Introduction to Invertebrate Animals

Invertebrate is the group of animals that neither possess a vertebral column nor do they develop a vertebral column in their lifetime. It is derived from the notochord. The Invertebrates animals are different from Vertebrate animals as the Vertebrate animals possess bony or cartilaginous vertebral columns. Among all the living beings, more than 90 percent of living beings are Invertebrates. Invertebrates animals include a diverse group including sea stars, earthworms, jellyfish, sea urchins, sponges, lobsters, crabs, insects, spiders, snails, clams, and squids. Invertebrate has their importance. Invertebrate animals are important as parasites or agents that transmit parasitic infections. These Invertebrates are also important as agricultural pests. Invertebrate zoology is the study of all the Invertebrates animals names, that is, it is the study of animals without a backbone( the structure which is found in fish, amphibians, reptiles, mammals, and birds). The division of Invertebrates into two groups was done by Carl Linnaeus.

Define Invertebrates:- 

Invertebrates are not Vertebrates since they lack a Spinal Cord or Backbone. Animals without a Spine are known as Invertebrates. Invertebrates make up roughly 97 percent of all living things. Porifera, Cnidaria, Platyhelminthes, Nematoda, Rotifera, Mollusca, Annelida, Arthropoda, and Echinodermata are the nine phyla that make up the Invertebrate kingdom. Invertebrates are the most diverse and abundant animal group on the planet. Invertebrates account for more than 140,000 species in the United States, a number that is increasing as additional species are discovered. Approximately 200 Invertebrates in the United States are on the endangered species list.

A cold-blooded animal with no backbone is known as an Invertebrate. Invertebrates, such as insects, spiders, and worms, can live on land or in water. Crustaceans (like crabs and lobsters), mollusks (like squids and clams), and coral are examples of Marine Invertebrates.

Types of Invertebrates:

Arthropods make up 85 percent of Invertebrates or 923,000 species. Mollusks have around 100,000 different species. The following are some of the most frequent Invertebrates:

  1. Protozoans- These are single-celled like an amoeba 

  2. Annelids-  These include earthworms and leeches

  3. Echinoderms- These include sea urchin, sea cucumber

  4. Molluscs- They include snails, Octopus

  5. Arthropods- Insects, spiders come under this category.

Where are they mostly found?

Invertebrates come in a wide variety of shapes and sizes. They can be found in freshwater, saltwater, on land, and in other animals as parasites. Invertebrates can be carnivorous (eat meat), herbivorous (consume plants), or omnivorous (eat both) (meat and plant eaters). Some Invertebrate species can produce their own food by growing bacteria and cells inside their bodies. Invertebrates can stay in one place or fly, swim, float, crawl, or burrow.

Characteristics of Invertebrates-

Invertebrates lack a backbone and have soft bodies due to the lack of an internal skeleton (endoskeleton) to provide support. Instead, many have external (exoskeleton) components that give support and protection. Invertebrates are also cold-blooded, which means they can't control their body temperature, which fluctuates depending on the surroundings.

Morphology of Invertebrates-

Most multicellular organisms have symmetrical body designs, whether they are radial, bilateral, or spherical. However, a small percentage of the population lacks symmetry. All gastropod species are examples of asymmetric Invertebrates. Snails and sea snails with helical shells are good examples of this. Slugs appear to be symmetrical from the outside, however, their pneumostome (breathing hole) is on the right side. Other gastropods develop outward asymmetry as they mature, such as Glaucus atlanticus, which has asymmetrical cerata. The genesis of gastropod asymmetry is a point of contention among scientists.

Nervous System of Invertebrates- 

Invertebrates have neurons that are different from mammalian ones. Invertebrates' cells fire in reaction to stimuli that are similar to those experienced by mammals, such as tissue injuries, high temperatures, or pH changes. The medical leech, Hirudo medicinalis, was the first Invertebrate in which a neuron cell was discovered.

Respiratory System of Invertebrates-

The open respiratory system of terrestrial arthropods, which consists of spiracles, tracheae, and tracheoles to carry metabolic gases to and from tissues, is one type of Invertebrate respiratory system. Although the number of spiracles varies widely among insect orders, each segment of the body can only have one pair of spiracles, each of which links to an atrium and has a rather big tracheal tube behind it. The tracheae are cuticular exoskeleton invaginations that branch (anastomose) throughout the body, with diameters ranging from a few micrometers to 0.8 mm. Tracheoles are the tiniest tubes that enter cells and act as diffusion sites for water, oxygen, and carbon dioxide.

