Cephalochordate

What are cephalochordates?

Cephalochordates are small, fishlike marine invertebrates that make up the subphylum Cephalochordata within the phylum Chordata. Often referred to as lancelets or amphioxus, these translucent animals typically live partially buried in the ocean floor.¹²³

They are considered the closest living relatives to vertebrates. Like vertebrates, cephalochordates possess a hollow dorsal nerve cord, pharyngeal gill slits, and a notochord¹. However, while vertebrates replace the embryonic notochord with bony or cartilaginous vertebrae, cephalochordates retain the notochord throughout their adulthood.¹²

The fossil record for these creatures dates back approximately 525 million years to the Cambrian Period. Because they share segmented musculature and specific circulatory patterns with vertebrates, they are often studied as “sister groups” that provide insight into the evolution of the vertebrate lineage.¹⁴

How are cephalochordates different from other chordates?

Cephalochordates, also known as lancelets, are distinguished from other members of the phylum Chordata—specifically tunicates and vertebrates—by several unique anatomical features and developmental characteristics.

Key Differences

• Notochord: In cephalochordates, the notochord is permanent and extends the entire length of the body, even reaching anterior to the dorsal nerve cord. In contrast, the notochord in tunicates is restricted to the tail region of the larva and disappears in adults, while in vertebrates, it is typically replaced by bony or cartilaginous vertebrae during development.⁴²⁵⁶
• Anatomy and Physiology: Unlike vertebrates, cephalochordates lack a heart, paired fins, and a well-developed head. They possess a single ventral atrium with a single opening, whereas vertebrates lack an atrium entirely.⁴²
• Relationship to Vertebrates: Cephalochordates and vertebrates are considered “sister groups” because they share specific traits not found in other chordates, such as segmented musculature, complex circulatory patterns, and biochemical similarities.⁷¹

While all chordates share a common body plan—including a hollow dorsal nerve cord, pharyngeal gill slits, and a notochord—these variations in structure define the distinct subphyla within the group.⁸²

What is the main body structure of a cephalochordate?

The body structure of a cephalochordate is characterized by a fishlike, segmented design that reflects its evolutionary proximity to vertebrates. Key structural components include:

• Notochord: Unlike vertebrates, cephalochordates retain a notochord throughout adulthood. It extends nearly the entire length of the body, providing essential structural support and preventing the body from shortening during locomotion.⁹¹⁰¹¹
• Musculature: The body is composed of serially repeated segments called metameres. These include chevron-shaped muscle blocks known as myotomes, which facilitate efficient side-to-side swimming movements.⁹¹⁰
• Nervous System: They possess a single, hollow dorsal nerve cord. While it features a slight swelling at the anterior end, it lacks the complex braincase or cranium found in vertebrates, leading to their classification as acraniates.⁹¹²
• Internal Cavities: A well-developed coelom exists as a system of spaces containing internal organs. Additionally, the pharynx is protected by an atrium, a large cavity with a single exit called the atriopore.⁹
• Circulatory System: They utilize a closed circulatory system driven by contractile vessels, such as the ventral aorta, rather than a heart.¹

Where do cephalochordates live?

Cephalochordates, commonly known as lancelets or amphioxi, are marine animals found widely in coastal waters across the globe. They primarily inhabit warmer tropical regions, though they are also found in temperate waters.¹⁴¹⁵¹⁶

Their preferred environment consists of soft-bottom habitats, including sand, gravel, or coarse shelly substrates in shallow coastal areas. As adults, they spend much of their time buried within these sediments, often with only the anterior portion of their bodies projecting above the surface to filter food particles from the water. While they are capable of swimming—often moving near the bottom at night—they are not buoyant and quickly return to the substrate when they stop moving.¹¹⁶¹⁴

In some regions, such as along the coast of China, these animals are so abundant that they support a local fishing industry.¹⁴¹⁶