The characteristics of this phylum are as follows:
- Presence of notochord
- Presence of dorsal hollow (tubular) nerve cord.
- Presence of pharyngeal gill slits.
- Post anal tail is present.
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This phylum is classified into four sub-phylum. They are as follows:
Sub- phylum 1: Hemichordata
- Marine animals.
- Simple worm-like soft bodied animal.
- Remain buried under mud or sand along the sea shore.
- Body is divisible into three regions. They are proboscis, collar and trunk.
- Notochord is restricted in the anterior region. Also called buccal diverticulum due to its doubtful nature.
- Many pairs of gill slits are present.
- Nervous system is simple consisting of subepideural nerve plexus.
- Blood vascular system is simple.
- Sexes are separate. Fertilization external in sea water.
- Free swimming tornaria larvae present which resembles to echinoderm larva.
- Includes about 80 species.
- Examples: Balanoglossus, Cephalodiscus, Atubaria, etc.
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Sub-phylum 2: Urochordata (Tunicata)
- Gr. Uros- tail +chordate– notochord.
- Marine animals found in all parts of the sea and at all depths.
- Commonly called “ascidians” because of general appearance to askidian.
- Usually sedentary or some are free living.
- Called tunicate as the entire body is covered by tunic or test made of tunicine.
- Distal end with two openings called bronchial or oral pore and the atrial siphons.
- Notochord and nerve cord are found in tail region of tadpole larvae.
- Adult animals show perforated pharynx.
- Adult body degenerate, sac-like, unsegmented usually without tail.
- Heart is ventral, tubular and enclosed in pericardium.
- No locomotory organ.
- Hermaphrodite in nature.
- Fertilization cross and external.
- Development indirect.
- Asexual reproduction by budding.
- Includes about 2100 species.
- Examples: Herdmania, Salpa, Doliolum, etc.
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Sub-phylum 3: Cephalochordata
- Gr. Kephalo- head + Chorde– cord.
- Small fish like animals, live in the sand of sea shore.
- Notochord and nerve cord are persistent and extend along the body.
- Mostly sedentary.
- Nerve cord is tubular and dorsal.
- Pharynx is large and sac like perforated by numerous gill slits.
- Ciliary feeding animals.
- Definite coelom is present.
- Excretion through definite nephridia.
- Lacks distinct head and paired appendages.
- Body has trunk and tail.
- Circulatory system well developed, closed and without heart.
- Respiratory pigment is absent.
- Sexes are separate.
- Fertilization external in sea water.
- Development is indirect including a free swimming larva.
- No asexual reproduction.
- Includes only 15 species.
- Examples: Amphioxus (Branchiostoma), Asynnetron, etc.
Sub-phylum 4: Vertebrata (Craniata)
- Notochord replaced by vertebrae and called vertebral column or back bone.
- Brain inside cranium so called craniata as well.
- Body triploblastic and bilaterally symmetrical.
- Lower vertebrates aquatic, higher terrestrial.
- Body consists of head, trunk and post-anal tail.
- Two pairs of lateral appendages in trunk region.
- In some cases appendages are reduced or absent.
- Skin consists of two layers outer epidermis and inner dermis.
- One pair of kidneys as excretory organ.
- Circulatory system is closed type.
- Blood is red and hemoglobin is present in RBCs.
- Organ-system grade of body organization.
- Endoskeleton of cartilages or bones or both.
- Endocrine glands secreting hormone to regulate body function.
- Sexes are separate.
- Examples: Labeo, Scoliodon, Homo sapiens, etc.
Phylum: Chordata, its classification and characteristics