Classify Gnathostomes with suitable examples
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General Characters of Gnathostomata:
1. Two well formed jaws encircle the mouth aperture in all members of this group.
2. Paired and in some cases jointed appendages are present in Gnathostomata.
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3. Calcified, bony skull and vertebra are characteristic features of Gnathostomata.
4. 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).
Class — Chondrichthyes:
General characters:
1. Most of the members are marine and are carnivorous.
2. Epidermis is covered by closely set placoid scales.
3. Endoskeleton is cartilaginous.
4. Median and paired fins are present. Fins are supported by cartilaginous fin rays.
5. Generally operculum is absent in these fishes, instead, gill slits are present.
6. Mouth is sub-terminal in position.
7. Jaws are armoured with rows of sharp teeth. These teeth are modified placoid scales.
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8. A spiral valve is present in the intestine.
9. Swim bladder is absent.
10. Caudal fin is of heterocercal type, i.e. two lobes of the fin are unequal.
11. Unisexual fish, males having clasper for reproduction. Clasper is the modified organ from pelvic fins.
12. Internal fertilisation is the characteristic of these fishes.
1. Two well formed jaws encircle the mouth aperture in all members of this group.
2. Paired and in some cases jointed appendages are present in Gnathostomata.
ADVERTISEMENTS:
3. Calcified, bony skull and vertebra are characteristic features of Gnathostomata.
4. 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).
Class — Chondrichthyes:
General characters:
1. Most of the members are marine and are carnivorous.
2. Epidermis is covered by closely set placoid scales.
3. Endoskeleton is cartilaginous.
4. Median and paired fins are present. Fins are supported by cartilaginous fin rays.
5. Generally operculum is absent in these fishes, instead, gill slits are present.
6. Mouth is sub-terminal in position.
7. Jaws are armoured with rows of sharp teeth. These teeth are modified placoid scales.
ADVERTISEMENTS:
8. A spiral valve is present in the intestine.
9. Swim bladder is absent.
10. Caudal fin is of heterocercal type, i.e. two lobes of the fin are unequal.
11. Unisexual fish, males having clasper for reproduction. Clasper is the modified organ from pelvic fins.
12. Internal fertilisation is the characteristic of these fishes.
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