60
THE BIOLOGY OF FISHES
sh ignore terrible injuries and even to overcome the impulse
to feed.: The digestive organs partially or Wholly degenerate, and the sh is then urged to seek relief by unusual
or extraordinary means, that is, to nd if possible external
conditions in better agreement With its changing or changed
constitution.
'
'
.
‘
,__
One cannot say denitely what happens in any particular
case, owing mainly to the protean nature of shes. A
Dog-sh, for example, is more in harmony
the surrounding medium than a Salmon, and a Stickleback will
survive under conditions that kill most shes.
These
phenomena depend apparently on their ability to alter the
blood—pressure, or concentration of salts, as they pass from
_
one set of conditions to another. And this ability, again,
upon how; far the“ gill—membrane and the skin
i
permit of the passage of…water outwards and inwards, a
question that has not yet been sufcieny investigated. ’
is possible, therefore, that the internal Stimulus to
migrati0n comes from some internal changé:”in“theblood; »
"
probably in the blood—pre‘ssure. This is alWayS much legs
'
than the pressure of the surrounding water, but it varies.
In the Salmon, Greene ”found that the concentration of
salts in the blood was far below that of the salt water in Which
î
the. shes were feeding voraciously, and gradually altered as
î
the sh entered fresh water.
It may be that a slight
_
reduction in the pressure Within the blood is a source of
'
'
discomfort-to the sh and urges .itto enter the rivers. In
.
thegcase'_of.theEel—,on the, other hand, there would seem to
theèiternal pressure accompanied by
‘an,increase in the amount
acid: at the…time when
.
'
thefshesarebeginning to :migrat6from fresh to Salt water.
"If—they aredetairifæd onth‘eir journey in: any way,— the=body
_
thickens and ,_ swells and
.
even increases greatly…in weight,
'.
"
thoügh the…—sh are: appareny not feeding. Petersen has
-
shown that the digestive tract is
Silver Belin its-' bræding-dress is aboùt7o cm.
,
,
(27-to 28 ins.) in length and weigh—s' about —1%…lb.“ The
THE BIOLOGY OF FISHES
sh ignore terrible injuries and even to overcome the impulse
to feed.: The digestive organs partially or Wholly degenerate, and the sh is then urged to seek relief by unusual
or extraordinary means, that is, to nd if possible external
conditions in better agreement With its changing or changed
constitution.
'
'
.
‘
,__
One cannot say denitely what happens in any particular
case, owing mainly to the protean nature of shes. A
Dog-sh, for example, is more in harmony
the surrounding medium than a Salmon, and a Stickleback will
survive under conditions that kill most shes.
These
phenomena depend apparently on their ability to alter the
blood—pressure, or concentration of salts, as they pass from
_
one set of conditions to another. And this ability, again,
upon how; far the“ gill—membrane and the skin
i
permit of the passage of…water outwards and inwards, a
question that has not yet been sufcieny investigated. ’
is possible, therefore, that the internal Stimulus to
migrati0n comes from some internal changé:”in“theblood; »
"
probably in the blood—pre‘ssure. This is alWayS much legs
'
than the pressure of the surrounding water, but it varies.
In the Salmon, Greene ”found that the concentration of
salts in the blood was far below that of the salt water in Which
î
the. shes were feeding voraciously, and gradually altered as
î
the sh entered fresh water.
It may be that a slight
_
reduction in the pressure Within the blood is a source of
'
'
discomfort-to the sh and urges .itto enter the rivers. In
.
thegcase'_of.theEel—,on the, other hand, there would seem to
theèiternal pressure accompanied by
‘an,increase in the amount
acid: at the…time when
.
'
thefshesarebeginning to :migrat6from fresh to Salt water.
"If—they aredetairifæd onth‘eir journey in: any way,— the=body
_
thickens and ,_ swells and
.
even increases greatly…in weight,
'.
"
thoügh the…—sh are: appareny not feeding. Petersen has
-
shown that the digestive tract is
Silver Belin its-' bræding-dress is aboùt7o cm.
,
,
(27-to 28 ins.) in length and weigh—s' about —1%…lb.“ The