Trolls don't have the need for nipples nor bellybuttons because of the way they grow.
Since they pupate, there's no need for nursing, therefore making nipples obsolete. Instead they have 'grub scars', small markings on their ribs that are literally scars from when their legs fall off when they were grubs. However, they are sensitive, like nipples. Seadwellers have an extra set on their neck that serve as gills (which is why Eridan wears his scarf; to cover them up).
Bellybuttons are also obsolete because, again, since they pupate, there's no umbilical cord, so therefore no bellybutton.
(( I didn't come up with this headcanon [except the seadweller part], but you can see the original here: http://ivettelc98.deviantart.com/art/Homestuck-Headcanon-Troll-Biology-The-torso-372454276 ))