Ctenophora, commonly known as comb jellies, are a rather perplexing phylum of beautiful pelagic creatures. Their evolutionary position has been debated for many years as is the origin of their nervous system (some scientists believe they are older than sponges and that sponges lost their nervous system, while others advocate the theory about the nervous system forming independently twice, once in cnidarians and once in ctenophores).
Ctenophora have two nerve nets: subepidermal and less organized subgastrodermal, which recent research identifies as a mesogleal nerve net. Nerve cells from this layer communicate with muscles by synapses and affect the locomotion of the body. The subepidermal net is denser around the mouth, the pharynx, and under the comb rows (comb rows are strips that run the length of the ctenophore body and contain cilia called “ctenes”). Ctenophore neurons can be iso- and multipolar.

They have sensory cells on the whole surface of the body and those correspond to vibrations and thermal and chemical stimuli: more receptors are located around the mouth and pharynx. Ctenophora also have an apical and aboral sensory organ. Such sensory organ consists of a statocyst, a sensor that contains a statolith that balances on four groups of long cilia connected to the comb rows. These organs help the orientation of the ctenophore body.
What’s extremely interesting is that ctenophores use different chemical signalling system than the ones described in the previous posts, mainly because these animals simply lack the neurotransmitters (and genes), such as serotonin, dopamine, noradrenaline, and acetylcholine; glutamate is the only neurotransmitter currently known to be present.

I gathered all this information from different resources, and some are sometimes contradictory or are generalizing conclusions about the whole phylum from the data of only one ctenophora species. This is the best overview I could manage, to show both the similarities and the differences of the ctenophora nervous system, when compared to the Cnidarian system. These lovely animals are not very well researched and I’m sure many wonderful breakthroughs about their anatomy, physiology, and their place in the evolutionary tree are to come.

Literature & more information:
^{Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
Norekian & Moroz Neural system and receptor diversity in the ctenophore Beroe abyssicola J Comp Neurol. 2019;1–23.
Ctenophores – quick guide
Did the ctenophore nervous system evolve independently?
Aliens in our midst}