The ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled pores. Each ampulla consists of a canal opening to the surface by a pore in the skin and ending blindly in a cluster of small pockets full of special jelly. The ampullae are mostly clustered into groups inside the body, each cluster having ampullae connecting with different parts of the skin, but preserving a left-right symmetry. The canal lengths vary from animal to animal, but the distribution of the pores is generally specific to each species. The ampullae pores are plainly visible as dark spots in the skin. They provide fish with a sixth sense capable of detecting electromagnetic fields as well as temperature gradients[WP]. [ http://en.wikipedia.org/wiki/Ampullae_of_Lorenzini ]