A particular kind of electroreceptors or mormyromasts, also called tuberous organ, ('large receptor'), consisting of one or more giant sensory cells with a diameter of up to 40-50 µm. In most genera the knollenorgans have up to 9 or 10 such sensory cells, but in Myomyrus and Petrocephalus there may be up to 60 – smaller ones – in one organ. Each giant cell lies in a tightly-fitting capsule, and only its base comes into contact with the wall of that capsule. The free membranes of the sensory cell are thickly covered with long microvilli. Beneath the membrane there are many layers of mitochondria. Supporting cells form the base of the structure, they produce a liquid rich in mucopolysaccharides and guide the nerve endings to the sensory cell. The whole structure is located in an evagination of the epidermis, i. e. in an ariole of the electroreceptor epidermis. There is no duct connecting the capsule to the outside. In some genera knollenorgans are arranged in conspicuous patterns, which may serve as a distinguishing feature of species. Knollenorgans are only found in Mormyridae, both Mormyrinae and Petrocephalinae, not in Gymnarchus, which has a particular kind of comparable organs. In mormyridae the function of the knollenorgans most probably is the communication between the animals.