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perch, common name for some members of the family Percidae, symmetrical freshwater fishes of N Europe, Asia, and North America. The perches belong to the large order Perciformes (spiny-finned fishes) and are related to the sunfishes and the sea basses. The best-known North American species is the yellow, or lake, perch (Perca flavescens), a popular game and food fish abundant in lakes and large streams, where it feeds on insects, crayfish, and small fish and grows to an average length of 1 ft (30 cm) and weight of 1 lb (.5 kg). The voracious walleye, or walleyed pike (Sander vitreus), another member of the family, is darker and larger (up to 10 lb/4.5 kg). Very similar to the walleye but slenderer and smaller is the sauger, or sand pike (S. canadensis). The native American darters (2–3 in/5–8 cm), found E of the Rockies, are widespread and of many species, most of them brilliantly colored. Of separate families are the pirate perch, a chubby little fish of sluggish streams and bayous (family Aphredoderidae), and the trout perch, a small fish abundant in the Great Lakes (family Percopsidae). See also surfperch. Perches are classified in the phylum Chordata, subphylum Vertebrata, class Actinopterygii, order Perciformes, family Percidae.
strabismus (strəbĭzˈməs), inability of the eyes to focus together because of an imbalance in the muscles that control eye movement; also called squint. It is a consequence of weakness or uneven development of one or more of the six small muscles that surround the eye. One or both eyes may be affected. Horizontal strabismus is caused when the eyes do not move together laterally; this condition is known as cross-eye if the eye turns inward or walleye if the eye turns outward. Vertical strabismus results when the eye rolls upward or downward in its socket. There is also torsional strabismus in which the eyes do not rotate together about their optical axes. Strabismus is usually present at birth and becomes apparent early in infancy, but it may also result from illness or injury. Because the condition results in perception of a double image, there is a tendency to use only one eye. It is important that treatment be started as soon as possible to prevent loss of sight in the unused eye. Corrective therapy includes exercise that strengthens eye muscles and prescription of corrective lenses. Sometimes a patch is placed alternately on each eye so that neither is allowed to become completely unused. If necessary, the eye muscles may be shortened or lengthened surgically.
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1. a divergent squint
2. opacity of the cornea
3. an eye having a white or light-coloured iris
4. (in some collies) an eye that is particoloured white and blue
5. a North American pikeperch, Stizostedion vitreum, valued as a food and game fish
6. any of various other fishes having large staring eyes
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005