Lancelets, often called Amphioxus, are small, translucent, marine animals with a simple, fish-like body plan. Typically measuring only a few centimeters, they spend much of their lives buried in the sandy bottoms of temperate and tropical ocean waters. They are classified within the phylum Chordata, specifically belonging to the subphylum Cephalochordata. Their anatomy represents a foundational structure that helps scientists understand the characteristics defining the entire phylum.
The Hallmarks of Chordate Anatomy
All animals in the phylum Chordata possess five distinct anatomical features at some point during their life cycle.
Notochord and Nerve Cord
The notochord is a flexible, rod-shaped structure that provides skeletal support along the body’s length. Lying above the notochord is the dorsal hollow nerve cord, which is unique because its nerve tissue is positioned on the back and is tube-shaped. This structure eventually develops into the central nervous system, including the brain and spinal cord, in more complex chordates.
Pharyngeal Slits and Post-Anal Tail
Another characteristic is the presence of pharyngeal slits, which are openings in the pharynx. In aquatic species, these slits initially function as a filter-feeding apparatus, but they later develop into the gill arches in fish. All chordates also share a post-anal tail, an extension of the body that continues past the anus. This tail contains skeletal elements and muscle blocks, providing locomotion, especially in aquatic forms.
Endostyle
The fifth feature is the endostyle, a ciliated groove located on the floor of the pharynx. This structure functions in filter feeding by producing mucus that traps food particles. The endostyle is considered homologous to the thyroid gland in vertebrates, highlighting an evolutionary connection. These five features collectively establish the blueprint for all chordate bodies.
Why Lancelets Fit the Chordate Profile
Lancelets retain all five chordate features throughout their entire adult life, unlike many other chordates. Their notochord runs the length of their body, providing the main axis of support. This rod extends well into the anterior tip of the body, which is the origin of their subphylum name, Cephalochordata, meaning “head chord.”
The dorsal hollow nerve cord persists as a simple, unspecialized tube above the notochord. While it shows a slight swelling at the anterior end, it does not form a true braincase, distinguishing the lancelet from vertebrates. Their pharynx is perforated by numerous pharyngeal slits that form a basket-like structure, crucial for filtering tiny food particles from the water current.
The endostyle secretes the mucus necessary to capture the food particles, which are then channeled to the digestive tract. Lancelets also possess a clear post-anal tail, which they use for swimming or burrowing into the sediment. The combination of all five defining features in their adult anatomy firmly establishes the lancelet’s position as a textbook example of a chordate.
Lancelets’ Place in the Evolutionary Tree
The subphylum Cephalochordata is recognized as a basal group within the phylum Chordata, representing one of the earliest lineages to diverge. Lancelets lack the complex traits associated with vertebrates, such as a true vertebral column, a skull, or well-developed sensory organs. They are considered the closest living invertebrate relatives to the vertebrates.
By studying their genetics and development, researchers gain insights into the changes that led to the evolution of fish, amphibians, reptiles, and mammals. The lancelet genome provides a simple point of comparison, clarifying which genetic control mechanisms were ancestral to all chordates and which developed later in the vertebrate lineage. For example, their simple body plan is studied to understand the origins of complex vertebrate structures like paired appendages and the specialized brain.
Molecular clock analysis suggests that the lineage leading to modern lancelets diverged over 520 million years ago, placing them near the origin of the vertebrate body plan. They are often used as a model organism in evolutionary developmental biology (evo-devo) because their anatomy illustrates fundamental chordate features in their most basic, unspecialized form. This makes the lancelet a living reference point for understanding the innovations that defined the vertebrate group.