9 
we must adopt another method to determine its growth. The otoliths, or 
earstones, also show zones of growth and the rate of growth of some fishes has 
been determined from them. Reibisch was the first to apply this method in 
determining the rate of growth. The method, however, presents some difficulties 
and is not so commonly used as the scale method. 
The labyrinths, with their contained otoliths, are situated at the base of the 
skull on either side of the brain. They can be exposed to view by making a 
transverse cut in the skull on the line connecting the anterior margins of the 
pectoral fins, cutting forward along the median axis of the skull, and then raising 
the two sections of the top of the skull thus formed on either side of the median 
axis. The lagena, or primitive cochlea, contains minute granules of calcareous 
matter and where the two semi-circular canals join the sac there is a very small 
otolith, but for determining the age of fishes only the large otolith, the so-called 
sagitta, situated in the sacculus, is used. 
The otoliths consist of calcareous matter and show a remarkable variety of 
form in different fishes. They have all, however, essentially the same structure. 
In the centre is an opaque kernel, oval or circular in outline, and around this 
kernal or nucleus are concentric lines marking off the successive zones of growth. 
During the rapid growth of summer a broad zone, which appears light under the 
microscope by transmitted light, is formed and this zone is separated from the 
zone of the following summer by a dark ring which represents the slow growth 
of winter. 
Fig. 4. Otolith one-twenty-fifth of an inch in diameter from Angler shown in figure 4. 
In the young Angler fish represented in Figure 3 the otolith is quite small 
(1 mm. or about 1/25 of an inch longest diameter), and is so thin that its structure 
can easily be made out. In the centre is an opaque oval kernel (Fig. 4) and 
around this there is a broad zone which, under higher magnification, is seen to 
have fine concentric laminae. Taking into consideration the time of spawning 
121 
