What is an Invertebrate?

Animals lacking the vertebral column or backbone are referred to as invertebrates. From earthworms and sea urchins, to jellyfish and squids, around 95% of all living species on earth are invertebrates, which means mammals, amphibians, reptiles, birds, and fishes, all being vertebrates, make up only about 5% of the animal kingdom population. Presently, there are 1.25 million recognized species of invertebrates, occupying all types of aquatic, land, and arboreal habitats, while the number is only 60,000 when it comes to vertebrates. Some common examples of invertebrates are insects like spiders, flies, beetles, butterflies, freshwater and ocean animals like snails, lobsters, mollusks, and larger animals like sponges, sea stars, and octopuses.


Invertebrates have been on the earth long before vertebrates came into existence. As the origin of life began with single-cell organism, they grew and developed into more complex, multi-celled small invertebrates. The first recognized invertebrates were soft-bodied marine animals, like trilobites, one of the earliest extinct groups of arachnomorph arthropods. Gradually, land invertebrates developed, with countless insects and annelids having been existed for thousands of years.

Scientific Classification

The word 'invertebrate' does not refer to a specific taxon or phylum; it just groups together all the land and water insects and animals without a backbone. So, they are divided into the following eight major phyla depending on their type, appearance and anatomy:


Includes all sponge species found in fresh and saltwater habitats, characterized by a long, cylindrical body full of pores (ostia) that allow water to run through all parts of the body. There are a number of different species of sponge, e.g. Venus' flower basket (Euplectella aspergillum), freshwater sponge (Spongilla lacustris), and the commercially used bath sponge (Spongia officinalis).


Consists of over 10,000 marine and freshwater animals including corals, sea anemones, and jellyfish. These animals are usually biradially (both bilaterally and radically) symmetric, with only one external opening in their ventral side, sometimes surrounded by tentacles, like in jellyfish. This phylum is further divided into four classes, Anthozoa (corals, sea anemones), Scyphozoa (jellyfish), Cubozoa (box jellies), and Hydrozoa (hydra). Another class, Staurozoa (stalked jellyfish), has recently been recognized.


Soft-bodied invertebrates like flatworms and tapeworms belong to this phylum, characterized by a bilaterally symmetrical, unsegmented body. The Platyhelminthes species are either aquatic or live in humid terrestrial habitats where sunlight doesn't reach. They can be of various colors from black and gray to bright shades of orange and green. This phylum is separated into two groups, Catenulida and Rhabditophora.


Includes over 25,000 recognized species, the nematoda phylum consists of various parasitic roundworms well-known for causing various diseases in humans and other mammals, including hookworm, common roundworm, pinworm, and trichina.


There are around 85,000 extant species belonging to this phylum, including the largest of all invertebrates, the octopus and squid. All snails, clams, slugs, and cuttlefish are mollusks. There are six extant classes in this phylum: Caudofoveata, Solenogastres, Polyplacophora (chitons), Monoplacophora (certain molluscs), Gastropoda (snails and slugs), Cephalopoda (octopuses, squids, nautilus), Bivalvia (oysters, mussels, clams), and Scaphopoda (tusk shells). Two extinct classes, Rostroconchia and Helcionelloida are also there. The cephalopods are considered the most intelligent among all invertebrates, with octopus thought to have the highest level of intelligence as they have a relatively large brain.


With over 17,000 extant species, this is a large phylum of invertebrates including all worms, like earthworms, tubeworms, and leeches. They are characterized by a segmented body. Annelida is classified into a few subgroups: the Polychaetes (lugworms, some plankton), Clitellates (earthworms), Siboglinidae (giant tubeworm), Echiura (spoonworms), and Myzostomida (crinoid worms).


The largest phylum consisting of a few million different species, the arthropods account for over 80% of all living entities on earth. Arthropods include various saltwater, freshwater, and land species ranging from all small and large insects, scorpions, centipedes to marine creatures like crustaceans. This phylum is divided into five subphyla: Trilobites, Chelicerates (spiders, mites, scorpions, horseshoe crab), Myriapods (centipedes and millipedes), Crustaceans (lobsters, shrimps, crabs), and Hexapods (all insects including butterfly, moth, bee, ant etc).


Another phylum of marine invertebrates, including around 7000 extant species, the Echinoderms are characterized by their five-point radially symmetrical body, with common examples including star fish, sand dollars, and sea cucumbers. This phylum belongs to the superphylum Deuterostomia, as does Chordata.


