THE "GENEALOGY OF FISHES
213
'
forms which have remained pelagic and free—swimming, such
'
as the Acanthodidæ and the primitive “.sharks.”
…
On the:
other hand, to broade'r forms tending to be, more sedentary'
witha strong development of the c‘alcication. in the anterior
region and paddle-likè paired ns, as inthe ancient Ganoids
‘and Dipteridæ;
'
‘
,
'
.
,
»
,
‘
The ossication of the notochordal sheath…(€ndostÿle‘)
seen in“ Palæospondylus and the larvalCluPeid hashot, beer-1
_
maintained in
majority of the early shes. Instead,
the cartilaginous arches from above‘and below havemore or
’
completely invested the sheath and
.it;
In
the
and Chimæfoids the notoch0rd;
persists_Withçthe arches
upon it above and below ;
in the other
the notochord is completely
inveSted by a complicated system of cartilages, which may
-
often show calcied rings internally. In the Teleosts the.
endostylic sheath “may persist, or the cartilaginous bases of
the arches may grow round and replace it, becoming ossied
later.
'
—
"
ï.
‘
The” gills and gill—arches
the earliest' shes:Were°
.
formed inside thei'skin from-the lower half of themüscular
‘
segmentsunder the -crànium and had an external cover Which
.
'
—
of Calcareous deposits, like the
the head andatèrior part of the
The_re may .
have been sèveral’openings, as apparentlyih
'
'
usually there was only one With a Well-developed…opercular'
'
cover over it.
In some pelagic
hoWever‘, which did
—
not acquire the strong head—calcication of the sedentary
forms, the gill-cover became shorter as the gill-arches
Out tothe open, as in Cladoselache (Fig. 47). In
«
soÏne Teleosts at the present day this uncovering of the gills
through
of the arches is notinfrequen‘c;
’
_
Ï and Tornier has been able to produce the condition experiÿ; '
,
metally. The retreat of the arches backWardsfromthe
:,
?
head and the develOpment of a
skin can-be follOwed in
Cladose—
-
'
lache down
Butlthasto
. î _
'
'
that the--Chiinærdidæ WhlCh
213
'
forms which have remained pelagic and free—swimming, such
'
as the Acanthodidæ and the primitive “.sharks.”
…
On the:
other hand, to broade'r forms tending to be, more sedentary'
witha strong development of the c‘alcication. in the anterior
region and paddle-likè paired ns, as inthe ancient Ganoids
‘and Dipteridæ;
'
‘
,
'
.
,
»
,
‘
The ossication of the notochordal sheath…(€ndostÿle‘)
seen in“ Palæospondylus and the larvalCluPeid hashot, beer-1
_
maintained in
majority of the early shes. Instead,
the cartilaginous arches from above‘and below havemore or
’
completely invested the sheath and
.it;
In
the
and Chimæfoids the notoch0rd;
persists_Withçthe arches
upon it above and below ;
in the other
the notochord is completely
inveSted by a complicated system of cartilages, which may
-
often show calcied rings internally. In the Teleosts the.
endostylic sheath “may persist, or the cartilaginous bases of
the arches may grow round and replace it, becoming ossied
later.
'
—
"
ï.
‘
The” gills and gill—arches
the earliest' shes:Were°
.
formed inside thei'skin from-the lower half of themüscular
‘
segmentsunder the -crànium and had an external cover Which
.
'
—
of Calcareous deposits, like the
the head andatèrior part of the
The_re may .
have been sèveral’openings, as apparentlyih
'
'
usually there was only one With a Well-developed…opercular'
'
cover over it.
In some pelagic
hoWever‘, which did
—
not acquire the strong head—calcication of the sedentary
forms, the gill-cover became shorter as the gill-arches
Out tothe open, as in Cladoselache (Fig. 47). In
«
soÏne Teleosts at the present day this uncovering of the gills
through
of the arches is notinfrequen‘c;
’
_
Ï and Tornier has been able to produce the condition experiÿ; '
,
metally. The retreat of the arches backWardsfromthe
:,
?
head and the develOpment of a
skin can-be follOwed in
Cladose—
-
'
lache down
Butlthasto
. î _
'
'
that the--Chiinærdidæ WhlCh