Phylum Platyhelminthes, also known as flatworms, consists of four distinct classes: Turbellaria, Monogenea, Trematoda (flukes), and Cestoda (tapeworms).

Today, I want to write more about Turbellarian nervous system, which is more advanced than the one found in Ctenophora or Cnidaria. Turbellaria are small animals (up to 20 mm in size, although there is one species that can be more than half a meter long, imagine that touching your foot) found in water and wet habitats. Turbellaria have a brain, both sensory and motor neurons, and a series of sensory receptors. Although bilateral animals, not all Turbellaria have a bilateral nervous system, with some of them still having a radial system characteristic for cnidarians.

The turbellarian nervous system, made of uni-, bi-, and multi-polar neurons, can be epidermal, sub-epidermal, and sub-muscular. However, only less advanced species have the epidermal nervous system, while all the others have both subepidermal and submuscular.

When discussing the radial nervous system, it is important to mention cerebral ganglion and three pairs of nerve cords (dorsal, lateral, and ventral). These cords are connected by annular commissures. In the bilateral system, on the other hand, we have a primitive brain made of several ganglia and only two ventro-lateral cords, mutually connected by transverse commissures. There are also sensory nerves, which extend forward from the brain.

Turbellaria have a whole myriad of sensory receptors: mechano-, chemo-, photo-, and balance receptors. Mechanoreceptors can be divided into two groups, thigmoreceptors and rheoreceptors. Both of these can be found on the whole area of Turbellaria body, and both contain cilia in order to sense outside stimulus. The difference between the two is that thigmoreceptors are specialized for touch, while rheoreceptors process water flow stimuli. Chemoreceptors are located in special grooves on the head, and serve to locate food or a mate. Photoreceptors are located in ocelli (ocelli are analogue of eyes) and although they usually have only a pair on the head, some species have couple of pairs or even many ocelli on the edge of their bodies. Statocysts serve as balance organs, although only some species have them. Statocysts are chambers filled with a fluid and also contain one statolith. It is actually unknown how statocysts receive stimuli.

Schmidtea mediterranea, an adorable flatworm of Tricladida class is one of the modal organisms in genetic and molecular research, because it has diploid genome and asexual and sexual strain. These characteristics make S. mediterranea a very popular choice among the scientists, especially since the discovery of its apparent immortality. Due to an abundancy of stem cells, almost any amputated part of this flatworm can regenerate into a full organism in a span of just several days. Yes, this little organism is literally Deadpooling its way through life!

Of course, the explanation behind this mechanism is all but simple; it seems that this regeneration ability depends much on the activity of an enzyme called telomerase, and not even this works the same in asexual and sexual strains of S. mediterranea. There are also many genes involved, but since some of the genes have orthologs in human, scientists are now trying to discover if they could somehow stop aging in our species.

So, what do you think about these small creatures? Do you like them, or do they frighten you a little bit?

Unfortunately, I couldn’t find many resources online regarding their general nervous system, so most of the information is sourced from one book, which is available only in Croatian. More research is needed regarding these creatures, and some are underway, especially regarding their astonishing regenerative capabilities.

Literature & more information:
^{Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
Moraczewski, Czubaj & Bgkowska Organization and Ultrastructure of the Nervous System in Catenulida (Turbellaria) Zoomorphologie 87, 87-95 (1977)
Tan et al: Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms PNAS vol. 109, no. 11, 4209–4214 (2012)
Handberg-Thorsager, Fernandez & Salo Stem cells and regeneration in planarians Frontiers in Bioscience 13, 6374-6394 (2008)}