Sometimes considered a subphylum of the Chordata
, the Cephalochordates
are composed of about
22 species of small fish-like organisms commonly called lancelets
. Amphioxus
(actually
Branchiostoma
sp.),
is an organism that is familiar to most biology students, as it is often used in the
laboratory to illustrate some of the basic chordate
features.
The Cephalochordates
have a notochord
, - but no vertebrae, cranium
, bone
, cartilage
,
or red corpuscles
in the blood
. They have an endostyle
, which is thought to be homologous
with the vertebrate
thyroid
gland. They don't have a true brain
, however they possess two pairs of cerebral
lobes and nerves.
They have separate sexes.
Lancelets
live mostly in shallow marine waters in tropical and subtropical areas, however, some species
extend into temperate seas as far north as Norway
and as far south as New Zealand
. They are especially
common in coastal
areas of China
. Present day lancelets
, such as
Branchiostoma
sp.,
bury in sand where they draw in water through their mouth and
filter out small organisms for food.
|
