In the framework of this article, it is necessary to list the ancient signs of modern cartilaginous fish. These include stingrays and sharks. It is about them that we will go further.
Sharks and stingrays: general description
As noted earlier, this is a class of fish, a group of cranial, bright representatives of which are stingrays and sharks. Several dozen species of this class belong to the remains of ancient groups. As previously assumed due to the lack of a skeleton, it does not make them a group of prehistoric animals. But for a number of signs, such a connection exists.
Ancient signs
Firstly, they have such a feature as the absence of a swimming bladder. Secondly, they do not have gill covers and the gills themselves have a primitive structure. All gill slots, in turn, open outward with an independent hole. All of these signs are characteristic of ancient fish. In this regard, they are forced to be in constant motion. The circulatory system should work more efficiently, the heart has the shape of an arterial cone, which enhances the efficiency of blood circulation. But the peculiarity of sharks is a powerful muscular body, which helps to pump blood throughout the body, since the heart of sharks is not adapted to heavy loads.
It is necessary to further further list the ancient features of modern cartilaginous fish. Thirdly, such an anatomical structure of the body reduces the force of friction against water, which allows to increase the speed of movement. In addition, the secret they secrete prevents the attachment of parasites to the body of the fish and to some extent protects them from certain types of bacteria.
Fourth, the cartilaginous fish has a mouth opening transversely, which is characteristic of ancient fish, in the front part, which is the base of the snout (rostrum), cartilage is formed, such a structure is characteristic, for example, of sharks. Fifthly, the body is covered with placoid scales.
Sixthly, it is not possible to list in full the ancient signs of modern cartilaginous fish in full, if you do not take into account their peculiarity of reproduction. Due to it, they are more adapted to the conditions of existence than, say, bony fishes, which throw out an unprotected game to their fate. A significant portion of caviar remains unfertilized. In most sharks and stingrays, the embryo’s body is connected to its mother through a specific “baby place” that is similar to the placenta of mammals. Or they lay large eggs dressed in thick shells, immerse them in the sand.
The internal structure of fish of this class
Let's talk about the internal structure of fish, which can be classified as cartilaginous, about the bright predator and representatives of this class, like a shark.
The muscular system of sharks consists of myomers, is developed. Myomers are muscle segments. Muscle is plentifully supplied with blood, because for a shark, movement is life. In order for the shark blood to return to the heart, only the pressure created by the heart itself will not be enough. The absence of urinary tract and kidney is a feature of the internal structure of these representatives (in particular, the polar shark). Toxins are washed away by blood and are removed simply through the skin of a predator. Gill slits located on the surface of the body and open outwards and into the pharynx. As described above: the shark does not have a swimming bladder, this feature is attributed to the ancient characteristics of modern sharks and stingrays listed above. In addition to its main function, the liver partially functions as a swim bladder. The size of the liver is significant. The pancreas and gall bladder also partially took over this function.
Conclusion
So, we have described and listed the ancient signs of modern cartilaginous fish, highlighted the structural features and adaptations to living in the external environment, similar to ancient fish.