Invertebrates are a diverse and fascinating group of animals that make up the vast majority of animal species on Earth. From the tiny insects that buzz around our gardens to the enormous squid that lurk in the depths of the ocean, invertebrates come in a staggering variety of shapes, sizes, and colors. Despite their incredible diversity, invertebrates are often overlooked or misunderstood, with many people unaware of the vital roles they play in ecosystems around the world.



The animals existing in nature are very numerous: it is estimated that their number amounts to about 4 million. Consequently, in order to study and describe them, it was necessary to divide them into groups taking into account the differences and similarities they present. To facilitate the study of animals, scientists have divided them into classes: a class is made up of all animals that have important common characteristics. Each animal belongs to only one class.

The first major division of animals is that which distinguishes them into vertebrates and invertebrates: Vertebrates are all those animals that are equipped with an internal bone skeleton whose main axis is the vertebral column. By the name of invertebrates, however, we mean all those animals that do not have an internal bone skeleton. Vertebrates are a separate type of animals, while invertebrates, given the considerable differences they have from each other, have been divided into various types.

Since the animals belonging to each type also have significant differences; the various types have been divided into smaller groups, which in turn have been divided into other subgroups and so on until finally there are small groups of animals all similar to each other. For example, if we want to classify a dog, we will say that it belongs to the vertebrate type, to the class of mammals, to the order of carnivores, to the family of canids, to the dog genus, and to the domestic species.

Groups of Invertebrates

Invertebrates are widespread in every environment and are divided into eight main groups:


Proferi or sponges live in the seabed. Water passes through the pores which bring nourishment and oxygen. They are aquatic and sessile organisms, they live anchored to bottoms of both seas and freshwaters. Most of them reproduce asexually (budding) giving rise to colonies of sponges. They have calcareous or siliceous spicules in the shape of needles and stars, arranged around the central cavity which has the function of maintaining the swollen shape of the body.




The coelenterates have a sack-shaped body with often stinging tentacles. They are:

  • Jellyfish
  • Anemones
  • Corals

Polyps form coral reefs, corals that can be seen in coral reefs (an example is the great barrier reef in Australia which is 2000km long, 100km wide, and 100m high) are the external skeletons of colonies formed by millions of polyps connected between theirs.

The life cycle of a jellyfish

During sexual reproduction, jellyfish females and males release in the water the eggs and spermatozoa that unite and produce a ciliated larva, which attaches itself to the ground and gives rise to a polyp. The polyp generates others by asexual reproduction, which grow, divide transversely, detach, turn upside down and start swimming becoming small jellyfish, which start the cycle again.



Platelminti, nematodes, and annelids are invertebrates called “worms” united by some elements:

  • The soft body of elongated shape
  • Bilateral symmetry
  • Creeping trend


Platelminti are called flatworms, they live in fresh and salty waters and are in some cases parasites of other animals. They have a branched digestive and excretory system but with only one opening that serves both for the introduction of food and for the expulsion of waste substances. They breathe through the skin and on the head have primitive sense organs, the eyespots, which serve as tactile receptors and for the search for food. They are planaria and tapeworms, a parasite that can also lurk in the intestine of humans (2m in length).



Nematodes or cylindrical worms are small animals with filiform bodies. They live in various environments and include eels, parasites of many plants and pinworms, parasites that can infect the intestines of children.



The annelids are worms with the cylindrical body divided into all equal parts, called metamers. This group includes terrestrial, marine, and other species of freshwater. The best known are the earthworm and the leech. The earthworm is a hermaphrodite and feeds on decaying animals, while the leech is a parasite that feeds on the blood of other animals.



Arthropods: arachnids, crustaceans, myriapods, insects. Arthropods, with over a million classified species, represent the largest group. The animals present:

  • Articulated legs
  • Bilateral symmetry
  • They have an exoskeleton made up of chitin, a hard substance
  • Body divided into segments


Arachnids are animals with 8 legs, without wings, and with a body divided into two parts: cephalothorax and abdomen. They are essentially carnivores and predators. It includes spiders, scorpions, mites, ticks.




Crustaceans include shrimps, lobsters, crabs, earth pigs, and sea ​​fleas. To this group belong both very small animals and animals of considerable size, like the Japanese crab, whose diameter reaches 3 meters.




The myriapods have elongated bodies divided into segments, with a high number of legs. Each segment can have a pair of legs (centipede), such as the scolopendra or a pair of legs (millipede).




Insects include various animals: fleas, dragonflies, beetles, grasshoppers, butterflies, ants, flies, mosquitoes. Their body is divided into head, chest, abdomen. On the head, there are the eyes, which can be simple that capture the intensity of the light, but not the image or composed (made up of many ommatidia) that give a mosaic vision. On the thorax, there are 6 legs, plus 4 wings in the dragonfly, two in the butterfly, and absent in the earwig.




Molluscs have a soft body and live mostly in water, others on land. Some are protected by a shell (snail), others by two (mussels, clams). Some do not (cuttlefish, squid, and octopus).



Bivalves and Gastropods

Mussels, oysters, clams are part of the bivalves, with a shell made of 2 valves and an ax-shaped foot to dig the sand. They have two muscles to keep the valves closed. Gastropods are limpets, sea snails, land snails (no shell), and snails. They have a developed foot and a mono valve shell usually wound in a spiral. The terrestrial ones breathe with the lungs, while the aquatic ones with the gills.


Among the mollusks, the group of cephalopods is the most evolved and complex. They have a foot divided into numerous tentacles equipped with suckers; they are agile and fast thanks to a propulsion system that pushes the water out of the body in order to make the animal move to the opposite side.


Echinoderms have their bodies covered with mobile spines which they use to move and defend themselves from predators. They are the stars and the sea urchins.

Starfish is a very common form of invertebrate; there are more than 200 species, more precisely called asteroids. Most starfish have a round body that extends into 5 identical arms, but some have more than 40 arms, while others have arms so small that they look like pads.


Sea urchins are marine invertebrates of echinoderms. They are characterized by a globular body covered with quills. There are about 850 species, different in shape, size, and texture of the shell. The quills of sea urchins can even exceed 30 cm in length!

Sea urchins