Classes

6: Defining the Lifestyle (Class)

Discover how the "Class" level of taxonomy organizes life. Learn the differences between Mammalia, Insecta, and the "True Dicots" of the plant world.

In the taxonomic hierarchy, Classes sit directly below Phylum and directly above Orders. While everyone in a Phylum shares a basic body plan, members of a Class share significantly more physical and genetic traits.

Think of it as a funnel: If you are in the Phylum Chordata, you are "in the club" with both a goldfish and a human. Once you move down to the Class Mammalia, you’ve narrowed the group to include humans, bats, and blue whales, while "kicking out" the goldfish.

1. Classes in the Animal Kingdom

Because animals are so diverse, their Classes are often defined by very obvious physical traits like how they breathe, move, or reproduce.

Phylum Arthropoda (Jointed Legs)

Classes here are defined by the number of legs and body segments.

Class Insecta: 6 legs and 3 body parts (e.g., ants, beetles, butterflies).
Class Arachnida: 8 legs and 2 body parts (e.g., spiders, scorpions, ticks).
Class Crustacea: Mostly aquatic with 2 pairs of antennae (e.g., crabs, lobsters).
Class Chilopoda: One pair of legs per body segment (e.g., centipedes).

Phylum Chordata (Spinal Cords)

This is the phylum humans belong to, divided by traits like body temperature and skin covering.

Class Mammalia: Possess hair and mammary glands; warm-blooded (e.g., humans, whales).
Class Reptilia: Covered in scales; lay eggs on land; cold-blooded (e.g., snakes, crocodiles).
Class Amphibia: Moist skin; live between water and land; eggs laid in water (e.g., frogs).
Class Aves: Possess feathers and beaks; lay eggs; warm-blooded (e.g., birds).
Class Chondrichthyes: Skeletons made of cartilage rather than bone (e.g., sharks, rays).

2. Classes in the Plant Kingdom

In botany, flowering plants (Phylum Magnoliophyta) were traditionally split into two iconic classes based on how they "sprout" their first leaves.

Class Monocotyledonae (Monocots): Produce one "seed leaf" when they sprout (e.g., grasses, lilies, corn).
Class Dicotyledonae (Dicots): Produce two "seed leaves" when they sprout (e.g., roses, oak trees, beans).

The Modern Update: DNA research in the 1990s revealed that while Monocots are a distinct group, "Dicots" was a "catch-all" category. Most were renamed Eudicots ("True Dicots"), while ancient plants like Magnolias were found to have branched off the tree much earlier.