INVERTEBRATES: CHARACTERISTICS, CLASSIFICATION, EXAMPLES, SYSTEMS - BIOLOGY - 2025

The invertebrates are abundant and diverse group of animals that have vertebrae. Although the figure may sound very large, since we are very familiar with vertebrate animals, invertebrates represent more than 90% of all animal species on the planet.

Due to the absence of a spinal column that helps them support the weight of their bodies, invertebrates cannot reach large sizes (only some marine invertebrates can reach more than one meter in length), contrary to what we observe in vertebrate animals.

Photograph of a beetle, an invertebrate animal from the arthropod group (Image by monikasmigielska at www.pixabay.com)

Invertebrates are found almost anywhere on earth imaginable; in fact, the famous biologist EO Wilson classified them as “little things that run through the world”, although not all are small and not all run, many swim, others fly and many others crawl on surfaces.

Within this group we can find creatures as different as butterflies and snails, spiders and bees, starfish and earthworms, among others, and they all constitute an indispensable part for the life of all the elements in an ecosystem.

Invertebrates are the oldest and most numerous animals that exist on earth. It is known that, of the 3 million living and currently known species, about 2 million correspond only to invertebrate animals.

However, scientists estimate that there are around 7 million species of invertebrates on the planet, which means that man only knows less than half of them.

Characteristics of invertebrates

Monarch butterfly, an invertebrate

Invertebrates are an incredibly diverse group of animals. In this one, animals as different as a fly and a sea jellyfish, for example, meet, so it is difficult to point out the common characteristics that are shared between them. However, here is a small list of the most outstanding ones:

- They are eukaryotic organisms, so their cells have, in addition to a nucleus that encloses the genetic material (DNA), internal membranous systems and other functional compartments.

- They are made up of animal cells, that is, they do not have organelles with pigments such as chlorophyll and they have a bare plasma membrane (without an accompanying cell wall).

- They are, for the most part, multicellular organisms.

- They are heterotrophic organisms, as they need to obtain their energy and carbon from other organisms (organic matter) and they are not capable of producing their own food.

- They do not have any support or internal skeleton, be it vertebrae, spinal column, cartilaginous skeleton or any other support structure. Inside they only have liquid, cavities or organs, depending on the species.

- Without bones or vertebrae, their bodies cannot support much weight and therefore do not reach large sizes. Only some marine invertebrates can reach several meters in length, since the lower density of the water helps them support a higher weight.

- Invertebrates are located in the first rungs of the food chain, since they feed on plants and other invertebrates, serving as food for vertebrate animals such as fish, amphibians, reptiles, birds and mammals.

- In this group are the most diverse, beautiful and striking forms of animals, some even unimaginable for the creativity of men.

- They are the most abundant animals in any ecosystem that can host life in the world.

Classification: types of invertebrates

The classification of animals as "invertebrates" is not, in fact, a valid taxonomic classification, since there is no common ancestor that relates all the groups of different organisms that are located within this group of animals.

However, in zoology courses a distinction is commonly made between vertebrate and invertebrate animals to facilitate their study.

To have more or less an idea of ​​the complexity and diversity that exists among invertebrates, let us bear in mind that the set contains, depending on the classification system used, approximately 30 different phyla. Here is a list of the 21 most popular phyla:

- Poriferous (phylum Porifera)

Photograph of a marine green sponge (Source: Steve Rupp, National Science Foundation / Public domain, via Wikimedia Commons)

They are aquatic animals shaped like a sponge. About 9 thousand species have been classified so far. They feed through filtration of the water where they live, in this way they trap particles, small larvae of other animals or substrates that are trapped in their porous bodies.

- Placozoa (phylum Placozoa)

They are shaped like flat discs and only about 100 species are known. They are little studied, but it is known that, for the most part, they are marine species, microscopic and flat-looking.

They have a very simple body organization, since they do not have specialized organs or tissues to perform specific functions. They are believed to feed on algae, larvae, protozoa, and other microscopic organisms.

They are marine animals very similar to jellyfish; They are gelatinous and have tentacles and cilia. They have been little studied, so that today little more than 150 species are known.

They are carnivorous animals that feed on plankton, small fish, larvae of other animals, etc. They generally live at the bottom of the oceans.

