CHAPTER IV 
INSTINCTS, HABITS, AND ADAPTATIONS 
E Habits of Fishes.—The habits of fishes can hardly 
be summarized in any simple mode of classification. 
In the usual course of fish-life the egg is laid in the 
early spring, in water shallower than that in which the parents 
spend their lives. In most cases it is hatched as the water 
grows warmer. The eggs of the members of the salmon and 
cod families are, however, mostly hatched in cooling waters. 
The young fish gathers with others of its species in little schools, 
feeds on smaller fishes of other species or of its own, grows and 
changes until maturity, deposits its eggs, and the cycle of life 
begins again, while the old fish ultimately dies or is devoured. 
Irritability of Animals.—All animals, of whatever degree of 
organization, show in life the quality of irritability or response 
to external stimulus. Contact with external things produces 
some effect on each of them, and this effect is something more 
than the mere mechanical effect on the matter of which the 
animal is composed. In the one-celled animals the functions 
of response to external stimulus are not localized. They are 
the property of any part of the protoplasm of the body. In the 
higher or many-celled animals each of these functions is spe- 
cialized and localized. <A certain set of cells is set apart for each 
function, and each organ or series of cells is released from all 
functions save its own. 
Nerve-cells and Fibres.—In the development of the indi- 
vidual animal certain cells from the primitive external layer 
or ectoblast of the embryo are set apart to preside over the rela- 
tions of the creature to its environment. These cells are highly 
specialized, and while some of them are highly sensitive, others 
are adapted for carrying or transmitting the stimuli received by 
the sensitive cells, and still others have the function of receiv- 
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