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General Chharacters and Classification of Chondrichthyes

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General Chharacters and Classification of Chondrichthyes GENERAL CHHARACTERS AND CLASSIFICATION OF CHONDRICHTHYES Subject – Zoology, Class - B.Sc. Part – II(Hons. and Subsi.) Paper – III(Hons.) & II(Subsi.) By:- KUNDAN PATEL Guest Faculty Department of Zoology, B.N. College, Patna INTRODUCTION Chondrichthyes includes cartilaginous fishes having cartilaginous endoskeleton. It includes some of the prominently known fishes like sharks, rays and chimaeras. Although these are cartilaginous, they have a wide range of adaptations on the earth. Let us see some of the prominent features and classification oh these fishes. GENERAL CHARACTERS OF CHONDRICHTHYES ● Comprise living sharks, rays and chimeras ● Marine, cold-blooded animals with streamlined body ● Have cartilaginous endoskeleton. The entire spinal column may be almost completely cartilaginous(Hexanchus and Heptranchias) ● Mouth is located ventrally ● Notochord is persistent throughout life ● Gill slits are separate and without operculum ● The skin is tough, containing placoid scales ● Teeth are modified placoid scales which are backwardy directed GENERAL CHARACTERS OF CHONDRICHTHYES ● Have powerful jaws ● Well-developed median and paired fins ● Caudal fin is large, and strongly heterocercal in the sharks and shark-like rays ● Predaceous animals ● They don’t have air bladder so they have to swim constantly to avoid sinking ● Have two-chambered heart; a rhythmically contractile conus arteriosus with several transverse rows of valves is always well- developed GENERAL CHARACTERS OF CHONDRICHTHYES ● Some have electric organs and some possess poison sting ● Sexes are separate. In males pelvic fins bear claspers ● Internal fertilization and many are viviparous ● Aperture of cloaca serves as a common outlet for the rectum and the renal and reproductive ducts ● The flattened form of the rays is an adaptation to bottom-dweling existence. E.g. – Manta – width of 22 feet, diamond-shaped body GENERAL CHARACTERS OF CHONDRICHTHYES ● In some cases, the head is produced forwards into a long rostrum. This is of great length and bordered with triangular teeth in the sawsharks(Pristiopherus) and saw fishes(Pristis) ● In the rays, dorsal fin is usually small, and the anal fin absent, pectoral fins are extremely large and much bigger than the pelvic fins, and the pectoral fins fringe the greater part of the length of the flattened body, and are prolonged forwards on either side and even in front of the head ● Along each side of the neck in the sharks, and on the ventral surface in the rays, there is a row of slit-like apertures – the branchial slits or clefts. These are usually 5 in number on each side but in Pliotrema, hexanchus, and chlamydoselachus, these are 6 in number and 7 in heptranchis GENERAL CHARACTERS OF CHONDRICHTHYES ● Truly cartilaginous fishes do not seem to have appeared before mid-Devonian ● It is likely that the elasmobranchs arose from a bony ancestor, and perhaps by neoteny ● Largest living shark – oviparous whale shark(Rhinodon) – nearly 70 feet, Basking shark(Cetorhinus) – app. 40 feet, great white shark or white pointer(Carcharodon carcharias) – man-eating – 36 feet; tiger shark(Galeocerdo) – 16 feet; grey nurse(Carcharias) – 9 feet; blue pointer or Mako(Isuropsis) – 13 feet GENERAL CHARACTERS OF CHONDRICHTHYES Placoid scales:- ● They are called dermal denticles ● Structurally homologous with vertebrate teeth, having a central pulp cavity supplied with blood vessels, surrounded by a conical layer of dentine, all of which sits on top of a rectangular basal plate that rests on dermis ● Cannot grow in size rather its number increases as the size of the fish increases ● It is a source of rawhide leather, called shagreen, used in making hand-grips for swords GENERAL CHARACTERS OF CHONDRICHTHYES GENERAL CHARACTERS OF CHONDRICHTHYES Sting:- ● A dangerous integumentary structure ● One or several deciduous spines, capable of independent movement, are attached some distance up the tail ● Grooves in the spine carry toxic secretions from continuous glands ● Sting-rays produce agonizing(causing great pain) wounds and hideous scarring ● A large ray may slash right down to the bone and often results into septicaemia. In 