The Nervous System Evolved

Porifera, known colloquially as the sea sponges, are composed of a diverse phylum of aquatic animals. They are also the host of the simplest form of nervous communication: cell-to-cell, sometimes called intercellular signalling.

An example of Porifera. "Aphrocallistes vastus" | by Dan Hershman from Wikimedia Commons | Licensed under CC BY-SA 2.0

Cell-to-Cell Communication

Cell-to-cell is not typically considered to be a nervous "system", as there is no level of organization to it, but rather a nervous function. This function is not unique to Porifera, but they are unique in being the only animals whose cells send signals by this method alone.

A diagram of cell-to-cell type communication.

With intercellular signalling, there are sensory proteins that penetrate the cell membrane and come into contact with other cells. There are also proteins on the inside of cells that detect chemicals, such as ligands, almost like a rudimentary endocrine system. These signals provide valuable insights, including information about water current activity, light levels, and what types of cells are nearby.

Even an apparently simple, crude function like intercellular signaling is essential in allowing eukaryotic organisms to coordinate actions.

Four different examples of the phylum Cnidaria. From top-left to bottom-right: a jellyfish, a soft coral, a rocky coral, and a sea anenome. By Frédéric Ducarme from Wikimedia Commons | Licensed under CC BY-SA 4.0

While the sponges were taking evolutionary big steps with its cell-to-cell communication, the phylum Cnidaria had gone a step further and formed the first dedicated nerve cells. While the "nerve net" of the Cnidaria's jellyfish and corals was not any more a 'system' than the signalling in Porifera, it was still an incredible stepping stone towards building the intricate nervous system us humans have.

Nerve Net

Again, this system was incredibly simple, but it contained the basic cell structure found in more complex nervous systems: the nerve cell. These specialized cells made intercellular communication more efficient and consistent than with cell-to-cell. In Porifera, chemical signals could be sent, but there was no guarantee that the signals would get to the right cells, or even be picked up by another cell at all. It was pivotal, yet faulty. The nerve net, on the other hand, acted like a set of telephone poles and wiring—albeit with a random distribution.

A nerve cell diagram with each part labelled. Source Bekolay, Trevor.

Nerve cells are made up of three parts: the soma, the axon, and the dendrite. The soma is the main part of the nerve cell; the base cell onto which the nervous equipment is mounted. It holds the cell's organelles and DNA, as well as other structures adapted for basic cell function and survival. The other parts allow the cell to transmit communicative electrical signals, the axon transmitting signals and the dendrite recieving them.

The nerve net was an important advancement for the nervous systems of all other animals, but especially to Cnidaria because they were moving away from the filter feeding lifestyle of sponges. These animals now required coordinated movement in order to hunt the microscopic planktonic organisms that were their prey.

They were able to achieve this with nerve cells and did not develop their nervous systems further like some other animal groups did.

Worms were the first animals to evolve a true central nervous system. There are three phyla of animals we traditionally call "worms": Platyhelminthes, Nematoda, and Annelida.

Platyhelminthes: Flatworms

A planaria, a type of common, non-parasitic flatworm. Image by me.

The simplest group of worms, the flatworm, is characterized by a large nerve cell mass in its head. This mass, called the ganglion, is divided into a top arch and two lumps of nerves called ganglia beneath the organism's eyes. Yes, this is also the first animal with complex eyes.

Flatworms, such as the planaria above, also have a pair of large sensory organs on their head. Called the auricles, they appear as triangular flaps on either side of the animal, and the sensitive skin allows the worm to feel where it's going. These worms also have two sets of muscles, a feature that the first two animal groups lacked.

Planarian nervous system diagram.

The flatworm peripheral nerve structure appears in a ladder shape, with two nerve cords running down either side of the body connected at several points by nerve "rungs". The shape of the nervous system allowed early flatworms to move in a much more controlled, purposeful manor than the animals before it. The centralized ganglion of the worm right next to its major sensory organs allowed it to process sensory input and supposedly to have basic thoughts.

These advancements allowed planaria (as well as other flatworm species) to become a more capable predator that could actively and nomadically hunt its prey.

Nematoda: Roundworms

In Nematoda, there is one less muscle set but four nerve cords in the nerve ladder instead of two, with the rungs connecting to make a circular shape. This nerve ring structure is useful to roundworms because it allows them to move more precisely despite having few muscles than the flatworms.

They also have a somewhat larger ganglion compared to the flatworms.

Annelida: Segmented Worms

True brain (pair of nerve clusters) with brain stem, ganglion controls muscles and organs in each body segment

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