For a long time after their discovery in the seventeenth century the lateral-line canals of fishes were considered to be mucus-secreting organs. In 1850 Leydig described sense organs in the lateral-line canals, and this discovery stimulated a keen interest in the investigation of both the morphological and functional features of the lateral-line system. Morphological studies have yielded a thorough understanding of the structure of these organs (Ewart and Mitchell 1892; Cole 1896; Johnson 1917; von Woellwarth 1933). Physiological studies, though numerous, have been less fruitful. An account of the older work was given by Baglioni (1913), and the more recent work is reviewed by Dykgraaf (1933). The only technique until recently available has been the elimination of the sensory system by nerve section and cauterization, and the comparison of the behaviour of intact and operated fishes in response to various stimuli. With so diffuse a structure as the lateral-line system, receiving its nerve supply from the fifth, seventh, ninth and tenth cranial nerves, this method is particularly inadequate, and involves a violent mutilation of the animal. When one considers the crudity of many of these operations, it is not the uncertainty of the results which is remarkable, but rather that some of the conclusions reached should remain valid to-day in the light of far more penetrating experimental analysis. This method of organ elimination could yield at best only an indication of the kind of stimulus that is effective in evoking the excitation of lateral-line receptors. In current textbooks the conclusion of Parker (1904) that the effective stimulus for the lateral line is low-frequency vibration, and that of Hofer (1907) that it is movement of water (i. e. local currents) have received most notice. The observations of Dykgraaf (1933), who employed the more refined methods of von Frisch’s
futterdressur
technique, support Hofer’s conclusion, and to some extent also Parker’s. Dykgraaf considers the lateral-line system to be an organ of
Ferntastsinn
, and if this is taken to mean a mechanoreceptor of such sensitivity that it can function both as a touch organ and as a receptor for disturbances coming from a distance, it is undoubtedly a true description, for it is fully confirmed by the direct electrophysiological studies of Hoagland (1933
a, b, c
and
d
) and of Schriever (1935). The latter, apparently unacquainted with Hoagland’s work, did little more than to confirm several of his observations.
Diversity and sexual dimorphism in the head lateral line system in North Sea populations of threespine sticklebacks, Gasterosteus aculeatus (Teleostei: Gasterosteidae)
