Cutaneous sensory organ located in an evagination of the epidermis into the cutis. In the head region often situated on a corium papilla. The organ consists of acyst or capsule without any connection to the outside, the intra-epidermal cavity, filled with a serous(?) fluid. The wall of this cyst again has evaginations, each surrounded by groups of sensory cells of type a . Another cavity is located centrally beneath the cyst, both are connected by a narrow duct. This cavity contains an acellular mass on top of another group of sensory cells, type b, which have a particular innervation, again by the lateral line nerves. Each group of sensory cells, type a as well as type b, is surrounded by supporting cells. The shape of these mormyromasts depends on their size, smaller ones are narrow and long, larger ones more spherical, and the number of groups of sensory cells depends on the size of the organ. Their function most probably is the electrolocation, they enable the fish to locate its position in the environment even in extremely turbulent water. As a matter of fact, electric fishes are very abundant in rapids, where the turbulences of the water may overcharge the mechanoreceptors of the lateral line system. In contrast, the electric field established by the fish is not impaired by the turbulences and electrolocation will function even under such extreme conditions.

Alternative forms for mormyromast type B : medium receptor, mormyromast type II.