The Lecithoepitheliata are an order of

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 ...

flatworms 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 ...

. They are free-living worms, found in freshwater, soil, and marine environments. However, it is still poorly known their roles in the natural food web.

Description

Members of the order Lecithoepitheliata are distinguished from other flatworms by the presence of four nerve cords and the fact that the ovary forms a single structure that produces both the eggs (

ovocyte An oocyte (, oöcyte, or ovocyte) is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female germ ...

s) and nourishing

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...

cells (vitellocytes). The vitellocytes form a follicle around the ovocyte, hence the name of the group, which means "with a yolk

epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

". In most other rhabditophorans, yolk cells, where present at all, are typically formed in glands derived from the ovaries, but separate from them, called vitellaria. Other diagnostic features of this order include the presence of a sharp stylet on the end of the penis, and a simple, unbranched, intestine. The penis lacks a proper pore and is protruded through the mouth.

Phylogeny

The order Lecithoepitheliata is usually classified as belonging to the clade Neoophora, a subgroup of

n flatworms distinguished by the presence of ectolecithal eggs, i.e., yolk is stored outside the ovocytes. However, recent molecular studies have grouped Lecithoepitheliata (at least the family Prorrhynchidae) with the order

Polycladida The Polycladida represents a highly diverse clade of free-living marine flatworms. They are known from the littoral to the sublittoral zone (extending to the deep hot vents), and many species are common from coral reefs. Only a few species are fo ...

, which has endolecithal eggs, i.e., yolk is stored inside the ovocytes as in most animals. Another previous study, which included representatives of both families of Lecithoepitheliata, concluded that Prorhynchidae is the basalmost clade of Neoophora, followed by Gnosonesimidae. In the former scenario, the ectolecithal condition would have evolved independently twice, and in the latter it would have arisen only once, but Lecithoepitheliata would be paraphyletic. Phylogeny of Lecithoepitheliata in relation to other rhabditophorans according to Laumer & Giribet (2011): Phylogeny according to Egger et al. (2015):

References

Rhabditophora Platyhelminthes orders {{Flatworm-stub