Chordates up to Class Level

The animal kingdom is vast and diverse, divided into multiple groups based on structural and functional characteristics. One of the most advanced and well-studied groups in the animal kingdom is the Phylum Chordata. The term Chordatacomes from the Greek words chorde (meaning “cord”) and ata (meaning “having”), referring to the presence of a notochord—a flexible, rod-like structure that supports the body.

1.0Introduction 

• The animal kingdom is incredibly diverse, with millions of species classified into various groups based on their anatomy and evolution. Among them, Chordates are among the most significant and advanced groups.
• Chordates are animals belonging to the Phylum Chordata, characterised by the presence of a notochord, dorsal hollow nerve cord, pharyngeal gill slits, and post-anal tail at some stage of their life. These key features differentiate them from non-chordates such as arthropods, molluscs, and annelids.
• In simple terms, Chordates are animals with a backbone, either in the form of a notochord or a vertebral column. Humans, fish, birds, reptiles, and mammals all belong to this group.
• Understanding the origin and classification of chordates up to the class level helps in studying the evolution and diversity of higher animals.

2.0Origin of Chordates

• The origin of chordates dates back to the early Cambrian period, around 540 million years ago. Fossil records suggest that chordates evolved from a group of invertebrate ancestors resembling modern echinoderms or hemichordates.
• Primitive chordates like Pikaia (a fossil found in the Burgess Shale) represent early forms with a notochord and nerve cord. Over time, these simple marine chordates gave rise to more complex vertebrates with well-developed skeletons, brains, and organ systems.
• The evolution from invertebrates to vertebrates marks one of the most crucial transitions in animal evolution, leading to the diverse and complex forms of life seen today.

3.0Chordates Characteristics

All members of the phylum Chordata share certain fundamental characteristics, either during embryonic development or throughout their life cycle.

1. Notochord

A flexible, rod-like structure located between the digestive tract and nerve cord. It provides skeletal support and is replaced by the vertebral column in higher chordates.

2. Dorsal Hollow Nerve Cord

Unlike non-chordates that have a solid, ventral nerve cord, chordates possess a hollow nerve cord on the dorsal (back) side. It later develops into the brain and spinal cord.

3. Pharyngeal Gill Slits

These are openings in the pharynx (throat area) used for filter feeding in aquatic chordates and respiration in fishes.

4. Post-Anal Tail

An extension of the body beyond the anus, which aids in locomotion in many chordates.

5. Endostyle or Thyroid Gland

The endostyle secretes mucus for trapping food particles in lower chordates and evolves into the thyroid gland in higher chordates, regulating metabolism.

These features collectively define what chordates are, forming the foundation for their classification.

4.0Chordates Classification

The classification of chordates up to class level is based on body structure, habitat, mode of reproduction, and degree of complexity.

1. Subphylum Urochordata (Tunicata)
2. Subphylum Cephalochordata
3. Subphylum Vertebrata (Craniata)

Each subphylum exhibits chordate features in different ways, and their evolutionary complexity increases progressively from urochordates to vertebrates.

1. Subphylum Urochordata (Tunicata)

Urochordates, commonly known as tunicates, are exclusively marine animals. The name “Tunicata” comes from the tunic-like covering (made of tunicin, a cellulose-like substance) around their body.

Features:

• The notochord and nerve cord are present only in the larval stage and disappear in adults.
• Adults are sessile (fixed) and often attached to rocks or surfaces in the sea.
• The larvae are free-swimming and resemble tadpoles, showing clear chordate characteristics.
• Adults feed by filter feeding, drawing water through a siphon.

Examples:

• Ascidia
• Salpa
• Doliolum

These organisms demonstrate the transition from invertebrate-like to vertebrate-like features, representing a key evolutionary stage.

2. Subphylum Cephalochordata

Cephalochordates are small, fish-like marine animals that exhibit all the chordate characteristics throughout their life. They are considered the true representatives of the basic chordate body plan.

