124
THE"BIOLÔGY'OF FISHES
;
,
external openings inthe Herring could be reduced to three,
and this in different ways. In one caSe the air—bladder“
and the genital ducts united at Some distance within the
body, in another there was a common urinogenital opening.
Ridewood (1898) found that the air-bladder did not open
.
'
to the exterior in the Twaite Shad (Clupea nta).
These and other variations are syinpt01atic of the '
position of the Clupeids. The Withdrawal of the rectum
"
within the body, which we-can see beginning in these forms, '
has'led to the formation of a denite cloaca in the=Sharks,
Rays, and Dipnoi. The persistence of a separate rectal
opening, butfusion of the kidney and genital ducts into a
comm0n 'urinogenital opening, is a characteristic of the
,
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The folds
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In many Teleost's,=
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other hand, the twisting and
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begin further forward in the pyloric
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region,- and coils or windings are
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