HIGHER INVERTEBRATES

Biology Notes — Evolution of Higher Invertebrates

Lower vs Higher Invertebrates

Feature

Lower Invertebrates

Higher Invertebrates

Segmentation

None (sponges, jellyfish)

Present (annelids, arthropods)

Symmetry

Asymmetrical or radial

Mostly bilateral

Coelom

Usually absent

True coelom present

Germ Layers

Diploblastic or triploblastic

All triploblastic

Lower invertebrates examples

  • Sponges (Porifera)

  • Jellyfish (Cnidaria)

  • Flatworms

  • Roundworms

Higher invertebrates examples

  • Mollusks

  • Annelids

  • Arthropods

  • Echinoderms

Evolutionary Mechanisms

Natural Selection

  • Organisms with advantageous traits survive and reproduce.

Speciation

  • Formation of new species through evolutionary change.

Fossils (~500 million years old) show early invertebrate diversity.

Examples:

  • Trilobites

  • Wiwaxia

  • Horseshoe crab (living fossil)

Key Evolutionary Features

1. Segmentation

  • Body divided into repeating segments.

  • First seen in annelids.

  • Highly developed in arthropods.

Examples:

  • Head

  • Thorax

  • Abdomen

Some groups lack segmentation:

  • Mollusks

  • Echinoderms

2. Cephalization

Development of a distinct head region with:

  • Brain

  • Sensory organs

  • Mouthparts

Absent in:

  • Sponges

  • Cnidarians

Primitive in:

  • Flatworms

  • Roundworms

Advanced in:

  • Annelids

  • Mollusks

  • Arthropods

3. Coelom Formation

Coelom

  • Fluid-filled body cavity lined with mesoderm.

Types:

Acoelomates

  • No body cavity (flatworms)

Pseudocoelomates

  • Partial cavity (roundworms)

Coelomates

  • True coelom (annelids, arthropods, mollusks, echinoderms)

Protostomes vs Deuterostomes

Feature

Protostomes

Deuterostomes

Blastopore becomes

Mouth

Anus

Cleavage

Spiral

Radial

Cell fate

Determinate

Indeterminate

Coelom formation

Schizocoelomate

Enterocoelomate

Examples:

Protostomes

  • Annelids

  • Mollusks

  • Arthropods

Deuterostomes

  • Echinoderms

Body Systems of Higher Invertebrates

Digestive system

  • Mouthparts (jaws, fangs, radula)

Respiration

  • Gills

  • Tracheal tubes

  • Book lungs

Circulation

  • Closed system (annelids)

  • Open system (arthropods)

Excretion

  • Nephridia (annelids, mollusks)

  • Malpighian tubules (arthropods)

Nervous system

  • Brain

  • Nerve cords

  • Ganglia

Reproduction

  • Sexual or asexual

  • Internal or external fertilization

Major Higher Invertebrate Phyla

Mollusks

Examples:

  • Snails

  • Clams

  • Squid

  • Octopus

Characteristics:

  • Soft body

  • Bilateral symmetry

  • True coelom

  • Protostomes

  • No segmentation

Body parts:

  • Foot

  • Mantle

  • Shell

  • Visceral mass

Special structure:
Radula

  • Tongue-like feeding organ.

Annelids

Examples:

  • Earthworms

  • Leeches

Characteristics:

  • Segmented bodies

  • Bilateral symmetry

  • True coelom

  • Protostomes

  • Cephalization

Importance:

  • Aerate soil

  • Improve soil fertility

Echinoderms

Examples:

  • Sea stars

  • Sea urchins

  • Sea cucumbers

Characteristics:

  • Marine animals

  • Radial symmetry (adults)

  • Spiny skin

  • No segmentation

  • Deuterostomes

Special system:
Water vascular system

  • Movement

  • Respiration

  • Circulation

Arthropods (Most Diverse Invertebrates)

~750,000 species.

Examples:

  • Insects

  • Spiders

  • Crabs

  • Centipedes

Reasons for evolutionary success:

1. Hard Exoskeleton

  • Made of chitin.

  • Protection and support.

  • Prevents water loss.

Molting

  • Shedding old exoskeleton to grow.

2. Jointed Legs

  • Flexible movement.

  • Used for walking, feeding, defense.

Word origin:

  • Arthro = joint

  • Pod = foot

3. Bilateral Symmetry

Left and right sides mirror each other.

4. Segmented Body

Segments often fused into:

  • Head

  • Thorax

  • Abdomen

5. Specialized Appendages

Examples:

  • Legs

  • Wings

  • Antennae

  • Claws

  • Mouthparts

Arthropod Subphyla

1. Uniramia (Insects)

Examples:

  • Bees

  • Butterflies

  • Beetles

  • Mosquitoes

Body divisions:

  • Head

  • Thorax

  • Abdomen

Features:

  • 3 pairs of legs

  • 1–2 pairs of wings

  • Antennae

  • Compound eyes

2. Crustacea

Examples:

  • Crabs

  • Lobsters

  • Crayfish

  • Barnacles

Features:

  • Mostly aquatic

  • Two pairs of antennae

  • Mandibles for feeding

Body regions:

  • Cephalothorax

  • Abdomen

Structures:

  • Swimmerets

  • Chelipeds (claws)

3. Chelicerata

Examples:

  • Spiders

  • Scorpions

  • Ticks

  • Horseshoe crabs

Characteristics:

  • No antennae

  • No wings

  • Two body segments:

    • Cephalothorax

    • Abdomen

Structures:

  • Chelicerae → stabbing mouthparts

  • Pedipalps → capture prey

  • Spinnerets → silk production (spiders)

Key Concepts to Remember

  • Higher invertebrates evolved segmentation, cephalization, and true coelom.

  • Two developmental groups: protostomes and deuterostomes.

  • Arthropods are the most successful and diverse animal phylum due to exoskeletons, jointed limbs, and body segmentation.