Invertebrates | Types, Characteristics and Facts

Invertebrates | Types, Characteristics and Facts

Invertebrates display an incredible diversity of forms and can be found in various environments, including terrestrial, aquatic, and aerial. They encompass a wide range of species, including insects, arachnids (spiders and scorpions), mollusks (snails, clams, octopuses), echinoderms (starfish, sea urchins), and many more.

What is Invertebrates?

Invertebrates are animals that make up the vast majority of the animal kingdom and are characterized by the absence of a vertebral column or backbone. In other words, invertebrates do not have an internal segmented spine made of bone or cartilage-like vertebrates (animals with backbones).

Instead, their bodies are typically supported by other means, such as exoskeletons, hydrostatic skeletons, or soft body structures. Invertebrates display an incredible diversity of forms and can be found in various environments, including terrestrial, aquatic, and aerial.

They encompass a wide range of species, including insects, arachnids (spiders and scorpions), mollusks (snails, clams, octopuses), echinoderms (starfish, sea urchins), and many more. They play crucial roles in ecosystems and contribute significantly to the biodiversity of life on Earth.

Invertebrates Classification

Invertebrates are a highly diverse group of animals, and their classification is based on various criteria such as body structure, symmetry, and evolutionary relationships. Here is a simplified overview of the classification of invertebrates:

Sponges (Phylum Porifera):

Sponges are simple, aquatic animals characterized by their porous bodies. They lack true tissues and organs.

Flatworms (Phylum Platyhelminthes):

Flatworms are flattened, unsegmented worms that can be free-living or parasitic. Examples include planarians and tapeworms.

Roundworms (Phylum Nematoda):

Roundworms are non-segmented, cylindrical worms found in various environments. They include both free-living and parasitic species.

Annelids (Phylum Annelida):

Annelids are segmented worms with a true coelom (body cavity). Examples include earthworms and leeches.

Mollusks (Phylum Mollusca):

Mollusks are soft-bodied animals often protected by shells. They are divided into classes, including gastropods (snails and slugs), bivalves (clams, mussels), and cephalopods (octopuses, squids).

Chordates (Subphylum Urochordata and Cephalochordata)

Some invertebrates belong to the phylum Chordata but lack a vertebral column. Urochordates, also known as tunicates or sea squirts, and cephalochordates, such as lancelets, are examples of these chordates.

Types of Invertebrates

  • Spiders
  • Slaters
  • Insects
  • Worms
  • Velvet worms
  • Millipedes
  • Centipedes
  • Landhoppers

List of Invertebrates that live in water:

  • Crabs
  • prawns
  • crayfish & lobsters
  • Snails
  • slugs
  • Anemones
  • Sea stars
  • sea urchins
  • corals
  • Sponges
  • Bluebottles
  • jellies

Groups of Invertebrates

  • Porifera (Sponges)
  • Cnidaria (Jellyfish, Coral, Sea Anemone)
  • Platyhelminthes (Flatworms)
  • Nematoda (Roundworms)
  • Annelida (Earthworms, Leeches)
  • Mollusca (Snails, Clams, Octopuses)
  • Arthropoda (Insects, Arachnids, Crustaceans, Myriapods)
  • Echinodermata (Starfish, Sea Urchins, Sea Cucumbers)
  • Urochordata (Tunicates or Sea Squirts)
  • Cephalochordata (Lancelets or Amphioxus)

Porifera (Sponges)

Simplest Animals

Sponges are considered one of the simplest and most primitive animals on Earth.

Aquatic Habitat

They are exclusively aquatic and are primarily found in marine environments, although some species can inhabit freshwater.

Lack of Tissues and Organs

Sponges do not have true tissues or organs. Instead, their bodies are composed of specialized cells that perform various functions.

Cnidaria (Jellyfish, Coral, Sea Anemone)

Radial Symmetry:

Cnidarians typically exhibit radial symmetry, meaning their body parts are arranged symmetrically around a central axis.

Aquatic Habitat:

Cnidarians are exclusively aquatic and are found in marine environments. Some species inhabit freshwater.

Platyhelminthes (Flatworms)

Flattened Body

Platyhelminthes are characterized by their flattened, ribbon-like body shape. They are often very thin, which allows for efficient gas exchange through their body surface.

