One of the most interesting aquatic animals are fish. Class Cartilaginous fish - sharks and stingrays - have significant differences from other representatives of this superclass.
Overclass Pisces
This group of animals has absolutely all the adaptations for a comfortable life in fresh and salt water. First of all, it is a streamlined body shape that allows you to easily move around. Their body is covered with scales. It protects fish from water friction and helps to glide in it. The skin secretes a lot of mucus, as an additional protection. Fins - this is a kind of steering wheel that determines the ability to move. And gill breathing, which allows oxygen to be released from water, finally determines the habitat of these animals.
Just a unique combination of features of the structure are fish. The class Cartilaginous fish and Bony are two classes of these organisms. Despite a single habitat, they have both similar and distinct features of the body.
The external structure of fish
All fish, class Cartilaginous fish and Bony, have the same parts of the body. This is a fixed head, trunk and tail. On the front are the eyes, mouth and nostrils.
The body of the fish is covered with scales, which are derivatives of horny skin formations. By increasing its layers, you can easily determine the age of the animal. This process can be compared with the formation of annual rings on tree trunks. After all, the scales cease to grow in the winter, and resumes this process with the onset of spring.
If you make a description of a shark, you need to indicate the presence of gill slits on the body of the fish. The same formations are found in stingrays. Unlike other fish, in which the gills are covered with lids.
Internal structure of fish
All the features of anatomy correspond to the aquatic lifestyle of these animals.
All bones of the skeleton are combined in 3 parts: head, trunk and fins. The skull provides reliable protection for the brain. It includes bones of gill covers and arches. Vertebrae, consisting of the body and arches, form the channel in which the spinal cord is located. The skeleton of the fins consists of rays, which are attached to the muscles, causing the fins to move.
All fish, including cartilage, have a similar structure of the digestive system. It is represented by the oral opening, oropharyngeal cavity, esophagus, stomach and intestines. In cartilaginous fish, the intestine ends in the anus, in bone fish - in the cloaca. The digestive glands - the pancreas and liver - secrete their enzymes into the ducts of the small intestine.
The excretory organs are represented by the kidneys - paired ribbon-like organs, in which metabolic products accumulate through the ureters going out.
Fish breathe with the help of gills, which are able to release oxygen from the water. They consist of arcs and petals. In bone fish, respiratory movements are carried out using gill covers. In cartilage, the respiratory organs open outward with slits - 6-7 pieces on each side.
A rare group of aquatic animals - double-breathing fish - in addition to the gills, has an extra lung. But they cannot live on land. The lung connects to the esophagus, allowing them to dwell in bodies of water with a low oxygen content.
The circulatory system is represented by a two-chamber heart and a closed system of blood vessels. All fish are cold-blooded animals. This means that their body temperature depends on the environment.
The nervous system consists of the brain and spinal cord. In the first of them, the development of the cerebellum, which determines the complex movements of fish, can be particularly distinguished.
The eyes of fish allow them to distinguish only the outlines of objects at close range. And the main role among the senses of the fish is played by the lateral line, with the help of which they perceive the water vibrations and its speed.
Bone fish
They constitute the vast majority of these aquatic animals. Bone fish are characterized by four main features:
- bone skeleton;
- the presence of a swimming bladder;
- gill covers;
- the skin is completely covered with scales.
The swim bladder is an outgrowth of the esophagus with thin walls, filled with gases. Due to its presence, the proportion of fish is reduced. This helps her to easily stay in the water column. In addition, with a lack of oxygen in the water, a certain amount of it can pass from the swim bladder to the blood.
Signs of cartilage fish
But this group of animals, on the contrary, has a cartilaginous skeleton and gill slits instead of covers. Cartilaginous fish lack a swimming bladder. This determines the lifestyle and habitat. So, stingrays lead a benthic lifestyle, and sharks are constantly in motion.
Their biology is very interesting. Cartilaginous fish, unlike bone fish, are not completely covered with scales. It is located on their body in separate rows and is formed by a durable substance - dentin. Externally, the scales of cartilaginous fish resemble teeth.
What are cartilaginous fish? Mostly predatory. Therefore, their teeth are arranged in several rows. Sharks do not chew food at all. They grab their prey, tear it up and swallow it whole.
The pectoral fins of cartilaginous fish are located in a horizontal plane. Therefore, their movements in the water resemble the soaring flight of birds.
Scary predators
The description of the shark will terrify any, even the most daring man. And those who survived the meeting with the predator are unlikely to want to remember about it. And this is no coincidence.
Sharks are completely illegible in food. And although some of them like to enjoy plankton, most prefer a larger treat.
The taxonomists know about 250 species of sharks. But the largest and most dangerous of them are the whale and giant, whose body length can reach 15 meters.
If most fish are characterized by external fertilization, then some species of sharks are ovoviviparous. Caviar lingers in the body of the female, and after internal fertilization, a young individual develops from it, which is completely viable.
Ramps
At first glance, these calm near-bottom representatives of cartilaginous fish pose no threat to others. But stingrays are predators, hunting for mollusks, echinoderms and crustaceans.
Stingrays are more common in nature , but freshwater species also exist. Among the types of stingrays - stingray , electric, manta ray, smooth.
Their appearance cannot but impress. The body is flattened in the dorso-abdominal direction. The fins grow together with him, which determines the name of this group of animals, which means "cloak".
Body size is also impressive. For example, the length of the sea devil (manti) can exceed 6 meters, the wingspan - up to 3. And they can weigh as much as 2 tons.
An interesting fact is that stingrays can fly. Not like birds, of course. But, jumping out of the water at 1.5 m, they create just such an impression.
The value of cartilaginous fish
In human economic activity, fish occupy a special place. The class of cartilaginous fish not only poses a certain threat. Bags and wallets are made of shark skin. Their meat has valuable nutritional properties, and fat is a source of vitamins and minerals.
Stingrays are also subject to fishing. But many of them are dangerous to humans. A shock from a stingray can be fatal, and exposure to an electric ramp can be paralyzed.
Cartilaginous fish are amazing representatives of aquatic animals that play an important role in the formation of food chains and are the object of artificial breeding.