Cephalochordates

Lancelets are common bottom-dwelling forms that possess all four chordate characteristics (a notochord, dorsal tail, etc; Fig 1). They dig into the sand and lie with their anterior end protruding from the burrow. Water with food is pumped past the oral tentacles (buccal cirri) and into the pharynx. The cirri strain out large particles and also have a sensory function. Food is separated from the in-coming water by the action of the pharyngeal gill slits (that also aid in respiration). The filtered water then passes into a cavity (atrium) and out the atriopore. The remainder of the digestive system consists of a tubular intestine with a ventral outpocketing (digestive caecum). Although lancelets are most often seen in their burrows, they can swim and posses several adaptations for their active lifestyle. Unlike the urochordates, for example, the notochord extends along the entire length of their body. This structure imparts rigidity to their body and permits more coordinated swimming movements. The muscles (myonemes) are <-shaped and run the length of the animal. The segmental arrangement of the myonemes facilitates the lateral movements used during swimming. A long dorsal fin, a posterior caudal fin, and the short ventral fin are supported by fibrous fin rays to increase their effectiveness during locomotion. Examine the preserved specimens, and slides (whole mounts and cross sections) of Branchiostoma (amphioxus) using your dissecting and compound microscopes.

Figure 1. Whole mount of the lancelet Branchiostoma (amphioxus). The inset shows a cross section through the pharynx. Fresh lancelets are tasty in soup (cook slowly for 30 min; add lots of cayenne pepper, some onion and garlic powder, green onion, kimchi, ramin noodles, and maybe a few shrimp).

Whole Mount of the Antterior Region.

        
Cross sections of the a cephalochordate, Lancet, through the region of the pharynx.