Reciprocal copulation of <i>Siphopteron quadrispinosum</i>.

<p>The bipartite penises, which are everted as largely translucent structures at the right front of the head (<b>h</b>), are reciprocally inserted into the partner. While the actual penis (<b>p</b>) is inserted into the gonopore (located behind the right parapod), the penile stylets (<b>s</b>) are hypodermically inserted into the foot of the partner. Markings are only shown for the lower animal.</p>

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