Characteristics Of Cosmoid Scale

Scales are the exoskeleton covering of the fishes found in most of the cartilaginous and bony fishes which provide protection to the body. Scales are small, thin, cornified, calcareous or bony plates which fit closely together or overlap. Scales are complex derivatives of the integument and are derived from the mesenchymal cells of the dermis. They vary greatly in size, shape, structure, and extent, ranging from rigid armour plates in fishes such as shrimp fishes and box fishes, to microscopic or absent in fishes such as eels and anglerfishes. Generally scales are transparent but in many species, scales have pigmented dots (e.g. rainbow trout). Many …show more content…
Cosmoid scales – *Cosmoid scales were found in ancient crossopterygian fishes of Devonian period. *The only living member with cosmoid scale is represented by the genus Latimeria. *Cosmoid scale is regarded as the precursor of the ganoid, placoid and the bony scales of the modern teleosts. *Each cosmoid scale consists of following three layers – a) Cosmine – It is the outer layer made of dentine type of material. b) Vascular layer – It is the middle layer consisting of a thick layer of spongy bone that carries blood vessels for the growth and nourishment of the scale. c) Isopedine – The last layer close to the body is called isopedine which is softer and spongy lamellar …show more content…
*These scales are found in modern teleosts. *They are thin, strong and extremely flexible. *They are large, oval in shape and made of only isopedine with an underlying layer of collagenous fibres that provide them with strength and flexibility. *The ctenoid scale differs from the cycloid scale in having comb like teeth (cteni) in its posterior margin. *They are arranged in an overlapping manner like tiles of a roof. They have following features – a) Focus – It is the central zone or nucleus and is the first part to develop. In some cases focus may be away from the