Cycloid scales cover these teleost fish (rohu). The skin of most fishes are covered with scales. Scales vary enormously in size, shape, structure, and extent, ranging from strong and rigid armour plates in fishes such as shrimpfishes and boxfishes, to microscopic or absent in fishes such as eels and anglerfishes.The morphology of a scale can be used to identify the species of fish it came from.Cartilaginous fishes (sharks and rays) are covered with placoid scales. Most bony fishes are covered with the cycloid scales of salmon and carp, or the ctenoid scales of perch, or the ganoid scales of sturgeons and gars.Some species are covered instead by scutes, and others have no outer covering on the skin.ost bony fishes are covered with the cycloid scales of salmon and carp, or the ctenoid scales of perch, or the ganoid scales of sturgeons and gars. Some species are covered instead by scutes, and others have no outer covering on the skin.
definition : hydrometer. A hydrometer or areometer is an instrument that measures the specific gravity (relative density) of liquidsâthe ratio of the density of the liquid to the density of water. A hydrometer is usually made of glass, and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright.
Reading of a hydrometer depends on the surface tension of the liquid in which it is used.This table gives an indication of possible errors, in the form of corrections which may be applied on account of difference between the surface tension of the liquid and the surface tension for which the hydrometer is graduated.