- Electric organ are found in Torpedo
(b) Subclass 2. Holocephali : (Gr., holos, entire + kephale, head)
(1) Single gill opening on either side covered by a fleshy operculum.
(2) No spiracles, cloaca and scales.
(4) Single nasal opening.
(5) Lateral line system with open groove.
(6) Examples : Rat fishes or chimaeras. About 25 species. Hydrolagus (= Chimaera).
Class 2. Osteichthyes (Bony fishes)
General Characters of Osteichthyes
(1) Inhabit all sorts of water-fresh, brackish or salt; warm or cold.
(2) Body spindle-shaped and streamlined.
(3) Fins both median and paired, supported by fin rays of cartilage or bone. Tail usually homocercal.
(4) Skin with may mucous glands, usually with embedded dermal scales of 3 types; ganoid, cycloid or ctenoid. Some without scales. No placoid scales.
(5) Endoskeleton chiefly of bone (Gr., osteon, bone + ichthyes, fish). Cartilage in sturgeons and some other. Notochord replaced by distinct vertebrae Pelvic girdle usually small and simple or absent. Claspers absent.
(6) Mouth terminal or sub terminal. Jaws usually with teeth. Cloaca lacking, anus present.
(7) Respiration by 4 pairs of gill on body gill arches, covered by a common operculum on either side.
(8) An air (swim) bladder often present with or without duct connected to pharynx. Lung-like in some (Dipnoi).
(9) Ventral heart 2-chambered (1 auricle + 1 ventricle). Sinus venosus and conus arteriosus present. Aortic arches 4 pairs. Erythrocytes oval, nucleated. Temperature variable (poikilothermous).
(10) Adult kidneys mesonephric. Excretion ureotelic.
(11) Brain with very small olfactory lobes, small cerebrum and well developed optic lobes and cerebellum. Cranial nerves 10 pairs.
(12) Well developed lateral line system. Internal ear with 3 semicircular canals.
(13) Sexes separate. Gonads paired. Fertilization usually external. Mostly oviparous, rarely ovoviviparous or viviparous. Eggs minute to 12 mm. Cleavage meroblastic. Development direct, rarely with metamorphosis.
Classification of Osteichthyes
(a) Subclass I. Sarcopterygii :
(1) Paired fins leg-like or lobed. With a fleshy, bony central axis covered by scales.
(2) Dorsal fins 2. Caudal fin hreterocercal with an epichordal lobe.
(3) Olfactory sacs usually connected to mouth cavity by internal nostrils or choanae, hence the previous name of subclass, choanichthyes (Gr., choana, funnel + ichthyes, fish).
(4) Popularly called fleshy or lobe-finned, or air breathing fish. Divided into 2 superorders or orders: Crossopterygii and Dipnoi.
Order 1. Crossopterygii
(Gr., crossoi, a fringe + pteryx, fin)
(2) Premaxillae and maxillae present.
(3) Internal nares present or absent. Spiracles present.
(4) Air bladder vestigial.
(5) Example– Primitive fleshy-finned extinct fishes. Single living genus Latimeria.
Order 2. Dipnoi
(Gr., di, double + pnoe, breathing)
(1) Median fins continuous to form diphycercal tail.
(2) Premaxillae and maxillae absent.
(3) Internal nares present and spiracles absent.
(4) Air bladder single or paired, lung-like
(5) Examples – Lung fishes. Only 3 living genera : Epiceratodus (Neoceratodus), Protopterus and Lepidosiren
(b) Subclass II. Actinopterygii
(Gr., actis, ray + pteryx, fin)
(1) Paired fins thin, broad, without fleshy basal lobes, and supported by dermal fin rays.
(2) One dorsal fin, may be divided.
(3) Caudal fin without epichordal lobe.
(4) Olfactory sacs not connected to mouth cavity.
(5) Popularly called ray-finned fishes. Divided into 3 infaclasses or superorders: Chondrostei, Holostei and Teleostei.
Superorder A. Chondrostei
(Gr., chondros, cartilage + osteon, bone)
(1) Mouth opening large.
(2) Scales usually ganoid.
(3) Tail fin heterocercal.
(4) Primitive ray-finned fish or cartilaginous ganoids.
(5) Examples – Acepenser (Sturgeon), Polyodon (paddlefish)
Superorder B. Holostei
(Gr., holos, entire + osteon, bone)
(1) Mouth opening small.
(2) Ganoid or cycloid scales.
(3) Tail fin heterocercal.
(4) Intermediate ray-finned fish, transitional between Chondrostei and Teleostei
(5) Examples –Lepisosteus (garpike)
Superorder C. Teleostei
(Gr., teleos, complete + osteon, bone)
(1) Mouth opening terminal, small.
(2) Scales cycloid, ctenoid or absent.
(3) Tail fin mostly homocercal.
(4) A hydrostatic swim bladder usually present.
(6) Examples – Harpodon (Bombay duck), Cyprinus (carp), Labeo rohita (rohu), Catla, Botia, Carassius (Goldfish), Clarius (Magur), Heteropneustes or Saccobranchus (singhi), Wallago (lachi), Mystus (tengra), Electrophorus (electric eel) Anguilla (freshwater eel), Muraena (moray) Hemirhamphus (half beak), Belone (garfish), Hippocampus (sea horse), Syngnathus (pipe fish), Fistularia (flute fish) Ophiocephalus or channa (snake head) Amphipbnous, Symbranchus (eels). Mastacembelus, Macrognathus, Pterois (scorpion fish), Pleuronectes, Synaptura, Solea, Echeneis or Remora (sucker fish), (porcupine fish), Tetrodon (globe fish)
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