for breathing; two- to four-chambered hearts or open circulation; and external or internal fertilization.
Classifying Living Things & Comparing Animal Organ Systems
Subtest 603 asks you to classify living things into major groups using the Linnaean system and to compare them by physical features, behaviors, and development. It also asks you to compare the structures and functions of human and other animal organ systems (for example, respiration, reproduction, and digestion). This section builds both skills.
The Linnaean Hierarchy
Swedish botanist Carl Linnaeus (1707–1778) introduced a nested system of ranks in Systema Naturae (first edition 1735) and standardized two-part scientific names. Each rank groups organisms that share more characteristics than the rank above it:
Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species (Mnemonic: Dear King Philip Came Over For Good Soup. The domain level was added in 1990 by microbiologist Carl Woese.)
| Rank | Human | American alligator |
|---|---|---|
| Domain | Eukarya | Eukarya |
| Kingdom | Animalia | Animalia |
| Phylum | Chordata | Chordata |
| Class | Mammalia | Reptilia |
| Order | Primates | Crocodilia |
| Family | Hominidae | Alligatoridae |
| Genus | Homo | Alligator |
| Species | Homo sapiens | Alligator mississippiensis |
The two organisms share ranks down through phylum and differ from class onward. The more ranks two organisms share, the more closely related they are.
Binomial Nomenclature
Every species has a two-part scientific name: the genus (capitalized) plus the specific epithet (lowercase), written in italics or underlined. Examples include Alligator mississippiensis (American alligator) and Quercus virginiana (southern live oak). Some species have subspecies names, such as the Florida manatee, Trichechus manatus latirostris. Scientific names avoid the confusion of common names: "puma," "cougar," and "Florida panther" all refer to Puma concolor.
Major Groups
- Three domains: Bacteria, Archaea (both prokaryotes), and Eukarya.
- Eukaryotic kingdoms commonly taught: protists, fungi, plants, and animals.
- Plants: nonvascular (mosses), seedless vascular (ferns), gymnosperms (conifers), and angiosperms (flowering plants); see Section 13.5.
- Invertebrate animals (no backbone; about 95% of animal species): sponges, cnidarians (jellyfish, corals), worms, mollusks (snails, clams, octopuses), echinoderms (sea stars, sand dollars), and arthropods. Arthropods include insects (3 body parts, 6 legs, usually wings and antennae), arachnids (2 body parts, 8 legs, no antennae), and crustaceans (crabs and shrimp, with gills and two pairs of antennae).
- Vertebrate classes:
| Class | Body covering | Breathing | Body temperature | Reproduction and development | Florida examples |
|---|---|---|---|---|---|
| Fish | Scales | Gills | Ectothermic | Most lay eggs, fertilized externally in water | Largemouth bass, snook |
| Amphibians | Moist, smooth skin | Gills as larvae; lungs and skin as adults | Ectothermic | Jelly-coated eggs in water; metamorphosis | Green tree frog, southern toad |
| Reptiles | Dry scales or plates | Lungs | Ectothermic | Internal fertilization; leathery-shelled eggs laid on land (most) | American alligator, gopher tortoise |
| Birds | Feathers | Lungs with air sacs | Endothermic | Internal fertilization; hard-shelled eggs | Brown pelican, Florida scrub-jay |
| Mammals | Hair or fur | Lungs | Endothermic | Internal fertilization; most give live birth and all nurse young with milk | Florida manatee, Florida panther |
Dichotomous Keys
A dichotomous key identifies organisms through a series of paired choices, each dividing the group in two:
- a. Has a backbone → go to 2 · b. No backbone → go to 3
- a. Has feathers → brown pelican · b. Has fur → raccoon
- a. Six legs → grasshopper · b. Eight legs → garden spider
Students can build their own keys for a set of leaves or classroom objects, practicing classification by observable characteristics.
Comparing Organisms by Behavior and Development
Classification can also use behaviors and life-cycle patterns, not just body parts:
- Development: Insects go through complete or incomplete metamorphosis, frogs through amphibian metamorphosis, and mammals develop directly without a larval stage.
- Behavior: Some animals are nocturnal or diurnal, some migrate or hibernate, some live in social groups and others alone, and some care for their young while others do not.
- Body temperature: Endotherms (birds and mammals) keep a steady body temperature using internal heat. Ectotherms (fish, amphibians, reptiles) depend on the environment, which is why alligators bask in the sun on cool mornings.
Teaching Classification
Start with objects students can sort themselves (buttons, leaves, shells) and have them explain their rules. Then have them sort picture cards of animals by one attribute at a time, and finally build a dichotomous key. A classic misconception is that whales and dolphins are fish because they live in water and have fins. Having students check the defining traits (lungs, hair at birth, milk, live birth) shows why they are mammals. Likewise, bats are mammals, not birds, and spiders are arachnids, not insects.
Comparing Human and Other Animal Organ Systems
| System | Humans (mammals) | Other animals |
|---|---|---|
| Respiration | Lungs with alveoli; the diaphragm drives breathing | Fish: gills take oxygen from water flowing over thin filaments. Insects: air enters spiracles and travels through tracheae directly to cells. Earthworms: gas exchange through moist skin. Birds: lungs plus air sacs keep air flowing one way through the lungs. |
| Circulation | Closed system; four-chambered heart separates oxygen-rich and oxygen-poor blood | Fish: two-chambered heart. Amphibians and most reptiles: three chambers (crocodilians have four). Birds: four chambers. Insects: an open circulatory system in which fluid bathes the organs. |
| Digestion | Mouth → esophagus → stomach → small intestine → large intestine | Cows and other ruminants: a four-compartment stomach and cud chewing to break down cellulose. Birds: a crop stores food and a muscular gizzard grinds it (no teeth). Carnivores have shorter digestive tracts than herbivores. |
| Support | Internal bony endoskeleton that grows with the body | Arthropods: an external exoskeleton of chitin that must be molted to grow. Earthworms: a fluid-filled hydrostatic skeleton. |
| Reproduction | Testes produce sperm and ovaries produce eggs; fertilization is internal and the embryo develops in the uterus | Most fish and frogs use external fertilization in water. Reptiles and birds use internal fertilization and lay shelled eggs. Monotremes (platypus) are egg-laying mammals, and marsupials (opossum) give birth to tiny young that develop in a pouch. |
Across these groups the functions are the same (obtain oxygen, move materials, break down food, support the body, reproduce), but the structures are adapted to each animal's environment. That idea connects directly to adaptations and natural selection (Section 13.4).
A student is told that a bobcat (Lynx rufus) and a Canada lynx (Lynx canadensis) are in the same genus, while a bobcat and a Florida panther (Puma concolor) are in the same family but different genera. What can the student correctly conclude?
A fourth-grade class observes an animal with smooth, moist skin that hatched from a jelly-coated egg laid in a pond, breathed with gills as a juvenile, and now breathes with lungs and through its skin. In which vertebrate class does it belong?
While comparing respiratory systems, a teacher asks how oxygen reaches the cells of a grasshopper. Which answer is accurate?