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Macrostomum hystrix is a [//en.wiktionary.org/wiki/free-living free-living] flatworm of the genus Macrostomum. It is small, transparent and a simultaneous hermaphrodite.

Biology
Macrostomum hystrix is capable of hypodermic insemination. Thereby sperm is injected through the epidermis into the parenchyma of the mating partner. This is done by a needle-like stylet, which is the male copulatory organ. The stylet has a rigid and pointed distal thickening as well as a subterminal stylet opening which can puncture the epidermis of the mating partner. The sperm have no flagellates and are highly motile, small and simple. Also the female antrum shows a simple anatomy and is involved only in egg laying. Macrostomum hystrix does not only use hypodermic insemination for outbreeding but also self-fertilization, also known as selfing, has been shown in isolated worms. Thereby they inject their own sperm mainly into their head due to physical constraints. From there the sperm apparently migrate to the site of fertilization. Worms that were isolated showed significantly more sperm in their head compared to worms that had the opportunity to cross fertilize. Macrostomum hystrix prefers outbreeding if partners are available because selfing has a cost. The amount of offspring of worms that selfed is reduced as well as the offspring survival. As a consequence Macrostomum hystrix shows delayed selfing, it only starts selfing in the extended absence of mating partners after a considerable delay. So delayed selfing represents a conditional reproductive strategy which allows them to deal with periodic conditions of low mate abundance.