Animals without brains

Have you ever wondered what animals that lack a brain are like, how do they perform their vital functions without a brain that allows them to recognize signals from the outside and give them a stimulus to move or feed? Once again, nature surprises us with different adaptations and the development of new tools and abilities within the animal world for those species that do not have a brain. It is interesting to discover how the majority of brainless animals are found in marine ecosystems around the world, generally having simpler and more primitive life forms than other animal species that, on the contrary, inhabit ecosystems. terrestrial.

Portuguese caravel

Known as one of the most amazing colonial organisms in the animal kingdom, the Portuguese man-of- war is characterized by the lack of a brain. Although by its appearance we could affirm that it is a jellyfish, the reality is that this animal is made up of four different species of hydroids ( hydrozoa belonging to the group of Cnidarians), which perform well differentiated functions within the correct functioning of the colonial organism. .

In addition to the lack of a brain , the Portuguese man of war stands out for the powerful poison present in its stinging cells, which it uses to immobilize its prey.

If you were wondering whether jellyfish have a brain or not , the answer is that they do not have a nervous system. Entering the world of jellyfish, the inverted jellyfish stands out as one of the most striking and particular species, due to its strange appearance and greenish color.

In the case of this species of jellyfish, its anatomy includes 8 long tentacles and a bell-shaped body that lacks a brain . On many occasions it is common to find inverted jellyfish that move on the shell of crabs, thus facilitating them to travel greater distances and, in turn, the jellyfish protect the crabs with the stinging cells of their tentacles, against possible predators.

Ctenophores, brainless animals that are part of plankton

The ctenóforos are among the animals that have brains smaller and curious there in the seas. They are one of the animals that are part of the well-known zooplankton , which is the basis of the diet of other well-known animals: baleen whales or mysticetes. Another curious fact about them is that they do have a very simple nervous system.

Another of the strange animals that do not have a brain are the sea ​​squirts of the Subaphylum Urochordata . These are marine animals that are found in all the seas of the world and that live on rocks and other surfaces, they feed on small particles that they trap through their absorption and filtering system. They do not have a brain, but neither do they have a nervous system or a heart.

Sea stars

One of the most emblematic animals in the entire marine ecosystem are, without a doubt, the precious starfish (Class Asteroidea , Phylum Echinoderms). Known for their surprising ability to regenerate the appendages of their body, starfish also stand out for being brainless animals .

Its anatomy is quite simple and it has other peculiarities, such as the ability to move thanks to the entry of water into its aquifer vascular system.

Sea urchins

Following within the group of echinoderms, sea ​​urchins (Class Echinoidea) are also characterized by the lack of a brain and nervous system in their anatomy.

The radial symmetry that characterizes all species of this class of echinoderms, guarantees a fairly rudimentary but effective anatomy, allowing sea urchins to develop their main vital functions even though they lack organs that at first glance seem essential for their survival, such as the brain or the heart.

Holothurians or sea cucumbers

Commonly known as sea ​​cucumbers, holothurians (Class Holothuroidea) are another of the classes of echinoderms that abound in marine ecosystems almost everywhere on the planet. They are characterized by their long and vermiform appearance, as well as their soft and muscular texture. They are quite simple organisms, lacking a brain and with the naked eye we can observe in their anatomy an oral opening in the anterior part and another posterior opening with anal function.

Sea lilies

Continuing with this list of brainless animals , we stop to meet this strange animal. Perhaps somewhat less well known than stars, sea urchins and cucumbers, sea lilies or feathered stars (Class Crinoids) are also included within the large and diverse group of echinoderms.

It is considered the oldest class of echinoderms and, in turn, the least abundant today. Only 600 species survive in almost all the seas of the world. Their anatomy gives them a branched appearance with numerous arms, lacking a well-defined body.

Sea sponges

Another of the most emblematic animals of the marine ecosystems and ocean floors are the curious marine sponges , belonging to the Poriferous group .

They are characterized by their ability to filter water and nutrients to survive, as well as having one of the simplest anatomies within the Animal kingdom. They lack both authentic tissues and limbs, as well as certain organs, including the brain.

Sea anemones

Although on many occasions, due to their appearance and way of life, it is difficult to distinguish and recognize these organisms as animals, sea ​​anemones fill the ocean floor with life, adhering to rocks and other substrates.

There are more than 1,200 species of sea anemones, all of them stand out for not having a brain , for their colors and tentacles of greater or lesser length, as well as their inability to move. Another aspect that allows us to recognize these animals quickly is the symbiosis they have with clown fish, since they live between the tentacles of anemones.

Marine corals, one of the most curious brainless animals

Among the most mysterious and surprising organisms that nature hides in the planet’s marine ecosystems , the marine corals (Phylum Cnidarians) stand out .

They are colonial organisms without brains that, grouped together to form the majestic coral reefs, fill with color and serve as shelter for numerous marine species, both small and large.

Lancet fish

Known as one of the organisms of greatest interest in the transition between invertebrate and vertebrate animals , the lancetfish inhabits the shallow waters of the Mediterranean and the Atlantic.

Its anatomy is a mystery: it lacks a skull and a well-differentiated brain, as well as a skeleton and jaws. However, it has a very simple nervous system, consisting of a dorsal cord protected under some vertebrae.


This group of invertebrates belonging to the Phylum Annelida, Subclass Hirudinea, we can find them both in marine, terrestrial and freshwater habitats , in which they are especially predominant.

Commonly known for their ability to suck blood from their prey, leeches are also notable for some curiosities of their anatomy, such as the absence of a well-differentiated brain centralized within the nervous system of the animal.


Like leeches, earthworms belong to the Phylum Annelida , but are included within the class Crassiclitellata, as they differ in various anatomical and lifestyle characteristics.

Earthworms are directly descended from marine ancestors, aquatic worms, so they currently have many characteristics of their ancestors, including the absence of a brain in their anatomy.

Both earthworms and leeches, mentioned above, are annelids that breathe through the skin

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