ECONOMY OF THE BODY—I
”137
kidneys. Since the renal portal system has become an
integral part of the organisation of the Higher VertebrateS,
it is not possible to explain the condition in Teleosts otherè
'
Wise than 28 a primitivefeature, Adaptation to any par—
ticular mode of life. it Cannot
it and
…
some have not, under the_same conditions.?
‘.
,
We can understand the change better :in'
way.
The rst attempt to form a kidney, anterioÿ, intérê
,
fered With the numerous blood—vessels that asèerble .juSt
under the shoulder—girdle and the skull posteriorly. At any
'
rate,-these get mixed up with the kidney tubules and in'soine
?
cases
dissolve the latter;
second set of tubules then
deVelops further back about the middle of the body. The
'
Teleosts represent various stages in the development of these
kidneys. As the posterior kidney grows in size, backwards
and downwards, it comes to envelop more of the caudal
veins, .also
__
found among the Teleosts, until there is a
,
complete fusion or interchange, as in the Elasmobranchs
and
In Higher
,
Vertebrates. a
third kidney is .
'
5
]
formed, still further»back in the b0dy, but this metanephros
'
_
does notappear
_.
,
,
:
_
.
Lastly,referencemay;beïmade to the rejprodubtive
only for the
'
and equipment of
but, also f0r producing internal
,
secretions
-
They have no organic connection
With the kidneys, but in the course of evolution
have become mixed up together. The Teleosts again repre—
sent the early condition.
The organs are developed in
pouches of the abdominal cavity, separated from the kidneys
-
by the peritoneum and air-bladder. The ducts from each
side unite to form a single external opening, quite distinct
,
from the urinary duct, in the Clupeids (Fig. 3 1). In the ,EelS,
.
Salmon, and many others the ducts do not develop,the genital
'
'
products simply drop into the abdominal cavity
‘
by the abdominal pores. As the
Ï
With greater restriction of the space_within, the génital duCt$j ; ‘ Ç
'
unite near their terminàtionwith
single urinogem'tal opening,_as in