The Tropical American region is the richest (about 1300 species); next follow the African region (about 1000), the Indian region (about 800), the Europaeo-Asiatic region (about 500), the North American region (about 400), the Tropical Pacific region (about 60); whilst the Antarctic region is quite insignificant. Of the migratory fishes, or fishes travelling regularly from the sea to fresh waters, most, if not all, were derived from marine forms. The anadromous forms, annually or periodically ascending rivers for the purpose of spawning, such as several species of _Acipenser_, _Salmo_, _Coregonus_, _Clupea_ (shads), and _Petromyzon_, are only known from the northern hemisphere, whilst the catadromous forms, spending most of their life in fresh water but resorting to the sea to breed, such as _Anguilla_, some species of _Mugil_, _Galaxias_ and _Pleuronectes_, have representatives in both hemispheres. (G. A. B.) FOOTNOTES: [1] For general anatomy of fishes, see T. W. Bridge, _Cambridge Natural History_, and R. Wiedersheim, _Vergl. Anat. der Wirbeltiere_. The latter contains an excellent bibliography. [2] Cf. J. Graham Kerr, _Proc. Camb. Phil. Soc._ x. 227. [3] For electric organs see W. Biedermann, _Electro-Physiology_. [4] J. Graham Kerr, _Quart. Journ. Micr. Sci._ vol. xlvi. [5] J. Graham Kerr, _The Budgett Memorial Volume_. [6] J. Phelps, _Science_, vol. N.S. ix. p. 366; J. Eycleshymer and Wilson, _Amer. Journ. Anat._ v. (1906) p. 154. [7] J. S. Budgett, _Trans. Zool. Soc. Lond._ xvi., 1901, p. 130. [8] L. Drüner, _Zool. Jahrbücher Anat._ Band xix. (1904), S. 434. [9] J. Graham Kerr, _Quart. Journ. Micr. Sci._ xlvi. 423. [10] J. S. Budgett, _op. cit._ [11] W. E. Agar, _Anat. Anz._, 1905, S. 298. [12] J. Graham Kerr, _The Budgett Memorial Volume_. [13] J. Phelps, _Science_, vol. N.S. ix. p. 366; J. Eycleshymer and Wilson, _Amer. Journ. Anat._, v. 1906, p. 154. [14]: F. Maurer, _Morphol. Jahrb._ ix., 1884, S. 229, and xiv., 1888, S. 175. [15] J. Rückert, _Arch. Entwickelungsmech_. Band iv., 1897, S. 298; J. Graham Kerr, _Phil. Trans._ B. 192, 1900, p. 325, and _The Budgett Memorial Volume_. [16] Cuvier et Valenciennes, _Hist. nat. des poiss._ xix., 1846, p. 151. [17] J. Rathke, _Üb. d. Darmkanal u.s.w. d. Fische_, Halle, 1824, S. 62. [18] Cf. W. Biedermann, Electro-Physiology. [19] Literature in N. K. Koltzoff, Bull. Soc. Nat. Moscou, 1901, P. 259. [20] J. Graham Kerr, _Proc. Zool. Soc. Lond._ (1901), p. 484. [21] J. S. Budgett, _Trans. Zool. Soc. Lond._ xv. (1901), vol. p. 324. [22] H. F. Jungersen, _Arb. zool. zoot. Inst. Würzburg_, Band ix., 1889. [23] E. J. Bles, _Proc. Roy. Soc._ 62, 1897, p. 232. [24] J. Graham Kerr, _Proc. Zool. Soc. Lond._ (1901) p. 484. [25] F. M. Balfour and W. N. Parker, _Phil. Trans._ (1882). [26] J. Graham Kerr, _Proc. Zool. Soc. Lond._ (1901), p. 495.