and receives it. The slugs often switch roles after sperm transfer for a second round of mating. Hence, as with simultaneous reciprocal mating, a mutual exchange of sperm usually but not always occurs. External sperm transfer During external sperm exchange, the penis of the donor slug is everted along with its ejaculate. The everted penis is several fold larger than its usual size, and appears as a bulbous, transparent structure. The ejaculate, an amorphous soft mass, is transferred onto the surface of the receiver slug's everted penis. At least one species, the grey field slug (Deroceras reticulatum), shows a primitive degree of sperm packaging. The retraction of the receiver's penis then takes the ejaculate with it. In most cases, both partners donate and receive simultaneously via intertwined penises; unilateral sperm transfer is only an occasional occurrence. Entwinement is allowed by the curvature present in slug penises; close entwinement aids successful sperm exchange, which occurs at the peak of penis eversion. When an individual of a species with external sperm transfer lacks a penis, it can still reproduce uniparenterally, such as by self-fertilisation, provided it possesses the other components of a normal reproductive system. Uniparental reproduction has been observed in the marsh slug (Deroceras laeve). When an appending penial gland, an accessory structure part of the male reproductive system, is present, it is also briefly everted during mating. If large enough, it is spread over the partner's body. This can occur along with or after sperm exchange. The penial gland is filled during early courtship and its secretions are transferred onto the skin of the partner slug. These secretions, like those of the sarcobelum during courtship, have several possible functions including acting as an allohormone which manipulates the partner to increase chances of paternity. This may be achieved through inhibiting additional mating, changing sperm transport or storage, or increasing egg laying in the partner. Penis retraction is usually a fast process, although much slower than eversion. It is common for both slugs to retract their penises simultaneously, pumping and rocking the anterior parts of their body while they do so. They separate and crawl away from each other as soon as their genitals are untangled. After this, it is common for one or both slugs to engage in mucus consumption; they may return to the mating site and lick the mucus off the ground, or lick it from their own body