Branchiostoma, an invertebrate chordate

In this lab, you'll examine several different specimens of Branchiostoma, which is also called Amphioxus. Although the name Amphioxus is commonly used, Branchiostoma is the correct scientific name for this genus, so I'll use that name on this page. This organism also goes by the common name "lancelet," supposedly because it resembles a small lance.

Objectives

• Recognize and name the anatomical features described on this page.
• Explain what these features do in Branchiostoma and what the homologous features are in vertebrates.
• Explain why Branchiostoma is a chordate, but not a vertebrate.

Specimens

• Microscope slide: Branchiostoma whole mount (may be labeled Amphioxus)
• Microscope slide: Branchiostoma oral region, cross section
• Microscope slide: Branchiostoma pharynx region, cross section
• Microscope slide: Branchiostoma gonad region, cross section
• Microscope slide: Branchiostoma posterior region, cross section

Why study Branchiostoma?

Branchiostoma is a member of the phylum Chordata, but it's not a vertebrate. As a member of our phylum, it shows all the fundamental chordate characteristics, but as an invertebrate, it displays these characteristics in the context of a simpler body structure. Thus, it provides a window into chordate evolution. Branchiostoma has no brain, no arms or legs, no jaws or backbone -- in fact, no bony skeleton at all --  but it gives us a way of looking at where these structures came from.

Branchiostoma whole mount

Cephalochordates are chordates, but they are not vertebrates. While it doesn't have a backbone (or any bones at all), Branchiostoma shows all the basic characteristics of the phylum Chordata, including:

• Notochord: a semi-rigid stiffening rod made of cartilage, running along the animal's back. In mammals, the notochord forms during early development but later is replaced with bony vertebrae; the only remnant of the notochord in adult mammals is the gel-like center (nucleus pulposus) of each intervertebral disk.
• Dorsal nerve cord: a thick cord of nerve cells, dorsal to the notochord; homologous to the vertebrate central nervous system, including the spinal cord and brain.
• Pharynx: a cartilaginous structure in the throat area. In Branchiostoma, it is used mainly for feeding. The pharynx consists of numerous pharyngeal bars, with pharyngeal slits between them. The animal sucks in water through its mouth and forces it out through the pharyngeal slits, capturing small food particles from the water. In vertebrates, some of the pharyngeal bars have evolved to become other structures, such as jaw bones.
• Tail: an extension of the body, including notochord and muscle, beyond the end of the digestive tract. This helps distinguish chordates from various kinds of worms, in which the anus is at the very back end of the body. In chordates, it's usually called a "post-anal tail," because it's posterior to the anus — but where else would a tail be?

Branchiostoma oral region

Compare the cross section and whole mount to get a better understanding of the 3-dimensional anatomy.

• The dorsal fin is stiffened with cartilage, which has mostly disappeared in this cross-section.
• The nerve cord is toward the animal's dorsal side, and it originally forms as a hollow tube. In this cross section, it appears as a large, pinkish area with dark dots (ocelli) in the center.
• There are several distinct blocks of muscle running along the sides of the body.
• The buccal cavity is simply the space inside the mouth. Branchiostoma lives in the ocean and burrows its tail into the sand; it then sucks in large amounts of water through the mouth in order to filter out small particles of food using its pharynx, which is shown below.
• Cirri are small tentacles surrounding the mouth.
• Ocelli (singular: ocellus) are clusters of light-sensitive cells within the nerve cord. These "eyes" don't form an image, but they may help the animal tell light from dark and may also help to set the animal's circadian rhythms.

Branchiostoma pharynx region

The pharynx plays a key role in Branchiostoma's feeding. As seawater is sucked in through the mouth, it is forced out the sides through the pharyngeal slits, which are spaces between the pharyngeal bars ( also called gill bars).

Other vertebrates also have structures homologous to the pharynx of Branchiostoma. In fish, some of the gill bars are cartilaginous structures that support the gills. In all jawed vertebrates (including fish and mammals) , some of the pharyngeal bars become the bones of the jaw. In mammals, some of the jaw bones found in reptiles have become the tiny bones of the middle ear.

This cross-section also shows one part of the coelom. Chordates have true coeloms, in which the major organs form.

The notochord consists of a tough outer sheath of connective tissue and collagen surrounding an inner core of loosely packed  cells and fluid-filled spaces. The outer sheath maintains fluid pressure in the notochord, resisting compression while allowing the notochord to flex. Thus, the notochord is a hydrostatic organ.

Branchiostoma gonad region

This image shows a cross-section of a mature female Branchiostoma. The cross-section is posterior to the one shown in the above picture.

Gonads produce eggs or sperm. Since this is a female, the gonads are ovaries, which produce eggs.

The hepatic cecum is part of the digestive tract. It apparently functions both to secrete digestive juices and to help absorb nutrients from the food. "Hepatic" means it relates to the liver. Although Branchiostoma doesn't have a liver, the hepatic cecum may be similar to the liver of vertebrates.

Branchiostoma posterior region

The intestine secretes digestive enzymes and absorbs nutrients from food.

This region of the body is mostly muscle. Note that the muscle is divided into discrete segments.

Branchiostoma swims like a fish, with side-to-side movements of the tail. The notochord in this part of the body is relatively narrow to allow this movement, but is tall to resist up-and-down motion. Note that the muscles are mostly along the sides to generate side-to-side movements.

In this image, you can clearly see the structure of the notochord. The outer sheath is dense and smooth, with a tight layer of collagen-rich extracellular matrix. The cells in the inner core are less densely packed, with few or no nuclei (dark dots) visible. Much of the inner core is filled with fluid and loosely packed collagen fibers.

The muscle tissue, on the other hand, is dense with nuclei and much darker.

Review

Recognize these features in cross sections and in longitudinal sections:

1. What is neurulation? What embryonic tissue is responsible? What structures are derived from neurulation? Which phyla perform neurulation?
2. What does the pharynx do in Branchiostoma? What structures in a fish or a human are homologous to the pharynx of Branchiostoma?
3. What does the notochord do in Branchiostoma? What structures in a human are homologous to the notochord of Branchiostoma?

References & further reading:

• Branchiostoma page on Invertebrate Anatomy Online.
• Branchiostoma on EOL, Encyclopedia of Life. Includes a few pictures showing what the animals look like when they're alive.
• Evolutionary crossroads in developmental biology: amphioxus. Stephanie Bertrand and Hector Escriva, 2011. Development 138, 4819-4830. This excellent article describes how Branchiostoma has been used in research and explains the significant discoveries that have been made.