Lancelets: Cephalochordata

Lancelets look like slender fish without eyes. They are 0.4 to 3 inches (1 to 8 centimeters) long and whitish to creamy yellow, sometimes with a tint of pink. Mucus secreted by cells in their body covering gives lancelets a pearly sheen. V-shaped lines on the outside of lancelets' bodies outline muscle blocks inside the body. Small bristles surround the mouth. A fin runs along the entire back of the animal and extends forward into a short fin over the snout and backward into a tail fin. There is a fin on the belly toward the rear of the animal. Paired fin-like folds in front of the belly fin extend to the front of the lancelet.

Lancelets have a notochord (NOH-tuh-koord), which is a flexible rod of cells supporting the body. The rest of the skeleton is made up of small, flexible rods between the gill slits and supporting the mouth bristles. A nerve cord runs along the top of the notochord, and various types of sensing cells are distributed in the body covering, especially near the snout. Lancelets have blood vessels but no heart. The chest has about two hundred gill slits that do not open to the outside but empty into a chamber inside the body wall. The chamber empties to the outside through a hole on the belly of the lancelet.