Reproductive system of planarians

The reproductive system of planarians is considerably regular among different families, although the structures associated can be more or less complex.

All planarians are hermaphrodites, 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.

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.

In land planarians, the structure of the male atrium may be quite variable between different genera. The most common constructions are:

The female part of the reproductive system in planarians is formed by two ovaries in the anterior region. Exiting the ovaries, a pair of oviducts (or, more precisely, ovovitelloducts) runs posteriorly towards the gonopore. A group of yolk glands also connects to these ducts, as planarians are neoophorans and thus yolk is not located inside the eggs.

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