The super class Pisces (L., Piscis, fish) includes all the fishes which are essentially aquatic forms with paired fins for swimming and gills for locomotion. About 40,000 species of fishes are known. Various workers have provided different schemes of their classification.
However, no classification had been universally accepted because of the confusion due to staggering numbers of fishes and great diversity in their shape, size, habits and habitat.
According to Parker and Haswell (1960) the super class Pisces if sub-divided into three classes.
(i) These include all extinct fishes found from early Devonian to Permian period.
(ii) Body heavily armoured with bony scales or plates.
(iii) Primitive jaws with teeth.
(iv) Paired or unpaired fins were present.
(v) Skeleton bony.
(vi) Notochord persistent throughout life.
(vii) They are immediate ancestors of astracoderms.
The class placodermi includes six subclasses.
(i) Sub-class – I – Acanthodii, Ex: Climatius
(ii) Sub-class – II – Arthrodin, Ex: Cocoosteus
(iii) Sub-class – III – Petalichthyda, Ex: Macropetalch
(iv) Sub-class – IV – Antiarchi, Ex: Pterich thyodes
(v) Sub-class – V – Rhenanida, Ex: Gemuendina
(vi) Sub-class – VI – Palaeospondylia, Ex: Palaeospondylus
CLASS-II – CHONDRICHTHYES:
(i) Mostly marine and predaceous
(ii) Body fusiform or spindle shaped.
(iii) Endeskeleton is cartilaginous
(iv) Skin with placoid scales.
(v) Fins both median and paired all, supported by fin rays. Pelvic fin bears claspers in males.
(vi) Notochord tail fin heterocereal persistant. Vertebrae complete and separate from notochord.
(vii) Mouth ventral in position, jaws p[resent. Teeth are modified placoid scales.
(viii) Digestive system complete, stomach J-shaped, intestine with spiral valves.
(ix) Gill slits separate, 5-7 pairs, laterally placed, without operculum.
(x) Air bladder and lungs absent.
(xi) Heart 2-chambered, poikilothermous.
(xii) Kidneys opisthonephrii, Ureotelic.
(xiii) Sexes separate, Gonads paired, fertilization internal, oviparous or ovoviviparous. It includes two sub-classes.
1. Sub-class-I-SELACHII (Elasmorbranchii
(i) Multiple gill slits on either side protected but individual skin flaps.
(ii) A spiracle behind each eye.
(iii) Cloaca present.
Ex: Scolodon (shark), Torpedo (Electric ray) pristis (saw fish)
(ii) sub-class-II – HOLOCEPHALUI
(i) Single gill opening on either side covered by a fleshy operculum.
(ii) No spiracles, cloaca and sales.
(iii) Single nasal opening.
Ex: Chimaeras or Rat fishes.
Class-III- OSTEICHTHYES (Teleostomi)
(i) Body is spindle shaped.
(ii) Median and paired fins are present.
(iii) Tail fin is usually Homocercal.
(iv) Endoskeleton is partly or wholly bony.
(v) Skin covered by 3 types of dermal scales ganoid, cycloid or ctenoid. Some without scales. No placoid scales.
(vi) Mouth terminal or subterminal Jaws usually with teeth. Cloaca lacking, anus present.
(vii) Gills are covered by a common operculum on either side.
(viii) An air (swim) bladder often present with or without duct connected t pharynx.
(ix) Adult kidneys mesonephric.
(x) Well developed lateral line system. Internal ear with 3 semicircular canals.
(xi) Sexes separate, Gonads paired fertilization usually external.
(xii) Mostly oviparous, rarely ovoviviparous or viviparous.
This subclass includes two sub-classes.
Sub-class-I – Sarcopterygii
(i) Paired fins are leg-like or lobed with a fleshy, bony central axis covered by scales.
(ii) Internal nares present.
(iii) All are fresh water forms.
Ex: – Latimeria (living fossil)
Protopterus & Lepidosiren
Sub-class II – Actinoptergii
(i) Paired fins thin, broad without fleshy basal lobes.
(ii) Double external nares are present
(iii) Popularly called ray-finned fish.
This subclass is divided into the infraclasses or super orders
(i) Chondrostei Ex: – Polypterus
(ii) Holostei Ex: – Lepidosteus
(iii) Telestoei Ex: – Labeo, Catla