The Lecithoepitheliata are an order of
rhabditophoran
flatworms. 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 ...
s) and nourishing
yolk 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 one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
". 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
Neoophora is a group of rhabditophoran flatworms with ectolecithal eggs, i.e., yolk is not present in the egg as in most animals, but rather is secreted by accessory glands called vitellaria or yolk glands. These glands have the same embryonic o ...
, a subgroup of
rhabditophoran 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, 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
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