The Polycladida represents a highly diverse

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

of free-living marine

flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegment ...

s. They are known from the

littoral The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely inundated), to coastal areas ...

to the sublittoral zone (extending to the deep hot vents), and many species are common from

coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Co ...

s. Only a few species are found in freshwater habitats.

Description

Polyclads range from to in length with a flattened, roughly oval, body shape and, in many cases, a pair of short tentacles on the head. They are distinguished from other related animals by the presence of a folded

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...

, an elongated intestine with numerous complex diverticula, and multiple ocelli. The etymology of the order name ''Polycladida'' corresponds to the two

ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...

words (), meaning "numerous", and (), meaning "branch". It refers to the ramified shape of the intestine in these flatworms. Most polyclads hide away from direct light. However, some of the brightly colored species often are active during the day. With their flamboyant coloring they advertise their potential toxicity to visual predators such as fish.

Development

Some polyclads develop through a Müller's larval stage.

Ecology

Often polyclads are associated with other

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s, such as bivalve

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...

s, or ascidians. In such associations, the worms may use the invertebrates as a source of food, or they may find protection from predators inside the structural framework of these hosts.

Systematics

The order Polycladida is divided into the two suborders, Cotylea and Acotylea, based on the character "presence/absence of a cotyl or sucker". Of the two, the Acotylea is the larger group with over 26 families worldwide. Acotyleans are major predators of sessile marine invertebrates such as all commercial bivalves species (including pearl and rock oysters), mussels, scallops and giant clams. Acotyleans are dull in coloration, and

cryptic Cryptic may refer to: In science: * Cryptic species complex, a group of species that are very difficult to distinguish from one another * Crypsis, the ability of animals to blend in to avoid observation * Cryptic era, earliest period of the Earth ...

in their behavior, hiding in crevices and under coral during the day. Cotyleans, on the other hand, with as many as 16 families, are prominent members of tropical coral reef communities and have bright, colorful bodies. Although cotylean flatworms are conspicuous predators in subtropical and tropical ecosystems, they are difficult to study. These worms are very fragile and when disturbed can break apart. Some examples of Polycladida families are as follows: *

Discocelidae Acotylea is a suborder of free-living marine turbellarian flatworms in the order Polycladida. Taxonomy * Superfamily Cryptoceloidea Faubel, 1984 ** Family Cryptocelidae Laidlaw, 1903 ** Family Discocelididae Laidlaw, 1903 ** Family Ilyplan ...

Cestoplanidae Cotylea is a suborder of free-living marine turbellarian flatworms in the order Polycladida. Families * Boniniidae Bock, 1923 * Cestoplanidae Lang, 1884 * Chromoplanidae Bock, 1922 * Dicteroidae Faubel, 1984 * Diposthidae Woodworth, 1898 ...

Planoceridae Acotylea is a suborder of free-living marine turbellarian flatworms in the order Polycladida. Taxonomy * Superfamily Cryptoceloidea Faubel, 1984 ** Family Cryptocelidae Laidlaw, 1903 ** Family Discocelididae Laidlaw, 1903 ** Family Ilyplan ...

Stylochidae Stylochidae is a family of polyclad flatworms. It includes the species '' Stylochus zebra'', which usually lives in shells occupied by the hermit crab ''Pagurus pollicaris ''Pagurus pollicaris'' is a hermit crab commonly found along the Atlant ...

Ditremageniidae Cotylea is a suborder of free-living marine turbellarian flatworms in the order Polycladida. Families * Boniniidae Bock, 1923 * Cestoplanidae Lang, 1884 * Chromoplanidae Bock, 1922 * Dicteroidae Faubel, 1984 * Diposthidae Woodworth, 1898 ...

Prosthiostomidae Prosthiostomidae is a family of free-living marine polyclad flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', ...

Opisthogeniidae Cotylea is a suborder of free-living marine turbellarian flatworms in the order Polycladida. Families * Boniniidae Bock, 1923 * Cestoplanidae Lang, 1884 * Chromoplanidae Bock, 1922 * Dicteroidae Faubel, 1984 * Diposthidae Woodworth, 1898 ...

Pseudocerotidae Pseudocerotidae is a family of flatworms which includes the Bedford's flatworm. Pseudocerotidae are simple organisms categorized by their oval bodies and tentacles and bright colors. They use the cilia to glide along surfaces. Most commonly refer ...

Phylogeny

In 2017, molecular analyses produced well-supported phylogenetic hypotheses that confirmed the monophyly of Polycladida, as well as that of Acotylea and of Cotylea.

Images

File:Pseudobiceros hancockanus.jpg, Marine flatworm,
'' Pseudobiceros sp.'' File:Yellow papillae flatworm (Thysanozoon nigropapillosum).jpg, Yellow papillae flatworm,
''

Thysanozoon nigropapillosum ''Thysanozoon nigropapillosum'' is a species of polyclad, polyclad flatworms belonging to the family Pseudocerotidae. Some common names include gold-speckled flatworm, marine flatworm, yellow papillae flatworm, yellow-spotted flatworm, and yello ...

References

{{Taxonbar, from=Q134765 Turbellaria