INTRODUCTION TO THE FISHES 207 



which empty directly into the heart. It lies dorsally to the 

 oesophagus, into which it opens ventrally by means of the curved 

 pneumatic duct. Geologically these Lung-fishes are of extreme 

 antiquity, going back in much their present form (the Ceratodus 

 which now exists in Australia is identical in shape with a fossil 

 form from the Trias of Europe) to the Devonian, and probably still 

 older Palaeozoic formations ; and their close relationship to the 

 true ancestors of the land vertebrates is confirmed by resemblances 

 in their anatomy and development. 



The organ of hearing is well developed in all fishes, the mem- 

 branous labyrinth having three semicircular canals, as in the land 

 vertebrates ; they communicate with a vestibule, in which are 

 contained the bony plates called otoliths ; there are usually two 

 of these flattened, somewhat oval organs, and in the cod family 

 one is strikingly larger than the other. In the Elasmobranchii the 

 otoliths are represented by calcareous particles. No fish possesses 

 a cochlea or true tympanic membrane, but sometimes there is a 

 connection between the labyrinth of the ear and the air-bladder, 

 made by a chain of small bones. The otoliths, or ear-stones, of the 

 plaice show alternate white opaque and dark semi-transparent 

 rings, the white rings being formed in spring and early summer, 

 the dark in late summer and autumn. In winter the growth of 

 the otolith, as of the fish, is generally in abeyance. Thus one 

 white and one dark ring is added to the otolith in each year, and 

 gives an accurate means of determining the exact age of the fish. 

 We owe this important discovery to Reibisch, and its confirmation 

 and practical application to Mr. W. Wallace, of the Marine Bio- 

 logical Association. It is a discovery of much importance in 

 relation to practical fishery problems, since it enables a more 

 accurate estimate to be made of the rate of growth of fishes in 

 different regions. By an examination of the otolith it is not only 

 possible to tell directly the age of the individual fish, but examina- 

 tion of the otolith, in conjunction with measurement, gives an 

 accurate insight into the range of size at a given age. 



The skin of fishes is rarely naked, in most species being covered 

 with scales which generally present the appearance of thin laminae 

 or plates, overlapping each other like the tiles on the roof of a 

 house, and embedded in furrows in the skin. Most Bony-fishes 

 possess flexible scales, marked with either concentric or radiating 