Respiratory System

The respiratory mechanism may vary between the phyla, but all invertebrates need oxygen to survive, and they use the basic method of diffusion (where substances move to a high-concentration from an area of low concentration) to get it. Poriferans (sponge), cnidarians (jellyfish), nematodes (roundworms), and platyhelminth (flatworm) species use diffusion as their primary means of absorbing oxygen from the environment. Their thin outer skin takes part in gas exchange, taking in oxygen and excreting carbon dioxide. In case of sponges, the countless pores covering their body take in oxygen from the water traveling through these pores. Species belonging to the other four phyla (mollusca, annelida, arthropoda, echinodermata) have slightly advanced respiratory systems. Two respiratory organs present in most of them are a trachea and gills. Some small animals, including scorpions and snails, have a diffusion lung (unlike the ventilation lungs in vertebrates). Trachea is the primary respiratory structure in insects, where the main tracheal tube divides into multiple tubes to supply to the different parts of the body. Echinoderms (star fish, sea cucumber) have gills, and they also use their tube feet to get oxygen.

Digestive and Excretory Systems

Invertebrate digestive system can be divided into two basic types, a gastrovascular cavity and alimentary canal. The gastrovascular cavity, present in jellyfish, sea anemones, flatworms, and coral, has just one opening. So, materials to be ingested enter the body through this opening, passes through the digestive tube where all the necessary chemicals and nutrients are absorbed, and then the waste product leaves the body through the same opening. The alimentary canal is a better-developed system where there is a digestive tube with two openings in both its ends, a mouth and an anus. Most nematodes, molluscs, annelids, arthropods, and echinoderms have an alimentary canal.

Circulatory System

There are two types of circulatory system - open and closed. Both are present in arthropods, depending on which phylum they belong in. As mentioned before, sponges get all they need from the water running through their pores, while animals belonging to the cnidaria, platyhelminthes, and nematoda phyla do not have a circulatory system at all. Molluscs, arthropods, and echinoderms have an open circulatory system, meaning that the heart pumps blood directly into the body cavity where it surrounds the organs for the process of diffusion to occur. Annelids, much like vertebrates, have a closed circulatory system, which means when their heart pumps blood, it circulates through a network of blood vessels to reach different parts of the body instead of just bathing all the organs.

Immune System

Most invertebrates use the phagocytosis method to fight diseases. So, their immune system functions in a similar manner as vertebrate immune system where certain cells in their body attack and destroy any potentially harmful cell or organism.

Excretory System

The two kidneys of Amphibians are located dorsally close to the roof of body cavity. The main function of the kidneys is filtering the blood of all metabolic waste and transporting the urine through the urinary bladder to be stored as urine before being excreted through cloacal vent. Nitrogen is excreted from the body as ammonia in dilute urine by the juveniles and many aquatic Amphibians. But the terrestrial species are known to pass the waste materials as urea which is comparatively less toxic.

Respiratory System

Their lungs are fairly primitive than those of amniotes, having few large alveoli and internal septa. Due to this reason, the lungs have a slow oxygen diffusion rate through the bloodstream. Buccal pumping is responsible for ventilation in these animals. The permeable property of the skin also helps with respiration. Some Amphibians, like the plethodontid salamanders breathe primarily through skin as they do not have lungs or gills. Various species carry out respiration through gills during the larval stage.

Nervous System

The basic anatomy of the nervous system is quite similar to that in other vertebrates. It comprises of a central brain, spinal cord and numerous nerves throughout their bodies. Their brains are not as developed as those of mammals, reptiles and birds. Rather, they bear similarities in function and morphology to the brains of fish. The brains are divided into three equal parts _ cerebrum, mesencephalon (midbrain) and cerebellum.


Aquatic (Marine & Freshwater) invertebrates:

Most ocean and freshwater species have cilia and flagella to help them move in water. Since the gravitational pull is less in aquatic habitats, they can move faster in water than they can on land. Larger animals like octopus and jellyfish have tentacles along with muscles that allow them locomote through jet propulsion. Some benthic (bottom-dwelling) invertebrates, like crabs and lobsters, have pointy legs to move around on the seabed, while molluscs often use an adhesive locomotion, crawling over solid surfaces under the water in the same manner as land snails do. Echinoderms use tube feet (star fish) or cilia-covered spines (sand dollar) to move slowly on the ocean floor. Invertebrate Locomotion There are some sessile invertebrates as well, like corals, barnacles, and most sponges, meaning that they attach themselves to something like a rock, a dead tree log, or even ruins of a sunken ship, and grow there, staying immobile all their life.

