Characters of Gnathostomata and Fishes

General Characters of Gnathostomata

  • Two well formed jaws encircle the mouth aperture in all members of this group.
  • Paired and in some cases jointed appendages are present in Gnathostomata.
  • Calcified, bony skull and vertebra are characteristic features of Gnathostomata.
  • All fishes, amphibians, reptiles, birds and mammals are members of this group.

Classification of Gnathostomata

The classification of gnathostomes is a major field of controversy among authors. Authors differ greatly in the way they arrange the main divisions of the Gnathostomata. The classificatory scheme of Gnathostoma adopted here is that of J.Z. Young (1981).


Living fishes with jaws mostly fall into two well marked classes, the cartilaginous fishes (Chondrichthyes) and the bony fishes (Osteichthyes) including the familiar ray- finned fishes and the lung fishes. These two groups arose in the late Devonian period of geological time scale. Before that time various other types of fish dominated the waters. Classificatory scheme of fishes by J. Z. Young (1981) is shown.

Classification of Gnathostomata

General characteristic features of superclass Pisces (Fish)

  • They are the first jawed vertebrates and the study of fish is called Ichthyology.
  • They are aquatic (may be fresh water or marine).
  • Fish are cold blooded animals (poikilothermic).
  • Their body is spindle shaped or streamlined to enable them to move rapidly in water.
  • Body is covered with water proof dermal scales; which may be of placoid, cycloid, ctenoid or ganoid type.
  • Fins are the locomotary organs. Pectoral and pelvic fins are paired whereas dorsal and caudal fins are unpaired.
  • Eyes are without eyelids. External and middle ears are absent; only internal ear is present.
  • They consist of 4-7 pairs of gills for respiration which may be naked or covered with an operculum.
  • Heart is two chambered, known as a venous heart. Heart consists of only the impure blood.
  • Circulation of blood is unicircuit with the presence of renal and hepatic portal
  • RBCs are nucleated.
  • Endoskeleton is made up of either cartilage or true bones.
  • Vertebrae in fish are amphicoelus, in which centrum is concave at both the surfaces.
  • Skull is monocondylice with only one occipital condyle.
  • 10 pairs of cranial nerves are present.
  • They have a lateral line sense organ system consisting of rheoreceptors.
  • They may be ammonotelic or ureotelic with mesonephric
  • Fish are unisexual. Fertilization may be internal or external.
  • Eggs are of mesolecithal or megalecithal