Diversity of The World of Life
APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE
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
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
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.
growth, development, and activities of lancelets are under genetic and
hormonal control, influenced by the environment.
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.