SUB-CLASS SLANCHINE OR WHOLESALE FISH (HOLOCEPHALI)
Possessing a number of unique features, representatives of this subclass of fish, in addition, in the most bizarre way combine the features of plate-gill (Elasmobranchii) and bone fish (Osteichthyes). It is no accident that the great Linnaeus gave one of the clans the name Chimera. With plate-gill (selachia), they are primarily related by the presence in males of paired copulatory organs (pterygopodia), the ability of females to lay large eggs enclosed in horn capsules, the presence of placoid scales (“skin teeth”) in the external skeleton, and the complete absence of ossification of the internal cartilage skeleton, some elements of which are sometimes strengthened by calcification (should not be mixed with ossification). In addition, in the heart of modern cochlear, like plate-gill, there is an arterial cone equipped with three rows of valves, a spiral valve passes in the intestine, large nostrils are brought closer to the transverse mouth and communicate with its posterior corners using grooves dissecting the upper lip, the outer fins of the fins are supported by a large number of thin elastotidinium filaments (elastotrichia), while in bony fishes homologous to them fin rays are replaced by bone rays (lep> On the other hand, in fusion, like and in bone fish, there is no cloaca (the anal and urogenital openings are separate) and a splasher, on each side of the body there is only one gill opening, and the skull is connected to the spine using the occipital condyles.
The dental apparatus of the cranial, as in the case of doubly breathing, is represented by strong chewing plates (two pairs on the upper jaws and one pair on the lower), often these plates are equipped with ridges or ridges. The upper jaw merged completely with the skull (hence their name Holocephali, i.e., whole-headed). The vertebral bodies are absent: the chord is preserved without segment-wise constrictions throughout life and in most forms is surrounded by calcified cartilages in the form of narrow open rings.
It is believed that cochlear cranes descended from extinct shark-like ancestors and represent a lateral phylogenetic branch, which is not connected with bone fish. This group is known from the Upper Devonian and flourished until the Cretaceous. All the few living cochlear-like ones belong to the chimera-like order.
SUB-CLASS WHOLESALE, OR SLITISH-CRANIAL FISH (HOLOCEPHALI)
The small and well-separated group of cartilaginous fish from the gill-gill fish is distinguished by a combination of primitive features and signs of narrow specialization arising in connection with adaptation to a deep-sea lifestyle. Most representatives have an elongated spindle-shaped body, noticeably thinning towards the tail. Rostrum is not developed in all species. The skin is bare, almost devoid of scales. The lateral line, which is an open furrow, is clearly visible. The external gill openings are covered with a leathery fold, and therefore only one slit is visible from the outside, leading to the cavity where the gill openings open. The axial skeleton is primitive, represented mainly by the chord. The skull is autostylic, i.e. palatine-square cartilage fused with the brain part of the skull. The teeth are small, lamellar in shape. The digestive tube is poorly differentiated. Fertilization is internal. Chimeras lay, as a rule, two eggs each, dressed in a thick complex shell.
Distribution and ecology. A small number of modern species (about 30) are combined into three families. They are common in the seas of the Atlantic, Indian and Pacific Oceans in the northern and southern hemispheres. Most species of chimeras are kept at great depths (1000 m and more) and feed on bottom invertebrates. The abundance of all species is small. They have no fishing value. In our waters near the Murmansk coast, there is a chimera, or sea cat (Chimaera monstrosa), about 1.5 m long, spread from Iceland, the Barents Sea to the Mediterranean Sea.
Classification of whole-headed and chimeric characteristics
The whole-headed subclass includes 11 orders and two superorders of Paraselachimorpha and Holocephalimorpha. The latter include chimera-like, which according to other classifications are classified as superorder or even a subclass. Modern whole-headed fish include approximately 50 species that live at depths of more than 500 m. The tail of these organisms is spider-like. They are distinguished by the presence of pterygoid pectoral fins of large sizes. Chimeras are dioecious organisms that feed on very solid foods, in particular shell mollusks, echinoderms. These fish are distinguished by the ability to compress the jaw with high strength. Among the chimera-like cannibalism occurs. Their diet also contains small bottom fish, crustaceans, polychaetes.
Common features of whole-headed with gill
The main similarity of whole-headed fish with sharks and stingrays is the presence of a completely cartilaginous skeleton without any bone elements. Some species have calcified elements. The body of Holocephali, like that of Elasmobranchii, is covered with placoid scales, which are distinguished by the presence of a tooth on top. It is formed from osteodentin - a bone substance. Instead of the spine, whole-headed fish have a chord surrounded by calcified cartilage in the form of open rings.
Holocephali, like sharks and stingrays, lacks a swim bladder. The heart of whole heads has an arterial cone, and a spiral valve is located in the intestine. The nostrils are brought closer to the transverse oral opening. They are connected to the mouth in the area of the posterior corners by grooves passing through the upper lip. The fins of the whole-headed subclass are formed not by bone rays, as in bony fishes, but by elastoidin filaments.
Similarities to Bonefishes and Lungfish
The subclass whole-headed is distinguished by the absence of cloaca and spatter. These features are characteristic of bone fish. Their urinary and genital openings are isolated. The skull with the spine Holocephali connects via the occipital condyles. These are the thickening of the pineal glands, which serve to strengthen the muscles.
Thus, whole-headed people are not just called chimeras. They incorporated features of several taxa of fish. For example, as with double-breathing, they are characterized by a powerful jaw apparatus. There is an assumption that the cranial descended from shark-shaped ancestors and are a special lateral branch. This group appeared in the Early Devonian, and its heyday fell on the Cretaceous.