Land Invertebrates:

Among land invertebrates, arthropods all have legs, and some also have wings (bee, butterfly, mosquito etc). Invertebrate Bees Flying Annelids and molluscs have a certain organ called parapodium (parapodia) to move from one place to another. Though it has the same name for both these phyla, in molluscs, this term refers to the fleshy part on their ventral side, used for adhesive locomotion. On the other hand in annelids, the paired leg-like protrusions that help them move are referred to as parapodia.


The primary difference between vertebrates and invertebrates is the lack of backbone (endoskeleton) in the latter. Apart from that, there are some other characteristic features that set invertebrates apart from the rest:

  • 1.All invertebrates are cold blooded. In fact there are many species, like sponges, flatworm, coral, and even jellyfish that do not have blood at all.
  • 2.The lack of endoskeleton is made up for in arthropods and molluscs with an exoskeleton instead, a tough chitinous layer covering the outer body. In case of mollusks like oysters and snails, the exoskeleton is referred to as shell.
  • Invertebrate Mollusc with Shell Invertebrate Exoskeleton in Arthropod
  • 3.Another feature that all invertebrates have in common is that they are all multi-cellular organisms, and since they belong to the animal kingdom, they do not have any cell walls.
  • 4.They do not really occupy multiple environments, with those living in water remaining in water, and land-dwelling species living on land for all their lives.
  • 5.Most invertebrates have a symmetrical body (except for a few, like sponge).
  • Invertebrate Symmetry


The mode of reproduction varies between different phyla and even species. Poriferans or sponges can reproduce both sexually, where males release sperms that fertilize the eggs in the female to give birth to an offspring, and asexually, where a bud develops and separates from the parent's body to attach itself to a solid surface to grow. Cnidaria animals like jellyfish reproduce sexually with a rather long life cycle where the larva settles to grow into a polyp, giving birth to juveniles. Platyhelminth animals and annelids have sexual as well as asexual reproduction, and they have both male and female reproductive organs (hermaphrodites). Nematodes, molluscs, arthropods, and echinoderms usually have separate sexes, and reproduce by fertilization of an egg. However, certain species in these 4 phyla may also show hermaphrodic features, developing both male and female sex organs.


With such a diverse population occupying all the possible corners of the world, the adaptive features of invertebrates vary from one species to another, depending on whether it's an insect or a large marine animal. Insects are the most successful living beings in the animal kingdom, having survived years and years of earth's evolution. Here are some of the primary adaptive features of invertebrates:

Quick reproduction:

One of the keys to their survival is the fact that they reproduce at an extremely fast rate. Most of them reproduce by egg-laying and the eggs of social insects like bees and ants can hatch and develop even without fertilization, giving birth to the worker bees or ants.

Opportunistic feeders:

Most insects are opportunistic feeders, meaning they eat whatever is available to them, from plants, fruits, seeds, flowers, to animal matters, and even decaying organic matters.

Generally hardy:

They are usually able to endure extreme weather conditions. There are many insects that can actually survive even when frozen solid. Once the ice melts, they carry on with their regular activities. Some invertebrates even have the ability to pull through extreme colds with the help of certain chemicals present in their body that protects their body from freezing.

Adept at escaping threats:

They are also good at camouflaging themselves to blend in to their environment, like a stick insect that looks exactly like a twig when hiding in a tree, or various species of octopus that can actually change their color to blend with the seabed.

Invertebrate Adaptation


Since habitat loss is affecting all marine, freshwater, terrestrial, and arboreal habitats, there is a cause for concern regarding these animals. However, most invertebrates, being small in size and having a fast rate of reproduction, are coping with the changing environment quite well. IUCN and other animal protection and preservation organizations have come forward to protect all these species, but it is often impossible to calculate the exact numbers of their population.

Invertebrates Conservation Marine species are even more difficult to monitor, with conservation status remaining unknown for many ocean animals, like star fish, and squids. But certain species, like an octopus from New Zealand, Cirroctopus hochbergi, are considered endangered.

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