Trachylina: The Group That Remains Enigmatic Despite 150 Years of Investigations

Trachylina is a group of cnidarians, a subclass of Hydrozoa. Despite the low species diversity of this group, its representatives are characterized by diversity of life cycles. Trachylina have populated various environments, from deep ocean to fresh water ecosystems. Polyps of Trachylina are either very small or absent in the life cycle, which distinguishes this group from the majority of other Hydrozoa. Trachylina are also highly diverse and have a number of features that are unusual for cnidarians. A number of representatives of this group are characterized by a small number of cells at the embryonic and larval stages. This phenomenon is well known for the representatives of phylogenetically distant taxa—Nematoda and Chordata (Tunicata). In addition, the development of Trachylina is characterized by a number of evolutionary changes that, apparently, make it possible to accelerate the formation of the definitive stage (medusa). Paradoxically, there is no one species among the representatives of this group that is studied in more or less detail. The purpose of our review is to summarize the scanty information on the Trachylina ontogeny and to demonstrate the importance of studying the ontogeny of this group for understanding the general rules of the evolution of development and life cycles of Metazoa.