146
BIOLOGY OF ’FISHES
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profoundly inuenced by these new ideas regarding the;;!
hormones.
Heredity can no longer be considered as the
transference of morphological or structural characters
the regulating inuence is likewise handed down from one
generation to another, and if we try to express __what this
means, we pass well beyond the mere physical
.
The hormones in fact» lie on the borderland between the
'physiological and the psychological worlds.
,
-2:5{
‘
2. MUSCULÀR SYSTEM AND ELECTRIC ORGANS
-Îg_
,
The conversion of potential energy within the body into
,
external work becomes most obvious to us in movement,
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5
_and this is
With the muscular system.
_
In _shes —
.
the muscles compose the greater part of the body, and they
are;
in the tail region, the. :
,
shes have no tails, it is
.
others î_-are;almost all tail*_of a_,slender kind, but
do not.a‘eCt the general rule.
:
forms like“ the Clupeids ,—th€
=
,
arranged in parallel series from head to tail, the position
of ‘
having little inùenœ on their
,
.pritivè __segmentation; , Of more
WidëÏ
separation of the lower half
the ,
development ofthe abdomrnalcavrtyTh1sdoesnot
:
-
the Symm_etry iii—thé ClupeldS(seePlVl)but1thasa
,
greatereect Inlaterf0rmsTheoretmallyasmthe
_,
ventra1halvesAssoonastheshbegmsto move, howeverâÏ
,
:
whthnthggfnthwtu primitive
th6— vertébral. columnf—Ï£1ÏÏÏ'ËÊË
?
_
1nchnebackwards,especrally in the càüdal- region,
,
the
Connection—With the__skiniatthe};_f:_Ï;ËË{:,}É
!Ï_Î
,
margins. Hèn“æ
._ of anySegrnent CQm_€ tohave
,—_
of
W(Ë) anda cross—sectionof _thebody cuts
îïçum_bçr of muscle
Further,