Explain on Class Chondrichthyes - QS Study
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Class Chondrichthyes are marine animals with streamlined body and have cartilaginous endoskeleton (Figure). Mouth is located ventrally. Notochord is persistent throughout life. Gill slits are separate and without operculum (gill cover).

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Fig: (a) Scoliodon (Dog fish), (b) Pristis (Saw fish)

The skin is tough, containing minute placoid scales. Teeth are modified placoid scales which are backwardly directed. Their jaws are very powerful. These animals are predaceous. Due to the absence of air bladder, they have to swim constantly to avoid sinking.

Heart is two-chambered (one auricle and one ventricle). Some of them have electric organs (e.g., Torpedo) anc some possess poison sting (e.g., Trygon). They are cold-blooded (poikilothermous) animals, i.e., they lack the capacity to regulate their body temperature. Sexes are separate. In males pelvic fins bear claspers. They have internal fertilisation and many of them are viviparous.

Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).