According to the latest scientific data ancestorsmodern fish - jawless animals, resembling their appearance - already lived in the early Cambrian, about 530 million years ago. It is assumed that such creatures found in 1999 in Yunnan Province may be the progenitors of all vertebrates.
At the moment, cartilage fish, bony fish(lobe-finned and ray-finned) make up more than half of all vertebrates living on the planet. In total, there are about 31 thousand species of the most diverse forms and sizes found in salt and fresh water bodies. The study of ancient beings is engaged in a separate science - ichthyology. Let us dwell in more detail on the classes, their features and differences.
The main feature of all members of the class inthat their skeleton consists of cartilage, which over time can become quite hard as a result of the deposition of minerals. Previously for this reason they were considered prehistoric animals. However, many of them are characterized by live birth, sometimes even with the formation of a biliary placenta - this is what makes cartilaginous fishes radically different from bony fish.
Besides this, they have several moreanatomical features of the structure. First, the lack of a swim bladder. Therefore, they have to move in order to sink to the bottom of the reservoir. Secondly, the cartilaginous fish lack gill covers, and the gills open to the outside with characteristic slots. Thirdly, they are all covered with placoid scales, which is similar to the teeth of vertebrates. It consists of dentin and a layer of enamel covering it. Such scales are not restored with the loss, however, with the growth of fish, its number increases.
Characteristics of cartilage and bone fish will notcomplete, if not to mention the main systems: circulatory, digestive and sexual, in which differences are observed. Cartilaginous have red blood (due to the presence of hemoglobin and red blood cells), which is produced by the spleen. The circulatory system itself is similar in structure to that of cyclostomes. Along the spine stretch in the form of two dark red stripes of the kidney. The intestine of cartilaginous fishes consists of three sections, it is: large and small intestines, rectum. Well developed liver, pancreas. But the most important thing is that the classes of cartilaginous fishes and bony fishes differ in the structure of the reproductive system. The former is characterized by internal fertilization with the formation of an egg, which can be deposited in the external environment or remain in the lower part of the oviduct. In the second case, the embryo begins to develop in the maternal organism.
All of the currently existing representatives of the class Cartilaginous fishes are divided into three super-orders.
For a long time, until the 21st century, cartilaginousfish, bony fish were considered as two classes. However, in the scientific community, a different point of view is becoming increasingly widespread. Thus, the Canadian zoologist, in his works, defines the blade-and-ray and ray-finned fish into separate classes, and the bone ones, respectively, into the superclass. These are the most diverse inhabitants of all types of reservoirs. Their mouth is formed by gagging jaws and teeth located on them, the gills are on the gill arches, and the nostrils are paired.
The most important difference between bone fish and cartilageclear from the name - the skeleton. He is from the bones. In the inner cavity are the circulatory system, excretion, reproduction and digestion. The scales are also characteristic, one of three types: cycloid, ctenoid, or ganoid.
The next difference is the presence of a swim bladder,located under the spine and filled with gases that release blood vessels. With an increase in its volume, the fish easily floats to the surface, with a decrease it goes to the depth.
Differences are not only external signscartilaginous and bony fishes, but also reproductive organs, as already mentioned. For most of the representatives of the second group is characterized by external fertilization that occurs in the aquatic environment. This process is called spawning, it occurs within a certain time frame and is accompanied by characteristic behavior.
This is the numerically predominant class in moderndiversity of fish, they number more than 20 thousand species, which is about 95%. They inhabit all corners of the planet, from the Arctic seas to the hot equator, the size ranges from 8 mm to 11 meters, and the weight of individual individuals reaches more than two tons. The name, as you might guess, is associated with the structure of paired fins, in which the basal axis is missing. The class, in turn, is divided into two groups: Fresh-fronted (most prosperous species) and Carnivorous fishes. The structure of the latter has distinctive features. They have a swim bladder, but at the same time, their skeleton consists mainly of cartilage tissue. Chord has only cartilaginous arches and is not dismembered, vertebral bodies as such are absent. A distinctive feature is the rostrum and lower mouth. Many of them are fishery, in particular sturgeon (in the photo below - the catch of beluga).
A small class of fish, based on the skeletonwhich is an elastic chord. They combine progressive and archaic features, all representatives belong to two modern super-detachments - Kisteperye and Lipose. Both groups combine ancient fish. The lungfish live in freshwater bodies of Australia, South America and Africa. They possess not only the gills, but also the lungs. This allows them to go without water for some time and feel free in the oxygen-depleted reservoirs. A total of 6 species are known: four African prototopters (photo below), the Australian cattail, and the South American flake maker.
It is considered almost extinct.Only one genus has reached our days - Latimeria (in the photo below), which has two species. And both of them were discovered relatively recently, the first specimen was caught in the Indian Ocean in 1938. It is believed that brushfish are inhabitants of freshwater bodies in which oxygen deficiency has been tested. In this regard, they have developed muscles at the base of the fins and a double breathing method (lungs and gills). This allowed some later to move back to the seas, and the freshwater eventually became extinct. There is an assumption that it was the cross-finned fish that gave rise to the class Amphibians.
Thus, cartilaginous fish, bony fish havea number of individual features. The main ones are observed in the structure of the skeleton (cartilage or bone), the presence or absence of a swim bladder, the type of scales, the reproductive system and the method of reproduction.
