262 
LIFE, 1ST ITS HIGHER FORMS. 
monly takes the temperature of the surrounding water; 
in some of the swift oceanic Fishes of the Mackerel family, 
however, such as the Tunny and the Bonito, the blood is 
found to be 10° higher than the temperature of the sur- 
face of the sea, even within the Tropics : the flesh of these 
Fishes is dark and dense. The blood-disks are sometimes 
circular, sometimes oval. They are larger than those of 
Mammalia aud Birds; smaller than those of Reptiles, 
and especially than those of Amphibia. 
The irritability of the muscular fibre is considerable, 
and is long retained. Fishmongers take advantage of this 
property, to produce rigid muscular contraction, after life 
has ceased, by transverse cuts and immersion of the 
muscles in cold water : by this operation, which is called 
“crimping,” the firmness aud density of the muscular 
tissue are increased. 
In our next chapter, we shall enter into some details of 
the instincts and habits of Fishes, and some other matters 
connected with the Class, of more popular interest than 
these structural peculiarities, which, we fear, will prove 
but a dry morsel to many of our readers. 
