Both the former classes "Polychaeta" (bristly marine worms) and "Oligochaeta" (including the earthworms) are paraphyletic: in each case the complete groups (clades) would include all the other groups shown below them in the tree. The Branchiobdellida are sister to the leech clade Hirudinida, which approximately corresponds to the traditional subclass Hirudinea. The main subdivision of leeches is into the Rhynchobdellida and the Arhynchobdellida, though the Acanthobdella are sister to the clade that contains these two groups. Evolution
The most ancient annelid group consists of the free-living polychaetes that evolved in the Cambrian period, being plentiful in the Burgess Shale about 500 million years ago. Oligochaetes evolved from polychaetes and the leeches branched off from the oligochaetes. The oldest leech fossils are from the Jurassic period around 150 million years ago, but a fossil with external ring markings found in Wisconsin in the 1980s, with what appears to be a large sucker, seems to extend the groups evolutionary history back to the Silurian, some 437 million years ago. Anatomy and physiology
Leeches show a remarkable similarity to each other in morphology, very different from typical annelids which are cylindrical with a fluid-filled space, the coelom (body cavity). In leeches, the coelom is reduced to four slender longitudinal channels, and the interior of the body is filled with a solid dermis in between the various organs. Typically, the body is dorso-ventrally flattened and tapers at both ends.
Such practice is known as content spamming and is used to attract search engines. An article published in 1897 in American Journal of Psychology noted "the absurd tendency to give Greek names to objects feared (which, as Arndt says, would give us such terms as klopsophobia – fear of thieves, triakaidekaphobia – fear of the number 13....)". Psychological conditions
Specialists may prefer to avoid the suffix -phobia and use more descriptive terms such as personality disorders, anxiety disorders, and avoidant personality disorder. Terms should strictly have a Greek prefix although many are irregularly formed with Latin or even English prefixes. Many use inaccurate or imprecise prefixes, such as aerophobia (fear of air) for fear of flying. A
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Cultural prejudices and discrimination
Ethnic prejudices and discrimination
The suffix -phobia is used to coin terms that denote a particular anti-ethnic or anti-demographic sentiment, such as Americanophobia, Europhobia, Francophobia, Hispanophobia, and Indophobia. Often a synonym with the prefix "anti-" already exists (e.g. Polonophobia vs. anti-Polonism). Anti-religious sentiments are expressed in terms such as Christianophobia and Islamophobia. Medical conditions
Cultural phenomena
-phobia in the natural sciences
In the natural sciences, words with the suffix -phobia/-phobic generally describe a predisposition for avoidance and/or exclusion. For antonyms, see here

Jocular and fictional phobias
Aibohphobia – a humorous term for the fear of palindromes, which is a palindrome itself. The term is a piece of computer humor entered into the 1981 The Devils DP Dictionary. Anatidaephobia – the fictional fear that one is being watched by a duck.