The Gnathostomata includes all vertebrates with upper and lower jaws. It comprises a wide range of animals, from fish to the various tetrapod classes, which have in turn been derived from a fish or fish-like ancestor. An entirely satisfactory classification of fishes is not easy to construct, because of the incompleteness of several parts of the fossil record, and the consequent lack of connecting links.
Complete agreement as to their arrangement, therefore, has not been reached, and the student will meet with a considerable choice of classifications. It is at least certain that within the old group of Pisces (fishes in the widest sense of the expression) there are fundamental divisions which are of great antiquity. Animals of these divisions appear to have little to do with one another beyond the possession of a possible, but unknown common ancestor at a very remote period. The old classification of the Gnathostomata into five classes-Pisces, Amphibia, Reptilia, Aves, and Mammalia-has therefore to be modified by the division of the fishes.
CLASSIFICATION OF GNATHOSTOMATA
The old classification of the Gnathostomata into five classes – Pisces, Amphibia, Reptilia, Aves, and Mammalia-has therefore to be modified by the division of the fishes. Here these will be divided into three classes, as follows :-
1. CLASS PLACODERMI (APHETOHYOIDEA) :- All the fishes of this group are extinct. Their epidermis were covered by heavy bony plates. Instead of gill slits they possessed spiracles for respiration.
2. CLASS ELASMOBRANCHII (CHONDRICHTHYES, SELACHII) :- Most of the members are marine and are carnivorous, Epidermis is covered by closely set placoid scales, Endoskeleton is cartilaginous, Median and paired fins are present. Fins are supported by cartilaginous fin rays, Generally operculum is absent in these fishes, instead, gill slits are present, Mouth is sub-terminal in position, Jaws are armoured with rows of sharp teeth. These teeth are modified placoid scales.
3.CLASS OSTEICHTHYES :- Small mouth aperture is guarded by jaws and lips, Plate like teeth firmly attached with the jaws. Holostylic upper jaw, i.e. rigidly attached to the skull, Gill slit partially covered by operculum, Cloaca absent, i.e., anus and urinogenital aperture separate, Skin is naked in adults, In addition to pelvic clasper males possess another clasper on the head, Spiracle absent. Examples :- Chimaera (Rat fish)
CHARACTERS OF GNATHOSTOMATA
Compared with the tetrapods the three classes mentioned above have certain characters in common. They have organs of respiration and locomotion related to a permanently aquatic life. The chief, and generally the only, respiratory organs are the gills. These are in the form of a series of vascular processes attached to the septa of the branchial arches and persistent throughout life.
The organs of locomotion are the paired pectoral and pelvic fins and the unpaired dorsal, anal, and caudal fins. These fins are all supported by fin-rays of dermal origin, in addition to the endoskeletal supports. The fin-rays are of diagnostic significance. A dermal exoskeleton is typically present, but is occasionally secondarily lost. In the endoskeleton the notochord is usually partly replaced by vertebræ, either of bone cartilage. There is a well-developed skull and a system of visceral arches, of which the first pair form the upper and lower jaws, the latter movably articulated with the skull. Both normally bear teeth.
The first pair of visceral arches mentioned above are actually the second pair, because primitively there was a premandibular arch that is represented in the Agnatha (p. 164; and see Petromyzon. There is no middle ear and no allantoic bladder, the latter structure arising for the first time in the Amphibia. These two negative characters are therefore also diagnostic.
There are other characters found in one or other of the three classes which, though highly characteristic when they occur, cannot be called diagnostic. It is the unequal distribution of these characters that makes the division of fishes into four classes desirable. A swim-bladder , for instance, is normally present (though it may be secondarily lost) in all fishes except elasmobranchs, which never at any time possessed one. In the placoderms and other extinct groups the presence of the swim-bladder can only be surmised.
The nasal capsules open by inhalent and exhalent apertures. These may be only partly separate, as in the elasmobranchs, or completely separate, as in the Actinopterygii, where they are both dorsal in position. In the Crossopterygii one pair is external, the other internal in the mouth, although in some very ancient forms the internal nares have been lost. The condition of the circulatory system, the structure of the brain, the urinogenital organs, fins, and scales all yield characters which help to diagnose the various groups.
The kidney is a mesonephros with an occasional persistence of a few pro nephric tubules (p. 153). As in all gnathostomes, there are three semicircular canals.
For convenience, some taxonomists further divide Gnathostomata into two superclasses. All the fishes and fish-like aquatic gnathostomes are placed in the superclass Pisces, whereas all the four-footed terrestrial gnathostomes in the superclass Tetrapoda. Their contrasting features are as follows :
Superclass 1. Pisces (L. piscis, fish)
- Fishes or fish-like aquatic forms with paired as well as median fins, gills and scaly skin.
Class 1. Placodermi
- Several extinct orders of primitive earliest jawed fishes of Palaeozoic with bony head shield movably articulated with trunk shield. Placoderms. Climatius, Dinichthys.
Class 2. Chondrichthves. (Gr. chondros, cartilage; ichthys, fish)
- Mostly marine. Cartilaginous endoskeleton. Skin with placoid scales. Gill-slits not covered by operculum. Pelvic claspers in male. Cartilaginous fishes. Approximately 600 species. Scoliodon (dogfish), Chimaera (ratfish).
Class 3. Osteichthyes. (Gr. osteon, bone; ichthys, fish)
- Freshwater and marine. Endoskeleton mostly bony. Skin having various types of scales (cycloid, ctenoid) other than placoid. Gill-slits covered by operculum. Males without claspers. Bony fishes. 20,000 species. Labeo (rohu), Protopterus (lungfish), Hippocampus (sea horse).
Superclass 2. Tetrapoda (Gr. tetra, four, podos, foot)
Land vertebrates with two pairs of pentadactyle limbs, cornified skin and lungs.
Class 1 Amphibia. (Gr. amphi, both; bios, life)
- Larval stage usually aquatic and breathes by gills. Adult typically terrestrial and respires by lungs. Skin moist, glandular and with no external scales. Heart 3-chambered. Amphibians. Approximately 2,500 species. Rana (frog), Bufo (toad), Ambystoma (salamander).
Class 2. Reptilia. (L. reptilis, creeping)
- Terrestrial tetrapods. Skin dry, covered by ectodermal horny scales or bony plates. Heart incompletely 4-chambered. Cold-blooded. Respiration by lungs. 7,000 species. Hemidactylus (wall lizard), Uromastix (spiny-tailed lizard), Naja (cobra), Sphenodon, Crocodilus (crocodile).
Class 3. Aves. (L. avis, bird)
- Typically flying vertebrates covered with feathers. Foreilmbs modified into wings. No teeth in beak. Heart 4-chambered. Warm blooded. Birds. About 9,000 species. Struthio (African ostrich), Columba (pigeon), Gallus (fowl)
Class 4. Mammalia. (L. mamma, breast)
Body covered by hair. Skin glandular. Female with mammary glands which secrete milk for suckling the young. Heart 4-chambered. Warm blooded, air breathing vertebrates. 4,500 species. Echidna (spiny anteater), Macropus (Kangaroo), Rams (rat), Homo (man).