Reproduction in Invertebrates- 

Most Invertebrates, like Vertebrates, reproduce in part by sexual reproduction. They make specialized reproductive cells that go through meiosis to make smaller, motile spermatozoa or larger, non-motile eggs. These join together to produce zygotes, which evolve into new people. Others can reproduce asexually, or in certain cases, both sexually and asexually.

Did You Know?

  1. In the 21st century, the two most commonly used model organisms for study purposes are Invertebrates. The fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans are very commonly used as model organisms. They were among the first life forms that were genetically sequenced. Due to the reduced state of their genomes, this sequencing of genes was facilitated.

  2. Starfish are also Invertebrates. They are called sea stars because of their arms which look like rays coming out of their bodies. Starfish live deep in the ocean and on the shore. There are over 1600 species of starfish. Starfish move with tube feet with the help of hydraulic pressure.


Invertebrates form a key element in the food chains. They serve as food for human beings. Invertebrates can support birds, fish, and many other species of Vertebrates. Invertebrates are often given less focus as compared to Vertebrates. They are often considered mere pests. With heavy use of pesticides in the 21st century all over the world, there has been a large decrease in the population of Invertebrates like bees, wasps, and other terrestrial insects. It was in the 20th century when Invertebrate zoology became popular in the fields of natural sciences, with prominent discoveries in different fields of medicines and genetics. Invertebrates are often used by scientists in aquatic biomonitoring to evaluate the different effects of water pollution and climate change.

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FAQs (Frequently Asked Questions)

1. Describe the social interaction of Invertebrates.

Cockroaches, termites, aphids, thrips, ants, bees, Passalidae, Acari, spiders, and other Invertebrates all have social behavior. Social interaction is most important in eusocial organisms, but it is also important in other Invertebrates. Insects are able to recognize information that has been communicated by other insects.

2. How have Invertebrates been helpful to humans?

Some Invertebrates help to clear and clean up the environment by eating fungi and bacteria, as well as decaying and dead waste, such as rotting animal carcasses and feces, as well as forest and garden leaf matter, and turning it into compost that serves to replenish the soil. Some Invertebrates are excellent aerators of soil as well as producers of it. Invertebrates, in other words, not only aid in the pollination of food crops, but also in the creation and maintenance of soil quality. Growing in agriculture, as well as gardens and allotments, need this. While a tiny number of Invertebrates are considered 'pests,' numerous Invertebrate species will happily devour such 'pests' if given the chance.

3. Write a Short Note on the Phyla Covered by Invertebrates.

Invertebrates are known to cover a lot of phyla. Sponges of Porifera is a common Invertebrate. Sponges have a very simple body organization. They have multiple cells that have differentiated but these cells have not developed into tissues. Sponges are known to feed through pores by drawing in water.Sea anemones, corals, and jellyfish of Ctenophora and Cnidaria have a radially symmetric body and have digestive chambers with single chambers. Echinoderms are radially symmetric. Arthropoda is the largest animal phylum followed by Nematoda. Platyhelminthes are flatworms that lack a body cavity. Some other Invertebrates include Nemertea which are roundworms and Sipunucula.

4. Give 2 Examples of Invertebrates and State Their Characteristics.

Two examples of Invertebrates are amoebas and squids.

  • Amoebas are Invertebrates. They are single-celled organisms. They belong to the simplest group of organisms. Amoebas have a cell membrane along with DNA and a nucleus. They have organelles. The difference between the body cells of human beings and amoebas is that amoebas can move with the help of their false feet called pseudopodia. Entameba histolytic is a species of amoeba that causes diarrhea.

  • Squids have ten tentacles and are thus decapods. Each tentacle has suckers that help squids to catch prey. The length of squids ranges from ¾ of an inch to 65 feet and more.

5. Mention the Differences Between Vertebrates and Invertebrates.

Invertebrates do not possess a backbone whereas Vertebrates have an internal skeleton. Invertebrates have an exoskeleton whereas Vertebrates do not possess any exoskeleton. Generally, the size of Invertebrates is smaller than Vertebrates. Invertebrates [possess an open circulatory system. Vertebrates possess a closed circulatory system. Most of the Invertebrates possess compound eyes, unlike Vertebrates. Invertebrates have the radial or bilateral type of body symmetry whereas Vertebrates have bilateral body symmetry. Invertebrates have a diverse population 95 percent of animal species are Invertebrates whereas 5 percent of animal species are Vertebrates. The modes of nutrition of Invertebrates include autotrophic, parasitic, and Heterotropic. The mode of Vertebrates is Heterotrophic.