122 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
consist of oil, and in some species of Gadoids the amount of oil 
may be so large that the liver will easily burn without much 
spluttering. 
The general metabolism, in so far as the storage of surplus 
fat is concerned, therefore differs notably in the Clupeoid and 
Gadoid fishes. The flesh of the former is rich in fat, that of the 
latter is poor in fat. Upon this difference depends the differ- 
ences in methods of curing and conservation. Why this difference 
in fat-metabolism should exist, we find it hard to explain. 
Both groups of fishes have the same general habits (except that 
the Gadoids are more demersal than Clupeoids) ; they frequent — 
the same waters and live, in general, on the same food animals— 
Crustacea. 
The natute @£°% we! ia be 
Opportunity for more than a very superficial examination 
of the chemical and physical nature of the fat has not yet been 
afforded. There are probably seasonal differences in com- 
position, and, of course, what one calls “ fat” or “ oil” is 
certainly a mixture. Expressed or extracted in such circum- 
stances as to avoid oxidation, it soon separates into constituents 
of different melting point and refractive index. Some part 
of it is very easily oxidised at the temperature of boiling water. 
Extracted in an atmosphere of H or CO, it is pale yellow in colour, 
but, heated to 100° C for a few minutes, it darkens greatly. 
Yet the oxidation has, for its main effect, the production of 
a small trace of this pigment, for the weight is not greatly 
increased. 
The fats of the food-animals eaten by both herring and cod— 
that is, of Crabs and other Crustacea (Cod), and Copepods and 
‘other micro-crustacea (Herring)—are apparently different in 
chemical composition from that of the fish : they are generally 
coloured, for instance. So also the fat of the food of the 
Copepods (that of Diatoms and Peridinians and Algae) is also 
