The origins of the monogeneans are obscure, but it is assumed that they evolved from free-living ancestors similar to modern turbellarians (aquatic flatworms). Their nearest relatives are the cestodes (tapeworms). The naming of their upper-level taxa (biological categories) is controversial. "Monogenea" is widely accepted as the name of the class itself, as is the subdivision of the class into two orders, namely the Monopisthocotylea and the Polyopisthocotylea. More recently, the name "Monogenoidea" has been proposed for the class, with the class being subdivided in turn into three subclasses. The first-mentioned, older, and more widely supported scheme is followed here. Some believe that the Monopisthocotylea and the Polyopisthocotylea have separate origins; that is, they think that Monogenea is not a monophyletic (descended from a common ancestor) class. As of 2003, 53 families of monogeneans are recognized.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.