Complete Exam Guide Subject-wise practice platform
Zoology Quiz

Larva of Cephalochordata

Cephalochordata is a subphylum of Chordata that includes small, fish-like marine invertebrates commonly known as lancelets or amphioxus. There are roughly 30 described species grouped mainly under the genera Branchiostoma and Asymmetron. Cephalochordates are among the closest living invertebrate relatives of vertebrates and retain all four defining chordate features throughout life: a notochord extending from head to tail, a dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail. Their larval stage is of special interest because the larva is asymmetrical, free-swimming, and planktotrophic, and it undergoes a distinct metamorphosis before settling into the adult burrowing lifestyle. This guide covers the general biology of cephalochordates, the structure and development of the larva, metamorphosis, and their evolutionary significance.

General Characteristics of Cephalochordata

Cephalochordates are small, elongated, lance-shaped marine animals that typically measure a few centimetres in length. They inhabit shallow coastal waters in warm temperate and tropical seas worldwide, where they bury themselves in sandy or muddy substrates with the anterior end exposed for feeding. Their body is laterally compressed and tapered at both ends, earning them the common name lancelets.

The body wall consists of a single-layered epidermis. Body muscles are arranged in V-shaped blocks called myotomes or myomeres, separated by connective tissue sheets called myosepta. Cephalochordates possess a low continuous dorsal fin but lack paired fins and jaws. They are the only invertebrate chordates that retain all four chordate features throughout adult life: a notochord made of striated muscle cells extending from head to tail, a dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail.

  • Small marine invertebrates, a few centimetres long, found in shallow tropical and temperate coastal waters
  • Body is lance-shaped, laterally compressed, and tapered at both ends
  • Muscles arranged in V-shaped myotomes separated by myosepta
  • Retain all four chordate features (notochord, nerve cord, pharyngeal slits, post-anal tail) throughout life
  • About 30 species in genera Branchiostoma, Asymmetron, and Epigonichthys

Feeding, Circulation, and Excretion

Cephalochordates are ciliary filter feeders. Water enters through the mouth, which is surrounded by an oral hood bearing filament-like projections called buccal cirri that filter out large particles. A wheel organ inside the oral hood creates water currents that direct food-laden water toward the pharynx. The endostyle, a glandular groove running along the floor of the pharynx, secretes mucus that traps microscopic food particles. Ciliated tracts then transport the mucus-bound food toward the intestine for digestion.

The circulatory system is closed (blood flows through vessels) and resembles the vertebrate plan, but there is no heart. A contractile ventral aorta and the pulsation of blood vessels drive circulation. The blood is colourless and lacks respiratory pigment; gas exchange occurs directly across the thin body wall and pharyngeal surfaces. Excretion is carried out by protonephridia equipped with solenocytes, flame-cell-like structures of ectodermal origin that handle waste removal and osmoregulation.

  • Filter feeders using cilia, buccal cirri, wheel organ, and the mucus-secreting endostyle
  • Closed circulatory system with no heart and colourless blood lacking respiratory pigment
  • Gas exchange occurs across the body wall and pharyngeal surfaces
  • Excretion via protonephridia with solenocytes

Reproduction and Early Development

Cephalochordates are dioecious, meaning sexes are separate. They possess multiple pairs of ductless gonads arranged metamerically (in segments) along the body. In Branchiostoma the gonads are present on both sides of the body, while in Asymmetron they are found only on the right side. Reproduction is by external fertilisation: males and females release sperm and eggs into the surrounding water, usually during evening hours triggered by light cues.

The fertilised egg undergoes holoblastic (complete) and roughly equal cleavage. Development proceeds through the blastula and gastrula stages. The embryo develops into a free-swimming, transparent, planktotrophic larva that feeds on phytoplankton and small organic particles in the water column. Development from fertilisation to a feeding larva takes roughly one to two days depending on species and temperature.

  • Dioecious with metamerically arranged ductless gonads
  • External fertilisation; spawning often occurs in the evening
  • Holoblastic and roughly equal cleavage of the fertilised egg
  • Development through blastula and gastrula to a free-swimming larva within one to two days

Larval Structure and Asymmetry

The cephalochordate larva is one of the most distinctive invertebrate larvae because of its pronounced left-right asymmetry. The mouth opens on the left side of the body, and the first gill slits form as a single row on the right side of the pharynx. As the larva grows, additional gill slits are added, and later a second row forms on the opposite side. The anus is positioned on the left side. Parts of the nervous system and the pharyngeal organs also show asymmetric positioning.

The larva is transparent and elongated, possessing a notochord, a dorsal nerve cord, a post-anal tail, and myotomes that allow it to swim by undulating its body. It feeds on phytoplankton trapped by mucus secreted from pharyngeal structures. The larva is planktotrophic, meaning it feeds and grows in the open water column for an extended period, typically one to three months depending on the species and environmental conditions. In Branchiostoma belcheri, for example, metamorphosis has been observed beginning around 60 days post-fertilisation.

