Amphioxus whole mount, young (Do not use high power—thick mount). In it you can identify the anterior or head end with tentacles and the narrower posterior end or tail; both are pointed. The three structures that are present at some stage in the life of all chordates can be seen: pharynx with gill slits; dorsal hollow nerve cord, and notochord.

Diagonal red-stained lines are the supports between gill slits. Amphioxus uses cilia to draw water in through the mouth, into the pharynx; the water passes through the gill slits, and strings of mucus travelling across them trap tiny food particles. The water collects in an atrium, the chamber outside the gills and then exits from the body through a posteroventral atriopore. The mucus containing food is carried by cilia posteriorly into the intestine and twirled; bits that break off enter the cecum, a sack on the right side of the pharynx. Here cells swallow the particles (endocytosis) and digest them. Waste travels through the intestine 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, usually with minute upright lines across it, that runs from the tip of the nose to the tip of the tail. This is the notochord which gives the creature its stiffness. The dorsal hollow nerve cord is immediately dorsal to the notochord and can be identified because its ventral part contains black, light-sensitive spots. 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 through the anterior part of the head reveals the notochord and an arch of animal with no floor; there are tentacles along its margin. This is called the oral hood region. The second section through the pharynx cuts across several gill slits, and the bars between them appear like a string of beads. The spaces between the beads are the slits. The atrium is the space between the pharyngeal gills and the outer body wall. In some specimens gonads occupy a part of the space (tiny cells inside testes are sperm; large cells inside ovaries are eggs). The body floor below the pharynx is expandable. The notochord makes a large, pink oval that is dorsal to the pharynx, and the dorsal hollow 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 hollow nerve cord above it. The fourth section through the tail is all meat (muscle), notochord and dorsal hollow nerve cord; it is posterior to the atriopore and anus, hence there are neither coelomic space nor organs here. You may be able to find small, circular sections of some blood vessels; the circulatory and digestive systems are separate.