1938, a Newzealand girl aged 18 was killed by stab-wounds from a fish that was almost certainly a sting-ray. In addition to inflicting three thigh wounds, the spine penetrated the thorax and both ventricles of the head GENERAL CHARACTERS OF CHONDRICHTHYES GENERAL CHARACTERS OF CHONDRICHTHYES CLASSIFICATION OF CHONDRICHTHYES The class chondrichthyes has been classified into two subclasses(Romer, 1959):- Subclass – 1. Selacchii:- ● It includes modern sharks ● The giils are placed in separate clefts and a spiracle is present behind each eye Order – 1. Cladoselachii:- ● Shark-like, fusiform ● Became extinct in Devonian to Permian periods and is the ancestor of all modern sharks ● Tail heterocercal with a large lower lobe, and a horizontal keel-like fin on each side of the caudal peduncle ● Pectoral, pelvic and two dorsal fins were very prominent ● The large sub-terminal mouth, long sharp teeth, relatively enormous forwardly directed eyes, body covered with small denticles, amphistylic jaw suspension, a well-developed postorbital process, paired nostrils and absence of claspers ● E.g. – cladoselache, cladodus, symmorium, diademonus, etc. CLASSIFICATION OF CHONDRICHTHYES CLASSIFICATION OF CHONDRICHTHYES Order – 2. Pleuacanthodii:- ● Slender, fresh water, shark-like fishes having dorsal fin extending along the body and tail ● It became extinct in Triassic period ● The pectoral fins are paddle-like, having long axis with pre- and post-axial rays ● The tail is diphycercal with equally developed blades of fin above and below the notochord ● A long spine on the head and claspers are present in male ● E.g. – Pleurocanthus CLASSIFICATION OF CHONDRICHTHYES Order – 3. Protoselachii:- ● The pectoral fin is notched at the posterior margin ● Claspers present in male ● Two types of teeth – one sharply pointed, located anteriorly and the other flat type placed behind for crushing molluscan shells ● They flourished in Palaeozoic seas and became extinct in Permian to Triassic period ● E.g. – Hybodus, Heterodontus, etc. Order – 4. Euselachii:- ● Shark and rays constituting this order are abundant in equatorial and temperate seas ● Teeth numerous, developed in continual succession ● Pectoral fins have three basal pieces, the pro-, meso- and metapterygium, from which a number of pre-axial radials spread out ● E.g. – chlamydoselachus, Heptranchius, Hexanchus, squalus, Pristis, Torpedo, Manta, etc. CLASSIFICATION OF CHONDRICHTHYES CLASSIFICATION OF CHONDRICHTHYES CLASSIFICATION OF CHONDRICHTHYES Subclass – 2. Bradyodonti:- ● Vertebrae are reduced to nodules ● Teeth are firmly attached to the jaws and modified into crushing plates ● Notochord remains unconstricted Order – 1. Eubradyonti:- ● Primitive bradyonts ● The teeth are few and fuse to form flattened crushing teeth ● The nature of vertebrae, mode of jaw suspension and cleavage pattern are primitive ● They became extinct at the end of Palaeozoic era ● E.g. – cochliodus, psammodus, helodus, etc. CLASSIFICATION OF CHONDRICHTHYES Order – 2. Holocephali:- ● Deep sea forms with claspers in male and large egg cases in females ● The skin is naked except for a few denticles restricted to the head claspers ● Jaw suspension holostylic, the upper jaw being immovably united with cranium ● Neither skull mor jaw are attached to hyoid arch ● The extra claspers are on the head and in front of the ordinary pair on the abdomen ● Cloaca and spiracle absent ● An operculum covers the gills ● The teeth are in the form of plates ● The first dorsal fin is far forward with strong spine ● The tail is slender and whip-like ● E.g. – chimaera, harriotta, callorhynchus, etc. CLASSIFICATION OF CHONDRICHTHYES CONCLUSION Cartilaginous fishes have a certain unique features compared to other fishes or other vertebrates. They have been classified into various subclasses and orders depending upon the variations in features and a large number of extinct species have also been included in the classification. They cannot be considered inferior in the race of evolution rather some of the forms like sharks seem to be very advanced in its size and strength. BIBLIOGRAPHY & REFERENCES 1. Chordate Zoology - E.L. Jordan and Dr. P.S. Verma 2. Vertebrates - R.L.Kotpal 3. A textbook of Zoology(Vol.II) – Parker and Haswell 4. Biology of Animals(Vol.II) - Sinha, Adhikari and Ganguli THANK YOU.
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