The Polycladida represents a highly diverse

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of free-living marine

flatworm Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...

s. They are known from the

littoral The littoral zone, also called litoral 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 i ...

to the

sublittoral The littoral zone, also called litoral 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 in ...

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 group ...

s. Only a few

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

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 (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

, an elongated intestine with numerous complex

diverticula In medicine or biology, a diverticulum is an outpouching of a hollow (or a fluid-filled) structure in the body. Depending upon which layers of the structure are involved, diverticula are described as being either true or false. In medicine, t ...

, and multiple

ocelli A simple eye or ocellus (sometimes called a pigment pit) is a form of eye or an optical arrangement which has a single lens without the sort of elaborate retina that occurs in most vertebrates. These eyes are called "simple" to distinguish the ...

. 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 classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

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 animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s, such as bivalve

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

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 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. Description As currently defined, Acotylea includes polyclads with a Lang's vesicle in the female reproductive system and the male copulatory apparat ...

* Cestoplanidae * Planoceridae *

Stylochidae Stylochidae is a family of Polycladida, polyclad flatworms. It includes the species ''Stylochus zebra'', which usually lives in shells occupied by the hermit crab ''Pagurus pollicaris'', though it can sometimes be found free living on rocks and p ...

Ditremageniidae Cotylea is a suborder of free-living marine turbellarian flatworms in the order Polycladida. Families *Anonymoidea Dittmann, Cuadrado, Aguado, Noreña & Egger, 2019 **Anonymidae Lang, 1884 **Chromoplanidae Bock, 1922 *Boninioidea Dittmann, Cua ...

* Prosthiostomidae * Opisthogeniidae * Pseudocerotidae

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''

References

Turbellaria {{Improve categories, date=June 2024