Phylum Chordata: Phylum Chordata belongs to the Kingdom Animalia and includes all the vertebrates, i.e., animals with a backbone, and several invertebrates, i.e., organisms without a backbone. They possess a bilaterally symmetrical body and are divided into three different sub-phyla.

Chordates show four features, at different stages in their life. They are:

• Notochord– It is a longitudinal rod that is made of cartilage and runs between the nerve cord and the digestive tract. Its main function is to support the nerve cord. In Vertebrate animals, the vertebral column replaces the notochord.
• Dorsal Nerve Cord – This is a bundle of nerve fibres which connects the brain to the muscles and other organs.
• Post-anal tail – This is an extension of the body beyond the anus. In some chordates, the tail has skeletal muscles, which help in locomotion.
• Pharyngeal slits–They are the openings which connect the mouth and the throat. These openings allow the entry of water through the mouth, without entering the digestive system.

Characteristics of Phylum Chordata

• Animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.
• They are bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation.
• They possess a post anal tail and a closed circulatory system.
• Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata.
• Subphyla Urochordata and Cephalochordata are often referred to as protochordates and are exclusively marine.
• In Urochordata, notochord is present only in larval tail, while in Cephalochordata, it extends from head to tail region and is persistent throughout their life.
• Examples: Urochordata – Ascidia, Salpa, Doliolum; Cephalochordata – Amphioxus or Lancelet.

Classification of Phylum Chordata

Phylum Chordata is classified into three subphyla, namely:

1. Urochordata (tunicates),
2. Cephalochordata (lancelets)
3. Vertebrata (vertebrates).

The subphylum Urochordata and Cephalochordata are collectively known as protochordates, which are marine animals. They are invertebrates but they share attributes of chordates.

Urochordata

• The adults are fixed to the substratum.
• It is also known as Tunicate because the body of an adult is enclosed within a tunic made up of cellulose-like substance known as tunicin.
• Notochord can be seen only in the larval stage and disappears in adults.
• The nerve cord present in larva is replaced by a dorsal ganglion in adults.
• The larva can move and undergoes a metamorphosis.

For e.g., Ascidia, Salpa, Doliolum.

Cephalochordata

• The atrium is present.
• Motile adult and larval stage.
• The tail is present throughout life.
• They show progressive metamorphosis.
• The notochord is found throughout life.
• Numerous well-developed pharyngeal gill slits are present.

For e.g., Lancelets possess the notochord and nerve cord throughout their life. However, they lack the brain and bony vertebral column like Branchiostoma.

Vertebrata

The characteristic features of vertebrates include:

• These are advanced chordates and have cranium around the brain.
• The notochord is replaced by a vertebral column in adults. This is why it is said that ‘all vertebrates are chordates but all chordates are not vertebrates’.
• A high degree of cephalization is observed.
• The epidermis is multi-layered.
• They consist of three types of muscles-striped, unstriped and cardiac.
• They have a well-developed coelom.
• The alimentary canal is complete.
• The heart is three or four-chambered.
• They have well-developed respiratory and excretory systems.
• Endocrine glands are present in all.
• They are unisexual and reproduce sexually, hagfish being an exception.

For e.g., humans.

Subphylum Vertebrata is further classified into seven classes. They are:

1. Cyclostomata
2. Chondrichthyes
3. Osteichthyes
4. Amphibia
5. Reptilia
6. Aves
7. Mammalia

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By Team Learning Mantras