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The Superclass Pisces (L. Piscis = fish) are the truly jawed vertebrates. They have organs of respiration and locomotion related to a permanently aquatic life. The respiratory organs are the gills and the organs of locomotion are paired and impaired fins. All are poikilothermous.
General Characters:
1. Aquatic, either freshwater or marine, herbivorous or carnivorous, cold blooded, oviparous or ovoviviparous vertebrates.
2. Body usually streamlined, spindle-shaped, some are elongated snake-like and a few are dorsoventrally compressed, and differentiated into head, trunk and tail.
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3. Locomotion by paired pectoral and pelvic fins along with median dorsal and caudal fins, supported by true dermal fin-rays. Muscular tail used in propulsion.
4. Exoskeleton of dermal scales, denticles or bony plates (in Placodermi) covering body surface. Placoid in Chondrichthyes and ganoid, cycloid or ctenoid in Osteichthyes.
5. Endoskeleton is cartilaginous or bony. The notochord in usually replaced by vertebrae, either bone or cartilage. Presence of well-developed skull and a system of visceral arches, of which the first pair forms the upper and lower jaws, the latter movably articulated with the skull.
6. Muscles arranged into segments called myotomes, with separate dorsal and ventral parts.
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7. Alimentary canal with definite stomach and pancreas and terminates into cloaca or anus.
8. Organs of respiration are gills. Gill-slits 5 to 7 pairs, naked or covered by an operculum.
9. Heart is venous and two chambered, i.e., one auricle and one ventricle. Sinus venosus and renal and portal systems present. Erythrocytes nucleated. Poikilothermous.
10. Kidneys mesonephros. Excretions ureotelic.
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11. Brain with usual five parts. Cranial nerves ten pairs.
12. Nostrils are paired but do not open into pharynx except Dipnoi. Nasal capsules are partly separate in Chondrichthyes and completely separate in Osteichthyes.
13. Tympanic cavity and ear ossicles are absent.
14. Internal ear with three semicircular canals.
15. Lateral line system is well developed.
16. Sexes separate. Gonads typically paired. Gonoducts open into cloaca or independently.
17. Fertilisation internal or external. Females of Chondrichthyes are oviparous or ovoviviparous and of Osteichthyes are mostly oviparous and rarely ovoviviparous or viviparous. Eggs with large amount of yolk. Cleavage meroblastic.
18. Extra-embryonic membranes are absent.
19. Development usually direct without or with little metamorphosis.
Classification:
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About 40,000 species of fishes are known. Various workers have proposed different schemes of classification of fishes. However, no classification has been universally accepted because of confusion due to large number of fishes and great diversity in their shape, size, habits and habitat.
J. Muller (1844) gave first classification of the lower vertebrates and divided Pisces into six subclasses:
1. Dipnoi
2. Teleostei,
3. Ganoidei,
4. Elasmobranchii,
5. Marsipobranchii (Cyclostomi) and
6. Leptocardii (Amphioxini).
He included cyclostomes in Marsipobranchii and cephalochordates in Leptocardii. Subsequently. Agassiz (1857) separated lampreys and hagfishes into a separate class Myzontes and divided fishes into four classes. Boulenger (1924) classified the teleostean fishes into thirteen orders. Regan (1906-1929) gave an elaborate classification of fishes in his several papers. Jordan (1923) divided fish-like vertebrates into six classes. Among the more recent authors Goodrich (1930), Berg (1940), Grasse (1958) and Romer (1959) have given a detailed classification of fishes.
Romer (1959) classified the fishes into two classes- Chondrichthyes (includes all cartilaginous fishes) and Osteichthyes (includes all bony fishes Dipnoi and Teleostomi). Osteichthyes is divided into two subclasses- Sarcopterygii and Actinopterygii. Parker and Haswell (1962) have further combined all the extinct jawed fishes under a single class- Placodermi or Aphetohyoidea.
This simple division of superclass Pisces into three classes- Placodermi, Chondrichthyes and Osteichthyes, has been followed more or less by all the eminent authors. This scheme of classification with slight modifications because of its simple and direct approach to our readers, while ignoring in other merits and demerits.