CHAPTER ITHE PROBLEM AND ITS SCOPEIntroduction:Fish, any of approximately 34,000C species of vertebrate animals (PhylumChordata) found in the fresh and salt waters of the world. The term fish applied to avariety of vertebrates of several evolutionary lines. It describes a life-form rather than ataxonomic group. As members of the Phylum Chordata, fish share certain features withother vertebrates. These features are gill slits, at some point in the life cycle, anotochord, or skeletal supporting rod, a dorsal hollow nerve cord and a tail (Parenti,Weitzman, 2010). Fish do not represent a monophyleticgroup, and therefore the"evolution of fish" is not studied as a single event. Early fish from the fossil record arerepresented by a group of small, jawless, armored fish known as ostracoderms. Jawlessfish lineages are mostly extinct. An extant clade, the lampreysmay approximate ancientpre-jawed fish. The first jaws are found in Placodermifossils. The diversity of jawedvertebrates may indicate the evolutionary advantage of a jawed mouth. It is unclear ifthe advantage of a hinged jaw is greater biting force, improved respiration, or acombination of factors. Fish may have evolved from a creature similar to a coral-like seasquirt, whose larvae resemble primitive fish in important ways. The first ancestors of fishmay have kept the larval form into adulthood(as some sea squirts do today), althoughperhaps the reverse is the case (Nelson, Joseph S., 2006).