Page 273 - (DK) Ocean - The Definitive Visual Guide

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Flatworms

POSSESSING VERY thin, sometimes
DOMAIN Eucarya
transparent bodies, flatworms are among
KINGDOM Animalia
the simplest of animals. Marine species
PHYLUM Platyhelminthes
mostly belong to a colorful group called
Xenacoelamorpha
polyclad flatworms—leaf-shaped animals,
CLASSES 8
common on coral reefs. Some are found in
SPECIES 20,430 fresh water, and many are parasitic. In the
FOOD SEARCH
oceans, parasitic flukes and tapeworms are This polyclad flatworm is
common in fish, mammals, and birds. Most flatworms belong to the searching for food using simple
eyespots and chemical receptors
phylum Platyhelminthes but acoel flatworms (see below) are now
on the margins of its head.
separated into the phylum Xenacoelomorpha (or Acoelomorpha).
Anatomy Reproduction
The flatworm has a simple, solid structure with no internal cavity. It is Most flatworms are hermaphrodites, so every individual has both
so thin that oxygen can diffuse in from the water, and there are no blood ovaries and testes. The reproductive system is complex for such a primitive
or circulatory systems. The head end contains sense organs; advanced animal and includes special chambers and tubules where the ripe eggs
species have primitive eyes. The gut opens to the outside at one end, the are fertilized. When two polyclad flatworms
opening serving as both mouth and anus. In polyclad flatworms, this meet, they may briefly touch heads and bodies
opening is in the middle underside in a short ritual before mating. After mating,
BODY SECTION of the body. When feeding, they the eggs are released into the water, laid in
In flatworms, the space between the internal
organs is filled with soft connective tissue extend a muscular tube (pharynx) sand, or stuck to rocks. In some flatworms,
crisscrossed by muscles. out of the mouth to grasp their the eggs develop directly into juvenile worms
food. Polyclad flatworms are but in others they develop initially into an
dorso-ventral
longitudinal
muscle gut muscle covered in tiny hairs (or cilia) eight-lobed planktonic larva. Called Müller’s
connective gut which, together with simple larva, it swims for a few
tissue branch muscles, help them to glide over days and then settles onto COMPLEX APPARATUS
almost any surface. The anatomy the seabed and flattens Some flatworms undetake
“penis fencing,” where
of tapeworms and flukes is adapted out into a young each tries to stab the
to suit their parasitic lifestyle. flatworm. other and inject sperm.

Most brightly colored flatworms are
CLASS ACOELOMORPHA CLASS RHABTITOPHORA
found on tropical reefs, but the candy
Acoel Flatworm Candy Stripe stripe flatworm is an exception and
Waminoa species Flatworm can be found as far north as Norway.
Generally a cream color, it is marked
1
LENGTH Less than / 4 in with reddish brown, lengthwise
Prostheceraeus vittatus
(5 mm) stripes. The head end of its flattened,
LENGTH
DEPTH Not recorded leaf-shaped body has a pair of distinct
Up to 2 in (5 cm) tentacles and groups of primitive
HABITAT On bubble
coral (Pleurogyra DEPTH eyes. As it crawls along, the flatworm
0–100 ft (0–30 m)
sinuosa) pushes the edges of its body up into
HABITAT
DISTRIBUTION Tropical Indian and Pacific oceans folds; it is also able to swim using
Muddy rocks
CLASS ACOELOMORPHA sinuous movements of the body.
These diminutive flatworms look like DISTRIBUTION Temperate waters of northeastern Usually found in rocky areas, it has
colored spots on the bubble coral on Green Acoel Atlantic and Mediterranean also been seen on sand.
which they live. Their ultra-thin
bodies glide over the coral surface as Flatworm
they graze, probably eating organic
debris trapped by coral mucus. Acoel Convoluta roscoffensis
1
flatworms have no eyes and instead of LENGTH Up to / 2 in
a gut, they have a network of digestive (1.5 cm)
cells. They are able to reproduce by DEPTH Intertidal
fragmentation, each piece forming a HABITAT Sheltered
new individual. The genus is difficult sandy shores
to identify to species level and the
distribution is uncertain. DISTRIBUTION Northeastern Atlantic; probably more
widespread than shown
flatworm on
bubble coral
Although difficult to see individually,
these flatworms show up when they
collect together in puddles of water
on sandy shores at low tide. Their
bodies harbor tiny, single-celled
algae (p. 248) that color them bright
green. In warm, sunlit pools the algae
can photosynthesize and pass some OCEAN LIFE
of the food they make to their host.
These flatworms are very sensitive to
vibrations and quickly disappear down
into the sand if footsteps approach.