The phylum Chordata is divided into three extant subphyla: the Urochordata, which includes the sea squirts, the Cephalochordata, which includes the lancelets (amphioxus), and the Craniata, which includes animals with backbones.

At some point in the life cycle of all chordates, the following four diagnostic characteristics are present:

1. A pharynx with gill slits.
2. A notochord.
3. A dorsal, hollow nerve tube.
4. A post-anal tail.

To these can be added, in most chordates, a circulatory system with a ventral heart that pushes blood forward to the gills, and then posteriorly along the dorsal side of the animal. Segmentally arranged muscles also occur along the body of most chordates. You will review these features in the cephalochordate Branchiostoma (also known as Amphioxus), and in the vertebrate lamprey larva (an agnathan known as ammocoete).