Animals > chordates > Branchiostoma
Cephalochordates: Branchiostoma
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.
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 humans, the notochord forms during early development but later is replaced with bone; the only remnant of the notochord in your body is the cartilaginous disks between your vertebrae.
- 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 gill bars, with gill slits between them. The animal sucks in water through its mouth and forces it out through the gill slits, capturing small food particles from the water. In vertebrates, some of the gill 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
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 image, it appears as a large, pinkish area with dark dots in the center. See below for an enlarged view.
There are several distinct blocks of muscle running along the sides of the body.
The buccal cavity is simply the space inside the mouth. Branchostoma 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.
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 gill slits, which are spaces between the 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 fish, mammals and other vertebrates, some of the gill bars have changed to become the bones of the jaw and ear.
This cross-section also shows one part of the coelom. Chordates have true coeloms, in which the major organs form.
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.
The notochord is made of cartilage, a type of connective tissue that consists mostly of extracellular matrix with widely scattered cells. Thus, in this picture the notochord appears wavy and smooth, and lacks the visible dark dots of cell nuclei. The muscle tissue, on the other hand, is dense with nuclei and much darker.
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This page updated September 17, 2011