The Diversity of Life: Animals – Structure, Function, and Evolution

The Diversity of Life: Animals

What Is an Animal?

Animals are a monophyletic clade of eukaryotes that share several defining characteristics. Understanding these traits helps distinguish animals from other life forms.

• Multicellularity: Animals are composed of multiple cells that lack cell walls and possess an extensive extracellular matrix.

• Heterotrophy: Animals obtain necessary carbon compounds from other organisms, primarily by ingesting food rather than absorbing it.

• Motility: Animals move under their own power at some point in their life cycle.

• All animals except sponges have neurons (for transmitting electrical signals) and muscle cells (for contraction and movement).

Example: Humans, insects, and fish all ingest food, are multicellular, and move independently.

Phylogenetic Relationships

Animals are closely related to choanoflagellates, forming sister groups within the eukaryotes. DNA sequence data supports the evolutionary relationships among major animal phyla.

• Choanoflagellates are the closest living relatives to animals.

• Animals, fungi, and choanoflagellates are grouped within the Opisthokonta.

Example: Phylogenetic trees based on DNA sequences show the evolutionary divergence of animal groups.

Key Characteristics of Animals

• Heterotrophs (not autotrophs)

• Multicellular

Test Yourself: Which of the following are characteristics of animals? (Answer: I and III only – Heterotrophs and Multicellular)

Animal Structure and Development

Embryonic Tissue Layers

Animals are classified based on the number of germ layers present in their embryos.

• Diploblasts: Have two germ layers:

  • Ectoderm (“outside-skin”)

  • Endoderm (“inside-skin”)

• Triploblasts: Have three germ layers:

  • Ectoderm

  • Mesoderm (“middle-skin”)

  • Endoderm

Example: Cnidarians are diploblastic; most other animals are triploblastic.

Origin and Diversification of Tissues

• Ectoderm: Produces the covering of the animal (skin, nervous system)

• Endoderm: Generates the digestive tract

• Mesoderm: Gives rise to tissues in between (circulatory system, muscle, internal structures such as bone and most organs)

Muscle

All animals share homologous genes for contractile proteins:

• Actin and myosin are the primary proteins involved in muscle contraction.

• In ctenophores and cnidarians, contractile cells derived from endoderm and/or ectoderm are called epitheliomuscular cells.

Animal Body Plans and Symmetry

Bilateral Symmetry, Cephalization, and the Nervous System

Body symmetry is a key morphological aspect of an animal’s body plan.

• Radial symmetry: At least two planes of symmetry (e.g., cnidarians, ctenophores, some sponges). Radial symmetry evolved independently in echinoderms.

• Bilateral symmetry: A single plane of symmetry, resulting in long, narrow bodies. Most animals exhibit this form.

The Nervous System

• Neurons transmit and process information as electrical signals.

• Symmetry and nervous system are related:

  • Sponges lack nerve cells and symmetry.

  • Radially symmetrical animals have a nerve net.

  • Bilaterally symmetric animals tend to have a more complex central nervous system (CNS).

  • Cephalization: Concentration of sensory and neural structures in the head.

  • Other neurons are clustered in masses called ganglia.

Animal Body Cavities and Segmentation

Origin of the Coelom

The basic bilaterian body shape is described as a tube within a tube:

• Inner tube: Gut with a mouth at one end and an anus at the other (derived from endoderm).

• Outer tube: Forms the nervous system and skin (derived from ectoderm).

• Mesoderm: In between, forms muscles and organs.

A coelom is an enclosed, fluid-filled body cavity between the tubes:

• Provides space for oxygen and nutrients to circulate.

• Enables internal organs to move independently of each other.

Types of Body Cavities

Type	Description	Example
Coelomates	Coelom completely lined with mesoderm	Annelids
Acoelomates	No coelom	Flatworms (Platyhelminthes)
Pseudocoelomates	Coelom partially lined with mesoderm	Roundworms (Nematoda), Rotifers

Origin of Protostomes and Deuterostomes

• Protostomes (“first-mouth”): Mouth develops before anus during embryonic development.

• Deuterostomes (“second-mouth”): Anus develops before mouth.

• Recent research shows development is variable in protostomes; the blastopore may become anus, mouth, both, or neither.

Origin of Segmentation

• Segmentation: Division of the body into a series of similar structures, enabling specialization.

• Segmented backbone is a defining characteristic of vertebrates (Chordata).

• Segmentation is also conspicuous in invertebrates such as annelids and arthropods.

Sensory Organs and Ecological Roles

Sensory Organs

Sense	Stimulus	Example
Sight	Light	Flies use compound eyes to find food and mates, escape predators
Hearing	Sound	Bats use hearing to find prey and avoid obstacles
Taste/Smell	Molecules	Moths detect chemical signals in air
Touch	Contact, pressure	Sea anemones detect and capture prey using touch

• Magnetic field: Some animals use magnetic fields for navigation.

• Electric field: Some aquatic predators detect electrical activity in prey.

• Barometric pressure: Some birds sense air pressure changes to avoid storms.

Diversification of Ecological Roles

Ecological Role	Example
Detritivores	Millipedes feed on decaying leaves
Herbivores	Pandas eat bamboo
Carnivores	Owls hunt and consume prey
Omnivores	Humans eat plants, animals, fungi, protists, archaea, and/or bacteria

• Predators: Kill and consume most or all of their prey, usually larger than prey, kill quickly.

• Herbivores: Consume plant tissue without killing the entire organism.

• Parasites: Harvest nutrients from hosts; usually smaller than hosts.

  • Endoparasites: Live inside hosts, often wormlike.

  • Ectoparasites: Live outside hosts, have limbs or mouthparts for grasping.

Animal Feeding Strategies

Table: Diversification of Feeding Strategies

Strategy	Example
Suspension feeders (filter feeders)	Barnacles use appendages to capture plankton
Deposit feeders	Sea cucumbers use tentacles to mop up detritus
Fluid feeders	Butterflies and moths drink nectar
Mass feeders	Lions bite off chunks of meat

How Animals Feed: Four General Strategies

• Suspension feeders: Capture food by filtering particles from water or air; commonly aquatic and often sessile (e.g., sponges, clams, barnacles, baleen whales).

• Deposit feeders: Digest organic matter in sediments (e.g., earthworms, segmented worms, sea cucumbers).

• Fluid feeders: Suck or mop up liquids such as nectar, plant sap, blood, or fruit juice; often have specialized mouthparts (e.g., butterflies, vampire bats).

• Mass feeders: Ingest chunks of food; mouthpart structure correlates with food type (e.g., lions, snails).

Animal Movement

Functions and Types of Locomotion

Animal locomotion serves several functions:

• Finding food

• Finding mates

• Escaping predators

• Dispersing to new habitats

Modes of movement include burrowing, slithering, swimming, flying, crawling, walking, or running, mostly powered by muscle.

Skeletal Systems

• Hydrostatic skeletons: Support from flexible body wall in tension surrounding fluid or soft tissue under compression.

• Endoskeletons: Support from rigid structures inside the body (e.g., bones in vertebrates, spicules in sponges).

• Exoskeletons: Support from rigid structures outside the body (e.g., external armor of arthropods).

*Additional info: The notes above are expanded with academic context and examples for clarity and completeness, suitable for college-level General Biology students.*