- Cnidarians (phylum Cnidaria)

Photograph of a jellyfish, a type of Cnidarian (Image by Samuele Schirò at www.pixabay.com)

All “true” jellyfish, corals and anemones belong to this edge. They are, for the most part, marine organisms and about 11 thousand species are known.

All species of the phylum have stinging cells called "cnidocytes", which are used to paralyze and trap the prey on which they feed.

- Nemerteos (phylum Nemertea)

They are mostly marine worms, although some species can be found in lakes, rivers and underground. They all feed on small invertebrates through their proboscis.

The marine species of this phylum can reach several meters in length. To date, some 2,000 species have been described in this group.

- Flatworms (phylum Platyhelminthes)

Photograph of a flatworm from the Mediterranean Sea (Source: PervyPirate / Public domain, via Wikimedia Commons)

They are flatworms that live in aquatic or very humid environments. They are carnivorous animals that feed on small insects and larvae. Some are parasites of vertebrate animals. Within this group, about 21 thousand different species are classified.

- Gnatostomúlidos (phylum Gnathostomulida)

It is also a group of small worms (between 0.1 mm and 20 mm). They live on the ground, especially in places that have an abundant organic layer; they can survive in the absence of oxygen and feed on roots, fungi and other microorganisms. About 150 species have been described.

- Nematomorphs (phylum Nematomorpha)

It is a group of small worms, many of them parasites of vertebrate animals. They measure between 2 and 10 cm in length. About 500 species are known in this group, all parasitic. They feed through the surface of their bodies, taking advantage of food already digested by their hosts.

- Nematodes (phylum Nematoda)

Schematic of the body of the nematode Ancylostoma duodenale (Source: Servier Medical Art / CC BY (https://creativecommons.org/licenses/by/2.0) via Wikimedia Commons)

These organisms are commonly known as "cylindrical worms" because their body looks like a sausage. In the group there are many aquatic species, but there are terrestrial or parasitic species of vertebrate animals. About 30 thousand species are known.

- Quinorrincos (phylum Kinorhyncha)

They are considered "marine microbes" that are part of plankton. They are commonly found near the sandy or muddy bottom of the oceans. Their body is divided into segments and they feed on protozoa and single-celled algae. Currently around 400 species are known.

- Gastroticos (filo Gastrotricha)

They are organisms with small cylindrical bodies, which have their bodies covered by cilia and feed on organic matter, larvae, algae, protozoa and particles that float in the bodies of water where they live. About 500 species are known.

- Rotifers (phylum Rotifera)

These are microbes in many different ways, similar to insects. They live in humid freshwater environments and measure between 0.5 mm and a few centimeters (the largest).

They feed on protozoa, algae and other microorganisms in their habitat. About 2 thousand different species are known.

- Entoproctos (Entoprocta phylum)

They are microscopic aquatic animals that are shaped like polyps or anemones. They are sessile (immobile) and have a filtering “crown” made up of cilia with which they feed on the substrates that float in the middle. About 20 different species have been described.

- Acantocephala (phylum Acanthocephala)

Acanthocephalus are parasitic worms of vertebrates. They have a specialized proboscis to adhere to the intestinal walls of the vertebrate animals that they parasitize.

These invertebrates feed by absorbing food already digested by their hosts through their integument (the tissue that covers them) and in animal taxonomists they recognize more or less a thousand species of these.

- Mollusks (phylum Mollusca)

Photograph of a snail-like mollusk (Image by Michael Strobel at www.pixabay.com)

Snails, octopuses, squid, oysters, clams, slugs and others belong to this group. Most are carnivorous animals or that feed on organic matter by filtration from their body surface. Within this group some 100,000 species are classified.

- Arthropods (phylum Arthropoda)

Photograph of some ants, a type of invertebrate animal from the phylum of arthropods (Image by monsterpong09 at www.pixabay.com)

This is the largest and most diverse group of animals on earth: more than 1 million different species are known. Within this phylum all insects, arachnids, mollusks, myriapods (centipedes) and many others are classified. They vary greatly in size, shape, and life and feeding cycle.

- Onychophores (phylum Onychophora)

Photograph of an onychophor from Peru (Source: Thomas Stromberg / Public domain, via Wikimedia Commons)

These animals have the appearance of legged worms or legged slugs. They live in very humid land areas; They are nocturnal and feed on small invertebrates. Most only live in tropical areas. Currently about 200 different species are known.

