250
'
THE BIOLOGY OF FISHES
«
_
.
a
“ special adjustment of structure or function to meet
partiCular conditions of life.”
"
_' Î.;
'
This denition ts in with the conclusion stated in
f i
previous chapters, that a pelagic form of sh, a
Clupeid, let us say, was the original parent of all shes, and
that from this type at various times all others have been Ï
:
derived, just by adapting themselves to other modes of life. [
Starting fr0m this point of view, we may trace the gradual‘
‘
attainment of some adaptations.
_'
'
E
'
_
1.
GROWTH OF ADAPTATIONS
_
'
;Ï
'
-
:
most of the Clupeids the Herring is a pelagic sh
'
that can rise to the surface or descend to conSiderabb
:'_ÇÎ';
depths, at least roofathoms. A few other forms can also
do this, and we may examine how it is done. The Herring
has -nOspecial adaptati0n of bone or gills for the purpose;
Its ability to rise and sink
a peculiarity in “the
ait-bladder. In addition to the communication with the
blind sac or stomach, this organ has also a posterior
,
opening direct to the exterior—the
“
safety opening,” as it:?
hasbeen called, which is formed in the late postlarval stages;
'
_
to take in water by this means the sh can adjust ':
its bâlànceto different depths without any effort. But
;; . pelagic ZOne is: invaded at times by other shes with
Ÿ
different structure.—_ The Mackerels and the Tunnies c0me‘
Ï up toçthe surfacezto spawn, and when they have done
:
'
intheir1hunger,theypurStre the pelagic forms of life,
Î '
…
0fthemtheHêtl‘mg, :-âhd*the“Tùnny 3180 the
Α f
_
andWhentheyhave fed thisyïere again
Waters;
' ai—It" seemscertamtha these;“_aregn-dtœauy adapted to '1ivinglï;
î atthesurfæeWha‘cenables
simply thé—ir
î ' Ï
they have attainedtd
adaptations tôÏ —;
:
mode,.0f'î‘îfeï
‘.
è!
region 70f'1£5
mfœsh waterwenämany le€rSef0mspV1ngundef
'
THE BIOLOGY OF FISHES
«
_
.
a
“ special adjustment of structure or function to meet
partiCular conditions of life.”
"
_' Î.;
'
This denition ts in with the conclusion stated in
f i
previous chapters, that a pelagic form of sh, a
Clupeid, let us say, was the original parent of all shes, and
that from this type at various times all others have been Ï
:
derived, just by adapting themselves to other modes of life. [
Starting fr0m this point of view, we may trace the gradual‘
‘
attainment of some adaptations.
_'
'
E
'
_
1.
GROWTH OF ADAPTATIONS
_
'
;Ï
'
-
:
most of the Clupeids the Herring is a pelagic sh
'
that can rise to the surface or descend to conSiderabb
:'_ÇÎ';
depths, at least roofathoms. A few other forms can also
do this, and we may examine how it is done. The Herring
has -nOspecial adaptati0n of bone or gills for the purpose;
Its ability to rise and sink
a peculiarity in “the
ait-bladder. In addition to the communication with the
blind sac or stomach, this organ has also a posterior
,
opening direct to the exterior—the
“
safety opening,” as it:?
hasbeen called, which is formed in the late postlarval stages;
'
_
to take in water by this means the sh can adjust ':
its bâlànceto different depths without any effort. But
;; . pelagic ZOne is: invaded at times by other shes with
Ÿ
different structure.—_ The Mackerels and the Tunnies c0me‘
Ï up toçthe surfacezto spawn, and when they have done
:
'
intheir1hunger,theypurStre the pelagic forms of life,
Î '
…
0fthemtheHêtl‘mg, :-âhd*the“Tùnny 3180 the
Α f
_
andWhentheyhave fed thisyïere again
Waters;
' ai—It" seemscertamtha these;“_aregn-dtœauy adapted to '1ivinglï;
î atthesurfæeWha‘cenables
simply thé—ir
î ' Ï
they have attainedtd
adaptations tôÏ —;
:
mode,.0f'î‘îfeï
‘.
è!
region 70f'1£5
mfœsh waterwenämany le€rSef0mspV1ngundef