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Introduction to Invertebrate Animals

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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 its 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.

Definition of Invertebrates

To define invertebrate we can simply say that these animals do not have a backbone and have a soft body due to a lack of internal skeleton. This internal skeleton is called the endoskeleton which is usually present in vertebrates for support. Invertebrates animals have many structures outside that form the exoskeleton that provide them with support and protection. 

The name “invertebrate” comes from the Latin word “vertebra” which generally means a joint. Specifically this joint means a joint of the vertebrate from the spinal column. The root word verto or vorto means to turn. Thus, the concept of the jointed vertebra is derived from the mechanism of turning. Now the prefix “in” means “not” or ”without”. Invertebrates meaning can be thus stated as animals without joints from the spinal cord.

To understand what are invertebrates we can say that invertebrates are cold-blooded animals, This means that their body temperatures change with the environment. They cannot regulate their body temperatures. The size of invertebrates ranges widely from 0.002 in rotifers to 30-33ft colossal squid.

Invertebrata is a term of convenience and not a taxon. The subphylum Vertebrata comprises a small portion of the Metazoa. Since the invertebrates do not possess any internal or external skeleton of bone, thus they possess different body plans. Jellyfish or worms have fluid-filled hydrostatic skeletons, Insects and crustaceans have a hard exoskeleton and outer shells. Some famous invertebrates are Protozoa, Porifera, Coelenterata, Platyhelminthes, Annelida, Nematoda, Echinodermata, Arthropoda, Mollusca. According to the estimation of IUCN, 66,178 extant vertebrate species have been described. This indicates that more than 95% of the animal species described in the world are invertebrates.

Related History of Invertebrates

 Interestingly, the earliest animal fossils ever found appear to be invertebrates. Early sponges are said to be found in Trezona Formation at Trezona Bore, West Central Flinders, South Australia which are 665-million-years old. According to many palaeontologists, these animals appeared some 1 billion years ago. Tracks and burrows and other such related track fossils in the Tonian era indicate the presence of triploblastic worms like metazoans.

Classification of Invertebrates

 It was Carl Linnaeus who divides the invertebrate animals into two groups as follows:

  1. Insect

  2. Vermes

After being appointed to the position of “Curator of Insects and Vermes” at the Museum National d’ Histoire Naturelle in 1793, Jeans- Baptiste Lamarck coined the term ” invertebrates”. 

It was concluded that invertebrates are animals without vertebral columns. This further led to the conclusion that vertebrates are extremely different from invertebrates. According to the theory of evolution of Lamarck, a progression to a higher form took place during the evolutionary process and thus the invertebrates are quite different from human beings whereas human beings are similar to vertebrates.

Types of Invertebrates and Examples of Invertebrates

Invertebrates are the most diverse animals on the planet earth. Multiple invertebrates examples can be stated. Invertebrates animals names include most animals except the subphylum Vertebrata. Several animals from different families are invertebrates. Families like arthropods( insects, arachnids, myriapods, crustaceans), molluscs (octopuses. squids, bivalves, snail, chitons), cnidarians(corals, sea anemones, jellyfishes, hydras) and annelid( earthworms and leeches).

Invertebrates belong to the kingdom of Animalia. Some of the groups of invertebrates are:

  1. Arthropoda

  2. Cnidaria

  3. Leptocardii

  4. Mollusca

  5. Tunicata

Tunicata and Cephalochordata are also invertebrates. However, their characteristic features are more similar to invertebrates than to vertebrates. As a result, they are paraphyletic. This term has almost no meaning in taxonomy.

Further many invertebrate taxa have several species which are much greater than the entire subphylum of Vertebrata.

Invertebrates can live in various conditions. Invertebrates can live in freshwater, saltwater. They can live on land and they can live as parasites in other animals. The eating habits of invertebrates also vary a lot. They can be carnivorous, that is, they can eat meat. They can be herbivores, that is, they can eat plants, they can be an omnivore, that is, they can eat both plants and meat. Some invertebrates examples can grow bacteria and cells inside their bodies. Thus, they can make food. Moreover, some invertebrates stay in one spot while other invertebrates can fly, swim, float, burrow and crawl.

85 percent of invertebrates, which makes up around 9,23,000 species, are arthropods. Molluscs have 100,000 distinct species approximately as far recorded. Protozoans are single-celled organisms. They are amoebas and paramecia. Earthworms and leeches belong to annelids. Starfish, sea urchins, sea cucumbers belong to echinoderms, Snails, octopus, squids, clams belong to molluscs, Insects, spiders, crustaceans like shrimps, crabs, lobsters belong to a group of arthropods. 

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Characteristics of Invertebrates

The absence of the vertebral column is the fundamental trait common to all invertebrates. This creates the main difference between vertebrates and invertebrates. This distinction is for convenience only, not based on any homologous traits. The common trait of the presence of wings unites bats, insects and birds functionally and those devoid of wings unites sponges, tortoises and snails. Invertebrates are heterotrophs and therefore nourishment in the form of consumption of other organisms. Apart from Porifera, which is an exception, all other invertebrates have their bodies composed of differentiated tissues.

Morphology and Symmetry

The body structure of the majority of multicellular organisms has some form of symmetry which may be radial, bilateral or spherical. Examples of invertebrates with no symmetry include gastropod species which can be easily seen in snails and sea snails. There are some which appear externally symmetrical but their breathing hole(pneumostome) appears on the right side. Glaucus atlanticus develops external asymmetry. Asymmetry is also found in fiddler crabs and hermit crabs. Their asymmetry is because of their claws: one claw is larger than the other. 


Nervous System

The neurons of invertebrates are quite different from that of mammalian cells. Invertebrate cells respond to similar stimuli like that of mammals. Their cells fire to stimuli such as tissue trauma, changes in pH and high temperature. Hirudo medicinalis, the medicinal leech, was the first invertebrate identified with a neuron cell. Tissue trauma and increasing pressure are detectable by the mollusc neurons. Neurons are therefore identified in a majority of invertebrates like molluscs, arthropods, nematodes and annelids.

Respiratory System

The open Respiratory System is one type of invertebrate respiratory system which is composed of trachea, spiracles and tracheoles which enables the terrestrial arthropods to transport the metabolic gases to and from tissues. Among the many orders of insects, spiracles distribution varies greatly, but generally, each body segment has only one pair of spiracles which is connected to the atrium and also has a very large tracheal tube. The trachea being invaginations of the cuticular exoskeleton almost branch throughout the entire body with diameters only a few micrometres up to 0.8 mm. 

Tracheoles, the smallest tubes pierces cells and helps in the diffusion of water, carbon dioxide and oxygen. Gases can also be conducted by the respiratory system by the process of passive diffusion. Insects do not carry oxygen in haemolymph, unlike the vertebrates. Ridge-like circumferential rings of taenidia of varying geometries like loops or helices may be present in the tracheal tube. The trachea may also be joined to air sacs in the thorax, abdomen and head. Insects like grasshoppers and bees can control the airflow throughout the body by pumping the air sacs in their abdomen. Aquatic insects exchange their gas through the body wall in the form of gills. 


The majority of the invertebrates reproduce partly through sexual reproduction just like the vertebrates. The invertebrates produce specialized reproductive cells which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These in turn fuse to form zygotes which further develop into new invertebrates. Apart from these, others are capable of asexual reproduction or sometimes both modes.


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 different effects of water pollution and climate change.

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.

FAQ (Frequently Asked Questions)

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

Ans. 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.

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

Ans. 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 diarrhoea.

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.

3. 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.