92
THE BIOLOGY OF FISHES
'
body which fuse together—except—‘in the case of abnormali"
ties where
nature of the ns. can be seen ; in the
-
paired ns the buds are most usually separated-from each
other and p'ressed out to the side. In
cases the
marginal skin-fold is also continued ,fo'rward under the
,
but this. seems to be a secondary condition, 6
f
due to the weak development of the ribs. As the latter grow
they usually reach down to near the ventral margin, and the
space forthe paired ns is thus restricted.
,
=The'possibility of ns developing anywhere along the
abdominal margin is shown, however, by what we nd in
‘
a number of cases.
It has been mentioned, that the muscle
buds when they get the chance form cartilage, and in many
of the Teleosts We nd cartilaginous bars along the ventral
'
margin of— the abdomeh,
These are most developed in
some Flat—shes where they extend along the whole distance
from the shoulder-girdle to the”anus. Then in forms like
Atheriria, and the Mugils the“ middle p0rtion
is ;ävell
€Gastrosteus, the Plectog“naths,{and
theancient fthrodirans (Fig. 48) , the bar extends from the
_
shoulder-girdle a long way back under the abdominal
_
cavity. In…
there is a imt’eworthy marginal n (or
fùsed Ventral ns) just behind the masSive, fused bars."
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These variations can be underst00d from the following
‘!considerations. … The[ muscle buds wh1chgo
the
'1car‘tilaginous bases— of'the paired ns are'preSSedgout'into the
-
'
_-;skin‘ ormembrane along the
,
is :no sp_ace{=f—orïthëm andthey come
abdôñal muscles and
girib‘s_andtheskin.£ » Conéeè1Uentlÿ;thèÿ have a better chance
,
of developing when
are but -feeble, as in the
=Fla‘t'—éhes and
forms 1menaäe—a ' But-ÿWheœ the
PmuSCles
ns may not be able
_
to
the
.
.
”ha-vé sùe'rè‘d
”But 1n some of the Eels and
-
_
Elatfshes the pkaCtorals‘mày be
even after they
_
have'begun
ï
'
a
..
'
'
'
7Ï;
These
along the ,Ventral margin