What function does the nervous system have? Any of us could answer this question by saying that the central nervous system mainly serves us to think, have ideas, consciousness and we would not be wrong, but the nervous system goes beyond.
The reason for its existence is to give animals the main characteristic that separates them from other groups of living beings, the locomotion. Animals are characterized, among other things, by our ability to move.
Animals without a central nervous system are not non-sentient animals, we must differentiate between perception of the environment and its possible threats and the presence or not from a central nervous system.
In this article on our site we will talk about animals without a central nervous system, starting with the definition of the system itself and the adaptations of animals that do not possess it.
What is the central nervous system?
The central nervous system is one of the two subdivisions of the nervous system, which is responsible for many tasks, such as controlling all the functions, organs and tissues of the body. The central nervous system is composed of the brain and spinal cord
The brain is found inside the skull in vertebrates and in the most anterior part of the body in other invertebrate animals. The brain is made up of the cerebro, the basis of memory and learning, the cerebellum, in charge of the motor functions of the body and the brainstem, from which a series of nerves that control organs located in the head come out, it is also in charge of the heart rate, breathing and other primary functions.
How, then, do animals without a central nervous system control all these vital functions?
What do animals that do not have a central nervous system possess?
One of the main characteristics that animals possess is the locomotion capacity, for this capacity to exist there must be a set of nerve cells or another system that allows them to react to the stimuli that occur in the environment, otherwise they would disappear.
Each group of animals has found the most suitable strategy for their way of life, so here is a list of some of the animals lacking a central nervous system:
Sea sponges
These animals do not have any type of tissue proper, nervous, digestive or respiratory. Instead they have several cell types with specific functions:
- Pinacocytes: cells that cover the body, there is no communication between the cells.
- Choanocytes: flagellated cells responsible for digestion.
- Mesohilo: space between the layer of Pinacocytes and choanocytes. The rudimentary skeleton of sponges and other lesser-known cell types are found here.
Sponges do not move, they do not require a central nervous system, it is their own cells that detect changes in the environment and reorganize according to these stimuli.
Jellyfish
Jellyfish, belonging to the cnidarian phylum, do have locomotion capacity, however, they do not have a central nervous system. So how do they move?
The truth is that jellyfish have little locomotion capacity, they can move within a column of water, up or down, but to move laterally they need water currents.
The nervous tissue of jellyfish is made up of a set of sensory cells that are embedded in the epidermis and gastrodermis (tissue that lines the gastrovascular cavity or "stomach" of jellyfish). These sensory cells are in communication with muscle cells and will inform the animal if there is any danger nearby, a food source or any other change in the environment.
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Acelomados
Acelomates are a group of very rudimentary animals but which are already beginning to show some cephalization, which is the evolutionary process by which the sensory organs are agglutinated at one pole of the body.
These animals, visually very similar to a worm or slug, have a nervous ring at one of the poles of their body, branching out in eight ribs longitudinal to the body. In addition, rudimentary eyes called ocelli appear for the first time in these animals.
Turbellarians
Turbellarians belong to the phylum Platyhelminthes. In this group of animals, the cephalization process is even clearer, but it is far from what is shown in more evolutionarily developed animals such as vertebrates.
The model of the nervous system is very basic, its "brain" is subepidermal with ring-shaped, with nerve cords (one or several pairs depending on the species) that extend along the body. Although, as we said, it has a more concentrated part (cephalization), it is still a diffuse set of nerve cells that run through the body.
Annelids
The defining characteristic of these animals is that their body is divided into metameres or segmentsIts nervous system is organized in such a way that we find a primitive brain in the segment corresponding to the head, from which two ventral nerve cords emerge that form anerve ganglion in each segment. Ganglia are groups of nerve cells.
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Mollusks
It is in this group that we find the point of inflection between the primitive and the most modern nervous systems. Molluscs have a proper cephalic area, with a brain, mouth and sensory organs.
They have a periesophageal ring and two pairs of nerves (tetraneuron), two pedal (locomotor) and two visceral (digestive, reproductive, etc.). In less active animals, such as bivalves (clams), it is poorly developed, but in snails, octopuses, cuttlefish and squid it is highly developed and has additional ganglia in the most active.
For all these reasons, we could say that molluscs have a central nervous system, whenever we talk about gastropods and cephalopods, and at a somewhat inferior in development than fish or mammals.
