In this process, sperm is injected through the epidermis into the parenchyma of the mating partner.

The female antrum shows a simple anatomy and is only involved in laying eggs.

[2] In isolated worms, it has been shown that Macrostomum hystrix does not only use hypodermic insemination for outbreeding but also self-fertilization, or "selfing".

[3] Macrostomum hystrix prefers outbreeding if partners are available because selfing also has costs.

As a consequence, Macrostomum hystrix only begins selfing in the extended absence of mating partners: delayed selfing represents a conditional reproductive strategy which allows them to deal with periodic conditions of low mate abundance.