Lancelets: Cephalochordata

Lancelets can swim vigorously forward and backward, but they spend most of their time buried halfway in the sand. They live in masses of more than nine thousand animals per square yard (meter). Depending on the coarseness of the sand, lancelets assume different feeding positions. In coarse sand they bury their entire body with only the head exposed to the water. In fine sand they lie on the bottom. Lancelets continuously produce a mucus net that they move over their gill slits to capture food particles in the water. The food particles and mucus net are rolled into a mass that passes into the digestive tract.

Lancelets have separate sexes. There are equal numbers of males and females in a population. Eggs and sperm are released into the water, where the fertilized (FUR-teh-lyzed) eggs, those that have united with sperm, develop into larvae. When twelve to fifteen pairs of gill slits have formed, the larvae sink to the bottom and transform into young lancelets. From that point on the animals grow, and additional gill slits and muscle segments develop while the reproductive organs grow to maturity.