Chordate Origins and Invertebrate Chordates

Core Concepts

The Four Defining Chordate Characteristics

All chordates possess the following four features at some stage of their life cycle. In some groups (e.g., adult tunicates), one or more of these features may be modified or lost in the adult but are present in the embryo or larva.

Notochord. A flexible, longitudinal rod located between the digestive tract and the dorsal nerve cord. It is composed of a core of fluid-filled cells surrounded by a fibrous sheath. The notochord provides structural support (resisting compression) and serves as an attachment site for muscles. In vertebrates, the notochord is largely replaced by the vertebral column during development, though remnants persist as the gelatinous intervertebral discs. In lancelets and tunicate larvae, the notochord persists throughout life.

Dorsal hollow nerve cord. A tube of nervous tissue running along the dorsal (back) side of the body, above the notochord. It develops from a plate of ectoderm that rolls into a tube. In vertebrates, the anterior end develops into the brain, and the remainder becomes the spinal cord. This is the central nervous system of chordates. The dorsal, hollow structure is unique to chordates — the nerve cords of protostome invertebrates (e.g., the annelid and arthropod ventral nerve cords) are ventral and solid.

Pharyngeal slits (pharyngeal clefts). Openings in the pharynx (the region behind the mouth) that connect the pharyngeal cavity to the outside. In aquatic chordates, pharyngeal slits are used for filter feeding (lancelets, tunicates) or respiration (fish gills). In terrestrial vertebrates, pharyngeal slits are present in embryonic development (pharyngeal arches) and give rise to structures of the jaw, inner ear, and neck. The presence of pharyngeal slits in the embryo is a key piece of evidence for chordate (and broader deuterostome) common ancestry.

Post-anal tail. A tail that extends beyond the anus. The post-anal tail contains muscle segments and, in many chordates, is used for locomotion (swimming). In many terrestrial vertebrates, the post-anal tail is reduced (e.g., the human coccyx), but its embryonic presence is evidence of chordate identity.

Additional Chordate Features

• Endostyle (or its derivative, the thyroid gland): A ciliated groove in the pharynx floor of invertebrate chordates and lamprey larvae. The endostyle secretes mucus that traps food particles (in filter feeders) and concentrates iodine — homologous to the vertebrate thyroid gland.

• Segmented muscle blocks (myomeres): Arranged in chevron-shaped blocks along the body. Visible in lancelets and fish.

Cephalochordata: Lancelets

Lancelets (genus Branchiostoma, formerly Amphioxus) are small (3–7 cm), fish-like marine animals that live buried in sand with only their anterior end exposed. They retain all four chordate characteristics throughout life and represent the most vertebrate-like invertebrate chordates.

Lancelet body plan:

• The notochord extends the entire body length (including anterior to the nerve cord — a unique cephalochordate feature).

• The dorsal hollow nerve cord lies above the notochord, with a slight anterior swelling (cerebral vesicle) but no true brain.

• Pharyngeal slits are numerous and used for filter feeding: cilia draw water into the mouth, food particles are trapped on mucus produced by the endostyle, and water exits through the slits. The pharyngeal basket is enlarged, reflecting the animal’s reliance on filter feeding.

• Segmented myomeres produce the side-to-side swimming motion characteristic of fish — evidence that the chordate body plan was originally adapted for locomotion.

• The post-anal tail is well-developed and used in swimming.

Lancelets share with vertebrates: segmented muscles, a circulatory system with a contractile ventral aorta (but no true heart), and a basic excretory system of protonephridia-like structures. They lack: a cranium, a vertebral column, paired sensory organs, a true heart, and a brain. Their phylogenetic position — sister group to vertebrates, or sister to tunicates plus vertebrates — is an active area of research.

Tunicata: Tunicates (Sea Squirts)

Tunicates (subphylum Tunicata, traditionally called Urochordata — “tail-chordates”) are marine animals that are radically different as larvae and adults.

Larval tunicate: The larva is tadpole-like, free-swimming, and possesses all four chordate features. The notochord is restricted to the tail (hence Urochordata = “tail-chordate”). The larva does not feed — it swims briefly, settles on a substrate, and metamorphoses.

