[Illustration: From Gegenbaur, _Untersuchungen zur vergleich. Anat. der Wirbeltiere_, by permission of Wilhelm Engelmann. FIG. 10.--View of _Torpedo_ from the dorsal side: the electric organs are exposed. I, Fore-brain. II, Mesencephalon. III, Cerebellum. IV, Electric lobe. br, Common muscular sheath covering branchial clefts (on the left side this has been removed so as to expose the series of branchial sacs). f, Spiracle. o.e, Electric organ, on the left side the nerve-supply is shown. o, Eye. t, Sensory tubes of lateral line system.] _Electrical Organs._[18]--It is characteristic of muscle that at the moment of contraction it produces a slight electrical disturbance. In certain fishes definite tracts of the musculature show a reduction of their previously predominant function of contraction and an increase of their previously subsidiary function of producing electrical disturbance; so that the latter function is now predominant. In the skates (_Raia_) the electrical organ is a fusiform structure derived from the lateral musculature of the tail; in _Gymnotus_--the electric eel--and in _Mormyrus_ it forms an enormous structure occupying the place of the ventral halves of the myotomes along nearly the whole length of the body; in _Torpedo_ it forms a large, somewhat kidney-shaped structure as viewed from above lying on each side of the head and derived from the musculature of the anterior visceral arches. In _Torpedo_ the nerve-supply is derived from cranial nerves VII. IX. and the anterior branchial branches of X. The electric organ is composed of prismatic columns each built up of a row of compartments. Each compartment contains a lamellated electric disc representing the shortened-up and otherwise metamorphosed muscle fibre. On one face (ventral in _Torpedo_, anterior in _Raia_) of the electric disc is a gigantic end-plate supplied by a beautiful, dichotomously branched, terminal nervous arborization. The development of the mesoderm of the head region is too obscure for treatment here.[19] The ventral portion of the trunk mesoderm gives rise to the splanchnocoel or general coelom. Except in the Myxinoids the anterior part of the splanchnocoel becomes separated off as a pericardiac cavity, though in adult Selachians the separation becomes incomplete, the two cavities being in communication by a pericardio-peritoneal canal.