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The reproductive system of
The male part of the reproductive system in planarians has a set of several testicles, 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 ducts which run posteriorly towards the gonopore.
In some groups, the sperm ducts met in their distal part, forming the ejaculatory duct, which then opens in a cavity called “male atrium”. In others, like land planarians, 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'', '' Obama'', ''
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'', ''
The reproductive system of planarian
A planarian is one of the many flatworms of the traditional class (biology), 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-li ...
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 taxonomic groups of animals (mostly invertebrates) do not have ...
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, 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 testicles, 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 ducts which run posteriorly towards the gonopore.
In some groups, the sperm ducts met in their distal part, forming the ejaculatory duct, which then opens in a cavity called “male atrium”. In others, like land planarians, 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'', '' Obama'', ''Cratera
A krater or crater ( grc-gre, , ''kratēr'', literally "mixing vessel") was a large two-handled shape of vase in Ancient Greek pottery and metalwork, mostly used for the mixing of wine with water.
Form and function
At a Greek symposium, kra ...
'', ''Paraba
''Paraba'' is a genus of land planarians from South America.
Description
Species of the genus ''Paraba'' have a slender body with nearly parallel margins while creeping. The largest specimens may reach about 8 cm in length. The copulato ...
'', ''Polycladus
''Polycladus'' is a genus of land planarians from South America.
Description
''Polycladus'' is a very understudied genus of land planarians. It was defined as land planarians with a wide, flat and leaf-like body, having the entire ventral sur ...
'', ''Microplana
''Microplana'' is a genus of 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 physi ...
'', 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
''Matuxia'' is a genus of land planarians from Brazil.
Description
The genus ''Matuxia'' is characterized by having a slender body with nearly parallel margins, reaching up to 12 cm in length, and being slightly convex on both dorsal and ...
''.
*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
''Choeradoplana'' is a genus of land planarians found in South America.
Description
Species of the genus ''Choeradoplana'' are characterized by the presence of a cephalic retractor muscle associated with cephalic glands, forming a cephalic mus ...
''.
*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
''Notogynaphallia'' is a genus of land planarians from South America.
Description
The genus ''Notogynaphallia'' is characterized by having a small-to-medium, slender body with nearly parallel margins. The eyes are arranged along the body marg ...
'').
Female part of the reproductive system
The female part of the reproductive system in planarians is formed by two ovaries in the anterior region. Exiting the ovaries, a pair ofoviduct
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, ...
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 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 ...
ns 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
''Artioposthia'' is a genus of land planarians from the Australasian and Indo-Pacific
The Indo-Pacific is a vast biogeographic region of Earth.
In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises ...
'', ''Arthurdendyus
''Arthurdendyus'' is a genus of land planarians from New Zealand. It was erected in 1999 and includes the invasive species ''Arthurdendyus triangulatus'', known as the New Zealand flatworm.
Description
Species of ''Arthurdendyus'' are charac ...
'', ''Coleocephalus
''Coleocephalus'' is a genus of land planarians that currently contains a single species, ''Coleocephalus fuscus'', from Enderby Island, New Zealand.
Description
The genus ''Coleocephalus'' is characterized by an anterior end that is curved ...
'' and ''Newzealandia
''Newzealandia'' is a genus of 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 phy ...
'') 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