Characteristics of Invertebrates with examples

Characteristics of Invertebrates with examples
Characteristics of Invertebrates with examples


  • 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.

About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He is doing his Ph.D. at the Central Department of Microbiology, Tribhuvan University, Kathmandu, Nepal. He was awarded the DAAD Research Grant to conduct part of his Ph.D. research work for two years (2019-2021) at Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. Sagar is interested in research on actinobacteria, myxobacteria, and natural products. He is the Research Head of the Department of Natural Products, Kathmandu Research Institute for Biological Sciences (KRIBS), Lalitpur, Nepal. Sagar has more than ten years of experience in blogging, content writing, and SEO. Sagar was awarded the SfAM Communications Award 2015: Professional Communicator Category from the Society for Applied Microbiology (Now: Applied Microbiology International), Cambridge, United Kingdom (UK).

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