Phylum Chordata

In the most important subphylum of Chordata Vertebrata (about 43,090 species) the notochord is replaced by cartilage or bone, forming the segmented vertebral column or backbone. The backbone forms part of internal skeleton called an endoskeleton. A skull surrounds a well-developed brain. They usually have a tail.

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 cartilaginous fish. The most widely known cartilaginous fish are sharks, skates and rays.

Sharks

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 speed is 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.

Date: 2014-12-22; view: 897