Invertebrates: Meaning, Classification, types, and examples

Did you know that invertebrates are the most diverse group in the animal kingdom? They are almost 95% of the animals on Earth. According to the five-kingdom classification, invertebrates belong to the Kingdom Animalia. In this blog, we will learn what invertebrates are, their classification, and important features of each phylum.

Basis of Classification of Invertebrates

Scientists classified invertebrates based on the following characteristics.

Body Symmetry

Asymmetrical invertebrates have no definite shape.

Radially symmetrical: Body parts are arranged around a central axis. When an animal is cut from anywhere from top to bottom, it generates identical body halves around a central axis.

Bilateral symmetry: Only one line can divide the organism into left and right halves.

Number of body layers

During embryonic development, the animal develops several germ layers.

Diploblastic: Two layers, ectoderm and endoderm, are formed during embryonic development.

Triploblastic: Three layers, ectoderm, endoderm, and mesoderm, are formed during embryonic development. Mesoderm is present between the ectoderm and the endoderm.

Presence of a coelom

The Coelom is a fluid-filled cavity between the outer body wall and the digestive tract. It is surrounded by mesoderm.

Acoelomate: No true body cavity is present.

Pseudocoelomate: False body cavity, not surrounded by mesodermal tissue, but between mesodermal and endodermal tissue.

Coelomate: True body cavity lined by mesoderm.

Segmentation

Unsegmented: An Animal’s body is not divided into sections.

Segmented: The Body is divided into interconnected segments.

Level of Organization

Invertebrates exhibit different levels of body organization, varying from the simplest cellular level to complex organ systems. 

Cellular level: Phylum Porifera has a cellular level of organization.

Tissue level: Phylum Cnidaria has a tissue level of organization

Organ-System level: Advanced invertebrates like platyhelminthes and beyond have an organ-system level of organization.

Major Phylum of Invertebrates

Phylum Porifera

They have specialized cells, but no tissues, that carry out important functions like digestion, etc.

Asymmetrical
Porous body
Filter feeders
Examples include sponges

Sponge

Phylum Cnidaria

True tissues are present.

Stinging cells are present.
Radial symmetry
Hollow body with a single opening
Examples include Jellyfish, Hydra, etc.

Jellyfish

Phylum Platyhelminthes

It consists of all flatworms.

Flat, thin, and unsegmented body.
Triploblastic
Bilateral symmetry
Acoelomate

Phylum Nematoda

It consists of round worms.

Cylindrical and unsegmented body, pointed at both ends.
Triploblastic
Bilateral symmetry
Pseudocoelomate
The first animals to have a complete digestive system.

Phylum Annelida

It consists of Segmented worms.

The body is segmented externally and internally.
Coelomate
The nervous system is present with a primitive brain.
The closed circulatory system is present.

Earthworm

Phylum Mollusca

Soft-bodied animals often have shells.

It is the second-largest phylum of the animal kingdom.
They have a muscular foot called a mantle used for locomotion.
A feeding structure within a mouth, called the radula, is present.
Examples: Octopus, squids, snails

Snail

Phylum Arthropoda

Largest phylum

They are the most diverse and successful group of invertebrates.
Segmented bodies
Jointed appendages
An Open-circulatory system is present.
Tough exoskeleton made of chitin.
It consists of all insects.

Housefly

Phylum Echinodermata

Closest to chordates

Unlike other invertebrates, they are deuterostomes. (those organisms in which opening of anus is formed before opening of mouth during embryonic development)
Radial symmetry in adults.
Spiny skin
Possesses a specialized endoskeleton made of calcium carbonate.
They have a water vascular system that pumps water instead of blood.
Starfish, sea urchin, and sea cucumber are examples.

Starfish

Conclusion

Although invertebrates lack a backbone, they are the most diverse group of animals on Earth. Each phylum has unique features from the simplest sponge to the complex arthropod. Understanding their classification based on body structure, organization, symmetry, and layers gives us a deep understanding of how life evolves.

Call to Action

If this blog helped, share it with your classmates and bookmark it for your next exam revision.

Want more Biology breakdowns? Comment with your next topic requests.