- Annelids (phylum Annelida)

Photograph of an earthworm, an annelid (Image by Wolfgang Eckert at www.pixabay.com)

Annelids are segmented worms found underground or in the oceans. Perhaps the best known animal of this group is the earthworm.

These animals have very diverse eating habits: some are filter feeders, others are carnivores and others feed on organic matter found in the soil. More or less 15 thousand different annelid species have been described.

- Bryozoans (phylum Bryozoa)

They are filter feeders that form small colonies of polyps. They are aquatic and sessile, as they live attached to substrates. They have a kind of specialized "tentacle" to filter the small organic materials from the water, on which they feed. There are about 6 thousand species.

- Priapulids (Priapulida phylum)

These are also marine worms that live buried at the bottom of the seabed. They only expose the front part of their bodies, where they have specialized organs for filtration.

They feed on plankton and organic material in suspension. Only about 50 species are known today.

- Hemichordates (phylum Hemichordata)

Another group of marine worms that live near the coast. They are known as "hemichordates" because they present the first sign of a spinal column. They have a kind of pharynx to feed on, filtering the ocean water. At least 150 species are recognized.

Examples of invertebrate species

The common black ant (

In almost all the cities, towns and hamlets of the world we can find small inhabitants who carry with their jaws small fragments of food, bread chips, pieces of leaves, etc. These are the common ants, which belong to the phylum of arthropods.

Photograph of a specimen of L. niger (Source: Python (Peter Rühr) / CC BY (https://creativecommons.org/licenses/by/3.0) via Wikimedia Commons)

Ants live in colonies of millions of individuals. These colonies are "societies" where different castes of individuals can be observed:

- there are workers who are in charge of getting food abroad

- There is the queen who is in charge of laying the eggs to generate new individuals for the colony, among others.

The giant squid of Antarctica (

Giant squid are mollusks that live deep in the ocean. They feed on fish, jellyfish, turtles and any animal of a size that it can devour, and they have the uncanny ability to communicate with each other through the change of color of their bodies.

Graphic schematic of the giant squid's body (Source: Rcidte / Public domain, via Wikimedia Commons)

The giant squid of Antarctica can reach up to 5 meters in length and videos have been observed of these mollusks being eaten by whales. It is believed that this species of mollusk inspired the tales of the "Kraken" in Greek mythology.

The sea wasp or bucket jellyfish (

This organism belongs to the phylum Cnidaria and is the most poisonous animal that exists on the face of the earth. It has been observed off many of the Australian coasts. Its bell is the size of a fingernail and its tentacles can reach up to 80 cm long.

Sea wasp photograph (Source: Guido Gautsch, Melbourne, Australia / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.0) via Wikimedia Commons)

It feeds on small fish that are trapped in its tentacles and only recently was it possible to understand what are the components that provide such lethality to its venom.

Today it is known that, at least for humans, its venom causes heart failure and diseases at the level of blood cells.

Invertebrate nervous system

The nervous system of invertebrates is quite "primitive," with notable exceptions in many species of mollusks and arthropods, for example. Like any nervous system, it is responsible for responding to stimuli that are perceived through the sensory organs of these animals.

There are phyla, such as that of arthropods and mollusks, in which there are neural systems with well-defined synapses, with a primitive type of brain, to which signals from external stimuli arrive to be processed before giving a response.

These "central nodes" usually group various senses of the animal, such as sight, taste and smell. Since these senses are "gathered" very close to what would be the central nervous system, some authors consider that it can be said that some invertebrates have heads.

Other invertebrates, on the other hand, have a much more basic nervous system than a centralized system can be, since their sensory organs are distributed throughout their body and are adapted to pick up stimuli in almost any direction in their environment, so that they act autonomously.

That is, the stimuli do not go to a central region that analyzes them to give a response, but, instead, the stimulus is captured by the receptors and the nervous system responds autonomously or instantaneously, without evaluating whether it represents a threat or benefit to the animal.

Invertebrate circulatory system

In invertebrates we observe two types of circulatory system:

- the closed circulatory system and

- the open circulatory system

In both systems, a fluid or "blood" is transported, which is in charge of carrying out the gas exchange with the environment, that is, expelling gaseous waste and obtaining oxygen for the body cells.

Closed circulatory system

Closed circulatory system

Closed circulatory systems keep "blood" or circulatory fluid separate from other fluids in the body.

This liquid travels through “pipes” to organs or specialized places for breathing, places that have a structure with little resistance to the entry of oxygen into the blood or circulatory fluid.

