Laboratory Notes for BIO 1003 & 3001
© 30 August 1999, John H. Wahlert & Mary Jean Holland
The Phylum Chordata contains three subphyla; we will look at the Cephalochordata, the lancelets. Another Subphylum is the Vertebrata; if you look around the room you will see examples of the group studying this sheet of information. Fish, amphibians, reptiles, birds, and mammals are all vertebrates. Amphioxus (Genus Branchiostoma) is most likely the descendant from an early stage in chordate evolution, and it shows all 5 of the defining characteristics* of chordates:
All chordates show these characteristics at some stage in their lifetimes. You will also notice features that are common among many animal phyla, including bilateral symmetry, coelom, and body segmentation (seen in the series of muscle blocks called myomeres). The circulatory and digestive systems are separate.
- pharyngeal slits
- endostyle or thyroid gland
- dorsal tubular nerve cord
- postanal tail
The example of a cephalochordate is called amphioxus which means both ends (amphi-) are sharp (-oxus). Amphioxus is a marine animal, and the several genera are distributed worldwide, especially in warm, shallow oceans where they burrow tail first into the sand and feed by filtering water. Preserved adult examples are available for you to examine.
Amphioxus whole mount (No high powerthick mount)
These are young individuals which are small enough to fit under a coverslip. Identify the cranial or head end with tentalcle-like buccal cirri and the narrower caudal or tail end; both are pointed. Four defining chordate structures can be seen: pharynx with gill slits; dorsal tubular nerve cord, notochord, and postanal tail.
Amphioxus uses cilia to move water into and through its digestive system. Overlapping buccal cirri from the two sides of the oral hood act as a strainer to keep out large particles; they probably have a sensory function, too. Water passes through the mouth into the pharynx; the water then goes through the gill slits where strings of mucus travelling across them trap tiny food particles. Diagonal red-stained bars are the supports between gill slits. The endostyle, along the base of the pharynx (this structure is best seen in cross section), secretes the muscus that is moved dorsally across the pharyngeal slits. After passing through the slits, the water collects in an atrium, the chamber outside the gills and then exits from the body through a posteroventral atriopore. The mucus and contained food is carried by cilia posteriorly along the epipharyngeal groove into the intestine. Here, a ring of cilia twirls the mucus; bits break off and enter the cecum, a laterally compressed sack on the right side of the pharynx. The columnar epithelial cells of the cecum secrete enzymes for preliminary extra-cellular digestion; the eipthelial cells then take up large quanties of particles (endocytosis) and digest them. Nutrients would be transferred to blood vessels at the base of the epithelial cells. Intracellualar digestion is unusual in animals. Waste travels through the intestine, where further absorption occurs, and out the anus, which is posterior to the atriopore but not near the tip of the tail.
Dorsal to the pharynx you can see a rod that runs from the tip of the nose to the tip of the tail. This is the notochord, which gives the creature its stiffness. Muscle alongside the notochord bends it into S-curves so that the body undulates. In many slides there are tiny vertical lines marking the notochord. The dorsal tubular nerve cord, which is the central nervous system, is immediately dorsal to the notochord and can be identified because its ventral part contains black, light-sensitive spots; the spots are most concentrated in the anterior part of the cord. The central nerve cord lacks a cranial swelling, so amphioxus lacks a brain. The blocks of the dorsal fin fold are above the nerve chord.
Amphioxus cross sections of representative regions (You may use high power; the slide is thin.)
- The first cross section cuts through the oral hood, which is the anterior part of the head. The notochord is dorsal and extends to the tip of the hood. The hood has no floor, and there are tentacle-like buccal cirri along its margin.
- The second section through the pharynx cuts across several gill slits; the slanted bars between them appear like a necklace of beads. The endostyle appears as an upward facing horshoe-shaped group of cells in the bottom of the pharynx; its cells secrete the mucus that is carried up across the gill slits and traps food particles; the endostyle is homologous in part to the thyroid gland in vertebrates. The atrium is the space between the pharyngeal slits and the outer body wall. In some specimens gonads occupy a part of the space (testes contain tiny sperm and have a stringy texture; ovaries contain large egg cells and have a blocky appearance). The body floor below the pharynx is expandable. The notochord makes a large, pink oval that is dorsal to the pharynx, and the dorsal tubular nerve cord (also pink) above it has either an indentation from the top or a hole in the center. Blocks of striated muscle in the body wall are used to bend the notochord from side to side (undulation) for swimming and burrowing.
- A third cross section farther posterior shows continuous muscle on the sides, a coelom containing the intestine, and the notochord and dorsal tubular nerve cord above it.
- The fourth section through the tail has only muscle, notochord and dorsal tubular nerve cord; it is posterior to the atriopore and anus, hence there are neither coelom nor organs here. You may be able to find small, circular sections of some blood vessels; the circulatory and digestive systems are separate.
Gas exchange occurs between water and cells at the organisms surface. There are no special respiratory structures; the function of the gills is to filter water and collect food.
- Why is the living amphioxus probably different from the common ancestor of cephalochordates and vertebrates?
- Why does this simple descendant of the common ancestor still exist?
* list of chordate characteristics is taken from K. Kardong, 2009, Vertebrates.
Details of amphioxus structure are from E. J. W. Barrington, 1965, The Biology of Hemichordata and Protochordata. Oliver & Boyd, Edinburgh and London. 176 pp.
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Last updated 10 December 2016 (JHW)