Children's Animal Encyclopedia - (Pocket Library)

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Publishing Date: 2006-08-01
Features: Marine chordates are transitional forms between vertebrates and invertebrates, including notochordates and cephalochordates. Another important class of marine animals is marine vertebrates, which mainly include fish, reptiles, birds, and mammals that rely on the ocean for survival. Fish are one of the most important marine animals. The earliest fish appeared over 500 million years ago, and at that time, they had neither jaws nor fins. Currently, there are approximately 25,000 species of fish worldwide, making them the most diverse group of vertebrates. Most fish have a protective layer of scales covering their bodies, and their internal organs are similar to those of other vertebrates. Their bodies are generally streamlined, they breathe through gills, and they swim using fins. Fish fins include the caudal fin, dorsal fin, and anal fin, among others. The pectoral and pelvic fins correspond to the limbs of vertebrates. Scientists classify fish into three major categories based on their characteristic features. One category is called jawless fish, such as lampreys and hagfish, which lack gills and have bodies resembling a section of a chimney. The second category is called bony fish, such as trout and cod, whose vertebrae, skull, ribs, and lower jaw are all made of hard bone. Their scales are called bony scales, some of which overlap and are arranged in an overlapping pattern beneath the epidermis. Most bony fish have a swim bladder, a gas-filled organ that controls buoyancy. The swim bladder acts like a variable bubble: when it expands, buoyancy increases, allowing the fish to float; when it contracts, buoyancy decreases, causing the fish to sink. 95% of fish are bony fish. The last category is cartilaginous fish, which are characterized by their soft bones; their scales are shield-like and embedded in the skin, creating a surface like sandpaper; they lack a swim bladder. Sharks belong to this category. To adapt to aquatic life, sharks have a large upper fin and a small lower fin, which helps them swim quickly and provides buoyancy during slow swimming. Currently, there are over 3,000 species of sharks worldwide. Among them, sharks living in tropical warm waters, such as the great white shark, bull shark, and tiger shark, are aggressive carnivorous fish known as "ocean hunters." P24