Bilateral Symmetry

They exhibit bilateral symmetry, meaning their bodies have a left and right side that are mirror images of each other.

Annelida (Earthworms, Leeches)

Segmented Bodies

Annelids have segmented bodies, with each segment separated by a septum (partition). This segmentation is a characteristic feature of the phylum.

Hydrostatic Skeleton

They possess a hydrostatic skeleton, which consists of fluid-filled coelomic cavities that provide support and allow for movement. This unique structure enables them to burrow through soil or swim through water.

Mollusca (Snails, Clams, Octopuses)

Soft-Bodied Animals with Shells

Mollusks are soft-bodied animals typically protected by a hard shell made of calcium carbonate. However, not all mollusks have shells, as some, like octopuses, have evolved without them.

Variety of Feeding Modes

Mollusks exhibit a wide range of feeding habits. Some are herbivores, feeding on plants and algae (e.g., snails), while others are carnivores (e.g., octopuses) or filter feeders (e.g., clams).

Arthropoda (Insects, Arachnids, Crustaceans, Myriapods)

Exoskeleton and Segmentation

Arthropods are characterized by their exoskeleton, which is a hard, external skeleton made of chitin. Their bodies are segmented, with jointed appendages, providing flexibility and support.

Largest Phylum

This group includes a tremendous diversity of forms and ecological roles.


Many arthropods undergo metamorphosis during their life cycle. This process involves distinct stages of development, such as egg, larva, pupa, and adult, with each stage having different body forms and functions.

Echinodermata (Starfish, Sea Urchins, Sea Cucumbers)

Water Vascular System

Echinoderms have a specialized water vascular system that helps them perform various functions, including locomotion, feeding, and gas exchange. It operates through a network of water-filled canals and tube feet.


Many echinoderms have remarkable regenerative abilities and can regenerate lost body parts, such as arms or even an entire body. This capacity for regeneration is one of their distinctive features.

Urochordata (Tunicates or Sea Squirts)

Sessile Adult Forms

Adult urochordates, or tunicates, are primarily sessile, meaning they are attached to a substrate and do not move. They often appear as bag-like structures or "tunics" attached to rocks or other surfaces.

Filter Feeders

Tunicates are filter feeders that draw water into their bodies through an inhalant siphon. They filter out small particles, including plankton, by passing water through a specialized filtering structure called a pharyngeal basket.

Chordate Characteristics

While adult tunicates may appear very different from typical chordates, they share chordate characteristics during their larval stage, including a notochord and a dorsal hollow nerve cord. These features are typically lost or modified in the adult stage.

Cephalochordata (Lancelets or Amphioxus)

Marine Filter Feeders

Cephalochordates are small, marine filter feeders that live in sandy or muddy ocean bottoms. They use cilia to create water currents that carry in tiny food particles, which they filter out using mucus-covered pharyngeal slits.

Chordate Characteristics

Cephalochordates are chordates, just like vertebrates, and retain several chordate characteristics throughout their life cycle. These include a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.

Simple Body Structure

They have a simple, elongated body that lacks a well-defined head. Cephalochordates are considered one of the most basal and primitive chordate groups and provide valuable insights into the evolution of chordates and vertebrates.

Invertebrates FAQS

What are invertebrates?

Invertebrates are animals that lack a vertebral column or backbone. They make up the majority of the animal kingdom and include a diverse range of species.

How do invertebrates differ from vertebrates?

Invertebrates lack a vertebral column (backbone), while vertebrates have a segmented spine made of bone or cartilage. Additionally, invertebrates often have simpler nervous systems and diverse body structures.

Where are invertebrates found?

Invertebrates inhabit a wide variety of environments, including marine, freshwater, terrestrial, and even aerial habitats. They can be found in oceans, rivers, forests, deserts, and more.

What are some common examples of invertebrates?

Common examples of invertebrates include insects, spiders, mollusks (e.g., snails, clams), crustaceans (e.g., crabs, lobsters), echinoderms (e.g., starfish, sea urchins), and many others.

How do invertebrates reproduce?

Invertebrates reproduce through various methods, including sexual reproduction and asexual reproduction. Some lay eggs, while others give birth to live young. Reproductive strategies vary widely among different groups of invertebrates.

Rate this post

Leave a Reply

Your email address will not be published. Required fields are marked *