This class has 2 subclasses, 13 orders, 45 families, 170 genera, about 846 species in the world;There are 217 species in 90 genera, 44 families, 13 orders, 13 subclasses, accounting for about 1/4 of the total.EndoskeletonCompletely composed of cartilage, often calcified, but without any realbone tissue；ExoskeletonUndeveloped or degraded, oftenShield scale。CraniumIt is the original skull, and the upper jaw is composed ofPalatine cartilageThe mandible is composed of Meier's cartilage.Gill aperture5~7 on each side, respectively opened in vitro;Or 1 pair of gill holes, covered with a pellicle.Male fishpelvic finInsidePedunclebyCopulator。Short intestine, withSpiral valve。heartConus arteriosusThere are several rows of valves.No swim bladder.No large otolith.Cloaca with or without.The egg is large, rich in yolk, discoid,In vivo fertilization。OogenesisEgg viviparityOr viviparous.[1]
Fish isVertebrateThe most primitive and lowest group of vertebrates are those that live in water for a long time, breathe with gills, and use fins to assist body balance and movement.Generally, it can be divided into two categories: jawed and jawed.The feeding habits of fish are generally divided into four types: filter feeding, herbivorous, carnivorous and omnivorous.Fish is rich in animal protein and phosphorus, rich in nutrition, delicious in taste, easy to be digested and absorbed by the human body, and plays a significant role in the development of human physical strength and intelligence.Other parts of the fish body can be made into cod liver oil, fish gelatin, fish meal, etc.Some fish, such as goldfish and tropical fish, are of high ornamental value because of their colorful bodies.Some fish, such as mosquito eating fish, can eliminate malaria, yellow fever and other infectious vectors, which is beneficial to human health.Chondrichthys is a class of vertebrates.It is mainly distributed in the ocean at low latitudes, and occasionally inhabits fresh water.This kind of organism does not have any real bone tissue, the exoskeleton is underdeveloped or degenerated, and the body is often covered with peltate scales.The cerebrum is the original skull, the maxilla is composed of palatine cartilage, and the mandible is composed of Meier's cartilage.There are 5-7 gill holes on each side, which are respectively opened in the body, or one pair of gill holes are covered with a membrane.The inner fin foot of the male's pelvic fin is the mating organ.The intestine is short with spiral valve.The conus arteriosus of the heart has several valves.No swim bladder.No large otolith.Cloaca with or without.The eggs are large, rich in yolk, discoid, and fertilized in vivo.Oviparous, oviparous, or viviparous.There are about 800 species in the world, and more than 200 known species in China[2]。
Because the bones of Chondrichthyes are cartilage, except for teeth and spinesFossilIt is extremely rare, and teeth are mostly preserved in a decentralized state, so it is difficult to understand their arrangement and combination forms.Many Paleozoic genera and species are only represented by individual teeth or fragments, and are classified morphologically, so the development history of some major groups and their genetic relationships are still unclear.[3]
Chondrichthyes Raja
Fossils of Chondrichthyes occur inEarly DevonianLate, thriving inCarboniferous, which has continued to modern times.Common in Carboniferous and Early Permian freshwater sedimentsHeterospinous shark（Costal spiny shark）Class, with well preserved individuals.It represents a branch that invades fresh water from sea water.Present-dayRed rayTraceableXijiang RiverArrive at Nanning.sincethe paleozoic eraSince Chondrichthyes appeared in freshwater sediments, there are many cases, so where Chondrichthyes originated is still a question.
The evolution of Chondrichthyes can also be divided into two directions, namely, branchia and holocephala, and they have long developed separately.GenerallyLamellibranchiaIts history can be divided into three stages: ① primitiveCladoselache The main stage isdevonian, continued to the late Paleozoic. ②Hybodus The stage starts from the Early Carboniferous tothe triassic period。③From the modern stagejurassic periodSince then, sharks and their relatives have developed.However, the three stages are not directly related to cohesion.
The second evolutionary route of Chondrichthyes is represented by holocephala.This can be done from the modernChimaerasMesozoicPolychaenoidstrace back toBuccal chimaeras。They are almost all benthic, with slow replacementToothed plate, basically eat food with shells, and grind it with tooth plates.HolocephalaIt reached its peak in the Carboniferous Period and occupied the originalScutellariaClass and replace it.There are two types of teeth in holocephala: "shark" type and "tooth plate" type.In the "shark" shapeLamellibranchiaContinuous teeth that are so separated;In the "tooth plate" type, the more synthetic the teeth are, the slower the grinding tooth plate is replaced[4]。
The body of Chondrichthyes consists of head, trunk and tail.Their bones and bones are all cartilage,Vertebral bodyDouble concave,notochordResidual, rosary.Both the upper and lower jaw have teeth, which become weapons of prey and attack.All kinds of fins exist, which are flexible.Caudal finMost of them are crookedTail typeSome of them are whip shaped.The branchial fissure is exposed separately, only existingHolocephalaThe chimaeras chimaeras have a total pellicle covering all 4 pairs of gill slits.The skin surface is covered with woodShield scaleIts morphology and structure are completely different fromTeleost fishOfSclerodermaandBone squama。Wooden Shield ScaleIt is a primitive scale. Its base is a plate of bone, with a medullary cavity connected with the dermis. The base plate is buried in the skin, with the dentine spines on it, leaning backward and exposed outside the skin.Chondrichthyes has developed true paired fins, namely pectoral fin and ventral fin, whose structures are respectively composed of shoulder girdle and waistband and their connected basal fin cartilage, radial fin cartilage and cortical fin strips.
Cartilaginous fish have no swim bladder, and use a fat liver to regulate body weight.
The skeleton is divided into three parts: skull, spine and appendage.The skull is divided into skull andPharyngeal boneTwo parts.The skull is a piece of cartilage formed by the fusion of several cartilage elements, which is called the protocranium orSoft cranium；The pharyngeal bone is composed of palatal cartilage of the upper jaw and Meier's cartilage of the lower jaw.The second is the lingual jaw cartilage, which is used to connect the pharynx and skull, and the second is used to connect the gill arc.
Chondrichthyes developed complete mandible and upper jaw.There are three types of connection between the maxilla and the skull: one is that the maxilla has two joints connected to the skull, one is the retroorbital joint, close to the back of the orbit, and the other joint is at the back of the skull. In the joint area at the back of the skull, the lingual jaw forms a connecting rod between the back of the maxilla and the skull. This connection is calledDouble connection type(amphistyly), which is an original connection mode;The second is the lingual type. The maxilla is connected to the back of the skull by the lingual jaw. The suspension mode of this type of jaw increases the mobility of the jaw, which is more advanced than the double connection type;ThirdSelf connected type(autostyly), maxilla and skull fused together, ChondroichthyesHolocephalaAnd highervertebrateThis connection mode is available.
Subordinate classification
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Chondrichthyes is divided into Lamellibranchia and Holocephala.Elasmobranchia is divided into 2 general orders, 12 orders, and holocephala has only 1 order of chimaeras.