  • Larva is markedly asymmetrical: mouth on the left, first gill slits on the right
  • Transparent, elongated body with notochord, nerve cord, and functional myotomes
  • Free-swimming and planktotrophic, feeding on phytoplankton for one to three months
  • Additional gill slits added progressively as the larva grows
  • In Branchiostoma belcheri, metamorphosis begins around 60 days post-fertilisation

Metamorphosis and Evolutionary Significance

Metamorphosis occurs when the larva has developed approximately 12 to 15 pairs of gill slits. The larva sinks to the bottom and transforms into a juvenile. The main change during metamorphosis is the formation of the atrium, an outer chamber created by an outgrowth of the epidermis that surrounds the pharynx and opens through a single ventral pore called the atriopore. The body becomes largely bilaterally symmetrical, the mouth shifts to a median ventral position, and the animal adopts its adult burrowing lifestyle in sandy sediment.

Cephalochordates hold great evolutionary significance because they are considered the best living model for the ancestral chordate body plan. Their simple anatomy, retention of all four chordate characters in the adult, and well-studied larval development make them a key reference organism for understanding how vertebrates evolved from invertebrate ancestors. Amphioxus is widely used in developmental biology and genomics research to study gene regulation, body axis formation, and the origin of vertebrate organ systems.

  • Metamorphosis triggered when 12 to 15 pairs of gill slits have formed
  • Key metamorphic change is formation of the atrium surrounding the pharynx
  • Body becomes bilaterally symmetrical; mouth moves to a median ventral position
  • Juvenile settles to the bottom and adopts a burrowing lifestyle
  • Considered the best living proxy for the ancestral chordate body plan
  • Important model organism for evolutionary developmental biology and genomics

Key Terms

Notochord
A flexible, rod-like support structure made of striated muscle cells that extends from head to tail in cephalochordates, providing structural support throughout the body and persisting into adulthood without being replaced by a vertebral column.
Endostyle
A glandular groove running along the floor of the pharynx that secretes mucus to trap food particles during filter feeding, considered the evolutionary precursor of the vertebrate thyroid gland.
Atrium
An outer chamber formed during metamorphosis by an epidermal outgrowth that surrounds the pharynx and gill slits, opening to the exterior through a ventral pore called the atriopore.
Protonephridia
Excretory structures equipped with specialised cells called solenocytes that carry out waste removal and osmoregulation in cephalochordates, functionally similar to flame cells found in flatworms.

Practice Quiz — 20 Questions

correct out of 20

  1. What are cephalochordates commonly known as?

    • A.Sea squirts
    • B.Lancelets or amphioxus
    • C.Hagfish
    • D.Lampreys
    B. Lancelets or amphioxus — Cephalochordates are commonly called lancelets (because of their lance-shaped body) or amphioxus (meaning sharp at both ends).

  2. Which of the following chordate features does a cephalochordate retain throughout its adult life?

    • A.Notochord only
    • B.Pharyngeal slits only
    • C.All four: notochord, nerve cord, pharyngeal slits, and post-anal tail
    • D.None of them
    C. All four: notochord, nerve cord, pharyngeal slits, and post-anal tail — Cephalochordates are unique among chordates in retaining all four defining features (notochord, dorsal hollow nerve cord, pharyngeal gill slits, and post-anal tail) throughout their adult life.

  3. What is the typical habitat of adult cephalochordates?

    • A.Deep ocean trenches
    • B.Freshwater rivers
    • C.Shallow coastal sandy or muddy substrates
    • D.Coral reef surfaces
    C. Shallow coastal sandy or muddy substrates — Adult cephalochordates live in shallow coastal waters in tropical and temperate regions, burrowing into sandy or muddy substrates with their anterior end exposed for filter feeding.

  4. What are myotomes in cephalochordates?

    • A.Sensory organs along the nerve cord
    • B.V-shaped blocks of striated muscle
    • C.Segments of the notochord
    • D.Pharyngeal gill pouches
    B. V-shaped blocks of striated muscle — Myotomes are V-shaped blocks of striated muscle fibres separated by connective tissue septa (myosepta) that enable the undulating swimming movements of cephalochordates.

  5. What structure in the pharynx secretes mucus to trap food particles?

    • A.Wheel organ
    • B.Buccal cirri
    • C.Endostyle
    • D.Atriopore
    C. Endostyle — The endostyle is a glandular groove in the floor of the pharynx that secretes mucus to trap microscopic food particles during filter feeding.

  6. What type of circulatory system do cephalochordates possess?

    • A.Open system with a two-chambered heart
    • B.Closed system without a heart
    • C.Open system without a heart
    • D.Closed system with a single-chambered heart
    B. Closed system without a heart — Cephalochordates have a closed circulatory system similar to the vertebrate plan, but they lack a specialised heart. Blood is driven by the contractile ventral aorta and vessel pulsation.