Features:

• The notochord extends the entire length of the body and persists throughout life.
• The dorsal nerve cord lies above the notochord.
• They possess pharyngeal gill slits used for filter feeding.
• They lack a well-defined head and a true heart.
• The body is elongated and laterally compressed.

Example:

• Branchiostoma (also known as Amphioxus or Lancelet)

Cephalochordates serve as a connecting link between the primitive urochordates and the more advanced vertebrates.

3. Subphylum Vertebrata (Craniata)

The subphylum Vertebrata represents the most complex and advanced group of chordates. Vertebrates are characterized by a backbone or vertebral column that replaces the notochord during development.

Features:

• The notochord is replaced by a vertebral column in adults.
• A well-developed brain is enclosed within a bony or cartilaginous skull (cranium).
• They possess complex organ systems such as a closed circulatory system, advanced nervous system, and efficient respiratory and excretory systems.
• Most vertebrates have paired limbs for locomotion.

The subphylum Vertebrata is further divided into two divisions:

1. Agnatha (jawless vertebrates)
2. Gnathostomata (jawed vertebrates)

5.0Division 1: Agnatha

Agnatha includes primitive, jawless fishes. These are the most ancient vertebrates, lacking both jaws and paired fins.

Features:

• The mouth is circular and jawless.
• The body is elongated and eel-like.
• Skeleton is made of cartilage.
• No paired appendages (fins).
• They are parasitic or scavengers.

Examples:

• Petromyzon (Lamprey)
• Myxine (Hagfish)

Agnathans are often referred to as Cyclostomes due to their round mouth.

6.0Division 2: Gnathostomata

Gnathostomes are jawed vertebrates. This division includes the majority of living vertebrate species. Gnathostomata is further divided into Pisces (fishes) and Tetrapoda (four-limbed vertebrates).

Superclass Pisces

Pisces includes all fishes, which are aquatic, cold-blooded vertebrates that breathe through gills and move with fins.

Key Features:

• Body is streamlined, covered with scales.
• Respiration occurs through gills.
• Heart is two-chambered.
• Cold-blooded (ectothermic).
• Reproduction is usually by external fertilization.

Classes under Pisces:

1. Class Chondrichthyes (Cartilaginous fishes)
2. • Skeleton made of cartilage.
   • Examples: Shark, Scoliodon, Torpedo, Trygon.
2. Class Osteichthyes (Bony fishes)
3. • Skeleton made of bone.
   • Have an air bladder for buoyancy.
   • Examples: Labeo (Rohu), Catla, Clarias (Magur).

Class Amphibia

Amphibians live both on land and in water.

Examples: Frog, Toad, Salamander, Ichthyophis.

Key Features:

• Skin is smooth and moist (without scales).
• Respiration through gills, lungs, and skin.
• Heart has three chambers.
• Cold-blooded.
• External fertilization and aquatic larval stage.

Class Reptilia

Reptiles are fully adapted to life on land.

Examples: Lizard, Snake, Turtle, Crocodile.

Key Features:

• Dry, scaly skin.
• Breathe through lungs.
• Heart mostly three-chambered, except in crocodiles (four-chambered).
• Internal fertilization with shelled eggs.
• Cold-blooded.

Class Aves (Birds)

Aves are warm-blooded, feathered vertebrates adapted for flight.

Examples: Pigeon, Crow, Parrot, Ostrich.

Key Features:

• Forelimbs modified into wings.
• Body covered with feathers.
• Bones are hollow and light.
• Four-chambered heart.
• Oviparous (egg-laying).
• Warm-blooded.

Class Mammalia

Mammals are the most advanced chordates.

Examples: Human, Cow, Elephant, Bat, Whale.

Key Features:

• Skin covered with hair.
• Presence of mammary glands for feeding young.
• Four-chambered heart.
• Warm-blooded.
• Give birth to young ones (viviparous, mostly).