The most common hemichordates
are the enteropneusts or acorn worms. Acorn worms have a ciliated proboscis
that has a mucous coating and cilia with which to trap food particles
and conduct the food along a ciliated groove into the mouth. Like the
echinoderms, the hemichordates have a ciliated larval stage, which many
systematists suggest is a strong phylogenetic link between the echinoderms
and hemichordates. Adult hemichordates are bottom-dwelling marine filter-feeders.
In addition to the general deuterostome characteristics, hemichordates
possess paired pharyngeal slits or pouches (in some) and most have
a dorsal nerve cord
which is sometimes hollow. Pharyngeal slits are essentially holes through
the walls of the digestive tract that open to the outside through the
body wall.
They are involved in food gathering: water is sucked in through the mouth,
and the food particles in it stick to a mucous coating on the proboscis
and the inside
of the pharynx. The filtered water then passes out through the pharyngeal
slits while the food particles are drawn into the digestive system
by cilia.
Pharyngeal slits (or pouches) and a dorsal (hollow) nerve cord are
found in all chordates as well. Some systematists point to these
shared characteristics
as
evidence of an evolutionary connection between hemichordates and chordates.
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