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78
‘
THE BIOLOGY OF FISHES
as if a globule of air or gas had been swallowed and stuck
there. In some forms, Pipe—shes for example, it swells
up almost at once into a large, thin—walled sac ; in others,,
like the Herring, the nger pushes backwards above the gut
for a short distance, then appears
to bend round or send
forward a branch, which gradually works its “way onward to
_Î
become connected With the statocyst (“ hearing
”
organ) in
.:
the head.
Its progress is rendered easy in these forms-by
the numerous lymph spaces. The posterior end meanwhile
'
expands backvvards—ùntil it occupies the wh01e. Space aboVe
'
the bind portion of the gut.
‘
Some interesting facts With regard to the development
and fate of the air—bladder have recently been discovered.
Moser has described how the digestive tract twists round
during , the development, so that the position of the duct
”
roundWith the gut.
In“?tlë Way we_zeanj__
.
Understand— how the ;duct disappears in the specialisedr f
where the gut shows a varying amount
fï
_
.
of twistirig-" In the Pipe—sh Tracy found that when the
£:
swelling
a certain length, the duct
'
%
”
its'hinder
the—digestive-traLct shrivelled up and
'
_
many, of the ClupeidS’ the. air—bladder
ôbtains Ïa‘ secOnd opening
?to
the
‘,Êg_S;afety valve,”as-de Beaufort
silä"t0 thebottbm and come under greatef’pressurŒ
ÆfÏÏ
,
*
4ORIGINŒ oss'xFä' -—SÎRÙCÎÜRËS
"
-
1
Îif
: Thepnc1palfeature1nthe
1 stages: of the
=
_
'
its
:
—
_— formatwnwehavetabormw from What:is known—regarding
;
_
the pà"stghi‘sæory
‘
?. ofshes
:i
;
*
'
l
—
"
it” was: probably
,
begining
Skull;
But _'
f evena WOÏm1$ à .-1iVing
bloodand 1uàç1e's' and
.
..