Cartilaginous fish appeared, of course, earlier than their bone counterparts. But does this fact allow us to assert that these ancient creatures are imperfect children of nature, losing in comparison with their more developed "colleagues"? Consider the main differences. The main sign, because of which the class got its name, is the cartilaginous skeleton. They also do not have a swimming bladder (however, some bone fish, for example, flounder, do not). And since the specific gravity of the animal is heavier than water, in order not to go to the bottom, it should always be in motion. Placed in the aquarium, they simply suffocate, even if the water is clean: in order to saturate the blood with oxygen, you need to pump a lot of water through the gills, which is achieved by continuous movement.
Further, cartilaginous fish do not have gill covers like bone. Slots of the respiratory tract open. They are located on the side of the head. Whole-headed (chimeras) have only one branchial fissure, while sharks and stingrays have several. In addition, behind the eye of the animal is another gill - a splatter. In sharks, this is a rudiment, and stingrays actively use spatter (they have it on top of their heads) when they are waiting for their prey, buried in the sand. While swimming, these creatures swallow water with their mouths and release it through openings.
What else makes the “cartilaginous fish” class different? Their general characteristic is that they all have placoid scales. These are special plates topped with a tooth with one or two points. They consist of stingrays spikes, fin spines of some species of sharks, “saw teeth” in saw fish and, by the way, real cartilage-class teeth located in the jaw. But here, the chimeras have a resemblance to doubly breathing - their chewing organs merged into two plates, grinding plant food. We also note that chimeras have traits that make them related to the bony population of the depths - they have a chord throughout their lives. The name itself (as the ancient Greeks christened mythical creatures combining the features of different creatures - the centaur, the sphinx) characterizes these strange creatures: they not only merged the anatomical features of the bony and cartilaginous, but also their tail resembles a rat, which is why they are also called "sea rats. "
The class "cartilaginous fish" is divided into two subclasses: whole-headed (chimeras) and plate-gill. This second is divided into units: sharks and stingrays. Their representatives have many similarities between themselves. One can say that stingrays are “flattened sharks”, although such a statement is too simplistic. Gill slits in them are located on the abdomen, while in dentate predators, on the sides. There are about 350 species of stingrays: among them there are sawtailed (not to be confused with sawnose sharks!), Stingray and high-voltage. If the stingers hit their prey with poison injected into the body through the spike, the electric ones kill it with a high-voltage discharge. The largest species on Earth - manta ray - with its gigantic size (wingspan reaches 8 meters) feeds on plankton.
But cartilaginous fish such as sharks are most popular in human culture. There are 250 species, and they are common in all oceans, from the equator to the polar latitudes. Their sizes range from 15 centimeters (feline) to 20 meters (whale). Not only the temperature of the water does not scare away sharks, but its salinity: they were met at the mouths and lower reaches of the rivers, where they sailed in the hope of attacking migratory herds of deer. In literature and cinema, they earned the not-so-flattering title of an eternally hungry stomach without brains, greedy and insatiable. The concept of “embryonic kainism” came into use after people discovered that in viviparous fish, embryos eat each other in the womb.
Are cartilaginous fish a class of prehistoric animals that have survived to our time, a kind of dead end in development, as some ichthyologists believe? Yes, they did not have the formation of a bone skeleton, but this does not mean that their bones are soft. Under the influence of mineralization, their skeleton is in no way inferior in hardness to other species. According to some characteristics (absence of gill slits and swim bladder), they show an evolutionary “weakness” compared to bone ones, however, they have internal fertilization, and many of their species are viviparous or ovoviviparous. And some sharks even develop a "child's place" like placental animals.