The Stylet

Diversity and Systematics of Acoela and Nemertodermatida

About Acoelomorpha

The classification of Acoela down to the “family level” was revised based on a phylogenetic study (U. Jondelius, A. Wallberg, M. Hooge, and O. I. Raikova, “How the Worm Got its Pharynx: Phylogeny, Classification and Bayesian assessment of Character Evolution in Acoela,” Systematic Biology, 2011.doi:10.1093/sysbio/syr073 )

Here is an overview of the current acoel classification (with diagnostic features of higher taxa in brackets). The suprafamilial clade names are informal, i.e. not part of a linnean classification.

1. Acoela (digestive parenchyma, biflagellate spermatozoa)
1.1. Diopisthoporidae Westblad, 1940
1.2. Bitesticulata (Paired or follicular testes, ventral gonopore. The most inclusive clade that contains Paratomella rubra and Symsagittifera roscoffensis but not Diopisthoporus longitubus)
1.1.1. Paratomellidae Dörjes, 1966
1.1.2. Bursalia (Copulatory bursa often present. The most inclusive clade that contains Oligofilomorpha interstitiophilum and Childia groenlandica but not Paratomella rubra) Prosopharyngida (Muscular pharynx in anterior part of body. The most inclusive clade that contains Hofstenia miamia and Oligofilomorpha interstitiophilum but not Haploposthia rubra) Hallangidae Westblad, 1946 Hofsteniidae Papi, 1957 Solenofilomorphidae Dörjes, 1968 Crucimusculata (Ventral crossover muscle fibres). The most inclusive clade that contains Actinoposthia beklemischevi and Childia groenlandica but not Haploposthia rubra) Dakuidae Hooge 2003 Isodiametridae Hooge & Tyler 2005 Otocelididae Westblad, 1948 Proporidae Graff, 1882 Aberrantospermata (Spermatozoa with 9+0 or 9+1 axonemes. The most inclusive clade that contains Neochildia fusca and Childia groenlandica but not Actinoposthia beklemischevi) Convolutidae Graff, 1905
     This family now includes species formerly classified in Anaperidae and Sagittiferidae, which have both been synonymized with Convolutidae Mecynostomidae Dörjes, 1968 This family now includes the species formerly classified in Childiidae as that taxon was synonymized with Mecynostomidae
1.3. Acoela Incertae cedis, no phylogenetic hypothesis available
1.1.3. Actinoposthiidae (poorly sampled in the phylogenetic study, indications that this may be a polyphyletic assemblage)
1.1.4. Anthroposthiidae (single species)
1.1.5. Antigonariidae (single species)
1.1.6. Nadinidae (single species)

1.1.7. Tauridiidae (single species)


The statocyst with a single highly refractile statolith is located anteriorly and can often be seen in the dissecting microscope. In many species there are prominent glands that open through an anteroterminal pore; these are the frontal glands. In mature animals, eggs are easily detected as very large cells with a large nucleus. Discerning the testes usually requires higher magnification. They are normally located anterolaterally to the ovaries, but there are exceptions. There are many different types of male copulatory organs including a simple pore, a ciliated antrum, a muscular penis or a sclerotized stylet. A female gonopore is present in most, but not all species. Female accessory organs may be present in the form of a bursa for storage of allosperm. In some species the bursa is equipped with a bursal nozzle, a narrow passage through which allospermatozoa have to pass in order to fertilize the oocytes. Other features of importance for identification of acoels are pigment patterns, presence of a pharynx (in a small number of species), presence of symbiotic algae, and body shape and size.

Magnesium chloride (MgCl₂) extraction of acoels and other meiofauna:

Acoels are common in marine sediments such as sand and mud. The MgCl₂ method is well-suited for the extraction of acoels and other meiofauna from samples taken from a sandy beach or sublitorally. This video demonstrates how to perform a magnesium chloride extraction.

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith