Sea slugs are hermaprodites. So each slug has both male and female reproductive organs. When two slugs meet, they fertilise one another.   The male organ is the white thing that emerges from the 'neck'!  More about this slug on the wildfacts sheets on wildsingapore.  For a high res version of this photo, please review the details on about using my photos. When making the request, please include this reference: 050529sjig7219