192 A HISTORY OF FISHES 



bladder, which is simply pushed inwards from the external 

 skin, a mode of development which is exactly similar to that of 

 the olfactory organs already described. At a later stage this 

 bladder takes on a more complicated structure, and the tube 

 by means of which it communicated with the exterior generally 

 becomes closed up in the adult fish, although in Selachians a 

 small opening on the surface of the skull is retained throughout 

 Hfe. 



The inner walls of the utriculus, sacculus, and lagena are 

 provided with patches or ridges of highly sensitive tissue, and 

 the cavities of the chambers are filled with a fluid known as the 

 endolymph : a similar fluid, the perilymph, occupies the spaces 

 between these parts and the walls of the containing auditory 

 capsule. In addition to the endolymph, the cavities also 

 contain certain bodies composed of limy matter secreted by their 

 walls. In the Selachians these take the form of small separate 

 particles connected with one another by mucus, but in most 

 Bony Fishes they form large, solid concretions or otoliths, a 

 sagitta in the sacculus, an asteriscus in the lagena, and a lapillus 

 in the utriculus. In nearly all fishes the sagitta is the largest 

 otolith (Fig. 75fl, b) and the lapillus is quite minute and of little 

 importance : in some, however, the asteriscus is relatively enor- 

 mous and the sagitta small. These otoliths or ear-stones have 

 enamelled surfaces, and are provided with peculiar grooves and 

 markings. They exhibit some variation in shape and size in 

 diflPerent fishes, and as the form is fairly constant in any par- 

 ticular species, they are of some importance in classification. 

 The otoliths grow by the deposition of lime in layers on the outer 

 surface, and as the rate at which this is laid down varies at 

 different seasons, if one is cut into thin sections and examined 

 under a lens or microscope the layers formed in successive 

 years are clearly visible as a series of alternately light and dark 

 concentric rings, similar to the "zones" on a scale or the rings 

 on a tree-trunk. Thus by a study of the otoliths it is possible 

 to ascertain the age of any particular fish, and this method 

 of age - determination has proved invaluable to those in- 

 vestigating the life-histories of our food fishes. The otoliths 

 of the Sciaenids or Drums [Sciaenidae) are very large, and in 

 ancient times were worn on a string round the neck as a 

 preventive and cure of colic. 



In some Bony Fishes the air-bladder is more or less intimately 

 connected with the internal ear. In many marine and a few 

 fresh-water forms there is an aperture in the hinder wall of the 



