Unit 2: Introduction to Chordates and Protochordates

1. Phylum Chordata
- 1.1 General Characteristics
- 1.2 Outline Classification
2. Protochordates (Acraniata)
3. Key Concepts in Chordate Evolution
- 3.1 Retrogressive Metamorphosis in Urochordata
- 3.2 Advanced Features of Vertebrates over Protochordata

1. Phylum Chordata

1.1 General Characteristics

Chordates are a diverse phylum of animals that includes vertebrates (fish, amphibians, reptiles, birds, mammals) and several groups of invertebrates. All chordates, at some point in their life cycle, possess four key diagnostic features.

The Four Diagnostic Features of Chordates:

Notochord: A flexible, rod-like structure providing skeletal support. In most vertebrates, it is replaced by the vertebral column in adults.

Dorsal Hollow Nerve Cord: A tube of nerve tissue located on the dorsal (back) side of the animal. In vertebrates, this develops into the brain and spinal cord.

Pharyngeal Slits or Clefts: A series of openings in the pharynx (the region just behind the mouth). In aquatic chordates, they function as gills for respiration. In terrestrial vertebrates, they are modified into other structures (e.g., parts of the ear and jaw).

Post-anal Tail: A muscular tail that extends posterior to the anus. This is used for locomotion in many aquatic species and is reduced or modified in others.

Other General Characters:

Symmetry: Bilateral symmetry.
Germ Layers: Triploblastic.
Body Cavity: True coelomates (enterocoelous or schizocoelous).
Circulatory System: Closed circulatory system.

1.2 Outline Classification of Chordates

The phylum is broadly divided into two main groups based on the presence or absence of a cranium (skull).

Group	Subgroup/Phylum	Key Characteristics	Examples
Group 1: Acraniata (Protochordata) (No cranium, no true brain, no vertebral column)	Subphylum Hemichordata*	Worm-like; body in 3 parts: proboscis, collar, trunk. Possess gill slits. (Doubtful chordate status).	Balanoglossus (acorn worm)
	Subphylum Urochordata	"Tunicates" or "Sea Squirts." Sessile adults. Larva is free-swimming and has all 4 chordate features.	Ascidia, Herdmania
	Subphylum Cephalochordata	"Lancelets." Fish-like, marine. Notochord and nerve cord persist throughout life.	Amphioxus (Branchiostoma)
Group 2: Craniata (Euchordata) (Cranium and true brain present)	Subphylum Vertebrata	Notochord replaced by a vertebral column. Distinct head with sense organs.	Fish, Amphibians, Reptiles, Birds, Mammals

*Note on Hemichordata: The syllabus lists Hemichordata under Protochordates. However, modern classification often places Hemichordata as a separate, independent phylum, as their "notochord" (stomochord) is now considered non-homologous. For the exam, follow the syllabus but be aware of this controversy.

2. Protochordates (Acraniata)

Protochordates are an informal group of "lower" chordates. They are crucial for understanding the origin of vertebrates.

2.1 General Characteristics of Hemichordata

Exclusively marine, worm-like animals.
Body is divided into proboscis (for burrowing and feeding), a short collar, and a long trunk.
They have pharyngeal gill slits, a feature shared with chordates.
They have a dorsal nerve cord, but also a ventral one (an invertebrate feature).
The "stomochord" in the proboscis was once thought to be a notochord, but this is now disputed.
Larva is called a tornaria larva, which resembles the larva of echinoderms, suggesting a close evolutionary link.

2.2 General Characteristics of Urochordata (Tunicates)

Adults are sessile (attached) and enclosed in a leathery or gelatinous covering called a tunic.
The adult body is a "filter-feeding bag" and shows only one chordate feature: a large, perforated pharynx with gill slits.
The free-swimming larva (ascidian tadpole) is crucial, as it clearly displays all four chordate characteristics: notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.

2.3 General Characteristics of Cephalochordata (Lancelets)

Marine, burrowing, fish-like animals.
The notochord, dorsal nerve cord, and pharyngeal slits persist throughout life.
The notochord extends the entire length of the body, even into the head region (hence "cephalo-" chordata).
They are filter feeders, using cilia to draw water into the pharynx.
They show simple segmented muscles (myotomes), similar to vertebrates.
Often considered the "archetypal" chordate, showing the basic body plan.

3. Key Concepts in Chordate Evolution

3.1 Retrogressive Metamorphosis in Urochordata

This is a unique type of development where the larva is more advanced and active than the adult, which is simpler and degenerate.

Advanced Larva: The ascidian tadpole larva is free-swimming and has a well-developed notochord, dorsal nerve cord, and tail. Its purpose is to find a suitable place to settle.
Settlement: The larva attaches to a substrate using adhesive papillae.
Metamorphosis (The "Retrogressive" Part):
- The tail, notochord, and nerve cord are lost and resorbed.
- The body rotates, and the pharynx enlarges dramatically.
- The animal loses its mobility and ability to sense the environment, becoming a sessile, bag-like filter feeder.

Evolutionary Significance: This process is a classic example of adaptation to a sessile lifestyle, where the complex locomotory and sensory structures of the larva are no longer needed by the adult and are discarded to save energy.

3.2 Advanced Features of Vertebrates over Protochordata

Vertebrates (Craniata) evolved from a protochordate-like ancestor and developed a suite of new features that allowed them to become larger, more active, and more dominant.

Feature	Protochordates	Vertebrates
Cranium & Head	Absent; no distinct head.	Cranium (skull) present, protecting a large, complex tripartite brain (forebrain, midbrain, hindbrain).
Skeletal Support	Persistent notochord.	Vertebral column (backbone) made of cartilage or bone, which replaces the notochord in adults.
Sense Organs	Simple sensory cells.	Complex, paired sense organs (eyes, ears, olfactory organs) concentrated in the head (cephalization).
Feeding Mechanism	Filter feeding (ciliary pump).	Active predation, supported by a muscular pharynx and (in most) jaws.
Circulatory System	Simple, often with a basic heart or contractile vessels.	Ventral, chambered heart (2, 3, or 4 chambers) and red blood cells with hemoglobin for efficient oxygen transport.
Excretory System	Simple protonephridia.	Paired kidneys for efficient osmoregulation and excretion.

Key Evolutionary Shift: The primary evolutionary shift was from a passive, filter-feeding lifestyle (seen in protochordates) to an active, predatory lifestyle (seen in early vertebrates). This shift drove the evolution of a better brain, sharper senses, a protective skull, and a powerful, mobile body.