REGULATIONOF FORM AND
89
in length and strength, the contihuitÿ is brol£en
and only isolated n-letsare able to persist. : In the Salmon
dorsal n has its» growth checked fand
adipose n. Ï '
_
,
’
Ï
Thè’Changing: modes of moveme'ntarcä
order in which the ns. appear. Thé caùdaPn is the
then among the Teleosts the second dorsal
together. But the cormon Clupeids
in that the rst dorsal developë before the anal
the caudal is still heterocercal (Fig. 20,' I). This’is "the ‘
condition seen“ in the earlièst fossils of shes, and it isŸ'also
present to sonîe extent in the Sturgeon(Fig. zo, 2). But
.
in the
and more especially in the Elasmobranchs
20, 3), we see that the body tends to
out,
'
oWig to the fact that it is not actiVely engaged in swimming,
and the ventral ns (V) thus appear early.
'
the movements are comparatively slow and
Sinuous, the dorsal and anal are long and develop
in the Flat—shæ and BlennieS. If
rapid, these
ns Îb1‘eàk'üp into parts Which“ dèvelOp separately
at a number of nodes and then gradually extend their bases :
‘
they meet. The Chimæroids and Dipnoi have also
_
_
retamedthis:jsiñuous form of movement, and if we draw
>
=
linefromthe;niduthialong the middle of the body—tb;thè
centre of the’it‘ailgÏWe can see the Condition that diaterrr—1ies]
it.
ThepartSjabdve and below this liñeare equal or approXië
mately so, in
gravity lies in the
middle line (cf.
;
.
’'_
.
:'
_
If now We”extend the'series of diverse forms, We— can
.s__ce
Shape Vof the sh and its structures change
5
_
hneInthe
the ceh*ëf of gavi‘tÿ ‘is£'bde
andtheanaln develops
‘
the‘dërSäl —nlreinain‘s
,
In '— the
PercheS, etc.) the :cénti:ebf grav1ty dies
above
the middle—line
'
"
whi‘lst the dor—Sal” developsalongthegreatergÏportionïof1the
7