The 25,000 known types of fish are separated into three principle bunches. They are the jawless, the cartilaginous (KAR-ti-LAJ-i-nus) and the hard fish. Jawless fish are the last survivors of the world's first vertebrate (VUR-ta-bret) creatures, which signifies "back-boned". They need both scales and jaws. Dating from more than 5000 million years back, just the hagfish and lampreys remain.
Cartilaginous fish created around 100 million years after the fact, predecessors of today's sharks. The skeleton of these fish is made of ligament (KART-laj), which is not as hard as bone. These fish have jaws, and also teeth which are typically hard and sharp. Their bodies are secured with hard scales.
Hard fish—angle with hard skeletons—showed up in the meantime as cartilaginous fish. They are the biggest gathering, with around 20,000 species. These fish have an organ called a swim bladder which gives the creature lightness (Kid en-see), the capacity to glide.
All fish live in water and inhale with gills. Fish are heartless, which implies their inside body temperature changes as the encompassing temperature changes.
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