Amphioxus (sometimes called Lancelets): These little guys are found burrowed under the sand in shallow areas of the tropical and temperate seas. In Asia, they are harvested as food for humans as well as animals. These fish-like vertebrates are very important because they give us more information on the origin of vertebrates. They grow between 5-7 centimeters long with a semi-translucent body and have no paired fins or limbs. A poor tail fin is present but swimming is not one of their strengths.
Some common findings within Amphioxus and vertebrates are a hollow nerve cord and blocked muscles. Some differences in Amphioxus are the dorsal nerve cord is not protected by bone but rather a notochord which is a tightly packed cylindar of cells that makes a tough rod. They do not have a true brain, eyes, or contain any complex sense organs like true vertebrates.
Amphioxus have cirri, the thin tentacle-looking strands coming from their mouth, that act as sensory device and filter the water passing into their bodies. They also do not have any respiratory system, a circulatory system is present but does not contain a heart or even any blood cells.
The differences in structure of the body for these vertebrates give clues to how vertebrates came about at first. It also give us an understanding of what type of systems within the body were necessary for early vertebrates and how they changed over time to the vertebrates we have today.