With Douglas Drenkow


The Diversity of

The World of Life

Featured Topics


About the Author

Legal Notices

The Diversity of The World of Life

Chordates (Chordata)




Lancelets (Amphioxus etc.)





Lancelets are marine species, often buried in the seafloor.


The "coelom" (the membrane-lined body cavity outside of the gut) is much reduced; and as in various life stages of other chordates, a strong, yet flexible, segmented "notochord" provides the main support for the body.  Lancelets are up to two inches long and indeed lance-like -- elongated, flattened from side to side, and tapered towards both ends.  Unlike in other chordates, however, the notochord extends virtually the entire length of the body -- hence the name "cephalochordates" ("head chordates").


Lancelets are filter-feeders.


The body has well-developed muscle tissue.  Moreover, the "metameric" (segmented) musculature and notochord allow for alternating waves of muscle contractions to flow from head to tail on opposite sides of the body, thus producing the typical swimming movements of a chordate.  Unlike true fish, however, lancelets are poorly finned, typically "corkscrewing" through the water when they swim (They are apparently more at home within burrows).


Typical of various life stages of other chordates, cephalochordates have "pharyngeal gill clefts" (gill slits in the "throat").  A current of water is drawn in past small "fingers" and tentacles around the mouth by cilia forming a "wheel organ" just ahead of the mouth and by cilia within the pharynx (throat).  The water current then moves through the gill slits in the pharynx and empties into a long "atrial cavity", within the ventral (lower) side of the body.  The water current finally exits out through a pore, towards the rear of the body.  Food particles snared by mucous in the pharynx enter the intestine (the gut), and smaller particles are diverted to and digested/sorted within the "midgut caecum" (an outpouching from the floor of the mid-gut).  The gut is "complete" -- it has both a mouth and an anus, the anus opening posterior to (behind) the exit pore of the atrial cavity and just anterior to (ahead of) the tail.


Although there are pharyngeal gill clefts, gases are exchanged mostly through the skin (the pharynx functions primarily in filter-feeding).


Dissolved gases and other materials are carried by a virtually "closed" circulatory system:  The blood is transported almost entirely within membrane-bound vessels.  The blood is pumped by the pulsing of "arteries" (out-going blood vessels):  There is no true, pulsing heart, only a collecting chamber, under the rear part of the pharynx.  From the collecting chamber, the blood moves forward through an "aorta" -- lying within the underside of the body -- and up through the bars of the gills.  From there, the blood is pumped back through a pair of aortas -- lying within the upperside of the body -- and into the spaces within the tissues of the body (the blood is not confined within "capillaries" -- tiny, membrane-lined vessels in intimate contact with the bodily tissues -- as in vertebrates).  Finally, the blood returns through "veins", to the collecting chamber.

Note that only cephalochordates and vertebrates have this pattern of blood circulation -- forwards in the ventral part (the underside) of the body and rearwards in the dorsal part (the upperside, or back) of the body (For example, compare segmented worms).  Also like vertebrates, cephalochordates have supplementary channels for "lymph" (fluid from blood filtered through bodily tissues).


Ciliated, blind-ended "protonephridia" (somewhat like those of flatworms), situated just above gill bars, collect excess salts and water and excrete them, as urine, into the atrium, flushed-out by the water current.


The growth, development, and activities of lancelets are under genetic and hormonal control, influenced by the environment.

Like other chordates, cephalochordates have a tubular "nerve cord", dorsal to (above) the supportive notochord.  Although lancelets are sensitive to such stimuli as light, chemicals, and touch, there is no true brain and no significant "cephalization" (development of a "head" end) -- these animals are more at home lying in their burrows than swimming through the water.


Lancelets reproduce sexually, with the (male and female) sexes separate.  Gametes (sperms or eggs) are discharged by the gonads simply rupturing, within the reduced coelom.  The gametes are then flushed out through the atrium.  The eggs are fertilized and the young develop in environmental water.

Chordates (Chordata)


(c) 2004 D.D.  All Rights Reserved.

Photo of Cells:  H.D.A. Lindquist, US EPA