Characteristics of Invertebrates with examples

Characteristics of Invertebrates


    • They are found in seas, freshwater, air, land from snow to desert.
    • 80% are found in terrestrial habitat.
    • Most successful invertebrates of the lands are Arthropods.
    • Protozoans are free-living, parasites or commensals.
    • Sponges and Coelenterates are aquatic animals.

Numerical Strength

    • Among 1.25 million animal species, 95% (1.2 million) are invertebrates.
    • Among 1.2 million invertebrates, 1 million are Arthropods.


    • Varied shape.
    • Amoeba are irregular every changing bodies.
    • Sponges and Coelenterates are plant-like.
    • Flatworms are leaf-like and ribbon-shaped.
    • Annelids, Nemerteans, and Nematodes are vermiform.
    • Starfishes are star-shaped.


    • Great variation in size.
    • Ranges from microscopic protozoans to large-sized cephalopods.
    • Malaria parasites (Plasmodium) are the smallest one (one-fifth of the human RBC) while the largest one is giant squids, Architeuthis with a body length of 16.5 meters including tentacles.


    • All types of symmetry.
    • Protozoans are bilateral or radial or asymmetrical.
    • Sponges are asymmetrical or radially symmetrical.
    • Coelenterates are radially symmetrical.
    • Ctenophores are biradial symmetry.
    • Some are spherical symmetry (Heliozoan and Radiolaria).

Grade of Organisation

    • All grades of the organization.
    • Protoplasmic grade- Protozoa.
    • Cellular grade- Sponges.
    • Cell-tissue grade- Coelenterates.
    • Tissue-organ grade- Flatworms.

Germ Layers

    • Absent in Protozoans.
    • Some are Diploblastic (derived from 2 germ layers), while others are triploblastic (3 germ layers).
    • Diploblastic- Sponges, Coelenterates.
    • Triploblastic- Other Invertebrates than Sponges & Coelenterates.

Simple Integument

    • Body covering is simple.
    • Protozoa- Plasma membrane
    • Other posses an outer protective layer called the epidermis.
    • Some have non-cellular cuticle or chitinous covering secreted by the epidermis.


    • Sessile- Sponges, Corals.
    • Pseudopodia, Cilia, Flagella- Protozoans.
    • Tentacular movements- Coelenterates, Molluscs.
    • Setae, Parapodia, Suckers- Annelids.
    • Jointed Legs- Arthropods.
    • Arms- Echinoderms.


    • Flatworms exhibit pseudo-segmentation.
    • True segmentation is found in Annelida and Arthropoda.

Living Endoskeleton

    • Do not possess a rigid internal skeleton.
    • Some like arthropods and molluscs possess hard exoskeleton for supporting and protecting the body.


    • Sponges and Coelenterates- Body is a double-layered sac surrounding a single cavity (Acoelomate- No Coelom).
    • Pseudocoelom- possess a cavity in between body wall and the gut (Nematodes).
    • Some possess true coelom.

Dorsal gut

    • The alimentary canal is either absent or partially formed or complete.
    • If present, lies dorsal to the nerve cord, runs anterior terminal mouth up to the posterior terminal anus.
    • Gill-slits are never formed in the pharyngeal wall.

Digestive System

    • Digestion takes place within the cell (intracellular digestion)- Protozoans, Sponges.
    • Digestion also takes place outside the cell (extracellular digestion).
    • Coelenterates exhibits both intracellular and extracellular digestion.

Circulatory System

    • A blood vascular system is well developed.
    • Open or lacunar circulatory system- Arthropods, Molluscs.
    • Closed circulatory system are also present.
    • The heart is dorsal to the gut.
    • The hepatic portal system is absent.

Respiratory System

    • Protozoans, Sponges, Coelenterates and many worms have a direct diffusion of gases.
    • Annelids exchange gases through moist skin.
    • Gills are present in higher invertebrates.
    • Echinoderms possess branchiae and tube feet for respiration.
    • In insects, the tracheal system is adapted for aerial respiration.

Excretory Mechanisms

    • Direct diffusion through cell membranes- Protozoans, Sponges, Coelenterates.
    • Flame cells- Flatworms.
    • True nephridia- Annelids and Molluscs.
    • Malpighian tubules- Insects.
    • Amoeboid Cells- Echinoderms.

Nervous System

    • Coelenterates (radially symmetry)- Head is absent, CNS is represented by a ring of nerve-tissue encircling the body.
    • In bilaterally symmetrical invertebrates- CNS is represented by a pair of nerve cord running along the mid-ventral line of the body.
    • In higher invertebrates- head ganglia form the brain.
    • Solid nerves, not hollow within.

Sense Organs

    • Protozoans- protoplast acts as a receptor.
    • Flagellates- stigma or eyespot acts as a photoreceptor.
    • Coelenterates- long sensory cells.
    • Flatworms- Eyespot, chemoreceptors.
    • Annelids- simple eyes.
    • Arthropods- compound eyes.
    • Arthropods and Molluscs- Statocyst (equilibrium), Tactile receptors, Chemoreceptors.


    • Asexual binary fission.
    • Sexual reproduction- Coelenterates, Platyhelminthes, Annelids, Crustaceans.
    • Fertilization may be internal as well as external.
    • Development is direct or indirect.
    • Parthenogenesis- Rotifers, Bees.

Cold-blooded animals

    • All invertebrates are cold-blooded.


  1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
  2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.

Characteristics of Invertebrates with examples

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