Phylum Chordata|Class 11 biology|Chapter no 10|Kingdom Animalia

Phylum Chordata – Detailed Explanation

Introduction: Phylum Chordata is one of the most advanced and diverse animal phyla. Members of this phylum are commonly known as chordates. This group includes animals ranging from simple tunicates to highly complex vertebrates like humans. All chordates share a set of key characteristics at some stage in their life cycle, making this phylum unique and easily distinguishable from others.


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Key Characteristics of Chordates:

1. Notochord:

A flexible, rod-like structure that runs along the length of the body.

Present at least in the embryonic stage.

In vertebrates, it is replaced by the vertebral column (backbone).



2. Dorsal Hollow Nerve Cord:

Unlike the ventral nerve cords in invertebrates, chordates have a single, tubular nerve cord situated dorsally (on the back).

It develops into the brain and spinal cord in vertebrates.



3. Pharyngeal Gill Slits:

Openings in the pharynx region.

Used for filter-feeding or respiration.

In aquatic chordates, they develop into gills, while in terrestrial forms they may be modified or disappear.



4. Post-Anal Tail:

An extension of the body beyond the anus.

It helps in locomotion in aquatic species.

In humans and other animals, it may be reduced or vestigial.



5. Endostyle or Thyroid Gland:

The endostyle is a mucus-secreting structure involved in filter feeding in lower chordates.

In higher chordates, it evolves into the thyroid gland, involved in metabolism regulation.





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Classification of Phylum Chordata:

Chordata is divided into three subphyla:

1. Subphylum Urochordata (Tunicata):

Marine, sessile or free-swimming animals.

Notochord is present only in the larval tail.

Examples: Ascidia, Salpa, Herdmania.


2. Subphylum Cephalochordata:

Small, fish-like marine animals.

Notochord extends from head to tail and remains throughout life.

Example: Amphioxus (Branchiostoma).


3. Subphylum Vertebrata:

Most advanced chordates with a well-developed brain and backbone.

Notochord is replaced by the vertebral column in adults.

Subdivided into two groups:

Agnatha – Jawless vertebrates (e.g., Petromyzon, Myxine)

Gnathostomata – Jawed vertebrates, further divided into:

Pisces – Fish (bony and cartilaginous)

Amphibia – Frogs, toads, salamanders

Reptilia – Snakes, lizards, crocodiles

Aves – Birds

Mammalia – Mammals including humans





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General Features of Vertebrates:

Bilaterally symmetrical and triploblastic.

Coelomates with a true body cavity.

Closed circulatory system.

Highly developed sensory organs and nervous system.

Separate sexes and mostly sexual reproduction.



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Evolutionary Importance: Chordates are significant in evolution because they show the development of complex organ systems and advanced nervous coordination. Vertebrates, especially mammals, are considered the pinnacle of animal evolution due to their intelligence, adaptability, and complex behaviors.


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Conclusion: Phylum Chordata represents the most structurally and functionally advanced group of animals. With characteristics like the notochord, dorsal nerve cord, and post-anal tail, chordates exhibit a perfect blueprint for the evolution of complex organisms. From simple sea squirts to humans, this phylum tells the story of evolutionary advancement and biological success.
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