  7. What colour is the blood of cephalochordates?

    • A.Red
    • B.Blue
    • C.Green
    • D.Colourless
    D. Colourless — The blood of cephalochordates is colourless because it lacks any respiratory pigment such as haemoglobin or haemocyanin.

  8. What excretory structures are found in cephalochordates?

    • A.Malpighian tubules
    • B.Nephrons
    • C.Protonephridia with solenocytes
    • D.Metanephridia
    C. Protonephridia with solenocytes — Cephalochordates possess protonephridia equipped with specialised cells called solenocytes that handle excretion and osmoregulation.

  9. Cephalochordates are dioecious. What does this mean?

    • A.They reproduce asexually
    • B.Sexes are separate
    • C.Each individual has both male and female organs
    • D.They undergo alternation of generations
    B. Sexes are separate — Dioecious means that sexes are separate, with each individual being either male or female. Cephalochordates release eggs and sperm into the water for external fertilisation.

  10. What type of cleavage does the fertilised egg of a cephalochordate undergo?

    • A.Meroblastic discoidal
    • B.Holoblastic and roughly equal
    • C.Superficial
    • D.Spiral
    B. Holoblastic and roughly equal — The fertilised cephalochordate egg undergoes holoblastic (complete) and roughly equal cleavage, dividing entirely into cells of approximately equal size.

  11. What is the most distinctive feature of the cephalochordate larva?

    • A.It has a vertebral column
    • B.It is markedly asymmetrical
    • C.It lacks a notochord
    • D.It has lungs for breathing
    B. It is markedly asymmetrical — The cephalochordate larva is notable for its pronounced left-right asymmetry: the mouth opens on the left side and the first gill slits form on the right side.

  12. On which side of the body does the mouth open in the cephalochordate larva?

    • A.Right side
    • B.Left side
    • C.Dorsal side
    • D.Ventral midline
    B. Left side — In the cephalochordate larva, the mouth opens on the left side of the body, contributing to the pronounced asymmetry that characterises this larval stage.

  13. Where do the first gill slits form in the cephalochordate larva?

    • A.Left side
    • B.Right side
    • C.Both sides simultaneously
    • D.Ventral surface
    B. Right side — The first row of gill slits forms on the right side of the pharynx in the larva. A second row on the opposite side develops later as the larva grows.

  14. What is the feeding mode of the cephalochordate larva?

    • A.Predatory
    • B.Parasitic
    • C.Planktotrophic
    • D.Lecithotrophic
    C. Planktotrophic — The larva is planktotrophic, meaning it actively feeds on phytoplankton and small organic particles in the water column during its free-swimming period.

  15. How long does the free-swimming larval period typically last before metamorphosis?

    • A.A few hours
    • B.One to two days
    • C.One to three months
    • D.Over one year
    C. One to three months — The free-swimming larval period typically lasts one to three months depending on the species and environmental conditions, after which the larva undergoes metamorphosis.

  16. Metamorphosis in cephalochordates is triggered when the larva has developed approximately how many pairs of gill slits?

    • A.3 to 5 pairs
    • B.6 to 8 pairs
    • C.12 to 15 pairs
    • D.20 to 25 pairs
    C. 12 to 15 pairs — Metamorphosis begins when the larva has developed approximately 12 to 15 pairs of gill slits. The larva then sinks to the bottom and transforms into a juvenile.

  17. What is the main structural change that occurs during cephalochordate metamorphosis?

    • A.Development of a vertebral column
    • B.Formation of the atrium around the pharynx
    • C.Loss of the notochord
    • D.Development of paired fins
    B. Formation of the atrium around the pharynx — The main metamorphic change is the formation of the atrium, an outer chamber created by an epidermal outgrowth that surrounds the pharynx and opens through a ventral pore called the atriopore.

  18. What is the atriopore?

    • A.The larval mouth opening
    • B.A ventral pore through which water exits the atrium
    • C.The anus of the larva
    • D.An opening for sperm release
    B. A ventral pore through which water exits the atrium — The atriopore is a single ventral pore through which water exits the atrium after passing over the gills. It forms during metamorphosis when the atrial chamber develops around the pharynx.

  19. Which genus of cephalochordates has gonads only on the right side of the body?

    • A.Branchiostoma
    • B.Asymmetron
    • C.Epigonichthys
    • D.Amphioxus
    B. Asymmetron — Asymmetron species have gonads exclusively on the right side of the body, while Branchiostoma species have bilaterally placed gonads on both sides.

  20. Why are cephalochordates considered important model organisms in evolutionary biology?

    • A.They are the ancestors of all insects
    • B.They are the best living proxy for the ancestral chordate body plan
    • C.They are the closest relatives of jellyfish
    • D.They have the largest genome among invertebrates
    B. They are the best living proxy for the ancestral chordate body plan — Cephalochordates are considered the best living model for the ancestral chordate body plan because of their simple anatomy, retention of all four chordate features, and key phylogenetic position between invertebrates and vertebrates.

References