In this article we will discuss about the classification of phylum chordata.
Classification of Phylum Chordata:
1. The chordates are bilaterally symmetrical, triploblastic, coelomate animals.
2. The notochord or chorda dorsalis is present either in the young stage or throughout life. In the higher chordates, this is replaced by a bony vertebral column at a later stage.
ADVERTISEMENTS:
3. The wall of the pharynx is perforated by a system of clefts, the branchial or gill clefts, either in the embryonic or larval condition only or throughout life.
4. A hollow dorsal nerve cord or spinal cord is always present. A cavity or a system of cavities, the neurocoel, is present within the central nervous system.
The phylum Chordata has been divided into three subphyla:
(A) Subphylum Urochordata:
ADVERTISEMENTS:
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.
3. The larval structures degenerate during retrogressive metamorphosis and are not found in the adult.
4. The ventral heart is tubular.
ADVERTISEMENTS:
Examples: Ascidia, Ciona, etc.
(B) Subphylum Acrania (Cephalochordata):
1. Lack distinct head, cranium, brain, eyes, auditory organs and jaws.
2. The nerve cord is dorsal and tubular.
3. Notochord is persistent and un-segmented and extends along the whole length of the body.
4. Gill slits are numerous and open in the atrium.
5. The body wall consists of over fifty myotomes.
6. Digestive, blood vascular and nervous system resemble those of the vertebrates.
7. The larval form is asymmetrical and the larval period is long.
ADVERTISEMENTS:
Example: Branchiostoma.
(C) Subphylum Craniata (Vertebrata):
1. Notochord does not proceed anteriorly beyond forebrain.
2. Pharynx perforated by not more than seven pairs of gill slits, except in some cyclostomes.
3. Heart muscular and chambered.
4. Renal tubules unite to form paired kidneys opening in ducts.
5. The complex brain is enclosed in a cranium.
6. Brain well-developed, cranial nerves at least ten pairs.
7. Sense organs paired and well-developed.
The subphylum Craniata has been divided into two super-classes:
a. Superclass 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. Examples: Petromyzon (Lampreys), Myxine, (Hag fishes,) etc.
b. Superclass Gnathostomata:
1. The mouth is provided with two jaws, the upper and lower.
2. Two pairs of limbs are present.
This superclass has been subdivided into following major classes:
Class PlacodermI (Aphetohyoidea) (Extinct):
1. Primitive Gnathostomata in which the hyoid arch persists as an effective branchial arch, preceded by a hyoid gill cleft with functional gill.
2. Jaws are attached directly to the cranium by ligaments.
Examples: Climatius, Dinichthys, etc.
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: Laticaudas, (Scoliodon), Skates, Rays, etc.
Class Osteichthyes (Teleostomi):
Fishes with bony skeleton.
Subclass Actinopterygii:
1. The skull is hyostylic, the jaws being supported by the hyomandibular arch.
2. Scales in the majority are cycloid and fins with fin rays.
3. Operculum always present, gill filaments project from the gill arches.
Examples: Channa, Labeo, Puntius, Clarias, etc.
Subclass Crossopterygii:
1. Lung partially used as respiratory organ.
2. Nostrils are placed within the mouth.
3. Scales cosmoid.
Examples: Latimeria, Dipnoi (lung fishes).
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 conusarteriosus.
4. A metamorphosis is usually present.
Examples: Rana, Bufo, 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 shell; embryo protected by amnion and allantois.
Examples: Lacerta, Calotes, Naja, etc.
Class Aves:
1. Warm-blooded, oviparous, feathered bipeds, forelimbs of which are modified as wings.
2. The skull is light with obliterated sutures and without teeth. A single occipital condyle is present.
3. The pectoral muscles are large and used in flight.
4. Air sacs are present, which continue to air spaces in some bones.
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. Heterodont and thecodont dentition.
6. The single, aortic arch curves to the left side.
7. Viviparous, and the embryos are nourished through placenta (except monotremes).
Examples: Cavia, Homo erectus (extinct), Homo sapiens, etc..