Adult tunicate: After metamorphosis, the adult becomes a sessile filter feeder attached to a substrate. Remarkably, the notochord, post-anal tail, and most of the dorsal nerve cord are resorbed or lost. The pharyngeal slits are greatly expanded into a pharyngeal basket — the adult’s primary feeding structure. Water enters through an incurrent siphon, passes through the pharyngeal slits (where food particles are trapped in mucus), and exits through an excurrent siphon. The body is enclosed in a tough outer layer — the tunic — composed of cellulose-like polysaccharides (tunicin), which is unusual among animals.

Tunicates have a simple circulatory system with a heart that periodically reverses the direction of blood flow — a unique feature among chordates. They are hermaphroditic and can reproduce both sexually (producing the tadpole larva) and asexually (budding in colonial species).

The Evolutionary Significance of Invertebrate Chordates

Invertebrate chordates demonstrate that the defining chordate features — notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail — evolved before the vertebral column and cranium. They also confirm that the chordate body plan was originally associated with a small, motile, filter-feeding organism. One hypothesis for vertebrate origins suggests that the vertebrate ancestor evolved through paedomorphosis — the retention of larval (motile, chordate-feature-bearing) characteristics into the adult stage, coupled with the evolution of a larger body and active predation. In this scenario, the tunicate larva represents the ancestral chordate body plan, and vertebrates evolved from an organism that never underwent the sessile adult transformation. Lancelets may represent a separate lineage that also retained the ancestral body plan. It is critical to remember that no living group is the direct ancestor of another — tunicates, lancelets, and vertebrates are all living lineages that diverged from common ancestors hundreds of millions of years ago.

ELI-10

Chordates — the phylum that includes us — are defined by four things: a flexible support rod (the notochord), a hollow nerve cord running along the back, slits in the throat area, and a tail that extends past the anus. Every animal with a backbone has these four features at some point in its life, even if only as an embryo.

But not all chordates have a backbone. The two groups that do not are lancelets and tunicates.

Lancelets look like tiny, simple fish. They keep all four chordate features their whole life. They burrow in sand and filter food from the water with their gill slits. They are the most vertebrate-like of the invertebrate chordates.

Tunicates (sea squirts) are the weird cousins. As larvae, they look like tiny tadpoles — they have a notochord, nerve cord, gill slits, and tail. They swim briefly, then attach to a rock and transform. The adult loses the notochord, nerve cord, and tail, and becomes a stationary filter feeder — a living water pump wrapped in a tough bag. It is one of the most dramatic metamorphoses in the animal kingdom: a mobile, chordate-featured larva turning into a sack with no brain to speak of.

The evolutionary story: the original chordate was probably a small, swimming, filter-feeding creature like a lancelet. Vertebrates kept the swimming lifestyle and added a backbone and a big brain. Tunicates took a different path — keep the larval body briefly for dispersal, then settle down and simplify.

ELI Example

Think of chordate evolution as three roommates who inherited the same family house. One roommate (lancelets) kept the house exactly as it was — small, simple, functional. Another (tunicates) renovated wildly — kept the plumbing (pharyngeal slits) but demolished the support beams (notochord) and rewired only the basics (reduced nerve cord). The third (vertebrates) kept the original floor plan but added a steel frame (backbone), a complex electrical system (brain and spinal cord), and built a much larger structure. All three share the original blueprint (the four chordate features), but they modified it in dramatically different directions. No one is the ancestor of another — they all inherited from the same architectural ancestor.

Do Not Confuse

• Notochord vs. Vertebral Column: The notochord is a flexible rod — the ancestral chordate skeletal support. The vertebral column is a series of cartilaginous or bony vertebrae — a derived vertebrate feature that largely replaces the notochord during development. A lancelet has a notochord but no vertebrae. A fish has both during early development.

• Cephalochordata vs. Tunicata vs. Vertebrata: Lancelets (Cephalochordata) retain all four chordate features as adults. Tunicates (Tunicata) have them as larvae, mostly lose them as adults. Vertebrates (Vertebrata) retain them and add a cranium and vertebral column.

• Tunicate Larva vs. Tadpole: Both are swimming chordates with tails, but a tadpole (amphibian larva) is a vertebrate with a cartilaginous skeleton, brain, and complex organs. A tunicate larva is an invertebrate with a notochord and simple nervous system. They are convergent in form, not closely related.

