Monogenean (flatworm) skin parasites – Entobdella

Monogeneans are flatworms (platyhelminths) belonging to the Phylum Platyhelminthes, a major sub-division of the Animal Kingdom. In contrast with protozoans, flatworms are metazoans, i.e. they have multicellular bodies. On an evolutionary scale of anatomical advancement their rather compact bodies, lacking a skeleton or blood system, are regarded as relatively primitive, although their three-layered (triploblastic) construction (ectoderm on the outside, endoderm lining the gut and mesoderm between) places them ahead of sponges and the cnidarians (sea anemones, jellyfish and relatives). There are many different kinds of free-living platyhelminths and although this is not a monophyletic assemblage its diverse members are commonly referred to as ‘turbellarians’. Freshwater habitats may yield black or white planarians (triclad ‘turbellarians’) measuring a centimetre or so in length and gliding gracefully and effortlessly over stones or waterweeds, propelled by a ventral ‘sole’ of beating cilia. Similar, but often larger, polyclads may be found beneath stones on a rocky shore. There is also an abundant microfauna, invisible without a microscope, living between sand grains on sandy shores, and tiny platyhelminths capable of swimming freely using their cilia make up a large proportion of this ‘interstitial fauna’.