+35 Invertebrate Animals: Examples and Characteristics

In the animal kingdom, invertebrates are often the most misunderstood group. Many people feel aversion or even fear toward them, without realizing that these organisms actually represent 95% of all known animal species on Earth. To put it into perspective, there are around 35 phyla in the animal kingdom, and all but one—the chordates—are invertebrates.

In this article, we’ll explore the main features, classifications, and examples of invertebrate animals, so you can better understand these incredibly diverse creatures.

 Related reading: Vertebrate Animals: Classification, Characteristics, and Examples

Main Characteristics of Invertebrate Animals

Although invertebrates are extremely diverse, they share some key traits:

1. No backbone or internal skeleton
   As the name suggests, invertebrates lack a vertebral column or notochord, unlike vertebrate animals.

2. Generally small body size
   Most invertebrates are small, but many have protective structures such as shells, exoskeletons, or hardened outer coverings.

3. Widespread distribution
   They inhabit nearly every environment on Earth—from deep-sea hydrothermal vents to frozen polar regions, from forest floors to freshwater lakes.

4. The majority of Earth’s biodiversity
   By 2005, scientists had catalogued about 1.7–1.8 million animal species, of which nearly 990,000 were invertebrates. The number continues to grow as new species are discovered.

For a long time, research on invertebrates was limited due to:

• The complexity of describing and classifying them.

• Their limited direct economic or food value (with exceptions in some cultures).

• Their often small, hidden lifestyles in soils, sediments, and marine environments.

Classification of Invertebrates

There are roughly 34 invertebrate phyla, with the arthropods being by far the largest, accounting for about 80% of all animal species.

Here are the main groups, with estimated species counts:

• Arthropods (>1,000,000 species): insects, arachnids, crustaceans, myriapods.

• Mollusks (100,000 species): snails, clams, octopuses, squids.

• Nematodes (25,000 species): roundworms.

• Flatworms (Platyhelminthes, 20,000 species): planarians, tapeworms.

• Annelids (16,500 species): earthworms, leeches.

• Cnidarians (10,000 species): jellyfish, corals, sea anemones.

• Sponges (Porifera, 9,000 species): sea sponges.

• Echinoderms (7,000 species): starfish, sea urchins.

Freshwater Macroinvertebrates

“Macroinvertebrates” are larger invertebrates commonly found in freshwater ecosystems such as rivers, ponds, and lakes. These include:

• Insects: dragonfly nymphs, mayflies.

• Crustaceans: freshwater shrimp.

• Mollusks: snails, mussels.

• Annelids: worms, leeches.

They are ecologically important as:

• Bioindicators of water quality.

• Key food sources for fish, birds, and amphibians.

• Decomposers that recycle organic matter.

Major Invertebrate Groups

1. Porifera (Sponges)

• The simplest and most primitive animals.

• Sessile (fixed in one place), with bodies full of pores.

• Filter feed using specialized cells called choanocytes.

2. Cnidarians

• Includes jellyfish, corals, sea anemones, hydras.

• Body is sac-like, with a single opening acting as mouth and anus.

• Tentacles equipped with stinging cells (cnidocytes) for defense and prey capture.

3. Flatworms (Platyhelminthes)

• Known as “flatworms,” such as planarians, flukes, and tapeworms.

• Some are free-living, others parasitic.

• Have a simple central nervous system.

• Show bilateral symmetry.

4. Mollusks

• Soft-bodied animals, often with calcium carbonate shells.

• Typical body structure includes a muscular foot, visceral mass, mantle, and shell.

• Major classes:

• Gastropods (75,000 species): snails, limpets.

• Bivalves (13,000 species): clams, mussels, oysters.

• Cephalopods (800 species): octopuses, squids, cuttlefish, nautilus.

5. Annelids

• Segmented worms like earthworms and leeches.

• Cylindrical body divided into repeated segments (metamerism).

• Secrete mucus to aid in movement.

6. Nematodes (Roundworms)

• Extremely abundant, found in soils and aquatic habitats.

• Unsegmented, cylindrical bodies.

• Can be free-living or parasitic.

• Possess a true body cavity (coelom) that allows complex internal organs.

7. Arthropods

The largest and most diverse animal group, making up about 80% of known species.

Key traits:

• Chitinous exoskeleton that requires molting for growth.

• Segmented bodies with specialized appendages (legs, antennae, mouthparts).

• Undergo metamorphosis in many species.

Major subgroups:

• Insects: butterflies, beetles, ants, bees, flies, grasshoppers.

• Arachnids: spiders, ticks, scorpions, mites.

• Crustaceans: crabs, lobsters, shrimp, barnacles.

• Myriapods: centipedes, millipedes.

8. Echinoderms

• Marine invertebrates like starfish, sea urchins, sea cucumbers.

• Five-part radial symmetry.

• Internal skeleton made of calcium plates.

• Unique water vascular system with tube feet for locomotion and feeding.

• Many have impressive regenerative abilities.

Conclusion

Invertebrates are the backbone of biodiversity:

• They thrive in nearly every environment.

• They play essential roles in food chains, nutrient cycling, and ecosystem balance.

• From sponges to insects, corals to jellyfish, they showcase nature’s adaptability and creativity.

To understand invertebrates is to understand the majority of life on Earth.

animal tags： invertebrate animals