2
THE BIOLOGY or FISHES
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behind the
The whale and the seal are not
shes ; they have entered the sea from the land at a different
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level of evolution, With very different structures to begin
With—nose opening into the mouth, and lungs, but no gills.
Their outer form may change to that of a sh and even the
legs disappear, but they have not retrograded to such an
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extent” as to lose their internal characteristics.
iiThe ancestors of the sh swam in the water before the
structures distinctive of a sh developed. The earliest sh
record of the rocks, perhaps 400 million years old, shows
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only the impression of muscles grouped as in a sh, nothing
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Solid about it.
But, it may he said, the worm is also a
muscular body _; how then could a
made ?__
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If we examine the description ofavvorm1ntext—books,
we nd that the muscles
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the form of circles found the body, theothe1rmmmg along
Ï the body;
,
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éhieÿ
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by the'digésŸÔ
;W1Ï1èngsîîch a ib,o_dy
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slip
:progress,… ; iIt '. Çänndÿt}
5biiigÏ "
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{s1oWfin spite
e0rts.
_' _,
_
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In
the space With the digestive tract
îoccupœs only the lower half of the body and even only a
part of the loWer half.
The rest_of_ the body .
_
,îîis:ÿall:boncî
? The d1gesvetractlsasüwere
Justasthebladesofaprepellœaxemchnedbaekwards
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Îeachmovement
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When no ,
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bone1spresen‘t, themuscles have qu1tea d1e1ent arrange'
ment_jfrom those of;a;wqrm,. In fact,_there is a very wide
î: ,î‘gap between them,and if_shes
from, the…WorniS,as
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Î
.
many suppose;_fs‘everal important Changes
structure had
before the
could
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there .is —Vefyf little? appearance ôf
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