Acoelomorpha is a

subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of " subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used th ...

of very simple and small soft-bodied animals with

planula A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and also in some species of Ctenophores, which are not related to cnidarians at all. Some groups of Nemerteans also produce larva ...

-like features which live in marine or

brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuary ...

s. They usually live between grains of

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

, swimming as

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

, or crawling on other organisms, such as

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

and

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.Cannon, L. R. G. (1986) ''Turbellaria of the World. A guide to families and genera''. Brisbane, Queensland Museum, 136 p. With the exception of two acoel freshwater species, all known acoelomorphs are marine.

Systematics

Etymology

The term "acoelomorph" derives from the

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 (), the ''

alpha privative An alpha privative or, rarely, privative a (from Latin ', from Ancient Greek ) is the prefix ''a-'' or ''an-'' (before vowels) that is used in Indo-European languages such as Sanskrit and Greek and in words borrowed therefrom to express negation or ...

'', expressing negation or absence, (), meaning "cavity", and (), meaning "form". This refers to the fact that acoelomorphs have a structure lacking a fluid-filled body cavity.

Classification

The subphylum Acoelomorpha is divided into two classes. There are at least 408 described species, with a majority of these falling within the Crucimusculata infraorder in Acoela. *

Acoela Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Historically they were treated as an order ...

comprise small flattened worms, classified into a dozen families. *

Nemertodermatida Nemertodermatida is a class of Acoelomorpha, comprising 18 species of millimetre-sized turbellariform, mostly interstitial worms. Taxonomy The order Nemertodermatida contains two families with 6 genera. The high level of cryptic diversity in ...

comprise millimetre-sized, mostly interstitial worms, distributed into two families.

Phylogeny

The soft bodies of acoelomorphs and the lack of some of the key bilaterian traits make them difficult to classify. Traditionally, based on phenotypic features, acoelomorphs were considered to belong to the phylum

Platyhelminthes Platyhelminthes (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called f ...

, which was long seen as the sister group to all other

bilateria Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left� ...

n phyla.Conway-Morris, S.; George, J. D.; Gibson R.; Platt, H. M. (1985) ''The Origins and relationships of lower invertebrates''. Oxford, Clarendon Press, 397 p. However, a series of molecular phylogenetics studies at the hinge between the 20th and 21st centuries demonstrated that they are fast-evolving organisms not closely related to platyhelminthes, therefore involving the

polyphyly A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as homoplasies, which ar ...

of flatworms. Actually, Acoelomorpha appeared to constitute a separate, deep-branching phylum, kingpin of bilaterian evolution. Yet their evolutionary affinities remain enigmatic as they might be the sister-group either to all other bilateral animals or to all deuterostomes. Resolving this debate would indicate whether acoelomorphs are simple or simplified. If they are the sister group to

Bilateria Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left� ...

, it would point to a simple

body plan A body plan, (), or ground plan is a set of morphology (biology), morphological phenotypic trait, features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many. This term, usually app ...

for the first bilaterian. Alternatively, if acoelomorphs are related to

deuterostomes Deuterostomes (from Greek: ) are bilaterian animals of the superphylum Deuterostomia (), typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia comprises three phyla: Chordata, Echinodermata, ...

, this would imply that their organisation is the result of secondary simplification. Xenoturbella japonica

In addition, comparative analyses of morphological, developmental, and molecular characters raised two points. *

Xenoturbellida ''Xenoturbella'' is a genus of very simple bilaterians up to a few centimeters long. It contains a small number of marine benthos, benthic worm-like species. The first known species (''Xenoturbella bocki'') was collected in 1878 and 1879 in the G ...

is the sister group to acoelomorphs, constituting the so-called

Xenacoelomorpha Xenacoelomorpha () is a small phylum of bilaterian invertebrate animals, consisting of two sister groups: xenoturbellids and acoelomorphs. This new phylum was named in February 2011 and suggested based on morphological synapomorphies (physica ...

clade. The close evolutionary relationship between Acoelomorpha and ''Xenoturbella'' is supported by the morphology (structure of epidermal

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

), the embryology (direct development without a feeding larval stage), and the concatenation of hundreds of proteins. * The phylogenetic placement of Xenacoelomorpha among bilaterian animals is not yet well defined, despite increased taxon and gene sampling, (re)-analyses of published data sets, and use of more sophisticated models of sequence evolution in phylogenomic studies. There is a conflict between two evolutionary hypotheses, with Xenacoelomorpha being the sister group to

Ambulacraria Ambulacraria , or Coelomopora , is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around ...

within

(i.e., the Xenambulacraria hypothesis) on the one hand, and Xenacoelomorpha as sister group to all other bilaterians (i.e., the

Nephrozoa Nephrozoa is a proposed major clade of bilaterian animals. Under this hypothesis, Xenacoelomorpha forms the earliest diverging branch of Bilateria, with all other bilaterians placed in Nephrozoa. It contrasts with the Xenambulacraria hypothesis, ...

hypothesis) on the other. However, the Nephrozoa hypothesis might reflect methodological errors resulting from model violations in the phylogenomic inference.

Anatomy

Acoelomorphs resemble flatworms in many respects, but have a simpler anatomy, not even having a gut. Like flatworms, they have no circulatory or respiratory systems, but they also lack an excretory system. They lack body cavities (acoelomate structure), a hindgut or an anus. The epidermal cells of acoelomorphs are unable to proliferate, a feature that is only shared with

rhabditophora Rhabditophora (from ''rhabdito''-, rhabdite + Greek -φορος ''phoros'' bearer, i.e., "rhabdite bearers") is a subphylum (previously a class) of flatworms. It includes all parasitic flatworms (clade Neodermata) and most free-living species ...

n flatworms and was for some time considered a strong evidence for the position of Acoelomorpha within Platyhelminthes. In both groups, the epidermis is renewed from mesodermal stem cells. The

nervous system In biology, the nervous system is the complex system, highly complex part of an animal that coordinates its behavior, actions and sense, sensory information by transmitting action potential, signals to and from different parts of its body. Th ...

of acoelomorphs is formed by a set of longitudinal nerve bundles beneath the ciliated epidermis. Close to the anterior end, these bundles are united by a ring commissure, but do not form a true brain, although it is hypothesized that such organization was the precursor of the

cephalization Cephalization is an evolutionary trend in animals that, over a sufficient number of generations, concentrates the special sense organ (biology), organs and nerve ganglia towards the front of the body where the mouth is located, often producing a ...

of the nerve system in more derived bilaterians. After decapitation, such a "brain" (rather, a ''cerebroid ganglion'') regenerates in a few weeks. The sensory organs include a

statocyst The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, crustaceans, and gastropods, A similar structure is also found in '' Xenoturbella''. T ...

– which presumably helps them orient to gravity –, and, in some cases,

ancestral An ancestor, also known as a forefather, fore-elder, or a forebear, is a parent or ( recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from ...

pigment-spot

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 ...

capable of detecting light. Acoelomorphs are simultaneous

hermaphrodite A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

s, but have no

gonad A gonad, sex gland, or reproductive gland is a Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...

s and no ducts associated with the female reproductive system. Instead,

gamete A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...

s are produced from the

mesenchymal Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood, or bone. The interactions between mesenchyme and epithelium help to form nearly ever ...

cells that fill the body between the epidermis and the digestive vacuole.

References

External links

Phylogeny of Lower Worms of the Meiofauna (Acoelomorpha)

Acoelomorpha at the Turbellarian taxonomic database

Acoelomorpha: A Phylogenetic Headache

Earthling Nature
' {{Taxonbar, from=Q131486 Animal subphyla