*2.1. General Information about Flatworms*

The Platyhelminthes, also known as flatworms, are dorsoventrally flattened organisms with bilateral symmetry [16] that morphologically constitute a heterogeneous group. Throughout history, the phylum Platyhelminthes has been the subject of multiple controversies, especially regarding its phylogeny since morphological data of some species can be troublesome [17]. We currently have nuclear and mitochondrial genomes and transcriptomic analyses that allow us to reach a consensus. The phylum Platyhelminthes is currently included within the Lophotrochozoa supergroup, which includes other invertebrates, as well as annelids and mollusks [18,19]. It is interesting to note that another phylum, which was traditionally thought to be close to flatworms, the phylum Nematoda, is included, together with arthropods and related groups, in the supergroup Ecdysozoa [18]. This is highly important to consider to understand the morphophysiological differences between flatworms and nematodes.

Already within the phylum Platyhelminthes is the paraphyletic group Turbellaria, which includes all free-living flatworms. These are found in aqueous environments, both in salt and fresh water, although there are some adapted to terrestrial environments with high humidity. An important characteristic that is present in the members of this group is that their external surface has a simple epidermis, composed of a single layer of columnar epithelium located on top of a basement membrane and several layers of muscle. This epithelium usually has cilia, which are used by these worms to swim in waterbodies or to glide over the substrate [20].

On the other hand, we have the monophyletic group Neodermata, whose innovation is replacement of the simple epidermis of the turbellarians by the presence of a syncytial-type tegument that is formed by extensions of cells that are below the basement membrane and that fuse together in the tegument creating a syncytium. This characteristic is considered an adaptation to parasitic life and is so important to understand their physiology that it is a criterion for defining this group, which is composed exclusively of parasitic organisms. Three types of flatworms with a clearer phylogenetic relationship are currently recognized [21]. On the one hand, we have the group of ectoparasitic flatworms of the class Monogenea, whose representatives are characterized by having a single host throughout their life cycle [22]; they usually live on the gills or skin of aquatic vertebrate animals. On the other hand, we have the flatworms that are endoparasites and are grouped within the class Cestoda and class Trematoda. These last two are considered the most successful parasites due to the great variety of vertebrates they infect; they are responsible for many diseases of livestock animals and humans [23–25]; thus, they are of great medical and economic relevance [26,27].
