A two-tailed monster

Despite some accident in its development, this slug had survived in the wild until adulthood. We found it while mapping the remarkably narrow contact zone between two species of slug that had previously been confused: Deroceras juranum and a species provisionally called Species A, which is closely related to D. praecox and D. rodnae. Deroceras juranum and Species A have distinct mating behaviours that prevent interspecific sperm exchange, but otherwise identification requires dissection so as to examine the genital anatomy. Neither species normally has two tails!

The details are explained in:
Hutchinson, J.M.C. & Reise, H. 2009. Mating behaviour clarifies the taxonomy of slug species defined by genital anatomy: the Deroceras rodnae complex in the Sächsische Schweiz and elsewhere. Mollusca 27:183–200.