There are cerebellum-like structures, including the valvula and the electroreceptive lateral line lobe (ELL), that are found in certain species of fish, as well as the mammalian dorsal cochlear nucleus, the architecture and molecular biology of which in some respects strikingly resemble that of the cerebellum of vertebrates. These merit separate discussion from the topics in Chapter 2 on comparative anatomy of the cerebellum.

The Valvula

In addition to a true cerebellum (corpus cerebelli), there are several structures in electroreceptive fish (particularly the mormyrids), including the valvula, that are recognizably cerebellar or cerebellar-like. This resemblance suggests that a comparative study of them could yield some general rules concerning the cerebellum (Bell and Szabo 1986). Thus, among the bony fishes (teleosts), the mormyrids, which comprise certain African fresh-water fish, possess a cerebellum that is divided into the usual parts, that is, the body (corpus cerebelli), and a part arranged quite differently, the valvula. This highly unusual structure is considered in some detail.

The valvula, which is actually unique to the mormyrids, can grow to relatively large dimensions, covering all other parts of the brain (Fig. 12.1). This enlargement has been considered to be related to the high degree of differentiation of the lateral line system in these fish, which have a weak electric organ in the tail. They also have electroreceptors, which to a large extent are constituted of lateral line organs distributed over the body, and which have become modified to be sensitive to electric fields in the water (Nieuwenhuys and Nicholson 1969a).