Spoon worm computer model
Echiurans: Spoon-worms
Above: a 3D computer model of a spoon-worm of the Bonellia type, rendered in Pov-Ray. The body
consists of the trunk and the long extensible proboscis with its T-fork. Just beneath the proboscis is
a pair of hooklike projections, the chaetae or setae (shown in black, the things you may have
thought were eyes!).
Echiura (Spoon-worms)

There are about 135 species of known echiuroid. These bizarre organisms typically live in burrows in the sand in salt
water (including the deep ocean and sea lochs). The proboscis, which is non-retractible but often highly extensible,
emerges from the burrow, typically at night, creeping over the surface of the substrate and sweeping it for food particles
that are wafted by minute beating cilia along the proboscis gutter to the mouth.

External Characteristics

Echiuroids are coelomate burrowing worms, and are unsegmented in the adult. The typical form is that of the female,
the male being minute (only a few millimetres long)! (This
extreme sexual dimorphism is characteristic of the
spoon-worms). The body may be up to 40 cm in length, excluding the proboscis. The body is rounded or a cylindrical
sausage-shape and is smooth or papillate. The proboscis may be very long and extensible; for example,
Ikeda has a
40-cm trunk and a 1.5-m trunk.
Bonellia has an 8cm trunk, but can extend its proboscis up to 2 m. Submersibles 5000
m down in the Pacific have seen proboscides emerging from the substrate that are more than 8 m long; we can only
guess how large the attached worms are! In some species, however, the proboscis is much shorter than the trunk, for
example in
Echiurus. The proboscis is actually a cephalic lobe (cf. the prostomium in annelids) containing the brain and
is non-retractable. It is able to extend by ciliary creeping and is very mobile. The proboscis is flattened and may fork
and usually has a ciliated ventral gutter. The mouth opens at the proboscis base.

Just posterior to the proboscis is one pair of ventral chaetae / setae, which are curved or hooked. In addition
Urechis
and
Echiurus also possess 1 or 2 circlets of setae around the posterior of the trunk. There is also one pair of anal
vesicles on the cloacal region of the intestine.

Echiuroids are usually drab gray or brown in colour. Some (e.g.
Bonellia) are green, while some are red or rose.
Nutrition

Most echiuroids burrow in mud, sand or else reside in small natural openings in rocks and among shells. Most
echiuroids are detritus feeders and obtain food from surrounding surfaces with their proboscis. The proboscis is
projected from the burrow and stretches over the substrate by ciiary creeping, due to the cilia on its ventral surface.
The extended proboscis sweeps the substrate and detritus adheres to secreted mucus covering the proboscis and is
driven back into the median ventral ciliated groove, which conveys the particles to the mouth. The anus is on the
posterior terminus. The gut, especially the intestine, is highly coiled.

The innkeeper worm,
Urechis caupo lives in a U-shaped burrow. The proboscis is very short. A circlet of mucous glands
girdles the anterior part of the trunk just behind the setae. The glands are brought into contact with the burrow wall and
their secreted mucus is spun-out as the worm backs-up, forming a funnel of mucus. Peristaltic action pumps water
through the burrow, all the water passes through the funnel. The mucous funnel traps particles, and when it is laden
with food this mucous net is detached from the body. The worm backs-up, seizes the net with its proboscis and swallows
it. About 30 litres of water pass through an
Urechis burrow each day.
Circulation & Coelom

The blood-vascular system is closed (except in Urechis) and similar to that of annelids. The blood is colourless, but
some coelomocytes contain haemoglobin. The
schizocoelous coelom is a large, fluid-filled cavity in which the fluid
and amoebocytes circulate. The circumintestinal vessels are contractile and body wall movements also aid the
circulation.

Excretion

Spoon-worms possess 1-100’s of pairs of large sac-like metanephridia. (Bonellia has one pair, Echiurus two pairs and
Ikeda 100’s of pairs). In Bonellia and Echiurus the anterior nephridiopores open to the outside just behind the anterior
setae. In
Thalassema the males have more nephridia than the females. Echiuroids also possess one pair of anal sacs,
which are simple or branched diverticula arising from each side of the rectum. These sacs possess numerous ciliated
funnels over their surface. These funnels open into the coelom, and their collected waste is eliminated through the
anus.
Respiration

In Urechis the hindgut pumps water in and out and is a major site for gas exchange. Gas exchange also occurs over
the body wall, especially the proboscis, which has a large surface area. Peristaltic waves ventilate the burrow. The
frequency of these waves increases in oxygenated water, suggesting that echiuroids are oxyconformers.

Movement

Spoon-worms move within their burrows by peristaltic contractions. The setae provide traction for gripping the burrow
walls. Some echiuroids can squeeze through very confined spaces. The body is very well muscularised and a spatious
coelom acts as a hydrostatic skeleton. However, these worms show no locomotory movements outside their permanent
residence. Longitudinal, circular and oblique muscles are present in the body wall.

Urechis controls its internal hydrostatic pressures by adjusting volume of water in the hindgut. When the internal
pressure increases due to a peristaltic pulse, the anus opens expelling water resulting in a drop in internal; pressure. If
internal pressure falls, then water is inhaled into the hindgut. Forced exhalent currents relieve pressure in the gut and
clear the burrow of its contents.
Nervous System

There is a single, solid ventral nerve cord, which bifurcates anteriorly into 2 circumpharyngeal connectives that unite in
the proboscis in the central nervous mass or brain. The ventral nerve cord may be sinuous, contains no ganglia and
gives off 100’s of nerves. A multicellular giant fibre in the ventral nerve cord conducts signals at 1.5 m/s in both
directions. Pacemakers in the nerve cord regulate anal pumping and peristalsis. Peristalsis is initiated in the proboscis,
which contains the brain.
Reproduction & Development

The sexes are separate. The gonads are in the peritoneum of the ventral mesentery in the trunk. Gametes are
released into the coelom where they mature and subsequently escape through the nephridia. Fertilisation is usually
external, except in
Bonellia. In this species the dwarf males are ‘parasitic’ on the females. The males are minute and
ciliated and have no proboscis, no blood system and a vestigial gut. The males are found in the female nephridia, in
the coelom, or in the oesophagus / pharynx or, as in
Pseudobonellia, in specialised male tubes. Developing males
may occur on the female proboscis. The eggs are fertilised in the nephridia.

In
Bonellia viridis the female trunk is up to 8 cm long, while the male is only 1-3 mm long and lives in the female
oesophagus or nephridia. Sex is determined at the larval stage. Any larva contacting and entering a female develops
into a male, under the influence of a female hormone. Initially the larva contacts the female proboscis, and attaches to
it via an adhesive secretion. The proboscis secretes the hormone that causes the larva to initiate male development.
After a few days on the proboscis, the male passes into the female oesophagus and into a nephridium or a special
male tube, where it matures in 1-2 weeks. One female may house about 20 males. Larvae that do not encounter a
female become females and require over one year to reach maturity.

Spiral cleavage gives rise to free-swimming trochophore larvae. These trochophores pass through a metamerically
segmented stage with 10 pairs of rudimentary coelomic pouches. Spoon-worms are thus thought to be related to
annelids and sipunculoids.

Ecology

Most species live in sand or mud burrows or in natural crevices or spaces among rocks and shells. Thalassema
inhabits dead sand dollar shells and becomes too large to leave its permanent abode. All are marine, most sub-littoral
and a few live in deeper waters. Spoon-worms are cosmopolitan and some species live in brackish water.