These types of circulatory systems are typical of animals that have highly developed body cavities, that is, they have a defined space in their body for each system separately. We can observe this in a worm and in an octopus, for example.

Open circulatory system

The open circulatory system does not efficiently separate body fluids into a single cavity and blood is not transported through pipes distributed throughout the body, so at some point waste, digested food and "blood" get mixed up, even partially.

This type of system imposes serious restrictions on the size of the body, since it takes a lot of energy to transport the fluids from one place to another inside. It is typical of animals such as insects, clams and others.

Digestive and excretory systems of invertebrates

There is a great diversity of digestive systems among invertebrates. However, many of these animals take a series of "basic" and common steps to feed themselves and activate their digestive system. They locate, select and capture their prey to later digest them and assimilate the nutrients.

Remember that digestion is the process by which food is broken down to assimilate it through the cells.

Many invertebrates carry out extracorporeal predigestion (outside the body), thanks to their ability to inject substances or microorganisms so that they degrade or “predict” their food before eating it.

- Structures involved

Generally, all invertebrates have some type of internal digestive tract or duct through which their food passes once it is ingested.

Single opening

In some groups, such as cnidarians and flatworms, for example, there is only one opening through which undigested food remains are ingested and removed or excreted; in simpler words, the anus and the mouth consist of the same opening.

Two openings

Other invertebrates have separate anus and mouth, that is, they have an opening through which they eat food and another to expel metabolic waste and food remains that are not digested and used by their body.

Having two separate openings for feeding and excretion provides these animals with great evolutionary advantages, since in the opening that functions as a "mouth" they can have separate and specialized regions or cavities for grinding, fluid secretion, storage, digestion and absorption of nutrients.

Likewise, after the assimilation of the nutrients, the wastes can be excreted independently of the new food ingested, avoiding contamination or the recirculation of already digested food.

Invertebrate respiratory system

Oxygen (O2) is necessary for the cellular respiration of all aerobic invertebrates, since few invertebrates can survive for long periods of time in anaerobic conditions (without oxygen) by reducing their metabolism and performing a type of anaerobic respiration.

All invertebrates take in oxygen from the environment and, at the same time, release carbon dioxide (CO ₂).

Gas exchange in invertebrates follows the common principles of all animals, despite the fact that some structural modifications serve to improve the process in the different conditions in which each species lives.

All strategies focus on the basic principle of bringing the environment, be it water or air, closer to the body fluid (blood or some similar fluid) so that both interact separated only by a thin wet membrane that allows the gas exchange of a place to another.

In other words: that oxygen (O ₂) can enter the body fluid while carbon dioxide (CO ₂) leaves it. The membrane must always be wet, so that the gases dissolved in the fluid that transports them can "pass" or diffuse from one place to another.

The diffusion of gases always depends on their relative concentrations between the two compartments that are in contact, that is, on the amount of one and the other that is on each side of the membrane. These gradients are maintained by the circulatory system.

Diffusion gradients

The gas in the highest concentration is always going to be transported to an area where its concentration is lower. In this way, the deoxygenated blood loaded with carbon dioxide releases it to the extracorporeal fluid and is loaded with oxygen, which is in higher concentration in the latter.

When this exchange occurs, the circulatory system "pushes" oxygenated blood through the body, so that it oxygenates the organs or tissues of the body. When part of the oxygenated blood is transported, its place is taken by new deoxygenated blood, loaded with CO2, thus repeating the process.

From all this it is understood that, as in vertebrate animals, the respiratory system and the circulatory system are closely related, since the blood or internal fluid is responsible for the transport of gases throughout the body.

References

1. Brusca, RC, & Brusca, GJ (2003). Invertebrates (No. QL 362. B78 2003). Basingstoke.
2. Hickman, CP, Roberts, LS, Hickman, FM, & Hickman, CP (1984). Integrated principles of zoology (No. Sirsi) i9780801621734).
3. Kotpal, RL (2012). Modern Textbook of Zoology: Invertebrates. Rastogi Publications.
4. Pechenik, JA (2010). Biology of the Invertebrates (No. 592 P3).
5. Tasch, P. (1973). Paleobiology of the invertebrates: data retrieval from the fossil record (Vol. 25, p. 946). New York: Wiley.
6. Wilson, EO (2001). Sociobiology.