The subphylum Cephalochordata
is a small group that includes the well-known amphioxus (Branchiostoma).
An amphioxus is pointed at both ends and is practically all trunk and
tail. The pharyngeal slits are very obvious. Cephalochordates like amphioxus
are found off sandy beaches where they burrow into the sandy bottom with
eel-like movements. They make a U-turn in their burrows and emerge with
just their anterior end protruding for filter-feeding. Cilia on the pharyngeal
surfaces create a steady current of water through the mouth into the
pharynx and out through the pharyngeal slits. Food particles trapped
in the mucus are directed into the digestive tract. The notochord extends
into the head region (hence "cephalo" chordata).
Compared with the urochordates, the cephalochordates are noteworthy in
retaining all major chordate characteristics throughout their
entire life cycle. Although primarily burrowing suspension feeders
they are much more capable (though slow) swimmers than urochordates.
Amphioxus swims by simple side-to-side undulations when leaving
its burrow to move to new territory.
Evidence from DNA sequences suggests that the Cephalochordata are the
"sister group" (nearest evolutionary relative) of the chordates
and are next most closely related to the urochordates. Knowing this,
one obvious
question is how did an organism that lacks chordate characteristics as
an adult (like a urochordate) evolve into an organism that displays
those characters in adulthood? One hypothesis suggests that this evolutionary
step involved paedomorphosis, the retention of larval
features in the sexually mature adult. In this case, paedomorphosis may
have involved changes in the genes that control timing of developmental
events such as the onset of sexual maturity. The early onset of sexual
maturity is a particular form of paedomorphosis called paedogenesis.
If a sexually mature organism that resembled a larval tunicate was successful
natural selection would have reinforced paedogenesis, ultimately eliminating
metamorphosis entirely, and giving us something similar to the cephalochordates
we see today.
Despite their similarities, there are several reasons why cephalochordates
are unlikely to be the direct ancestors of vertebrates, one of them
being
the lack of a complex brain and cephalic sense organs homologous to those
of vertebrates. A second reason is the extension of the notochord into
the snout (probably an adaptation for burrowing in marine sediment).
Thus, although the cephalochordates give us insights into the likely
ancestral vertebrate body plan, they and the vertebrates probably diverged
from a common ancestor far back in time.
|
Subphylum Cephalochordata:
the lancelet Branchiostoma (or amphioxus). This
small invertebrate displays all four chordate characteristics.
The pharyngeal
slits
function in suspension feeding. Water passes into the pharynx and
through slits into the atrium, a chamber that vents to the outside
via the atriopore. Food particles trapped by mucus are swept by
cilia into the digestive tract. The muscle segments you can see
in this photo of a lancelet produce the sinusoidal swimming of
these animals. (From Campbell
6th edition's Fig
34-4a.)
Subphylum Cephalochordata: the
lancelet Branchiostoma (or amphioxus). The
muscle segments you can see in this photo of a lancelet produce
the sinusoidal swimming of these animals. (From Campbell
6th edition's Fig
34-4b.)
Amphioxus (Branchiostoma sp). The
anterior end of an immature adult (60X).
|
Labs
