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The
Diversity of The World of Life
Animals
(Metazoa)
Acorn Worms etc.
(Hemichordata)
Representative
Acorn
Worm
Biology
APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE
90
ENVIRONMENTS
Hemichordates are marine species, living under stones or shells, in burrows in the
seafloor, or sometimes in tubes.
DESCRIPTION
Acorn worms are the typical form.
The wormlike body, from one to one-hundred inches long, is composed of
three major divisions: In front, is the round to conical, sometimes acorn-shaped
"proboscis"; in the middle is the "collar"; and to the
rear is the long, tapering
"trunk".
The body is given shape in large part by "hydrostatic" support
(water pressure) within a cavity lying inside the proboscis and within
the "coelom" (the membrane-lined body cavity outside the gut) in the
collar and trunk.
Apparently segmented "hepatic caecae" (liver sacs) may be visible on the
trunk of an acorn worm. Sand may adhere to
mucous secreted by the animal and form a camouflaging tube in which it
lives. Many species stink!
Atypical hemichordates are minute, vase-like, and like
bryozoans (Unless otherwise indicated, the following discussion
pertains only to typical hemichordates, the acorn worms).
FEEDING HABITS
Acorn worms are scavengers and filter-feeders.
MOTION
Hemichordates have muscle tissue, although they are
apparently very "sedentary" animals (that is, most do not move
about very much).
DIGESTION
Like various life stages of true chordates, hemichordates
typically have "pharyngeal gill clefts", which perforate both
sides of the front of the trunk. A water current is drawn-in by cilia through the mouth (at
the base of the proboscis, at the front of the collar) and through the
pharynx and out through the externally opening pores of the gill slits (In
addition, there is a "buccal pouch", from the mouth, up inside
the proboscis). Food
particles that stick to the mucous secreted on the proboscis are also
carried by the cilia through the mouth and into the pharynx, from which
the food passes into the rest of the gut. The digestive system is "complete",
with both a mouth and (near the rear end of the trunk) an anus.
Hemichordates living in "U-shaped" burrows on the
beach swallow both sand and food (similar to the feeding habits of earthworms) and pile-up coils of their sandy "fecal castings" at
the exit of their burrows.
Some deep-water, tube-dwelling hemichordates bear
tentacles; and some of these creatures also lack gill slits. The
creatures are presumably similar to the
"missing links" between hemichordates and their ancestors, prehistoric sea-lily-like
animals.
RESPIRATION
Dissolved gases and presumably other materials are
typically exchanged across the surface of the gill clefts (which are not
fully developed gills, as in true fishes).
CIRCULATION
There is an "open" circulatory system (unlike in
true fishes).
As in annelids but not in
vertebrates, blood flows anteriorly
(forwards) within a
dorsal vessel (above the gut) through the collar and into a "heart
vesicle", within the base of the proboscis. Probably aided by the contraction of
large vessels, the proboscis pumps the blood even farther forward, to the
"glomerulus" (See "Excretion", below) and then down
into the ventral vessel, which runs posteriorly (backwards) below the gut
within the collar; once back in the trunk, the blood circulates freely within the coelom.
Some branches of the vessels pass near the gill slits.
EXCRETION
Soluble wastes are probably collected from the blood by the
"glomerulus", lying within the proboscis cavity, and excreted
from that cavity through a dorsal pore (an opening on the upperside) to the outside
(Water may also be drawn in through the dorsal pore, to make the proboscis
firm, for burrowing).
COORDINATION
The
growth, development, and activities of acorn worms are under genetic and
hormonal control, influenced by the environment.
Unlike the various life stages of true chordates, hemichordates have a
"diffuse" nervous system, with no "dorsal nerve
cord" (or supportive "notochord") running the length of the
body; although in some hemichordates there is a rather tubular dorsal
nerve structure within the collar (Animals like hemichordates are the presumed
ancestors of us chordates).
REPRODUCTION
Asexual reproduction is possible in the atypical,
moss-animal-like forms, via specialized "buds"; and the typical
hemichordates, acorn worms,
can at the very least re-grow lost parts.
Acorn
worms have separate (male and female) sexes, each typically with several pairs of gonads, each discharging gametes through pores
perforating the sides
of the front of the trunk. The masses of mucous-covered eggs are fertilized in the open
sea. Many hemichordates
develop through a planktonic "tornaria" larva (with the mouth
anterior (at the front end), the anus posterior (at the rear end), and
bands of cilia winding around the body -- very similar to the larvae of
echinoderms, with whom hemichordates probably share a common ancestor
(something like a sea-lily). Other
acorn worms develop "directly" -- the young just look like small
adults.
Animals
(Metazoa)
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