Cold-blooded vertebrates. Fish
Fish can be divided into three classes: Angatha (the jawless fish), Chondrichtyes (cartilaginous fish) and Osteichthyes (bony fish). The first fish ware jawless and had the strong notochord running the length of their bodies. Today these jawless fish are represented only by the hagfish and lampreys which are the most primitive living vertebrates. The lampreys are highly predatory, attaching to other fish by sucker-like mouths and rasping through the skin into the viscera of their host, use its body fluids for food. In similar manner, the hagfish uses its well-developed toothlike tongue to bore into its prey for food.
According to present evidence, fish evolved in freshwater. The chondrichthyans return to the sea early in their development, while the bony fish went through most of their evolution in freshwater and spread to the seas at a much later period. Some still recapitulate in each lifetime this difficult physiological transition. Salmons, for example, return to freshwater to spawn, while eels leave the freshwaters of the continents to return to the Sargasso Sea at breeding time, from this distant point the young begin the long, difficult journey, often lasting many years, back to the rivers and lakes.
Fish are ectothermic vertebrates. This means that their temperature change with their surroundings. When it is cold outside, body heat produced when food is burned, is lost to the surroundings and the fish becomes cool. When it is hot outside, the body temperature of the fish goes up. Many ectothermic vertebrates must change their activity according to the outside temperature. For example, some fish become sluggish when the weather is cold.
5.9.1. Cartilaginous fish
Some fish have skeleton made of cartilage. Most of these fish are known as cartilaginousfish. The most widely known cartilaginous fish are sharks, skates and rays.
They are able to swim very fast because they have a streamlined body and a large caudal fin.
The mouth of a shark is on the underside of its head. In the mouth of a shark there are five or six rows of sharp teeth. All the rows except one or two lie out of sight along the inside of the jaws.
Different species of sharks are different in size. A whale shark may be eighteen meters long. Whale sharks are not harmful to people. They eat only plankton and small fish.
However, sharks may be useful to people. Some people use sharks for food. A shark’s skin can be used as leather. Sharks are even used to study certain diseases because sharks are disease-resistant.
The sharks and skates, the Chondrichthyes, have a completely cartilaginous skeleton. Their skin is covered with small pointed teeth (denticles), which resemble vertebrate teeth structurally and give the skin the texture and abrasive capacity of the coarsest grade of sandpaper.
Sharks, skates and rays have five to seven pairs of gills and two-chambered heart. A spiral membrane or valve extends through the intestine. The valve delays the passage of food through the intestine, and furthers digestion. The fishes have separate sexes and fertilization of eggs is internal.
Sharks are carnivorous, or meat-eating, fish with torpedo-like bodies that are well-adapted to a predatory life. Sharks live in every ocean and are particularly abundant in warm seas. The whale shark is the largest of all fishes. It grows to about 18 m. long and weighs about 15.4 tons. Sharks are powerful swimmers that can move rapidly when excited. Their rapid speedis made possible by strong muscles and streamlined bodies that are stabilized by two pairs of fins. The pectoral fins grow on the sides just behind the head. The pelvic fins grow farther back on the underside of the body. The dorsal fins extend along the back, one behind the other. Sharks also have a vertical caudal fin,which is an expansion of the tail. Because sharks are heavier than water, they will sink if they do not continually move forward.
Cone-shaped tooth-like structures called placoid scales cover the shark’s tough leathery skin. Each scale consists of an enamel-like outer layer and a tooth, called a denticle that projects from the center. Modified scales form the two rows of backward-pointing teeth in the animal’s jaws. The backward slant of the teeth makes it possible for the shark to hold food more securely in its jaws.
Senses.Sharks have keenly developed senses. As much as two-thirds of the shark’s brain is given over to the sense of smell. Sharks can smell the odor of blood as far away as 0.4 km (0.25 mi.), in concentrations as small as one part of blood in 1 million parts of water. The eyesight of sharks is not as well developed as their sense of smell, but they can see moving objects as far away as 15 m (50 ft.).
Sharks also detect movement and locate objects through a system of fluid-filled canals called lateral line system.
Reproduction. The eggs of sharks are fertilized inside the female’s body. The male grasps the female with his teeth and inserts the sperm into her with two organs called claspers.
Rays and skates
Unlike sharks, rays and skates have flat, broad bodies well suited for life on the bottom of the ocean. Their pectoral fins are greatly enlarged and flap like wings when the fish are swimming. The gill openings are on the underside of the head.
Rays are typically less than 1 m long. The giant mantra ray measures as much as 7 m in breadth. Most of the 350 species of rays are harmless to humans. Most rays eat smaller fishes and invertebrates. Some have unique methods of defense and attack. The string ray has a long, slender tail with a saw like spines that can inject poison.
5.9.2. Bony Fish
The third group in the vertebrate fish class is the bony fish, or true fish, group. The fish in this group have jaws and paired fins. Most bony fish have scales that protect their skin. Some fish, such as the porcupinefish, have spines instead of scales.
Bony fish vary greatly in size. The goby fish can be as small as one and a half centimeters. The swordfish, one of the largest bony fish, may reach a length of over 11 meters. The vast majority of the world’s fishes belong to the class Osteichthyes. Osteichthyes means “bony fishes.” As the name indicates, these fishes have skeletons made of bone instead of cartilage. Like sharks, skates and rays, however, bony fishes also have jaws and scaly skin.
Mouth, nostrils and eyes without lids are located on the head of a typical bony fish. It has two sets of gills, one on either side of the body. A protective flap of tissue called an operculum covers the gills. The fins stabilize and maneuver the fish and propel it forward. The forward most pair are the pectoral fins. Midway along the sides of its body are the paired pelvic fins. Two dorsal fins extend from the trout’s back. An anal fin extends from its ventral surface, near its anal opening. A fish swims by moving its body from side to side while swimming its tail in the opposite direction. The paired fins help change course. Most fishes have skin covered by thin overlapping segments called scales. Fish are born with certain number of scales. Though the scales enlarge throughout the life of the fish, new ones are never grown.
Skeletal and digestive systems
The fish skeleton is composed almost entirely of bone. The anterior end of the vertebral column is the skull, which covers and protects the brain. Ribs project from the backbone. The spinal cord runs parallel to the backbone and nerves branch from it to various parts of the body. Fish feed on insects and fish eggs. The food moves from the throat cavity down the esophagus. In stomach the food is stored and digestion begins. Near the stomach is the liver, a large organ that secretes bile, a substance that breaks down the fats in the food.
Respiratory and circulatory systems
Like most fishes, bony fish obtains oxygen by means of gills. It has 4 gills, 2 in each gill chamber on either side of the head. Each gill consists of a bony gill arch fringed with thin-walled tissues called gill fragments. The gill filaments contain many small blood vessels. A two-chambered heart pumps blood through a series of vessels to all parts of the body. Arteries carry blood away from the heart; veins carry blood back to the heart. The small vessels form the connecting network between arteries and veins.
Fish reproduce by spawning
Fertilizationin most fish is external and takes place in water. During the process of spawning,the female lays hundreds or thousands of eggs in the water. The male fish then spreads sperm – called milt – over the eggs. The fertilized eggs develop into embryos and then into young fish. In some fish, such as the seahorse, the female deposits her eggs into a pouch in the male. The eggs hatch in the pouch and young seahorses emerge into their watery environment.
Fish eggs do not have protective shells and would dry out if they were not in water. The yolk sac of a fish egg provides food for the fish as it develops.
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