Amphioxus

How does the anatomy of amphioxus compare to other chordates?

Amphioxus (or lancelets) are members of the subphylum Cephalochordata and serve as vital models for understanding the evolution of chordates and vertebrates.

Key Anatomical Comparisons

• Shared Chordate Features: Like all chordates, amphioxus possesses a notochord, a hollow dorsal nerve cord, and pharyngeal gill slits. Unlike many vertebrates, where the notochord is replaced by a vertebral column, amphioxus retains its notochord throughout adulthood.¹²³
• Musculature and Segmentation: Amphioxus shares a segmented body plan with vertebrates, characterized by blocks of muscles called myotomes. This segmentation, which is absent in tunicates and hemichordates, is considered a shared ancestral trait between cephalochordates and vertebrates.⁴⁵
• Circulatory System: Unlike vertebrates, amphioxus lacks a centralized heart. Instead, it utilizes accessory branchial hearts and contractions in various vessels to move blood. Its circulatory pattern—blood flowing forward ventrally and backward dorsally—is a feature shared with vertebrates.⁶⁵
• Endocrine and Sensory Structures: Amphioxus lacks a true brain or cranium. However, its endostyle functions similarly to the vertebrate thyroid, and structures like Hatschek’s pit show evolutionary links to the vertebrate pituitary gland.²⁷

What are the key characteristics of amphioxus that classify them as cephalochordates?

Amphioxus, commonly known as lancelets, are classified as cephalochordates due to a specific set of anatomical features that distinguish them from other members of the phylum Chordata.

The key characteristics defining their classification include:

• Notochord Extension: The defining feature of the subphylum is that the notochord extends the entire length of the body, reaching well into the anterior portion, even extending beyond the nerve cord. Unlike vertebrates, this structure is retained throughout adulthood and is never replaced by a vertebral column.⁵³⁸⁹
• Acraniate Status: They lack a braincase or cranium, which is why they are often referred to as acraniates.⁵³
• Segmented Body Plan: Their bodies exhibit well-developed, serially repeated segments known as metameres. This segmentation includes blocks of muscles (myotomes) and extends to nerves, excretory structures, and body cavities, a trait shared with vertebrates.⁵³
• Atrial Structure: They possess a large cavity called an atrium that surrounds the pharynx and its gill slits, featuring a single ventral opening known as the atriopore.⁵³
• Lack of Specialized Organs: Cephalochordates do not possess a heart or paired fins, further distinguishing them from the subphylum Vertebrata.³

What role does the notochord play in the development and life of amphioxus?

The notochord is a fundamental anatomical structure for amphioxus, serving as a stiffening rod that provides essential support throughout the animal’s life cycle.¹⁰²

Role in Structure and Locomotion

• Central Support: The notochord runs the entire length of the body, extending from the tip of the tail into the anterior region. It consists of a turgid core enclosed within a firm, fibrous sheath, which maintains the body’s structural integrity.^1011239
• Preventing Shortening: A primary function of the notochord is to prevent the body from shortening during locomotion. When the animal swims, it contracts its myotomes—chevron-shaped muscle blocks—to produce side-to-side movements. The notochord acts as a rigid axis that resists the compressive forces generated by these muscular contractions, allowing for efficient swimming.³⁹¹⁰
• Internal Composition: Unlike the notochord in many other chordates, the notochord of amphioxus contains muscle fibers within its core, which are believed to help maintain its stiffness.⁹

Because amphioxus lacks a bony skeleton or a vertebral column, the notochord remains the primary skeletal element from the larval stage through adulthood.¹⁰²

What is the phylogenetic relationship between amphioxus and vertebrates?

Amphioxus, or lancelets, are considered the closest living relatives of vertebrates, representing a collateral branch on the vertebrate lineage. They are classified as “sister groups” because they share several unique features not found in other chordates, including segmented musculature (myotomes), specific patterns of circulation, and various biochemical traits.⁵⁶²

While both groups possess a notochord, a dorsal nerve cord, and pharyngeal gill slits, their developmental paths diverge significantly. In vertebrates, the embryonic notochord is typically replaced by a vertebral column, whereas in cephalochordates, the notochord persists throughout adulthood. Furthermore, amphioxus lacks a cranium and a true heart, distinguishing them as acraniates.⁵⁴⁶

Although some researchers have debated whether the ancestral chordate resembled a cephalochordate or a tunicate, the morphological and molecular evidence strongly supports the close evolutionary proximity between amphioxus and vertebrates.²⁶