, the cartilaginous fishes, is numerous, consisting of sharks (Figure 15.38a), rays, and skates, along with sawfishes and some dozen species of fishes known as chimaeras, or ghost sharks. Chondrichthyes have paired fins and a skeleton made from cartilage.
In primitive vertebrates, the mouth is an easy oral opening that results in the gill slits and digestive system. As the vertebrates continued to evolve, the oral cavity was changed by a extra specialised mouth and gill equipment.
The vertebrate circulatory system is at all times closed, however this is not unique to the group. However, the vertebrate heart is all the time positioned in a ventral place and the digestive system is complete. Surprising to many, sure vertebrates, especially extinct varieties, have an exoskeleton . Early fishes, referred to as ostracoderms and placoderms, had bony exoskeletons that protected their head and sensory areas.
How Many Vertebrates Are
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