In the UK, the term "learning disability" refers to a range of developmental disabilities or conditions that are almost invariably associated with more severe generalized cognitive impairment. The Lancet defines learning disability as a "significant general impairment in intellectual functioning acquired during childhood", and states that roughly one in 50 British adults have one. Japan
In Japan, acknowledgement and support for students with learning disabilities has been a fairly recent development, and has improved drastically in the last decade. The first definition for learning disability was coined in 1999, and in 2001, the Enrichment Project for the Support System for Students with Learning Disabilities was established. Since then, there have been significant efforts to screen children for learning disabilities, provide follow-up support, and provide networking between schools and specialists. Effects
The effects of having a learning disability or learning difference are not limited to educational outcomes: individuals with learning disabilities may experience social problems as well. Neuropsychological differences can affect the accurate perception of social cues with peers. Researchers argue persons with learning disabilities not only experience negative effects as a result of their learning distinctions, but also as a result of carrying a stigmatizing label. It has generally been difficult to determine the efficacy of special education services because of data and methodological limitations. Emerging research suggests adolescents with learning disabilities experience poorer academic outcomes even compared to peers who began high school with similar levels of achievement and comparable behaviors.
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.