The

reproductive system The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are als ...

planarian A planarian is one of the many flatworms of the traditional class Turbellaria. It usually describes free-living flatworms of the order Tricladida (triclads), although this common name is also used for a wide number of free-living platyhelmint ...

s is broadly similar among different families, although the associated structures can vary in complexity. All planarians are

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...

s, so their reproductive system has a male and a female part. Both parts communicate with the surface of the body via a single opening called

gonopore A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects have a single common gonopore, except mayflies, which have a pair of gonopores. More specifically, in the unmodified female it is the ...

, which is located on the ventral side of the posterior half of the body.

Male part of the reproductive system

The male part of the reproductive system in planarians has a set of several

testicle A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...

s, distributed throughout the body in two or more rows. They are usually concentrated in the anterior two thirds of the body, although they can reach close to the posterior end. The testicles are connected to a pair of

sperm duct The vas deferens or ductus deferens is part of the male reproductive system of many vertebrates. The ducts transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. The vas deferens is a partially coiled tube w ...

s which run posteriorly towards the gonopore. In some groups, the sperm ducts met in their

distal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

part, forming the ejaculatory duct, which then opens in a cavity called “male atrium”. In others, like

land planarian Geoplanidae is a family of flatworms known commonly as land planarians or land flatworms. These flatworms are mainly predators of other invertebrates, which they hunt, attack and capture using physical force and the adhesive and digestive prope ...

s, both open in the prostatic vesicle, a glandular organ which then runs posteriorly and opens in the male atrium, usually through an ejaculatory duct. The male atrium is located anteriorly to the gonopore. Usually associated to the ejaculatory duct, there is a structure formed basically by a fold of the walls of the male atrium called “penis papilla”. The penis papilla is a permanent structure that occupies part or the whole male atrium and is protruded during copulation, thus also called a protrusible penis. In species without a penis papilla, the penis is formed only during copulation by a projection of the walls in the male atrium and called an eversible penis.Ogren, R. E. and Kawakatsu, M. (1990). ''Index to the species of the family Geoplanidae (Turbellaria, Tricladida, Terricola) Part I: Geoplaninae.'' Bulletin of Fujis Women's College. 29: 79-166. In land planarians, the structure of the male atrium may be quite variable between different genera. The most common constructions are: *Permanent or true penis papilla: A big penis-like fold occupies the whole male atrium or most of it and is crossed by an ejaculatory duct. It is found in the genera ''

Geoplana ''Geoplana'' is a genus of land planarians found in South America. Taxonomic history The genus ''Geoplana'' was erected in 1857 by William Stimpson and included most land planarians with several eyes distributed along the body.Stimpson, W. (1 ...

'', ''

Obama Barack Hussein Obama II ( ; born August 4, 1961) is an American politician who served as the 44th president of the United States from 2009 to 2017. A member of the Democratic Party, Obama was the first African-American president of the U ...

'', '' Cratera'', '' Paraba'', '' Polycladus'', '' Microplana'', among others. *Apparent penis papilla: Similar to a true penis papilla, but with a wide ejaculatory cavity instead of an ejaculatory duct. It is found in the genus '' Matuxia''. *Intra-antral penis papilla: A much smaller version of the true penis papilla, perceived as a small conical structure in the male atrium, which is mostly occupyed by irregular folds. It is found in the genera '' Amaga'' and ''

Endeavouria ''Endeavouria'' is a monotypic genus of land planarians from the Pacific region. It contains a single species, ''Endeavouria septemlineata''. Description The genus ''Endeavouria'' is similar to other closely related genera, such as '' Caenopl ...

'', among others. *Inverted penis: A structure that fills most of the male atrium, leaving a narrow canal connecting to the prostatic vesicle that resembles an ejaculatory duct. This canal, however, is pushed outwards during copulation, becoming the outer wall of the penis. It is found in the species '' Choeradoplana minima''. *Absent penis papilla: The male atrium lacks a permanent penis-like structure, i.e., contains an eversible penis, and is usually filled by irregular folds. An ejaculatory duct may be present (as in ''

Pasipha ''Pasipha'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and bel ...

'', ''

Imbira ''Imbira'' is a genus of land planarians found in South America. Description The genus ''Imibira'' is characterized by having a large, slender and flat body with parallel margins, reaching up to 14 cm in length. The eyes are arranged alon ...

'' and ''

Luteostriata ''Luteostriata'' is a genus of land planarians from Brazil characterized by a yellow body with dark longitudinal stripes. Description The genus ''Luteostriata'' is characterized by the presence of a cephalic retractor muscle, which allows thos ...

'') or not (as in '' Notogynaphallia'').

Female part of the reproductive system

The female part of the reproductive system in planarians is formed by two

ovaries The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...

in the anterior region. Exiting the ovaries, a pair of

oviduct The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...

s (or, more precisely, ovovitelloducts) runs posteriorly towards the gonopore. A group of

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 ...

glands also connects to these ducts, as planarians are neoophorans and thus yolk is not located inside the eggs. Close to the gonopore, the ovovitelloducts reach a cavity called “female atrium”, which is located posteriorly to the gonopore. The ovovitelloducts may or not meet and become a single duct (common ovovitelloduct) before entering the female atrium. The distal part of the ovovitelloducts is usually sorrrounded by shell glands, which secrete the material that forms the shell of the eggcase (also called cocoon).

Accessory structures

Among the accessory structures of the reproductive system of planarians, one usually found in freshwater and marine species is the ''bursa copulatrix'', which has the function to store sperm after mating.Kaburaki, T. (1922). ''One some Japanese Tricladida Maricola, with a Note on the Classification of the Group''. Journal of the College of Science, Tokyo Imperial University. 54: 1-54. Some planarians (such as the genera ''

Planaria ''Planaria'' is a genus of planarians in the family Planariidae. When an individual is cut into pieces, each piece has the ability to regenerate into a fully formed individual. Description Currently the genus ''Planaria'' is defined as fresh ...

'', '' Artioposthia'', '' Arthurdendyus'', '' Coleocephalus'' and '' Newzealandia'') have a series of finger-like projections, called adenodactyls, near the male and female atria. Adenodactyls contain glands and produce several secretions believed to be useful during reproduction.Graff, L. v. (1896). ''Über die Morphologie des Geschlechtesapparates der Landplanarien''. Verhandlungen der Deutschen Zoologische Gesellschaft: 75-93.

References

{{Animal sexual behavior Tricladida Animal reproductive system Platyhelminth anatomy