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 al ...

planarian Planarians (triclads) are free-living flatworms of the class Turbellaria, order Tricladida, which includes hundreds of species, found in freshwater, marine, and terrestrial habitats.pp 3., "Planarians (the popular name for the group as a whole ...

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

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, 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 ...

, 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 ( testes) is the gonad in all male bilaterians, including humans, and is Homology (biology), homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of Androgen, androgens, p ...

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 (: vasa deferentia), ductus deferens (: ductūs deferentes), or sperm duct is part of the male reproductive system of many vertebrates. In mammals, spermatozoa are produced in the seminiferous tubules and flow into the epididyma ...

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 describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

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 proper ...

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 from 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. (1857). ...

'', '' Obama'', '' 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'', 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'', '' Imbira'' and '' Luteostriata'') 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 a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are endocr ...

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

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

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'', '' 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