Amphioxus
Amphioxus, or Lancelet, a small marine animal found in shore waters in temperate and tropical regions. There are about 30 species of amphioxuses. The animal is seldom more than two inches (5 cm) long; the American species of the Pacific coast, however, sometimes grows to four inches (10 cm) in length. The amphioxus is rather fishlike in appearance. On the back it has a fin that runs almost the entire length of the body; on the underside near the tail it has a short fin. The sharp-edged body gave it the name amphioxus (“double edged”), and the lance-shaped tail the name lancelet.
The amphioxus is an intermediate form between invertebrates (animals without backbones) and vertebrates (animals with backbones). Instead of a backbone, the amphioxus has a notochord —a firm, flexible rod of cartilage-like material that extends from the tip of its head to the tip of its tail. It has a nerve cord lying above and parallel to the notochord, but has no specialized brain.
The amphioxus swims well, but spends most of its time buried tail down in coarse, clean sand or gravel in shallow water. It feeds on microscopic organisms, which it strains from the seawater through its gill slits.
Amphioxuses make up the subphylum Cephalochordata of the phylum Chordata. The amphioxus of the American west coast is Branchiostoma californiense; that of the east coast, B. virginiae.