In this article we will discuss about the classification of Phylum Chordata.
1. Subphylum and Class Hemichordata:
1. The body is divisible into three regions, a proboscis, a collar and a trunk, the anterior end of which bears gill slits.
2. The nerve cord is dorsal and arises from the ectoderm along the middle line.
3. A notochord is always present.
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4. Coelomic pouches are present.
Examples – Balanoglossus, Cephalodiscus, etc.
2. Subphylum and Class Urochordata:
1. The adult animals are invested in thick cuticular tests and are mostly sedentary. They exhibit a divergence from the chordate type.
2. The dorsal nervous system, notochord, gill slits and brain-eye develop in the free-swimming larval stage.
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3. The larval structures degenerate during retrogressive metamorphosis and are not found in the adult.
4. The ventral heart is tubular.
Examples – Ascidia, Ciona, etc.
3. Subphylum and Class Cephalochordata:
1. The nerve cord is dorsal and tubular.
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2. Notochord is persistent and un-segmented.
3. Gill slits are numerous and open in the atrium.
4. The body wall consists of over fifty myotomes.
5. The larval form is asymmetrical and the larval period is long.
Example – Branchiostoma.
4. Subphylum Agnatha:
1. The mouth is round and suctorial and without jaws.
2. Paired fins and scales are absent. Unpaired fins are without fin rays.
3. Skeleton cartilaginous and the notochord persists.
4. Nasal organ unpaired, no sympathetic nervous system, no conus arteriosus, no distinct pancreas, no spleen, no genital ducts.
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Examples – Lampreys, Hag fishes, etc.
5. Subphylum Gnathostomata:
1. The mouth is provided with two jaws, the upper and lower.
2. Two pairs of limbs are present.
This subphylum has been subdivided into following major classes.
Class Chondrichthyes (Elasmobranchii):
1. Scales are placoid.
2. Skeleton entirely cartilaginous with no true bones.
3. The operculum is absent and the gill slits open directly to the exterior.
4. The venous system is expanded in places into large sinuses.
5. A spiral valve is present in the intestine.
Examples – Scoliodon, Skates, Rays, etc.
Class Aphetohyoidea (Extinct):
Primitive gnathostomata in which the hyoid arch persists as an effective branchial arch, preceded by a hyoid gill cleft with functional gill.
Jaws are attached directly to the cranium by ligaments.
Examples – Climatius, Dinichthys, etc.
Class Actinopterygii (Teleostomi):
1. Fishes with bony skeleton.
2. The skull is hyostylic, the jaws being supported by the hyomandibular arch.
3. Scales in the majority are cycloid and fins with fin rays.
4. Operculum always present, gill filaments project from the gill arches.
Examples – Channa, Labeo, etc.
Class Choanichthyes:
1. Lung partially used as respiratory organ.
2. Nostrils are placed within the mouth.
3. Scales cosmoid. Examples. Lung fishes, Dipnoi.
Class Amphibia:
1. The larva possesses gills and the adult bears lungs.
2. The skull has two occipital condyles.
3. Heart with one sinus venosus, two auricles, one ventricle and a conus arteriosus.
4. A metamorphosis is usually present.
Examples – Rana, Bufa, etc.
Class Reptilia:
1. The body is covered with epidermal scales.
2. The skull bears a single occipital condyle.
3. Heart with two auricles, one partly divided ventricle and a sinus venosus.
4. The eggs are large, land eggs with calcareous shells; embryo protected by amnion and allantois.
Examples – Lacerta, Calotes, Naja, etc.
Class Aves:
1. Warm-blooded, oviparous, feathered bipeds, fore limbs of which are modified as wings.
2. The pectoral muscles are large and used in flight.
3. Air sac’s are present, which continue to air spaces in some bones.
4. The skull is light with obliterated sutures and without teeth. A single occipital condyle is present.
5. Heart with two auricles and two ventricles. The single aortic arch curves to the right side.
Examples – Columba, Gallus, etc.
Class Mammalia:
1. The body is covered with hairs at least in the early stage.
2. Mammary glands are universally present.
3. A diaphragm separates the thoracic cavity from the abdomen.
4. The skull bears two occipital condyles, sutures are present.
5. Viviparous, and the embryos are nourished through placenta (except monotremes).
6. Heterodont and thecodont dentition.
Examples – Cavia, Homo sapiens, etc.