Hemipenis

A hemipenis (plural hemipenes) is one of a pair of intromittent organs of male squamates (snakes, lizards and worm lizards). Hemipenes are usually held inverted within the body, and are everted for reproduction via erectile tissue, much like that in the human penis. They come in a variety of shapes, depending on species, with ornamentation, such as spines or hooks.

The hemipenis is the intromittent organ of squamata, which is the second largest order of vertebrates with over 9000 species distributed around the world. They differ from the intromittent organs of most other amniotes such as mammals, archosaurs and turtles that have a single genital tubercle, as squamates have the paired genitalia remaining separate. Squamate hemipenes also develop from a different cell origin, originating from the same embryonic cells that produce the limbs, whereas mammalian penises arise from the embryonic cells that develop the tail.

Much debate continues regarding the evolutionary origin of hemipenes and its relationship to the intromittent organs of other species. However, embryonic and molecular research is beginning to shed light on the origin of the hemipenis.

This theory proposes that the single unpaired penis is the ancestral state for amniotes, and that this trait was retained by most amniotes today. A look at the embryonic underpinning of hemipenes and penises of other animals suggests that there are fundamental differences in their developmental stages, particularly their origin of development relative to the embryonic cloaca. Specifically, the hemipenes of squamata are found to develop on the posterior side, while the paired genitals of non-squamata amniota develop on the anterior side. This developmentally significant difference suggests that the two types of penises could have distinct homologies, and it is thought that this could be attributed to variance of signaling genes during embryological development.

Hemipenes are also being used to study speciation among squamata, especially in identifying cryptic diversity and understanding taxonomy at a species level. One study conducted in 2015 investigated anole speciation through hemipenis variation, and found that anole hemipenial morphology evolved six times faster than other non-genital morphological features. Such studies can help researchers understand adaptive radiation and recover phylogenetic relationships, especially between species that are morphologically very similar.

...
Wikipedia