Animal kingdom | Part -4

Phylum Chordata and Its Divisions

Phylum Chordata represents one of the most highly evolved and diverse groups in the animal kingdom. Organisms within this phylum share key anatomical features at some point in their life cycle, making them distinct from all other animals.

What Are Chordates?

Chordates are animals that, during some stage of their development, possess the following four fundamental features:

• Notochord: A flexible, rod-like structure that provides skeletal support.
• Dorsal hollow nerve cord: Located above the notochord; develops into the brain and spinal cord in vertebrates.
• Pharyngeal slits: Openings in the pharynx that may develop into gills or other structures.
• Post-anal tail: An extension of the body beyond the anus, which may be present or reduced in adults.

Divisions of Phylum Chordata

Chordates are divided into three major subphyla based on how and when these features are expressed:

1. Urochordata (Tunicata)

• Marine and mostly sessile as adults.
• Notochord present only in the larval tail.
• Adults lose many chordate features.
• Body covered with a tunic made of tunicin.
• Examples: Ascidia, Salpa, Doliolum

2. Cephalochordata

• Marine, fish-like burrowing animals.
• Chordate features present throughout life.
• Notochord extends from head to tail.
• Example: Branchiostoma (also known as Amphioxus)

3. Vertebrata (Craniata)

• Most advanced chordates.
• Notochord partially or fully replaced by a vertebral column.
• Possess a well-developed brain encased in a cranium (skull).
• Highly developed organ systems.

Vertebrates are further divided into:

a. Agnatha (Jawless Vertebrates)

• Lack jaws and paired fins.
• Examples: Lamprey, Hagfish

b. Gnathostomata (Jawed Vertebrates)

• Possess jaws and paired limbs.
• Major classes include:
• Pisces: Shark, Rohu
• Amphibia: Frog, Salamander
• Reptilia: Snake, Lizard
• Aves: Eagle, Parrot
• Mammalia: Human, Tiger, Whale

Characteristics of Chordates

• Bilateral symmetry
• Triploblastic body plan
• Coelomate (true body cavity)
• Closed circulatory system
• Segmented body (in some)
• Mostly dioecious (separate sexes)

Phylum Chordata includes an extraordinary range of animals — from simple tunicates to complex mammals. Despite their diversity, they all share a common set of features that reveal a shared evolutionary origin. Chordates include animals that live in oceans, fly in the skies, and walk on land, making them central to the study of biology.