Sea Slugs? Flat Worms! IX.

In a sea slugs videos, I showed exactly 50 species of mollusks we encountered! In part 9, I want to show not less bright and attractive sea inhabitants - flatworms - 16 species, which, I hope, I correctly identified. The Platyhelminthes (Greek: platys-flat, helminthes-worm) which belong in the kingdom Animalia are unsegmented flat worms with a head and a tail end. They are considered the most primitive bilaterally symmetrical animals. Bilateral symmetry means that their body exists in mirror images about a long anterior-posterior axis with definite upper and lower surfaces and anterior and posterior ends. The bilateral shape of the body is an important feature because it permits cephalization, the concentration of sensory structures and nervous function (head ganglion) in the head end. This is an important trend in evolution. Furthermore, flatworms are triploblastic, which means that body structure is based on three fundamental cell layers (endoderm, mesoderm and ectoderm). As a third characteristic, they hav