Lab Link

When observing invertebrate chordate specimens in the laboratory, examine a preserved lancelet to identify the notochord (the translucent, flexible rod visible through the body wall), the dorsal nerve cord (above the notochord), the pharyngeal slits (in the pharyngeal basket), and the segmented myomeres (chevron-shaped muscle blocks). Observe a preserved adult tunicate and note the incurrent and excurrent siphons, the pharyngeal basket, and the loss of the notochord and tail. If tunicate larvae are available, compare their chordate features with those of the adult.

High-Yield Memory Anchors

• Chordate 4: Notochord (flexible rod), Dorsal hollow nerve cord, Pharyngeal slits, Post-anal tail. All present at some life stage.

• Lancelet (Cephalochordata) = all 4 retained in adult. Tunicate (Tunicata) = all 4 in larva, mostly lost in adult. Vertebrate = all 4 + cranium + vertebral column.

• Tunicate metamorphosis: tadpole larva (chordate features) → sessile adult (pharyngeal basket, tunic).

• No living invertebrate chordate is the direct ancestor of vertebrates. They are sister lineages sharing common ancestry.

Quick Check

Q1: Which of the following is NOT one of the four defining chordate characteristics?

A) Notochord

B) Dorsal hollow nerve cord

C) Cranium

D) Post-anal tail

Q2: An adult tunicate lacks a notochord, dorsal nerve cord, and post-anal tail, yet it is classified as a chordate. Explain on what basis this classification is justified.

Q3: Compare the life-history strategies of lancelets and tunicates. How might the tunicate life cycle inform hypotheses about vertebrate origins?

Quick Check Answers

A1: C. Cranium. A cranium is a vertebrate feature, not a defining chordate feature. The four chordate characteristics are notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail.

A2: The classification is based on the tunicate larva. The free-swimming tadpole larva possesses all four chordate characteristics: notochord (in the tail), dorsal hollow nerve cord, pharyngeal slits, and post-anal tail. Phylogenetic classification is based on shared derived characters and evolutionary relationships, not solely on adult morphology. Molecular evidence confirms that tunicates are chordates — they are more closely related to vertebrates than to any non-chordate phylum. The loss of chordate features in the adult is a derived condition (secondary simplification), not evidence of non-chordate identity.

A3: Lancelets retain the motile, chordate-featured body plan throughout life — they are filter feeders that burrow in sand with their anterior end exposed. Tunicates have a biphasic life cycle: a motile, chordate-featured larva that disperses and a sessile adult that filters food. The hypothesis of vertebrate origins by paedomorphosis suggests that the vertebrate ancestor evolved from an organism that retained the larval chordate body plan (motile, notochord-bearing, dorsal nerve cord) into adulthood — effectively, a tunicate-like ancestor that never metamorphosed into the sessile adult form. In this scenario, the motile tunicate larva approximates the ancestral chordate body plan, and vertebrates represent the retention and elaboration of that larval body into a sexually mature adult through changes in developmental timing (heterochrony). Lancelets follow a different path — they retain the ancestral body plan without such a dramatic metamorphosis.

Chapter Summary

Chordates are defined by four features: notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail. Invertebrate chordates — lancelets (Cephalochordata) and tunicates (Tunicata) — demonstrate that these features evolved before the vertebral column. Lancelets retain all four features as adults. Tunicates have them as larvae but largely lose them during metamorphosis to a sessile adult. Both groups are sister lineages to vertebrates, not direct ancestors. The chordate body plan was originally associated with a small, motile, filter-feeding organism.

Common Mistakes

• “All chordates have a backbone.” The defining feature of chordates is the notochord, not the vertebral column. Lancelets and tunicates are chordates without backbones. The vertebral column is a vertebrate feature.

• “Lancelets are the ancestors of vertebrates.” Lancelets are a living sister lineage, not direct ancestors. They share a common ancestor with vertebrates. Their body plan approximates the ancestral chordate condition, but they have been evolving independently for hundreds of millions of years.

• “Tunicates are ‘simplified’ because they are ‘primitive.’” Adult tunicates are simplified relative to their larvae — this is secondary simplification, not an ancestral state. Their body plan is highly derived for a sessile, filter-feeding lifestyle.

• “Pharyngeal slits are gills.” In aquatic vertebrates, pharyngeal slits form gills. But the slits are present in all chordate embryos, including human embryos (as pharyngeal arches), where they give rise to jaw, inner-ear, and neck structures — not gills.