Branchiostoma

Branchiostoma is commonly called a lancelet

Branchiostoma, formerly referred to as amphioxus, is the living invertebrate chordate that most closely resembles vertebrates.  The ancestors to the first vertebrates may well have been similar to this animal.  The animal is most commonly referred to as the lancelet, undoubtedly because of its long pointed, cigar-shaped, appearance.  These small, translucent animals are one to several inches (up to 50 mm) in length and inhabit the coastline of marine waters.  Their notochord, the most fundamental characteristic of chordates, is unusual because it extends well into the animal’s head.  This is believed to be an adaptation better enabling this animal to burrow in the sand where it spends most of its life.  Despite its appearance the lancelet lives a sedentary life with only its head protruding from the soft sediment that is its home. 

Head shows tentacles around mouth to help feeding

Branchiostoma is a filter feeder much like another protochordate, or non-vertebrate chordate, the tunicates, or sea squirts.  Food particles are swept in with the sea water that enters the mouth.  The water continues into the pharynx, or branchial basket, which filters out the minute bits of food.  The water is strained through the pharyngeal slits, and then enters into an atrium where it is expelled to the outside through the atriopore.  The remaining food particles are snared by a mucous that is secreted from a gland called the endostyle.  The endostyle is located beneath the pharynx and it is this gland that serves as the basis for the thyroid that develops later in vertebrate animals.  The beating of cilia forms the mucous into a food band that gradually moves up the pharynx and into the intestine for the process of digestion. 

Lancelets spend most of their life burrowed

While Branchiostoma lives mostly a sedentary life its body shape and musculature give it the appearance of an active swimmer.  Its segmented musculature, or myotomes, enables it to disperse from its point of birth as well as provide the propulsion needed for burrowing.  Movement is accomplished by waves of muscle contractions that alternate from side to side and extend the length of the body from the head to the tail.  The resulting lateral undulations propel the lancelet forward. 

The notochord extends the full length of the head

Lancelets’ musculature, notochord, dorsal nerve tube, pharyngeal slits and a circulatory system are all characteristics it shares with vertebrates.  Its body plan probably resembles that of the most primitive of vertebrates but it lacks a number of characteristics distinctive of the modern vertebrate.  Its circulatory system is not closed.  Arteries and veins are linked by sinuses.  There is no heart or capillaries.  There are no blood cells.  Oxygen is transported in a solution containing no oxygen-carrying pigment.  There are no gills.  The entire body surface is sufficient for the exchange of respiratory gases.  The animal has bilateral symmetry but there are no true paired fins.  Instead, Branchiostoma has ventrolateral tissue folds that extend from the pharynx to the atriopore. 

Segmented musculature is like that of vertebrates

There is little fossil record of the cephalochordates that include Branchiostoma and the ones discovered only superficially resemble the modern lancelet.  Their soft body makes fossilization extremely rare.  What does exist from the early Cambrian period of more than 500 million years ago reveals a simple, bilaterally symmetrical animal that could conceivably provide the basis for the lineage that arrives at the modern lancelet as well as the lineage the developed into the diverging forms of modern-day vertebrates.