Cephalochordata probably represents an offshoot of the evolutionary line that led to the vertebrates. In this group of burrowers, the notochord extends into the head (which is adaptive for burrowing, but leaves little room for the brain). In other features of their organization, though, the cephalochordates may resemble the ancestor of vertebrates.

(A) Amphioxus ( also known as lancelets, or Branchiostoma)

            Observe living Amphioxus  (Rust, fig. 76; V & C fig. 7.162-7.166). These animals live on the sea bottom, burrowing through the sediments. With their anterior end projecting above the sediments, they feed by filtering water through their pharynx. Much like clams, they use their "gills" to trap food particles, which are then wrapped up in mucous and passed down a ciliated groove to the stomach. This mode of filter feeding was inherited from the tunicate-like ancestors of Amphioxus. We will observe in the Vertebrate section that the ammocoete larva of lampreys also lives in sediments and filter feeds.

            Now study a slide of Amphioxus, finding the tail with its segmental muscles (myotomes), the stiffening notochord, and the dorsal nerve tube. These features are all associated with locomotion, and are in large part responsible for the tremendous evolutionary success of chordates. Observe the pharynx with gill slits separated by gill bars. If possible, find the ventral and dorsal aortas in the cross-section of Amphioxus. Set this slide aside to compare to the lamprey larva (ammocoete)